Static Analysis with PMD

PMD is a utility for finding problems in Java code. PMD does this using static analysis; that is, analyzing the source code without actually running the program. PMD comes with a number of ready-to-run rules that you can run on your own source code to find unused variables, unnecessary object creation, empty catch blocks, and so forth. You can also write your own rules to enforce coding practices specific to your organization. For example, if you're doing EJB programming, you could write a PMD rule that would flag any creation of Thread or Socket objects. If you're feeling generous, you can donate that rule back to PMD for anyone to use.

PMD was initially written in support of Cougaar, a Defense Advanced Research Projects Agency (DARPA) project billed as "An Open Source Agent Architecture for Large-scale, Distributed Multi-Agent Systems." DARPA agreed that the utility could be open sourced, and since its release on SourceForge, it has been downloaded over 14,000 times and has garnered over 130,000 page views. More importantly, though, numerous PMD rule suggestions and IDE plugins have been written by open source developers and contributed back to the core PMD project.

C:\data\pmd\pmd>java -jar lib\pmd-1.02.jar c:\j2sdk1.4.1_01\src\java\util 
   text rulesets/unusedcode.xml

c:\j2sdk1.4.1_01\src\java\util\AbstractMap.java 650 
    Avoid unused local variables such as 'v'    

c:\j2sdk1.4.1_01\src\java\util\Date.java        438  
    Avoid unused local variables such as 'millis'   

// etc, etc, remaining errors skipped

Built-in Rules

So what rules come with PMD? Well, here are some examples:

• Unused code is always bad:

  public class Foo {
     // an unused instance variable
     private List bar = new ArrayList(500);
  }

• Why are we returning a concrete class here when an interface--i.e., List--would do just as well?

  public ArrayList getList() {
     return new ArrayList();
  }

• Nothing's being done inside the if success block ... this could be rewritten for clarity:

  public void doSomething(int y) {
     if (y >= 2) { 
     } else {
        System.out.println("Less than two");
     }
  }

• Why are we creating a new String object? Just use String x = "x"; instead.

  String x = new String("x");

Related Reading Java Enterprise Best Practices By The O'Reilly Java Authors

There are many other rules, but you get the idea. Static analysis rules can catch the things that would make an experienced programer say "Hmm, that's not good."

At the heart of PMD is the JavaCC parser generator, which PMD uses in conjunction with an Extended Backus-Naur Formal (EBNF) grammar and JJTree to parse Java source code into an Abstract Syntax Tree (AST). That was a big sentence with a lot of acronyms, so let's break it down into smaller pieces.

Java source code is, at the end of the day, just plain old text. As your compiler will tell you, however, that plain text has to be structured in a certain way in order to be valid Java code. That structure can be expressed in a syntactic metalanguage called EBNF and is usually referred to as a "grammar." JavaCC reads the grammar and generates a parser that can be used to parse programs written in the Java programming language.

There's another layer, though. JJTree, an add-on to JavaCC, enhances the JavaCC-generated parser by decorating it with an Abstract Syntax Tree (AST)--a semantic layer on top of the stream of Java tokens. So instead of getting a sequence of tokens like System, ., out, ., and println, JJTree serves up a tree-like hierarchy of objects. Here's a simple code snippet and the corresponding AST:

Source Code

public class Foo {
	public void bar() {
		System.out.println("hello world");
	}
}

Abstract Syntax Tree

CompilationUnit
 TypeDeclaration
  ClassDeclaration
   UnmodifiedClassDeclaration
    ClassBody
     ClassBodyDeclaration
      MethodDeclaration
       ResultType
       MethodDeclarator
        FormalParameters
       Block
        BlockStatement
         Statement
          StatementExpression
           PrimaryExpression
            PrimaryPrefix
             Name
            PrimarySuffix
             Arguments
              ArgumentList
               Expression
                PrimaryExpression
                 PrimaryPrefix
                  Literal

Your code can traverse this tree using the Visitor pattern--the object tree generated by JJTree supports this.