Deep Typechecking and Refactoring

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Deep Typecheckingand Refactoring

Zachary Tatlock, Chris Tucker, David Shuffleton, Ranjit Jhala, Sorin Lerner

University of California, San Diego

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Communicating with Databases

String based queries are prevalent:JPA, Hibernate, TopLink

JAVA DBStrings

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Example: Using JPA to query DB

“SELECT w FROM Weblog w WHERE w.id = ?1 AND w.link.id = ?2”

Query in JPA Query Language:

Mapping Java Classes to DB Tables: Expressed in Object Relational Mapping (ORM)

Java syntax in query String

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Example: Using JPA to query DBString getText(String id, Link link) { String qStr; Query q;

qStr = “SELECT w FROM Weblog w ”; qStr += “WHERE w.id = ?1 ”; qStr += “AND w.link.id = ?2”;

q = createQuery(qStr);

q.setParam(1, id); q.setParam(2, link.id);

Weblog w = (Weblog) q.execQuery();

return w.text;}

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return w.text;}

1. Build query string

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return w.text;}


2. Create query

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return w.text;}


2. Create query

3. Set parameters

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return w.text;}


2. Create query

3. Set parameters

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return w.text;}


2. Create query

3. Set parameters

4. Execute query

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return w.text;}


2. Create query

3. Set parameters

4. Execute query

• Efficient• Flexible

Unsafe

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Uncaught ErrorsString getText(String id, Link link) { String qStr; Query q;





return w.text;}

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q.setParam(1, id); // q.setParam(2, link.id);


return w.text;}

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q.setParam(1, id); // q.setParam(2, link.id);


return w.text;}

1. Unset parameter

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return w.text;}

1. Unset parameter

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q.setParam(1, new Weblog()); q.setParam(2, link.id);


return w.text;}

1. Unset parameter

2. Unsafe param type

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return w.text;}

1. Unset parameter


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Warranty w = (Warranty) q.execQuery();

return w.text;}

1. Unset parameter


3. Unsafe downcast

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return w.text;}

1. Unset parameter


3. Unsafe downcast

Java compiler does not reason about the query strings; cannot typecheck.

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Refactor: Weblog.id Weblog.nameString getText(String id, Link link) { String qStr; Query q;





return w.text;}

1. Unset parameter


3. Unsafe downcast

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return w.text;}

1. Unset parameter


3. Unsafe downcast

Refactor

Don’t Refactor

Need to know type of w and w.link to

refactor safely.

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return w.text;}

1. Unset parameter


3. Unsafe downcast

4. Refactoring difficult

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String Based Query ChallengesString getText(String id, Link link) { String qStr; Query q;





return w.text;}

1. Unset parameter


3. Unsafe downcast


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Contributions

Deep Typechecking: All parameters set Parameters correctly set Results safely downcast

Deep Refactoring: Builds on Deep Typechecking Enables class and field renaming

1. Unset parameter


3. Unsafe downcast


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Outline

1. Introduction and Motivation2. Deep Typechecking and Refactoring

Straight Line Code Control Flow Multiple Methods

3. Experimental Results4. Related Work and Conclusion

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Deep Typechecking ExampleString getText(String id, Link link) { String qStr; Query q;





return w.text;}

Query safety:

• All params set

• Params safely set

• Result safely downcast

Is this query exec safe?

Need to know:

1. query string

2. param types

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Deep Typechecking ExampleString getText(String id, Link link) { String qStr; Query q;





return w.text;}

Bound Query:• query string• param types

query : “SELECT … ?1 … ?2 … ?3 …”?1 : String?2 : Weblog?3 : unknown

Example :

At each program point map each var to a set of BQs.

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Bound Query AnalysisString getText(String id, Link link) { String qStr; Query q;





return w.text;}

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return w.text;}

qStr = “SELECT … ?2”

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return w.text;}


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return w.text;}


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return w.text;}


query : “SELECT …”?1 : unknown?2 : unknown

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return w.text;}



query : “SELECT …”?1 : String?2 : unknown

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return w.text;}



query : “SELECT …”?1 : String?2 : unknown

query : “SELECT …”?1 : String?2 : int

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return w.text;}

Checking param types:

1. Parse query string

2. Check all params set

3. Check param types

Bound Queries:


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return w.text;}

Checking result type:

1. Infer result type

2. Propagate result type

3. Check downcasts

Bound Queries:


query : “SELECT …”?1 : String?2 : intresult : Weblog

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return w.text;}

If a query passes Deep Typechecking, then it will not cause an error at runtime.

Therefore, Bound Query Analysis has no silent failures.

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Deep Refactoring ExampleString getText(String id, Link link) { String qStr; Query q;





return w.text;}

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return w.text;}

Refactor Weblog field:

id name

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return w.text;}


id name

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return w.text;}


id name

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Deep Refactoring ExampleRefactor Weblog field:

id name

String getText(String id, Link link) { String qStr; Query q;





return w.text;}

query : SELECT w FROM Weblog w WHERE w.id = ?1 AND w.link.id = ?2?1 : String?2 : int

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return w.text;}

query : SELECT w FROM Weblog w WHERE w.id = ?1 AND w.link.id = ?2?1 : String?2 : int


id name

1. Refactor full query

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return w.text;}

query : SELECT w FROM Weblog w WHERE w.name = ?1 AND w.link.id = ?2?1 : String?2 : int


id name


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return w.text;}



id name


2. Propagate changes

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qStr = “SELECT w FROM Weblog w ”; qStr += “WHERE w.name = ?1 ”; qStr += “AND w.link.id = ?2”;




return w.text;}



id name


2. Propagate changes

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Outline




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Flow SensitivityString getText(String id, Link link) { String qStr; Query q;





return w.text;}

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Flow SensitivityString getText(String id, Link link) { String qStr; Query q; qStr = “SELECT w FROM Weblog w ”; qStr += “WHERE w.id = ?1 ”;

qStr += “AND w.link.id = ?2”; q = createQuery(qStr); q.setParam(1, id); q.setParam(2, link.id);

Weblog w = (Weblog) q.execQuery(); return w.text;}

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qStr += “AND w.link.id = ?2”; q = createQuery(qStr); q.setParam(1, id); q.setParam(2, link.id);


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if(link != null) { qStr += “AND w.link.id = ?2”; q = createQuery(qStr); q.setParam(1, id); q.setParam(2, link.id); } else {

}


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if(link != null) { qStr += “AND w.link.id = ?2”; q = createQuery(qStr); q.setParam(1, id); q.setParam(2, link.id); } else { q = createQuery(qStr); q.setParam(1, id); }


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query : “SEL … ?2”?1 : String?2 : int


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query : “SEL … ?1”?1 : String



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query : “SEL … ?1”?1 : String

“SEL … ?2”?1 : String?2 : int

“SEL … ?1”?1 : String




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“SEL … ?2”?1 : String?2 : int

“SEL … ?1”?1 : String

As before, for each bound query:

1. Check param types

2. Check result type

In general, we express Bound Query Analysis as a dataflow analysis.

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LoopsString getText(String id, Link link) { String qStr; Query q;

qStr = “SELECT w FROM Weblog w ”; qStr += “WHERE w.id = ?1 ”; qStr += “OR w.link.id = ?2”;




return w.text;}

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LoopsString getText(String id, List<Link> links) { String qStr; Query q;





return w.text;}

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return w.text;}

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qStr = “SELECT w FROM Weblog w ”; qStr += “WHERE w.id = ?1 ”;

for(int i = 0; i < links.size(); i++) { qStr += “ OR w.link.id = ?” + i; }


...}

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String getText(String id, List<Link> links) { String qStr; Query q;




...}

Loops

qStr = ???qStr = “SELECT … w.id = ?1” ( “ OR w.link.id = ?#” )*

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...}

Loops

qStr = “SELECT … w.id = ?1” ( “ OR w.link.id = ?#” )*

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...}

Loops

qStr = “SELECT … w.id = ?1” ( “ OR w.link.id = ?#” )*

Deep Refactoring:• know query structure• know fragment locs• refactor across loops

Deep Typechecking:• unknown # of params• do not check params• can still check result

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Outline




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Multiple MethodsString mainQueryStr() { return “SELECT ... ?1”;}

Object getMain() { String qStr = mainQueryStr();

Query q = createQuery(qStr);

q.setParam(1, “main”);

return q.execQuery();}

String mainId() { return ((Weblog) getMain()).id;}

String Analysis : • interprocedural• compute regexps

Bound Query Analysis : • intraprocedural• no complex aliasing

Result Analysis :• interprocedural• propagate result type

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Outline




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Results Implementation: Quail

5700 lines of Java in Eclipse plugin

Benchmarks:Application LOC DescriptionRoller 82,900 Blogging frameworkPlanet 14,500 News aggregatorPetStore 5,440 Example JPA applicationCaveatEmptor 3,370 Online auction systemTotal 106,000

Create Query Set Param Exec Query163 199 136

JPA Calls:

Tested,Stable,

Deployed

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Deep Typechecking ExperimentCheck for all query execution sites:

All parameters set Parameters safely set Query results safely downcast

Show 84% of query executions are safe

Remaining 16% due to imprecision: Queries built with loops or data structures Path sensitivity Reflection

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Deep Refactoring Experiment Rename 16 most frequently appearing classes / fields Correctly refactor 98% of refactorable locations Remaining 2% from heap based strings in query

String[] blacklist = getBlacklist();

String qStr = “SELECT w FROM Weblog w WHERE w.id = ?1 ”;

for(int i = 0; i < blacklist.length; i++) {

qStr += “AND w.id != ” + blacklist[i] + “ ”;

}

...

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Results

Typechecking and Refactoring Experiments: No silent failures Show user exactly where to check 2% vs 16% imprecision:

Refactoring does not depend on parameter types

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Outline


Straight Line Code Control Flow Mutliple Methods


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Related Work

Query Extraction [Wiedermann, Ibrahim, Cook 08] Infer query from Java code that uses DB root object Not applicable to legacy code

Static Query Checking [Gould, Su, Devanbu 04] Typechecks JDBC queries against DB schema Not applied to source language type system

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Related Work

Language Based [Matthes 95, Schmidt 94] Syntax extension for queries Not applicable to legacy code

Orthogonal Persistence [Atkinson 96, Liskov 96] Map DB to collection of persisted objects Must express queries in non-query language

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Conclusion

String based queries can be safe and flexible.

Deep Typechecking ensures: All parameters are set Parameters are set to correct type Query results are safely downcast

Deep Refactoring enables: Class and field renaming

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Thank You

Deep Typechecking and Refactoring

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