Java Enterprise Edition Programming Page 1 of 9Configuring Servlets Web Application Context Name In...

Java Enterprise Edition Programming

of 9Configuring Servlets

Web Application Context Name

In multiple web applications, a “context name” is used to access a specific Web application.

The general syntax of a servlet Web application URL is:

http://host:port/context/path/file



Accessing Servlet Using the Fully-Qualified Class

The problem in accessing servlet using the fully-qualified class is that the names can be very long.

It reveals the implementation details of about the Web application.

Example

http://localhost:8080/ValidateUser/servlet/adprog1.web.ValidateUserServlet



Servlet Mapping

In the deployment descriptor, servlet mapping is performed in two steps.

1. A servlet definition is configured which names a particular servlet and specifies the fully qualified Java technology class that implements that servlet.

2. A servlet mapping is created which identifies a URL structure that maps to the named servlet definition.



Example of Deployment Descriptor

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD

Web Application 2.3//EN" "http://java.sun.com/dtd/web-

app_2_3.dtd"><web-app> <display-name>ADPROG1 Web Application

Example</display-name> <description> This Web Application

demonstrates a simple deployment descriptor.

It also demonstrates a servlet definition and servlet mapping.

</description <servlet> <servlet-name>Hello</servlet-

name> <servlet-class>adprog1.web.

HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>Hello</servlet-

name> <url-pattern>/greeting</url-

pattern> </servlet-mapping></web-app>



Deployment Environment

When a Web application is deployed to the Web container, the directory structure must follow a particular format:

The static HTML files are stored in the top level directory of the Web application.

The servlet and related Java technology class files must be stored in the WEB-INF/classes directory.

The auxiliary Java Archive (JAR) files must be stored in the WEB-INF/lib directory.

The deployment descriptor must be stored in the file called web.xml in the WEB-INF directory.



Servlet Life Cycle Overview

The Web container manages the life cycle of a servlet instance by calling three methods defined in the Servlet interface: init, service and destroy.

<<interface>>Servlet

initservicedestroy

Readyinit

service

destroy



init Life Cycle Method

The init method is called by the Web container when the servlet instance is first created.

The Servlet specification guarantees that no requests will be processed by this servlet until the init method has completed.



service and destroy Life

Cycle Method

The service method is called by the Web container to process a user request.

The destroy method is called by the Web container when the servlet instance is being destroyed.

The Servlet specification guarantees that all requests will be completely processed before the destroy method is called.



Servlet Configuration

When a servlet is initialized, a common task is to load any external resources into memory.

The Web container uses a configuration object to pass the initialization parameters to the servlet at runtime.



ServletConfig API

HelloServlet

greetingText: String

init( )doPost(request,response)

HttpServlet{abstract}

init(config:ServletConfig)service(request, response)destroy( )


GenericServlet

{abstract}

init(config:ServletConfig)init( )service(request,response)destroy( )getInitParameter(name:String) : StringgetInitParameterNames( ) :EnumerationgetServletName( ) : String

getInitParameter(name:String) : StringgetInitParameterNames( ) : EnumerationgetServletName( ) : String


getInitParameter(name:String) : StringgetInitParameterNames( ) : EnumerationgetServletName( ) : String

VenderServletConfigImpl

delegate



Initialization Parameters

Servlet initialization parameters are name-value pairs that are declared in the deployment descriptor. The names and values are arbitrary strings.

Example:

<servlet><servlet-name>MsgHello1</servlet-name><servlet-class>adprog1.web.HelloServlet</servlet-class><init-param>

<param-name>msgText</param-name>

<param-value>Hello</param-value></init-param><init-param>

<param-name>msgText</param-name>

<param-value>Hello</param-value></init-param>

</servlet>



ServletContext Object

A Web application is a self-contained collection of static and dynamic resources: HTML pages, media files, data and resource files, servlets (and JSP pages), and other auxiliary Java technology classes and objects.

A ServletContext object is the runtime representation of the Web application.



ServletContext API

Read-only access to application scoped initialization parameters

Read-only access to application-level file resources

Read-write access to application scoped attributes

Logging functionality



ServletContext API

<<abstract>>GenericServlet

getServletContext() : ServletContextlog(message: String)log(message:String, Throwable)

getInitParameter(name:String) : StringgetInitParameterNames( ) : EnumerationgetAttribute(name: String) : ObjectsetAttribute(name: String, value:Object)getAttributeNames() : EnumerationgetResource(path) : URLgetResourceAsStream(path) : InputStreamlog(message: String)log(message: String. Throwable: excp

<<interface>>ServletContext



Context Initialization Parameters

A Web application may have one or more initialization parameters.

Context initialization parameters are accessed using the getInitParameter method on the ServletContext object.

The context-param element tag is used to configure the parameters in the deployment descriptor.



Example

<web-app><display-name></display-name><description></description><context-param>

<param-name>catalogFileName</param-name><param-value>/WEB-INF/catalog.txt</param-value

</context-param></web-app>



Retrieving Context Parameters

To retrieving a context initialization parameter, invoke the getInitParameter() method from the ServletContext instance.

ServletContext context = sce.getServletContext()

String catalogFileName = context.getInitParameter(“catalogFileName”);



Access to File Resources

The ServletContext object provides read-only access to file resources through the getResourceAsStream() method that returns a raw InputStream object.

Example

ServletContext context = sce.getServletContext();

String catalogFileName = context.getInitParameter(“catalogFileName”);

InputStream is = null;BufferedReader catReader = null;

try{is =

context.getResourceAsStream(catalogFileName);

catReader = new BufferedReader(new InputStreamReader(is));



Writing to the Web Application Log File

The ServletContext object provides write-only access to log file: The log(String) method writes a

message to the log file. The log(String, Throwable)

method writes a message and the stack trace of the exception or error to the log file.

Example

context.log(“The ProductList has been initialized.”);



Accessing Shared Runtime Attributes

The ServletContext object provides read-write access to attributes shared across all servlets through the getAttribute and setAttribute methods.

Storing an Application Scoped Attribute

context.setAttribute(“catalog”,catalog);

Retrieving an Application Scoped Attribute

ServletContext context = (ProductList)

context.getAttribute(“catalog”);



Web Application Life Cycle

When the Web container is started, each Web application is initialized. When the Web container is shut down, each Web application is destroyed.

Ready

New

initialize destroy

Destroyed



ServletContextListener API

<<interface>>ServletContextListener

contextInitialized(event)contextDestroyed(event)

getServletContext() : ServletContext

ServletContextEvent

Java Enterprise Edition Programming Page 1 of 9Configuring Servlets Web Application Context Name In...

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