JDBC Interview Questions With Answers Page I

7/30/2019 JDBC Interview Questions With Answers Page I

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JDBC Interview Questions with Answers

Page I

1. What is JDBC?

JDBC may stand for Java Database Connectivity. It is also a trade mark. JDBC is a layer ofabstraction that allows users to choose between databases. It

allows you to change to a different database engine and to write to a single API. JDBC allows

you to write database applications in Java without having to

concern yourself with the underlying details of a particular database.

2. What's the JDBC 3.0 API?

The JDBC 3.0 API is the latest update of the JDBC API. It contains many features, includingscrollable result sets and the SQL:1999 data types.

JDBC (Java Database Connectivity) is the standard for communication between a Java

application and a relational database. The JDBC API is released in two

versions; JDBC version 1.22 (released with JDK 1.1.X in package java.sql) and version 2.0

(released with Java platform 2 in packages java.sql and

javax.sql). It is a simple and powerful largely database-independent way of extracting andinserting data to or from any database.

3. Does the JDBC-ODBC Bridge support the new features in the JDBC 3.0 API?

The JDBC-ODBC Bridge provides a limited subset of the JDBC 3.0 API.

4. Can the JDBC-ODBC Bridge be used with applets?

Use of the JDBC-ODBC bridge from an untrusted applet running in a browser, such as NetscapeNavigator, isn't allowed. The JDBC-ODBC bridge doesn't allow

untrusted code to call it for security reasons. This is good because it means that an untrusted

applet that is downloaded by the browser can't circumvent

Java security by calling ODBC. Remember that ODBC is native code, so once ODBC is called

the Java programming language can't guarantee that a security


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violation won't occur. On the other hand, Pure Java JDBC drivers work well with applets. They

are fully downloadable and do not require any client-side

configuration.

Finally, we would like to note that it is possible to use the JDBC-ODBC bridge with applets thatwill be run in appletviewer since appletviewer assumes

that applets are trusted. In general, it is dangerous to turn applet security off, but it may be

appropriate in certain controlled situations, such as

for applets that will only be used in a secure intranet environment. Remember to exercise caution

if you choose this option, and use an all-Java JDBC

driver whenever possible to avoid security problems.

5. How do I start debugging problems related to the JDBC API?

A good way to find out what JDBC calls are doing is to enable JDBC tracing. The JDBC trace

contains a detailed listing of the activity occurring in the

system that is related to JDBC operations.

If you use the DriverManager facility to establish your database connection, you use the

DriverManager.setLogWriter method to enable tracing of JDBC

operations. If you use a DataSource object to get a connection, you use theDataSource.setLogWriter method to enable tracing. (For pooled connections,

you use the ConnectionPoolDataSource.setLogWriter method, and for connections that can

participate in distributed transactions, you use the

XADataSource.setLogWriter method.)

6. What is new in JDBC 2.0?

With the JDBC 2.0 API, you will be able to do the following:

Scroll forward and backward in a result set or move to a specific row(TYPE_SCROLL_SENSITIVE,previous(), last(), absolute(), relative(), etc.)

Make updates to database tables using methods in the Java programming languageinstead of using SQL commands.(updateRow(), insertRow(), deleteRow(),


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etc.)

Send multiple SQL statements to the database as a unit, or batch (addBatch(),executeBatch())

Use the new SQL3 datatypes as column values like Blob, Clob, Array, Struct, Ref.

7. How to move the cursor in scrollable resultsets?(new feature in JDBC 2.0)

a. create a scrollable ResultSet object.

Statement stmt = con.createStatement

(ResultSet.TYPE_SCROLL_SENSITIVE,

ResultSet.CONCUR_READ_ONLY);

ResultSet srs = stmt.executeQuery("SELECT COLUMN_1,

COLUMN_2 FROM TABLE_NAME");

b. use a built in methods like afterLast(), previous(), beforeFirst(), etc. to scroll the resultset.

srs.afterLast();

while (srs.previous()) {

String name = srs.getString("COLUMN_1");

float salary = srs.getFloat("COLUMN_2");

//...

c. to find a specific row, use absolute(), relative() methods.

srs.absolute(4); // cursor is on the fourth row

int rowNum = srs.getRow(); // rowNum should be 4

srs.relative(-3);


srs.relative(2);


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d. use isFirst(), isLast(), isBeforeFirst(), isAfterLast() methods to check boundary status.

8. How to update a resultset programmatically? (new feature in JDBC 2.0)

a. create a scrollable and updatable ResultSet object.

Statement stmt = con.createStatement

(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);

ResultSet uprs = stmt.executeQuery("SELECT COLUMN_1,

COLUMN_2 FROM TABLE_NAME");

b. move the cursor to the specific position and use related method to update data and then, callupdateRow() method.

uprs.last();

uprs.updateFloat("COLUMN_2", 25.55);//update last row's data

uprs.updateRow();//don't miss this method, otherwise,

// the data will be lost.

9. How can I use the JDBC API to access a desktop database like Microsoft Access over the

network?

Most desktop databases currently require a JDBC solution that uses ODBC underneath. This is

because the vendors of these database products haven't

implemented all-Java JDBC drivers.

The best approach is to use a commercial JDBC driver that supports ODBC and the database you

want to use. See the JDBC drivers page for a list of

available JDBC drivers.

The JDBC-ODBC bridge from Sun's Java Software does not provide network access to desktop

databases by itself. The JDBC-ODBC bridge loads ODBC as a local


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DLL, and typical ODBC drivers for desktop databases like Access aren't networked. The JDBC-

ODBC bridge can be used together with the RMI-JDBC bridge,

however, to access a desktop database like Access over the net. This RMI-JDBC-ODBC solution

is free.

10. Are there any ODBC drivers that do not work with the JDBC-ODBC Bridge?

Most ODBC 2.0 drivers should work with the Bridge. Since there is some variation infunctionality between ODBC drivers, the functionality of the bridge

may be affected. The bridge works with popular PC databases, such as Microsoft Access andFoxPro.

11. What causes the "No suitable driver" error?

"No suitable driver" is an error that usually occurs during a call to theDriverManager.getConnection method. The cause can be failing to load the

appropriate JDBC drivers before calling the getConnection method, or it can be specifying an

invalid JDBC URL--one that isn't recognized by your JDBC

driver. Your best bet is to check the documentation for your JDBC driver or contact your JDBC

driver vendor if you suspect that the URL you are

specifying is not being recognized by your JDBC driver.

In addition, when you are using the JDBC-ODBC Bridge, this error can occur if one or more thethe shared libraries needed by the Bridge cannot be loaded.

If you think this is the cause, check your configuration to be sure that the shared libraries areaccessible to the Bridge.

12. Why isn't the java.sql.DriverManager class being found?

This problem can be caused by running a JDBC applet in a browser that supports the JDK 1.0.2,such as Netscape Navigator 3.0. The JDK 1.0.2 does not

contain the JDBC API, so the DriverManager class typically isn't found by the Java virtual

machine running in the browser.

Here's a solution that doesn't require any additional configuration of your web clients. Remember

that classes in the java.* packages cannot be


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downloaded by most browsers for security reasons. Because of this, many vendors of all-Java

JDBC drivers supply versions of the java.sql.* classes that

have been renamed to jdbc.sql.*, along with a version of their driver that uses these modified

classes. If you import jdbc.sql.* in your applet code

instead of java.sql.*, and add the jdbc.sql.* classes provided by your JDBC driver vendor to your

applet's codebase, then all of the JDBC classes needed

by the applet can be downloaded by the browser at run time, including the DriverManager class.

This solution will allow your applet to work in any client browser that supports the JDK 1.0.2.

Your applet will also work in browsers that support the

JDK 1.1, although you may want to switch to the JDK 1.1 classes for performance reasons. Also,

keep in mind that the solution outlined here is just an

example and that other solutions are possible.

13. How to insert and delete a row programmatically? (new feature in JDBC 2.0)

Make sure the resultset is updatable.

1. move the cursor to the specific position.

uprs.moveToCurrentRow();

2. set value for each column.

uprs.moveToInsertRow();//to set up for insert

uprs.updateString("col1" "strvalue");

uprs.updateInt("col2", 5);

...

3. call inserRow() method to finish

the row insert process.

uprs.insertRow();

To delete a row: move to the specific


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position and call deleteRow() method:

uprs.absolute(5);

uprs.deleteRow();//delete row 5

To see the changes call refreshRow();

uprs.refreshRow();

14. What are the two major components of JDBC?

One implementation interface for database manufacturers, the other implementation interface for

application and applet writers.

15. What is JDBC Driver interface?

The JDBC Driver interface provides vendor-specific implementations of the abstract classes

provided by the JDBC API. Each vendor driver must provide

implementations of the java.sql.Connection,Statement,PreparedStatement, CallableStatement,ResultSet and Driver.

16. How do I retrieve a whole row of data at once, instead of calling an individual

ResultSet.getXXX method for each column?

The ResultSet.getXXX methods are the only way to retrieve data from a ResultSet object, which

means that you have to make a method call for each column

of a row. It is unlikely that this is the cause of a performance problem, however, because it is

difficult to see how a column could be fetched without

at least the cost of a function call in any scenario. We welcome input from developers on this

issue.

17. What are the common tasks of JDBC?

Create an instance of a JDBC driver or load JDBC drivers through jdbc.drivers

Register a driver

Specify a database


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Open a database connection

Submit a query

Receive results

Process results

18. Why does the ODBC driver manager return 'Data source name not found and no default

driver specified Vendor: 0'

This type of error occurs during an attempt to connect to a database with the bridge. First, note

that the error is coming from the ODBC driver manager.

This indicates that the bridge-which is a normal ODBC client-has successfully called ODBC, so

the problem isn't due to native libraries not being

present. In this case, it appears that the error is due to the fact that an ODBC DSN (data source

name) needs to be configured on the client machine.

Developers often forget to do this, thinking that the bridge will magically find the DSN they

configured on their remote server machine

19. How to use JDBC to connect Microsoft Access?

There is a specific tutorial at javacamp.org. Check it out.

20. What are four types of JDBC driver?

Type 1 Drivers

Bridge drivers such as the jdbc-odbc bridge. They rely on an intermediary such as ODBC totransfer the SQL calls to the database and also often rely on

native code. It is not a serious solution for an application

Type 2 Drivers

Use the existing database API to communicate with the database on the client. Faster than Type1, but need native code and require additional permissions

to work in an applet. Client machine requires software to run.


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Type 3 Drivers

JDBC-Net pure Java driver. It translates JDBC calls to a DBMS-independent network protocol,

which is then translated to a DBMS protocol by a server.

Flexible. Pure Java and no native code.

Type 4 Drivers

Native-protocol pure Java driver. It converts JDBC calls directly into the network protocol used

by DBMSs. This allows a direct call from the client

machine to the DBMS server. It doesn't need any special native code on the client machine.

Recommended by Sun's tutorial, driver type 1 and 2 are interim solutions where direct pure Javadrivers are not yet available. Driver type 3 and 4 are

the preferred way to access databases using the JDBC API, because they offer all the advantagesof Java technology, including automatic installation. For

more info, visit Sun JDBC page

21. Which type of JDBC driver is the fastest one?

JDBC Net pure Java driver(Type IV) is the fastest driver because it converts the jdbc calls intovendor specific protocol calls and it directly interacts

with the database.

22. Are all the required JDBC drivers to establish connectivity to my database part of the JDK?

No. There aren't any JDBC technology-enabled drivers bundled with the JDK 1.1.x or Java 2

Platform releases other than the JDBC-ODBC Bridge. So,

developers need to get a driver and install it before they can connect to a database. We are

considering bundling JDBC technology- enabled drivers in the

future.

23. Is the JDBC-ODBC Bridge multi-threaded?

No. The JDBC-ODBC Bridge does not support concurrent access from different threads. The

JDBC-ODBC Bridge uses synchronized methods to serialize all of


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the calls that it makes to ODBC. Multi-threaded Java programs may use the Bridge, but they

won't get the advantages of multi-threading. In addition,

deadlocks can occur between locks held in the database and the semaphore used by the Bridge.

We are thinking about removing the synchronized methods in

the future. They were added originally to make things simple for folks writing Java programs

that use a single-threaded ODBC driver.

24. Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection?

No. You can open only one Statement object per connection when you are using the JDBC-ODBC Bridge.

25. What is the query used to display all tables names in SQL Server (Query analyzer)?

select * from information_schema.tables

1. Why can't I invoke the ResultSet methods afterLast and beforeFirst when the method nextworks?

You are probably using a driver implemented for the JDBC 1.0 API. You need to upgrade to aJDBC 2.0 driver that implements scrollable result sets. Also be

sure that your code has created scrollable result sets and that the DBMS you are using supports

them.

2. How can I retrieve a String or other object type without creating a new object each time?

Creating and garbage collecting potentially large numbers of objects (millions) unnecessarily canreally hurt performance. It may be better to provide a

way to retrieve data like strings using the JDBC API without always allocating a new object.We

are studying this issue to see if it is an area in which the

JDBC API should be improved. Stay tuned, and please send us any comments you have on thisquestion.

3. How many types of JDBC Drivers are present and what are they?

There are 4 types of JDBC Drivers


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Type 1: JDBC-ODBC Bridge Driver

Type 2: Native API Partly Java Driver

Type 3: Network protocol Driver

Type 4: JDBC Net pure Java Driver

4. What is the fastest type of JDBC driver?

JDBC driver performance will depend on a number of issues:

(a) the quality of the driver code,

(b) the size of the driver code,

(c) the database server and its load,

(d) network topology,

(e) the number of times your request is translated to a different API.

In general, all things being equal, you can assume that the more your request and response

change hands, the slower it will be. This means that Type 1

and Type 3 drivers will be slower than Type 2 drivers (the database calls are make at least three

translations versus two), and Type 4 drivers are the

fastest (only one translation).

5. There is a method getColumnCount in the JDBC API. Is there a similar method to find the

number of rows in a result set?

No, but it is easy to find the number of rows. If you are using a scrollable result set, rs, you can

call the methods rs.last and then rs.getRow to find

out how many rows rs has. If the result is not scrollable, you can either count the rows byiterating through the result set or get the number of rows by

submitting a query with a COUNT column in the SELECT clause.

6. I would like to download the JDBC-ODBC Bridge for the Java 2 SDK, Standard Edition(formerly JDK 1.2). I'm a beginner with the JDBC API, and I


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would like to start with the Bridge. How do I do it?

The JDBC-ODBC Bridge is bundled with the Java 2 SDK, Standard Edition, so there is no need

to download it separately.

7. If I use the JDBC API, do I have to use ODBC underneath?

No, this is just one of many possible solutions. We recommend using a pure Java JDBC

technology-enabled driver, type 3 or 4, in order to get all of the

benefits of the Java programming language and the JDBC API.

8. Once I have the Java 2 SDK, Standard Edition, from Sun, what else do I need to connect to a

database?

You still need to get and install a JDBC technology-enabled driver that supports the database that

you are using. There are many drivers available from a

variety of sources. You can also try using the JDBC-ODBC Bridge if you have ODBCconnectivity set up already. The Bridge comes with the Java 2 SDK,

Standard Edition, and Enterprise Edition, and it doesn't require any extra setup itself. The Bridgeis a normal ODBC client. Note, however, that you

should use the JDBC-ODBC Bridge only for experimental prototyping or when you have no

other driver available.

9. What is the best way to generate a universally unique object ID? Do I need to use an external

resource like a file or database, or can I do it

all in memory?

1: Unique down to the millisecond. Digits 1-8 are are the hex encoded lower 32 bits of the

System.currentTimeMillis() call.

2: Unique across a cluster. Digits 9-16 are the encoded representation of the 32 bit integer of theunderlying IP address.

3: Unique down to the object in a JVM. Digits 17-24 are the hex representation of the call to

System.identityHashCode(), which is guaranteed to return

distinct integers for distinct objects within a JVM.


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4: Unique within an object within a millisecond. Finally digits 25-32 represent a random 32 bit

integer generated on every method call using the

cryptographically strong java.security.SecureRandom class.

Answer1

There are two reasons to use the random number instead of incrementing your last. 1. The

number would be predictable and, depending on what this is used

for, you could be opening up a potential security issue. This is why ProcessIDs are randomized

on some OSes (AIX for one). 2. You must synchronize on

that counter to guarantee that your number isn't reused. Your random number generator need not

be synchronized, (though its implementation may be).

Answer2

1) If your using Oracle You can create a sequence ,by which you can generate unique primarykey or universal primary key. 2) you can generate by using

random numbers but you may have to check the range and check for unique id. ie randomnumber generate 0.0 to 1.0 u may have to make some logic which

suits your unique id 3) Set the maximum value into an XML file and read that file at the time ofloading your application from xml .

10. Whan happens when I close a Connection application obtained from a connection Pool? How

does a connection pool maintain the Connections that I

had closed through the application?

Answer1

It is the magic of polymorphism, and of Java interface vs. implementation types. Two objects canboth be "instanceof" the same interface type, even

though they are not of the same implementation type.


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When you call "getConnection()" on a pooled connection cache manager object, you get a

"logical" connection, something which implements the

java.sql.Connection interface.

But it is not the same implementation type as you would get for your Connection, if you directlycalled getConnection() from a (non-pooled/non-cached)

datasource.

So the "close()" that you invoke on the "logical" Connection is not the same "close()" method as

the one on the actual underlying "physical" connection

hidden by the pool cache manager.

The close() method of the "logical" connection object, while it satisfies the method signature of

close() in the java.sql.Connection interface, does not

actually close the underlying physical connection.

Answer2

Typically a connection pool keeps the active/in-use connections in a hashtable or other

Collection mechanism. I've seen some that use one stack for

ready-for-use, one stack for in-use.

When close() is called, whatever the mechanism for indicating inuse/ready-for-use, thatconnection is either returned to the pool for ready-for-use or

else physically closed. Connections pools should have a minimum number of connections open.Any that are closing where the minimum are already available

should be physically closed.

Some connection pools periodically test their connections to see if queries work on the ready-for-

use connections or they may test that on the close()

method before returning to the ready-for-use pool.

11. How can I know when I reach the last record in a table, since JDBC doesn't provide an EOFmethod?

Answer1


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You can use last() method of java.sql.ResultSet, if you make it scrollable.

You can also use isLast() as you are reading the ResultSet.

One thing to keep in mind, though, is that both methods tell you that you have reached the end of

the current ResultSet, not necessarily the end of the

table. SQL and RDBMSes make no guarantees about the order of rows, even from sequential

SELECTs, unless you specifically use ORDER BY. Even then, that

doesn't necessarily tell you the order of data in the table.

Answer2

Assuming you mean ResultSet instead of Table, the usual idiom for iterating over a forward only

resultset is:

ResultSet rs=statement.executeQuery(...);

while (rs.next()) {

// Manipulate row here

}

12. Where can I find info, frameworks and example source for writing a JDBC driver?

There a several drivers with source available, like MM.MySQL, SimpleText Database, FreeTDS,and RmiJdbc. There is at least one free framework, the

jxDBCon-Open Source JDBC driver framework. Any driver writer should also review ForDriver Writers.

13. How can I create a custom RowSetMetaData object from scratch?

One unfortunate aspect of RowSetMetaData for custom versions is that it is an interface. Thismeans that implementations almost have to be proprietary.

The JDBC RowSet package is the most commonly available and offers thesun.jdbc.rowset.RowSetMetaDataImpl class. After instantiation, any of the


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RowSetMetaData setter methods may be used. The bare minimum needed for a RowSet to

function is to set the Column Count for a row and the Column Types for

each column in the row. For a working code example that includes a custom RowSetMetaData,

14. How does a custom RowSetReader get called from a CachedRowSet?

The Reader must be registered with the CachedRowSet using

CachedRowSet.setReader(javax.sql.RowSetReader reader). Once that is done, a call to

CachedRowSet.execute() will, among other things, invoke the readData method.

15. How do I implement a RowSetReader? I want to populate a CachedRowSet myself and the

documents specify that a RowSetReader should be used. The

single method accepts a RowSetInternal caller and returns void. What can I do in the readData

method?

"It can be implemented in a wide variety of ways..." and is pretty vague about what can actuallybe done. In general, readData() would obtain or create

the data to be loaded, then use CachedRowSet methods to do the actual loading. This wouldusually mean inserting rows, so the code would move to the

insert row, set the column data and insert rows. Then the cursor must be set to to the appropriate

position.

16. Can I set up a connection pool with multiple user IDs? The single ID we are forced to use

causes probelems when debugging the DBMS.

Since the Connection interface ( and the underlying DBMS ) requires a specific user and

password, there's not much of a way around this in a pool. While

you could create a different Connection for each user, most of the rationale for a pool would then

be gone. Debugging is only one of several issues that

arise when using pools.However, for debugging, at least a couple of other methods come to

mind. One is to log executed statements and times, which should

allow you to backtrack to the user. Another method that also maintains a trail of modifications is

to include user and timestamp as standard columns in

your tables. In this last case, you would collect a separate user value in your program.


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17. How can I protect my database password ? I'm writing a client-side java application that willaccess a database over the internet. I have

concerns about the security of the database passwords. The client will have access in one way or

another to the class files, where the connection string

to the database, including user and password, is stored in as plain text. What can I do to protectmy passwords?

This is a very common question.

Conclusion: JAD decompiles things easily and obfuscation would not help you. But you'd havethe same problem with C/C++ because the connect string would

still be visible in the executable.

SSL JDBC network drivers fix the password sniffing problem (in MySQL 4.0), but not the

decompile problem. If you have a servlet container on the web

server, I would go that route (see other discussion above) then you could at least keep peoplefrom reading/destroying your mysql database.

Make sure you use database security to limit that app user to the minimum tables that they need,then at least hackers will not be able to reconfigure

your DBMS engine.

Aside from encryption issues over the internet, it seems to me that it is bad practise to embed

user ID and password into program code. One could

generally see the text even without decompilation in almost any language. This would be

appropriate only to a read-only database meant to be open to the

world. Normally one would either force the user to enter the information or keep it in aproperties file.

18. Detecting Duplicate Keys I have a program that inserts rows in a table ...

Detecting Duplicate Keys I have a program that inserts rows in a table. My table has a column'Name' that has a unique constraint. If the user attempts

to insert a duplicate name into the table, I want to display an error message by processing theerror code from the database. How can I capture this


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error code in a Java program?

A solution that is perfectly portable to all databases, is to execute a query for checking if that

unique value is present before inserting the row. The

big advantage is that you can handle your error message in a very simple way, and the obvious

downside is that you are going to use more time for

inserting the record, but since you're working on a PK field, performance should not be so bad.

You can also get this information in a portable way, and potentially avoid another database

access, by capturing SQLState messages. Some databases get

more specific than others, but the general code portion is 23 - "Constraint Violations". UDB2, for

example, gives a specific such as 23505, while others

will only give 23000.

19. What driver should I use for scalable Oracle JDBC applications?

Sun recommends using the thin ( type 4 ) driver.

On single processor machines to avoid JNI overhead. On multiple processor machines, especially running Solaris, to avoid synchronization

bottlenecks.

20. Can you scroll a result set returned from a stored procedure?...

Can you scroll a result set returned from a stored procedure? I am returning a result set from a

stored procedure with type SQLRPGLE but once I reach the

end of the result set it does not allow repositioning. Is it possible to scroll this result set?

A CallableStatement is no different than other Statements in regard to whether related ResultSetsare scrollable. You should create the CallableStatement

using Connection.prepareCall(String sql, int resultSetType, int resultSetConcurrency).

21. How do I write Greek ( or other non-ASCII/8859-1 ) characters to a database?


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From the standard JDBC perspective, there is no difference between ASCII/8859-1 characters

and those above 255 ( hex FF ). The reason for that is that

all Java characters are in Unicode ( unless you perform/request special encoding ). Implicit in

that statement is the presumption that the datastore can

handle characters outside the hex FF range or interprets different character sets appropriately.

That means either:

The OS, application and database use the same code page and character set. For example,a Greek version of NT with the DBMS set to the default OS

encoding.

The DBMS has I18N support for Greek ( or other language ), regardless of OS encoding.This has been the most common for production quality databases,

although support varies. Particular DBMSes may allow setting the encoding/code page/CCSID at

the database, table or even column level. There is no

particular standard for provided support or methods of setting the encoding. You have to check

the DBMS documentation and set up the table properly.

The DBMS has I18N support in the form of Unicode capability. This would handle anyUnicode characters and therefore any language defined in the Unicode

standard. Again, set up is proprietary.

22. How can I insert images into a Mysql database?

This code snippet shows the basics:

File file = new File(fPICTURE);

FileInputStream fis = new FileInputStream(file);

PreparedStatement ps =

ConrsIn.prepareStatement("insert into dbPICTURE values (?,?)");

// ***use as many ??? as you need to insert in the exact order***


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ps.setString(1,file.getName());

ps.setBinaryStream(2,fis,(int)file.length());

ps.close();

fis.close();

23. Is is possible to open a connection to a database with exclusive mode with JDBC?

I think you mean "lock a table in exclusive mode". You cannot open a connection with exclusivemode. Depending on your database engine, you can lock

tables or rows in exclusive mode.

In Oracle you would create a statement st and run

st.execute("lock table mytable in exclusive mode");

Then when you are finished with the table, execute the commit to unlock the table. Mysql,

informix and SQLServer all have a slightly different syntax for

this function, so you'll have to change it depending on your database. But they can all be done

with execute().

24. Update fails without blank padding. Alhough a particular row is present in the database for agiven key, executeUpdate() shows 0 rows updated

and, in fact, the table is not updated. If I pad the Key with spaces for the column length (eg if the

key column is 20 characters long, and key is msgID,

length 6, I pad it with 14 spaces), the update then works!!! Is there any solution to this problem

without padding?

In the SQL standard, CHAR is a fixed length data type. In many DBMSes ( but not all), that

means that for a WHERE clause to match, every character must

match, including size and trailing blanks. As Alessandro indicates, defining CHAR columns tobe VARCHAR is the most general answer.

25. What are the standard isolation levels defined by JDBC?


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The values are defined in the class java.sql.Connection and are:

TRANSACTION_NONE TRANSACTION_READ_COMMITTED TRANSACTION_READ_UNCOMMITTED TRANSACTION_REPEATABLE_READ TRANSACTION_SERIALIZABLE

1. What isolation level is used by the DBMS when inserting, updating and selecting rows from a

database?

The answer depends on both your code and the DBMS. If the program does not explicitly set the

isolation level, the DBMS default is used. You can determine

the default using DatabaseMetaData.getDefaultTransactionIsolation() and the level for the

current Connection with Connection.getTransactionIsolation(). If

the default is not appropriate for your transaction, change it with

Connection.setTransactionIsolation(int level).

2. How can I determine the isolation levels supported by my DBMS?

Use DatabaseMetaData.supportsTransactionIsolationLevel(int level).

3. Connecting to a database through the Proxy I want to connect to remote database using a

program that is running in the local network behind the

proxy. Is that possible?

I assume that your proxy is set to accept http requests only on port 80. If you want to have a local

class behind the proxy connect to the database for

you, then you need a servlet/JSP to receive an HTTP request and use the local class to connect tothe database and send the response back to the client.

You could also use RMI where your remote computer class that connects to the database acts as

a remote server that talks RMI with the clients. if you

implement this, then you will need to tunnel RMI through HTTP which is not that hard.In

summary, either have a servlet/JSP take HTTP requests, nstantiate


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a class that handles database connections and send HTTP response back to the client or have the

local class deployed as RMI server and send requests to

it using RMI.

4. How do I receive a ResultSet from a stored procedure?

Stored procedures can return a result parameter, which can be a result set. For a discussion of

standard JDBC syntax for dealing with result, IN, IN/OUT

and OUT parameters, see Stored Procedures.

5. How can I write to the log used by DriverManager and JDBC drivers?

The simplest method is to use DriverManager.println(String message), which will write to thecurrent log.

6. How can I get or redirect the log used by DriverManager and JDBC drivers?

As of JDBC 2.0, use DriverManager.getLogWriter() and

DriverManager.setLogWriter(PrintWriter out). Prior to JDBC 2.0, the DriverManager methods

getLogStream() and setLogStream(PrintStream out) were used. These are now deprecated.

7. What does it mean to "materialize" data?

This term generally refers to Array, Blob and Clob data which is referred to in the database via

SQL locators "Materializing" the data means to return

the actual data pointed to by the Locator.

For Arrays, use the various forms of getArray() and getResultSet().

For Blobs, use getBinaryStream() or getBytes(long pos, int length).

For Clobs, use getAsciiStream() or getCharacterStream().

8. Why do I have to reaccess the database for Array, Blob, and Clob data?

Most DBMS vendors have implemented these types via the SQL3 Locator type


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Some rationales for using Locators rather than directly returning the data can be seen most

clearly with the Blob type. By definition, a Blob is an

arbitrary set of binary data. It could be anything; the DBMS has no knowledge of what the data

represents. Notice that this effectively demolishes data

independence, because applications must now be aware of what the Blob data actually

represents. Let's assume an employee table that includes employee

images as Blobs.Say we have an inquiry program that presents multiple employees withdepartment and identification information. To see all of the data

for a specific employee, including the image, the summary row is selected and another screenappears. It is only at this pont that the application needs

the specific image. It would be very wasteful and time consuming to bring down an entire

employee page of images when only a few would ever be selected

in a given run.Now assume a general interactive SQL application. A query is issued against theemployee table. Because the image is a Blob, the

application has no idea what to do with the data, so why bring it down, killing performance along

the way, in a long running operation? Clearly this is

not helpful in those applications that need the data everytime, but these and other considerations

have made the most general sense to DBMS vendors

9. What is an SQL Locator?

A Locator is an SQL3 data type that acts as a logical pointer to data that resides on a database

server. Read "logical pointer" here as an identifier the

DBMS can use to locate and manipulate the data. A Locator allows some manipulation of the

data on the server. While the JDBC specification does not

directly address Locators, JDBC drivers typically use Locators under the covers to handle Array,

Blob, and Clob data types.

10. How do I set properties for a JDBC driver and where are the properties stored?

A JDBC driver may accept any number of properties to tune or optimize performance for the

specific driver. There is no standard, other than user and


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password, for what these properties should be. Therefore, the developer is dependent on the

driver documentation to automatically pass properties. For a

standard dynamic method that can be used to solicit user input for properties, see What

properties should I supply to a database driver in order to

connect to a database?In addition, a driver may specify its own method of accepting properties.

Many do this via appending the property to the JDBC

Database URL. However, a JDBC Compliant driver should implement the connect(String url,Properties info) method. This is generally invoked through

DriverManager.getConnection(String url, Properties info).

11. What is the JDBC syntax for using a literal or variable in a standard Statement?

First, it should be pointed out that PreparedStatement handles many issues for the developer and

normally should be preferred over a standard Statement.

Otherwise, the JDBC syntax is really the same as SQL syntax. One problem that often affectsnewbies ( and others ) is that SQL, like many languages,

requires quotes around character ( read "String" for Java ) values to distinguish from numerics.So the clause:

"WHERE myCol = " + myVal

is perfectly valid and works for numerics, but will fail when myVal is a String. Instead use:

"WHERE myCol = '" + myVal + "'"

if myVal equals "stringValue", the clause works out to:

WHERE myCol = 'stringValue'

You can still encounter problems when quotes are embedded in the value, which, again, a

PreparedStatement will handle for you.

The passed properties are ( probably ) stored in variables in the Driver instance. This, again, is up

to the driver, but unless there is some sort of

driver setup, which is unusual, only default values are remembered over multiple instantiations.


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12. How do I check in my code whether a maximum limit of database connections have beenreached?

Use DatabaseMetaData.getMaxConnections() and compare to the number of connections

currently open. Note that a return value of zero can mean unlimited or,

unfortunately, unknown. Of course, driverManager.getConnection() will throw an exception if aConnection can not be obtained.

13. Why do I get UnsatisfiedLinkError when I try to use my JDBC driver?

The first thing is to be sure that this does not occur when running non-JDBC apps. If so, there is

a faulty JDK/JRE installation. If it happens only when

using JDBC, then it's time to check the documentation that came with the driver or thedriver/DBMS support. JDBC driver types 1 through 3 have some

native code aspect and typically require some sort of client install. Along with the install, various

environment variables and path or classpath

settings must be in place. Because the requirements and installation procedures vary with the

provider, there is no reasonable way to provide details

here. A type 4 driver, on the other hand, is pure Java and should never exhibit this problem. The

trade off is that a type 4 driver is usually slower.

14. DB2 Universal claims to support JDBC 2.0, But I can only get JDBC 1.0 functionality. Whatcan I do?

DB2 Universal defaults to the 1.0 driver. You have to run a special program to enable the 2.0driver and JDK support. For detailed information, see

Setting the Environment in Building Java Applets and Applications. The page includesinstructions for most supported platforms.

15. How to get a field's value with ResultSet.getxxx when it is a NULL? I have tried to execute a

typical SQL statement:

select * from T-name where (clause);

But an error gets thrown because there are some NULL fields in the table.


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You should not get an error/exception just because of null values in various columns. This

sounds like a driver specific problem and you should first

check the original and any chained exceptions to determine if another problem exists. In general,

one may retrieve one of three values for a column that

is null, depending on the data type. For methods that return objects, null will be returned; for

numerics ( get Byte(), getShort(), getInt(), getLong(),

getFloat(), and getDouble() ) zero will be returned; for getBoolean() false will be returned. Tofind out if the value was actually NULL, use

ResultSet.wasNull() before invoking another getXXX method.

16. How do I insert/update records with some of the columns having NULL value?

Use either of the following PreparedStatement methods:

public void setNull(int parameterIndex, int sqlType) throws SQLException

public void setNull(int paramIndex, int sqlType, String typeName) throws SQLException

These methods assume that the columns are nullable. In this case, you can also just omit the

columns in an INSERT statement; they will be automatically

assigned null values.

17. How do I disallow NULL values in a table?

Null capability is a column integrity constraint, normally aplied at table creation time. Note thatsome databases won't allow the constraint to be

applied after table creation. Most databases allow a default value for the column as well. Thefollowing SQL statement displays the NOT NULL constraint:

CREATE TABLE CoffeeTable (

Type VARCHAR(25) NOT NULL,

Pounds INTEGER NOT NULL,

Price NUMERIC(5, 2) NOT NULL


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18. Why can't Tomcat find my Oracle JDBC drivers in classes111.zip?

TOMCAT 4.0.1 on NT4 throws the following exception when I try to connect to Oracle DB

from JSP.

javax.servlet.ServletException : oracle.jdbc.driver.OracleDriver

java.lang.ClassNotFoundException: oracle:jdbc:driver:OracleDriver

But, the Oracle JDBC driver ZIP file (classes111.zip)is available in the system classpath.

Copied the Oracle Driver class file (classes111.zip) in %TOMCAT_HOME%\lib directory and

renamed it to classess111.jar.

Able to connect to Oracle DB from TOMCAT 4.01 via Oracle JDBC-Thin Driver.

19. I have an application that queries a database and retreives the results into a JTable ...

I have an application that queries a database and retreives the results into a JTable. This is thecode in the model that seems to be taken forever to

execute, especially for a large result set:

while ( myRs.next() ) {

Vector newRow =new Vector();

for ( int i=1;i


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newRow stores each row of the resultset and allRows stores all the rows. Are the vectors here the

problem?

Is there another way of dealing with the result set that could execute faster?

java.util.Vector is largely thread safe, which means that there is a greater overhead in calling

addElement() as it is a synchronized method. If your

result set is very large, and threading is not an issue, you could use one of the thread-unsafe

collections in Java 2 to save some time.

java.util.ArrayList is the likeliest candidate here.

Do not use a DefaultTableModel as it loads all of your data into memory at once, which will

obviously cause a large overhead - instead, use an

AbstractTableModel and provide an implementation which only loads data on demand, i.e. when

(if) the user scrolls down through the table.

20. Is there a way to find the primary key(s) for an Access Database table? Sun's JDBC-ODBC

driver does not implement the getPrimaryKeys() method

for the DatabaseMetaData Objects.

// Use meta.getIndexInfo() will

//get you the PK index. Once

// you know the index, retrieve its column name

DatabaseMetaData meta = con.getMetaData();

String key_colname = null;

// get the primary key information

rset = meta.getIndexInfo(null,null, table_name, true,true);


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while( rset.next())

{

String idx = rset.getString(6);

if( idx != null)

{

//Note: index "PrimaryKey" is Access DB specific

// other db server has diff. index syntax.

if( idx.equalsIgnoreCase("PrimaryKey")) {

key_colname = rset.getString(9);

setPrimaryKey( key_colname ); }

}

}

21. How does one get column names for rows returned in a ResultSet?

ResultSet rs = ...

ResultSetMetaData rsmd = rs.getMetaData();

int numCols = rsmd.getColumnCount();

for (int i = 1; i


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}

22. What are the considerations for deciding on transaction boundaries?

Transaction processing should always deal with more than one statement and a transaction isoften described as a Logical Unit of Work ( LUW ). The

rationale for transactions is that you want to know definitively that all or none of the LUW

completed successfully. Note that this automatically gives

you restart capability. Typically, there are two conditions under which you would want to use

transactions:

Multiple statements involving a single file - An example would be inserting all of a groupof rows or all price updates for a given date. You want all

of these to take effect at the same time; inserting or changing some subset is not acceptable.

Multiple statements involving multiple files - The classic example is transferring moneyfrom one account to another or double entry accounting; you

don't want the debit to succeed and the credit to fail because money or important records will be

lost. Another example is a master/detail relationship,

where, say, the master contains a total column. If the entire LUW, writing the detail row and

updating the master row, is not completed successfully, you

A) want to know that the transaction was unsuccessful and B) that a portion of the transaction

was not lost or dangling.

Therefore, determining what completes the transaction or LUW should be the deciding factor for

transaction boundaries.

23. How can I determine where a given table is referenced via foreign keys?

DatabaseMetaData.getExportedKeys() returns a ResultSet with data similar to that returned by

DatabaseMetaData.getImportedKeys(), except that the

information relates to other tables that reference the given table as a foreign key container.

24. How can I get information about foreign keys used in a table?


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DatabaseMetaData.getImportedKeys() returns a ResultSet with data about foreign key columns,

tables, sequence and update and delete rules.

25. Can I use JDBC to execute non-standard features that my DBMS provides?

The answer is a qualified yes. As discussed under SQL Conformance: "One way the JDBC API

deals with this problem is to allow any query string to be

passed through to an underlying DBMS driver. This means that an application is free to use asmuch SQL functionality as desired, but it runs the risk of

receiving an error on some DBMSs. In fact, an application query may be something other thanSQL, or it may be a specialized derivative of SQL designed

for specific DBMSs (for document or image queries, for example)." Clearly this means either

giving up portability or checking the DBMS curently used

before invoking specific operations.

1. What is DML?

DML is an abbreviation for Data Manipulation Language. This portion of the SQL standard is

concerned with manipulating the data in a database as opposed to

the structure of a database. The core verbs for DML are SELECT, INSERT, DELETE,

UPDATE, COMMIT and ROLLBACK.

2. What is the significance of DataBaseMetaData.tableIndexStatistics? How to obtain and use it?

To answer the second question first, the tableIndexStatistic constant in the TYPE column will

identify one of the rows in the ResultSet returned when

DatabaseMetaData.getIndexInfo() is invoked. If you analyze the wordy API, atableIndexStatistic row will contain the number of rows in the table in the

CARDINALITY column and the number of pages used for the table in the PAGES column.

3. What types of DataSource objects are specified in the Optional Package?

Basic - Provides a standard Connection object. Pooled - Provides a Connection pool and returns a Connection that is controlled by the

pool.


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Distributed - Provides a Connection that can participate in distributed transactions ( morethan one DBMS is involved). It is anticipated, but not

enforced, that a distributed DataSource will also provide pooling.

However, there are no standard methods available in the DataSource class to determine if onehas obtained a pooled and/or distributed Connection.

4. What is a JDBC 2.0 DataSource?

The DataSource class was introduced in the JDBC 2.0 Optional Package as an easier, more

generic means of obtaining a Connection. The actual driver

providing services is defined to the DataSource outside the application ( Of course, a production

quality app can and should provide this information

outside the app anyway, usually with properties files or ResourceBundles ). The documentation

expresses the view that DataSource will replace the common

DriverManager method.

5. Does the database server have to be running Java or have Java support in order for my remoteJDBC client app to access the database?

The answer should always be no. The two critical requirements are LAN/internet connectivity

and an appropriate JDBC driver. Connectivity is usually via

TCP/IP, but other communication protocols are possible. Unspoken, but assumed here is that theDBMS has been started to listen on a communications port.

It is the JDBC driver's job to convert the SQL statements and JDBC calls to the DBMS' nativeprotocol. From the server's point of view, it's just another

data request coming into the port, the programming language used to send the data is irrelevant

at that point.

6. Which Java and java.sql data types map to my specific database types?

JDBC is, of necessity, reliant on the driver and underlying DBMS. These do not always adhere

to standards as closely as we would like, including

differing names for standard Java types. To deal with this, first, there are a number of tables

available in the JDK JDBC documentation dealing with


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types.

7. When an SQL select statement doesn't return any rows, is an SQLException thrown?

No. If you want to throw an exception, you could wrap your SQL related code in a custom classand throw something like ObjectNotFoundException when the

returned ResultSet is empty.

8. Why should I consider optimistic versus pessimistic approaches to database updates?

In a modern database, possibly the two most important issues are data integrity and concurrency

( multiple users have access to and can update the data

). Either approach can be appropriate, depending on the application, but it is important to beaware of possible consequences to avoid being blindsided.

A pessimistic approach, with locks, is usually seen as good for data integrity, although it can be

bad for concurrency, especially the longer a lock is

held. In particular, it guarantees against 'lost updates' - defined as an update performed by one

process between the time of access and update by

another process, which overwrites the interim update. However, other users are blocked from

updating the data and possibly reading it as well if the read

access also tries to acquire a lock. A notorious problem can arise when a user accesses data for

update and then doesn't act on it for a period of time.

Another situation that occurred with one of my clients is that a batch ( non-interactive ) process

may need to update data while an interactive user has

an update lock on the same data. In that case, data integrity goes out the window and, dependingon how the application is written, more problems may be

introduced. ( No, we did not write the interactive update program and yes, we had recovery

procedures in place. )An optimstic approach can alleviate lock

concurrency problems, but requires more code and care for integrity. The "optimistic" definitionusually says that expectations of update clashes are

rare, but I view them as normal occurrances in a heavily used database. The basics are that anychanges between time of access and time of update must be


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detected and taken into account. This is often done by comparing timestamps, but one must be

sure that the timestamp is always changed for an update and,

of course, that the table contains a timestamp column. A more involved, but more complete

method involves saving the original columns and using them in

the 'Where' clause of the Update statement. If the update fails, the data has changed and the latest

data should be reaccessed.

9. What is optimistic concurrency?

An optimistic approach dispenses with locks ( except during the actual update ) and usuallyinvolves comparison of timestamps, or generations of data to

ensure that data hasn't changed between access and update times. It's generally explained that the

term optimistic is used because the expectation is

that a clash between multiple updates to the same data will seldom occur.

10. What is pessimistic concurrency?

With a pessimistic approach, locks are used to ensure that no users, other than the one who holdsthe lock, can update data. It's generally explained

that the term pessimistic is used because the expectation is that many users will try to update the

same data, so one is pessimistic that an update will

be able to complete properly. Locks may be acquired, depending on the DBMS vendor,automatically via the selected Isolation Level. Some vendors also

implement 'Select... for Update', which explicitly acquires a lock.

11. Can I get information about a ResultSet's associated Statement and Connection in a method

without having or adding specific arguments for the

Statement and Connection?

Yes. Use ResultSet.getStatement(). From the resulting Statement you can useStatement.getConnection().

12. What is the most efficient method of replicating data between databases using JDBC?


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Within Java, the most efficient method would be, opening connections using the JDBC and

inserting or updating the records from one database to the other

database, but it depends upon the databases being replicated. If you are using Oracle databases, it

has standard methods for replication, and you do not

need the JDBC for the replication. Use snapshots like updateable and read-only.

There are different kind of replication. Let us consider the most widely used ones:

A) One Master - One slave

I) If there is not a significant difference between the structure of the database tables, the

following method would be useful.

FromDatabase=A; ToDatabase=B

1) Open JDBC connections between the databases A and B.

2) Read a record (RA ) from A using an SQL query.

3) Store the values in the local variables in the Java program.

4) Insert the record in B if PK does not exist for the record RA in B.

5) If the PK exists in B, update the record in B.

6) Repeat the steps 2-5 'til all the records are read by the query.

7) If there are multiple tables to be replicated, repeat steps 2-7 using the different queries.

II)If there is significant difference between the structure of the database tables, the following

method would be useful.

FromDatabase=A; ToDatabase=B

1) Open the JDBC connections to the databases A.

2) Read a record ( RA ) from A using an SQL query.

3) Write the output to an XML file-XMLA, according to the DTD for the records for thedatabase A structure.

4) Repeat steps 2 & 3 'til all the records are written to XMLA.


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5) If there are more queries, repeat steps repeat steps from 2-4 and write the records to the

different entities in the XML file.

6) Transform the XMLA file using the XSL and XSLT to the format useful for the database B

and write to the XML file-XMLB.

7) Open the second JDBC connection to the Database B.

8) Read the XMLB file, one record at a time.

9) Insert the record in B if PK does not exist for the record RA in B.

10) If the PK exists in B, update the record in B.

B) One Master - Multiple slaves

The difference here is to open multiple JDBC connections to write to the different databases onerecord at a time.

C) Multiple Masters:

For multiple masters, use timestamps to compare the times of the records to find out which is the

latest record when a record is found in all the master

databases. Alternatively, create a column to store the time and date a record is inserted or

updated. When records are deleted, record the event in a log

file along with the PK.

Prepared statements and batch updates should be used wherever possible in this scenario.

13. How can I tell if my JDBC driver normalizes java.sql.Date and java.sql.Time objects?

To actually determine the values, the objects must be converted to a java.util.Date and examined.

See What does normalization mean for java.sql.Date and

java.sql.Time? for the definition of normalization. Notice that even a debugger will not show

whether these objects have been normalized, since the

getXXX methods in java.sql.Date for time elements and in java.sql.Time for date elements throw

an exception.

So, while a java.sql.Date may show 2001-07-26, it's normalized only if the java.util.Date value

is:


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Thu Jul 26 00:00:00 EDT 2001

and while a java.sql.Time may show 14:01:00, it's normalized only if the java.util.Date value is:

Thu Jan 01 14:01:00 EST 1970

14. What is the difference between setMaxRows(int) and SetFetchSize(int)? Can either reduce

processing time?

setFetchSize(int) defines the number of rows that will be read from the database when the

ResultSet needs more rows. The method in the java.sql.Statement

interface will set the 'default' value for all the ResultSet derived from that Statement; the method

in the java.sql.ResultSet interface will override

that value for a specific ResultSet. Since database fetches can be expensive in a networkedenvironment, fetch size has an impact on performance.

setMaxRows(int) sets the limit of the maximum nuber of rows in a ResultSet object. If this limit

is exceeded, the excess rows are "silently dropped".

That's all the API says, so the setMaxRows method may not help performance at all other than to

decrease memory usage. A value of 0 (default) means no

limit.Since we're talking about interfaces, be careful because the implementation of drivers is

often different from database to database and, in some

cases, may not be implemented or have a null implementation. Always refer to the driver

documentation.

15. What is JDO?

JDO provides for the transparent persistence of data in a data store agnostic manner, supportingobject, hierarchical, as well as relational stores.

16. When I intersperse table creation or other DDL statements with DML statements ....

When I intersperse table creation or other DDL statements with DML statements, I have aproblem with a transaction being commited before I want it to be.

Everything ( commit and rollback ) works fine as long as I don't create another table. How can Iresolve the issue?


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While the questioner found a partially workable method for his particular DBMS, as mentioned

in the section on transactions in my JDBC 2.0 Fundamentals

Short Course:

DDL statements in a transaction may be ignored or may cause a commit to occur. The behavioris DBMS dependent and can be discovered by use of

DatabaseMetaData.dataDefinitionCausesTransactionCommit() and

DatabaseMetaData.dataDefinitionIgnoredInTransactions(). One way to avoid unexpected results

is to separate DML and DDL transactions.The only generally effective way to "rollback" table

creation is to delete the table.

17. What's the best way, in terms of performance, to do multiple insert/update statements, a

PreparedStatement or Batch Updates?

Because PreparedStatement objects are precompiled, their execution can be faster than that ofStatement objects. Consequently, an SQL statement that is

executed many times is often created as a PreparedStatement object to increase efficiency.A

CallableStatement object provides a way to call stored

procedures in a standard manner for all DBMSes. Their execution can be faster than that of

PreparedStatement object.Batch updates are used when you want

to execute multiple statements together. Actually, there is no conflict here. While it depends onthe driver/DBMS engine as to whether or not you will

get an actual performance benefit from batch updates, Statement, PreparedStatement, and

CallableStatement can all execute the addBatch() method.

18. I need to have result set on a page where the user can sort on the column headers. Any ideas?

One possibility: Have an optional field in your form or GET url called (appropriately) ORDER

with a default value of either "no order" or whatever you

want your default ordering to be (i.e. timestamp, username, whatever). When you get your

request, see what the value of the ORDER element is. If it's

null or blank, use the default. Use that value to build your SQL query, and display the results to

the page. If you're caching data in your servlet, you


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can use the Collection framework to sort your data (see java.util.Collections) if you can get it

into a List format. Then, you can create a Collator

which can impose a total ordering on your results.

19. What are the components of the JDBC URL for Oracle's "thin" driver and how do I use

them?

Briefly: jdbc:oracle:thin:@hostname:port:oracle-sid

1. in green the Oracle sub-protocol (can be oracle:oci7:@, oracle:oci8:@, racle:thin:@, etc...) is

related on the driver you are unsign and the protocol

to communicate with server.

2. in red the network machine name, or its ip address, to locate the server where oracle isrunning.

3. in blue the port (it is complementary to the address to select the specific oracle service)

4. in magenta the sid, select on which database you want to connect.

example:

jdbc:oracle:thin:@MyOracleHost:1521:MyDB

IHere's an example:

jdbc:oracle:thin:scott/tiger@MyOracleHost:1521:MyDB

where user=scott and pass=tiger.

20. Why doesn't JDBC accept URLs instead of a URL string?

In order for something to be a java.net.URL, a protocol handler needs to be installed. Since thereis no one universal protocol for databases behind

JDBC, the URLs are treated as strings. In Java 1.4, these URL strings have a class calledjava.net.URI. However, you still can't use a URI to load a JDBC

driver, without converting it to a string.

21. What JDBC objects generate SQLWarnings?


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Connections, Statements and ResultSets all have a getWarnings method that allows retrieval.

Keep in mind that prior ResultSet warnings are cleared on

each new read and prior Statement warnings are cleared with each new execution. getWarnings()

itself does not clear existing warnings, but each object

has a clearWarnings method.

22. What's the fastest way to normalize a Time object?

Of the two recommended ways when using a Calendar( see How do I create a java.sql.Time

object? ), in my tests, this code ( where c is a Calendar and t is

a Time ):

c.set( Calendar.YEAR, 1970 );

c.set( Calendar.MONTH, Calendar.JANUARY );

c.set( Calendar.DATE, 1 );

c.set( Calendar.MILLISECOND, 0 );

t = new java.sql.Time( c.getTime().getTime() );

was always at least twice as fast as:

t = java.sql.Time.valueOf(

c.get(Calendar.HOUR_OF_DAY) + ":" +

c.get(Calendar.MINUTE) + ":" +

c.get(Calendar.SECOND) );

When the argument sent to valueOf() was hardcoded ( i.e. valueOf( "13:50:10" ), the time

difference over 1000 iterations was negligible.

23. What does normalization mean for java.sql.Date and java.sql.Time?


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These classes are thin wrappers extending java.util.Date, which has both date and time

components. java.sql.Date should carry only date information and a

normalized instance has the time information set to zeros. java.sql.Time should carry only time

information and a normalized instance has the date set to

the Java epoch ( January 1, 1970 ) and the milliseconds portion set to zero.

24. How do I create a java.sql.Date object?

java.sql.Date descends from java.util.Date, but uses only the year, month and day values. There

are two methods to create a Date object. The first uses a

Calendar object, setting the year, month and day portions to the desired values. The hour, minute,

second and millisecond values must be set to zero. At

that point, Calendar.getTime().getTime() is invoked to get the java.util.Date milliseconds. That

value is then passed to a java.sql.Date constructor:

Calendar cal = Calendar.getInstance();

// set Date portion to January 1, 1970

cal.set( cal.YEAR, 1970 );

cal.set( cal.MONTH, cal.JANUARY );

cal.set( cal.DATE, 1 );

cal.set( cal.HOUR_OF_DAY, 0 );

cal.set( cal.MINUTE, 0 );

cal.set( cal.SECOND, 0 );

cal.set( cal.MILLISECOND, 0 );

java.sql.Date jsqlD =

new java.sql.Date( cal.getTime().getTime() );


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The second method is java.sql.Date's valueOf method. valueOf() accepts a String, which must bethe date in JDBC time escape format - "yyyy-mm-dd". For

example,

25. How do I create a java.sql.Time object?

java.sql.Time descends from java.util.Date, but uses only the hour, minute and second values.There are two methods to create a Time object. The first

uses a Calendar object, setting the year, month and day portions to January 1, 1970, which isJava's zero epoch. The millisecond value must also be set

to zero. At that point, Calendar.getTime().getTime() is invoked to get the time in milliseconds.

That value is then passed to a Time constructor:

Calendar cal = Calendar.getInstance();

// set Date portion to January 1, 1970

cal.set( cal.YEAR, 1970 );

cal.set( cal.MONTH, cal.JANUARY );

cal.set( cal.DATE, 1 );

cal.set( cal.MILLISECOND, 0 );

java.sql.Time jsqlT =

new java.sql.Time( cal.getTime().getTime() );

The second method is Time's valueOf method. valueOf() accepts a String, which must be the

time in JDBC time escape format - "hh:mm:ss". For example,

java.sql.Time jsqlT = java.sql.Time.valueOf( "18:05:00" );

creates a Time object representing 6:05 p.m. To use this method with a Calendar object, use:

java.sql.Time jsqlT = java.sql.Time.valueOf(


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cal.get(cal.HOUR_OF_DAY) + ":" +

cal.get(cal.MINUTE) + ":" +

cal.get(cal.SECOND) );

which produces a Time object with the same value as the first example.

java.sql.Date jsqlD = java.sql.Date.valueOf( "2010-01-31" );

creates a Date object representing January 31, 2010. To use this method with a Calendar object,use:

java.sql.Date jsqlD = java.sql.Date.valueOf(

cal.get(cal.YEAR) + ":" +

cal.get(cal.MONTH) + ":" +

cal.get(cal.DATE) );

which produces a Date object with the same value as the first example.

1.What does ResultSet actually contain? Is it the actual data of the result or some links to

databases? If it is the actual data then why can't we

access it after connection is closed?

A ResultSet is an interface. Its implementation depends on the driver and hence ,what it

"contains" depends partially on the driver and what the query

returns.

For example with the Odbc bridge what the underlying implementation layer contains is an

ODBC result set. A Type 4 driver executing a stored procedure that

returns a cursor - on an oracle database it actually returns a cursor in the databse. The oracle

cursor can however be processed like a ResultSet would be

from the client. Closing a connection closes all interaction with the database and releases any

locks that might have been obtained in the process.


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2. How do I extract a BLOB from a database?

A BLOB (Binary Large OBject) is essentially an array of bytes (byte[]), stored in the database.

You extract the data in two steps:

1. Call the getBlob method of the Statement class to retrieve a java.sql.Blob object

2. Call either getBinaryStream or getBytes in the extracted Blob object to retrieve the java byte[]which is the Blob object.

Note that a Blob is essentially a pointer to a byte array (called LOCATOR in database-talk), so

the java.sql.Blob object essentially wraps a byte

pointer. Thus, you must extract all data from the database blob before calling commit or

private void runGetBLOB()

{

try

{ // Prepare a Statement:

PreparedStatement stmnt = conn.prepareStatement("select aBlob from BlobTable");

// Execute

ResultSet rs = stmnt.executeQuery();

while(rs.next())

{

try

{

// Get as a BLOB

Blob aBlob = rs.getBlob(1);


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byte[] allBytesInBlob = aBlob.getBytes(1, (int) aBlob.length());

}

catch(Exception ex)

{

// The driver could not handle this as a BLOB...

// Fallback to default (and slower) byte[] handling

byte[] bytes = rs.getBytes(1);

}

}

// Close resources

rs.close();

stmnt.close();

}

catch(Exception ex)

{

this.log("Error when trying to read BLOB: " + ex);

}

}

3. How do I extract the SQL statements required to move all tables and views from an existingdatabase to another database?

The operation is performed in 9 steps:


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1. Open a connection to the source database. Use the DriverManager class.

2. Find the entire physical layout of the current database. Use the DatabaseMetaData interface.

3. Create DDL SQL statements for re-creating the current database structure. Use the

DatabaseMetaData interface.

4. Build a dependency tree, to determine the order in which tables must be setup. Use the

DatabaseMetaData interface.

5. Open a connection to the target database. Use the DriverManager class.

6. Execute all DDL SQL statements from (3) in the order given by (4) in the target database to

setup the table and view structure. Use the

PreparedStatement interface.

7. If (6) threw exceptions, abort the entire process.

8. Loop over all tables in the physical structure to generate DML SQL statements for re-creating

the data inside the table. Use the ResultSetMetaData

interface.

9. Execute all DML SQL statements from (8) in the target database.

4. How do I check what table types exist in a database?

Use the getTableTypes method of interface java.sql.DatabaseMetaData to probe the database for

table types. The exact usage is described in the code

below.

public static void main(String[] args) throws Exception

{

// Load the database driver - in this case, we

// use the Jdbc/Odbc bridge driver.

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");


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// Open a connection to the database

Connection conn = DriverManager.getConnection("[jdbcURL]",

"[login]", "[passwd]");

// Get DatabaseMetaData

DatabaseMetaData dbmd = conn.getMetaData();

// Get all table types.

ResultSet rs = dbmd.getTableTypes();

// Printout table data

while(rs.next())

{

// Printout

System.out.println("Type: " + rs.getString(1));

}

// Close database resources

rs.close();

conn.close();

}

5. What is the advantage of using a PreparedStatement?

For SQL statements that are executed repeatedly, using a PreparedStatement object would almostalways be faster than using a Statement object. This is

because creating a PreparedStatement object by explicitly giving the SQL statement causes thestatement to be precompiled within the database


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immediately. Thus, when the PreparedStatement is later executed, the DBMS does not have to

recompile the SQL statement and prepared an execution plan -

it simply runs the statement.

Typically, PreparedStatement objects are used for SQL statements that take parameters.However, they can also be used with repeatedly executed SQL

statements that do not accept parameters.

6. How do I find all database stored procedures in a database?

Use the getProcedures method of interface java.sql.DatabaseMetaData to probe the database for

stored procedures. The exact usage is described in the code

below.


{


// use the Jdbc/Odbc bridge driver.Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");






// Get all procedures.

System.out.println("Procedures are called '"

+ dbmd.getProcedureTerm() +"' in the DBMS.");

ResultSet rs = dbmd.getProcedures(null, null, "%");


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while(rs.next())

{

// Get procedure metadata

String dbProcedureCatalog = rs.getString(1);

String dbProcedureSchema = rs.getString(2);

String dbProcedureName = rs.getString(3);

String dbProcedureRemarks = rs.getString(7);

short dbProcedureType = rs.getShort(8);

// Make result readable for humans

String procReturn = (dbProcedureType == DatabaseMetaData.procedureNoResult

? "No Result" : "Result");

// Printout

System.out.println("Procedure: " + dbProcedureName

+ ", returns: " + procReturn);

System.out.println(" [Catalog | Schema]: [" + dbProcedureCatalog

+ " | " + dbProcedureSchema + "]");

System.out.println(" Comments: " + dbProcedureRemarks);

}


rs.close();


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conn.close();

}

7. How can I investigate the physical structure of a database?

The JDBC view of a database internal structure can be seen in the image below.

Several database objects (tables, views, procedures etc.) are contained within a Schema. Several schema (user namespaces) are contained within a catalog. Several catalogs (database partitions; databases) are contained within a DB server (such

as Oracle, MS SQL

The DatabaseMetaData interface has methods for discovering all the Catalogs, Schemas, Tables

and Stored Procedures in the database server. The methods

are pretty intuitive, returning a ResultSet with a single String column; use them as indicated inthe code below:


{


// use the Jdbc/Odbc bridge driver.

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");






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// Get all Catalogs

System.out.println("\nCatalogs are called '" + dbmd.getCatalogTerm()

+ "' in this RDBMS.");

processResultSet(dbmd.getCatalogTerm(), dbmd.getCatalogs());

// Get all Schemas

System.out.println("\nSchemas are called '" + dbmd.getSchemaTerm()

+ "' in this RDBMS.");

processResultSet(dbmd.getSchemaTerm(), dbmd.getSchemas());

// Get all Table-like types

System.out.println("\nAll table types supported in this RDBMS:");

processResultSet("Table type", dbmd.getTableTypes());

// Close the Connection

conn.close();

}

public static void processResultSet(String preamble, ResultSet rs)

throws SQLException

{


while(rs.next())

{


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// Printout

System.out.println(preamble + ": " + rs.getString(1));

}


rs.close();

}

8. How does the Java Database Connectivity (JDBC) work?

The JDBC is used whenever a Java application should communicate with a relational databasefor which a JDBC driver exists. JDBC is part of the Java

platform standard; all visible classes used in the Java/database communication are placed in

package java.sql.

Main JDBC classes:

DriverManager. Manages a list of database drivers. Matches connection requests from thejava application with the proper database driver using

communication subprotocol. The first driver that recognizes a certain subprotocol under jdbc(such as odbc or dbAnywhere/dbaw) will be used to establish

a database Connection.

Driver. The database communications link, handling all communication with thedatabase. Normally, once the driver is loaded, the developer need not

call it explicitly.

Connection. Interface with all methods for contacting a database

Statement. Encapsulates an SQL statement which is passed to the database to be parsed,compiled, planned and executed.

ResultSet. The answer/result from a statement. A ResultSet is a fancy 2D list whichencapsulates all outgoing results from a given SQL query.


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9. What is Metadata and why should I use it?

Metadata ('data about data') is information about one of two things:

1. Database information (java.sql.DatabaseMetaData), or

2. Information about a specific ResultSet (java.sql.ResultSetMetaData).

Use DatabaseMetaData to find information about your database, such as its capabilities and

structure. Use ResultSetMetaData to find information about the

results of an SQL query, such as size and types of columns.

10. How do I create a database connection?

The database connection is created in 3 steps:

1. Find a proper database URL (see FAQ on JDBC URL)

2. Load the database driver

3. Ask the Java DriverManager class to open a connection to your database

In java code, the steps are realized in code as follows:

1. Create a properly formatted JDBR URL for your database. (See FAQ on JDBC URL for more

information). A JDBC URL has the form

jdbc:someSubProtocol://myDatabaseServer/theDatabaseName

2.

try {

Class.forName("my.database.driver");

}

catch(Exception ex)

{


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System.err.println("Could not load database driver: " + ex);

}

3. Connection conn = DriverManager.getConnection("a.JDBC.URL", "databaseLogin","databasePassword");

11. Can I use the JDBC-ODBC bridge driver in an applet?

Short answer: No.

Longer answer: You may create a digitally signed applet using a Certicate to circumvent the

security sandbox of the browser.

12. Which is the preferred collection class to use for storing database result sets?

When retrieving database results, the best collection implementation to use is the LinkedList.

The benefits include:

Retains the original retrieval order Has quick insertion at the head/tail Doesn't have an internal size limitation like a Vector where when the size is exceeded a

new internal structure is created (or you have to find out

size beforehand to size properly)

Permits user-controlled synchronization unlike the pre-Collections Vector which isalways synchronized

Basically:

ResultSet result = stmt.executeQuery("...");

List list = new LinkedList();

while(result.next()) {

list.add(result.getString("col"));

}


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If there are multiple columns in the result set, you'll have to combine them into their own datastructure for each row. Arrays work well for that as you

know the size, though a custom class might be best so you can convert the contents to the proper

type when extracting from databse, instead of later.

13. The java.sql package contains mostly interfaces. When and how are these interfaces

implemented while connecting to database?

The implementation of these interfaces is all part of the driver. A JDBC driver is not just one

class - it is a complete set of database-specific

implementations for the interfaces defined by the JDBC.

These driver classes come into being through a bootstrap process. This is best shown by steppingthrough the process of using JDBC to connect to a

database, using Oracle's type 4 JDBC driver as an example:

First, the main driver class must be loaded into the VM:Class.forName("oracle.jdbc.driver.OracleDriver");

The specified driver must implement the Driver interface. A class initializer (static code block)

within the OracleDriver class registers the driver with

the DriverManager.

Next, we need to obtain a connec

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