##### For example

- Run PMD on a Sourceforge project

## How to write a PMD rule

Writing PMD rules is cool because you don't have to wait for us to get around to implementing feature requests.

### Get a development environment set up first

Here's some initial information on compiling PMD

### Java or XPath?

There are two way to write rules:

- Write a rule using Java
- Write an XPath expression

We'll cover the Java way first and the XPath way second. Most of this documentation is applicable to both methods, too, so read on.

### Figure out what you want to look for

Lets's figure out what problem we want to spot. We can use "While loops must use braces" as an example. In the source code below, it's easy to get lost visually - it's kind of hard to tell what the curly braces belong to.

    class Example {
     void bar() {
      while (baz)
       buz.doSomething();
     }
    }

So we know what an example in source code looks like, which is half the battle.

### Write a test-data example and look at the AST

PMD doesn't use the source code directly; it uses a `JavaCC` generated parser to parse the source code and produce an AST (Abstract Syntax Tree). The AST for the code above looks like this:

    CompilationUnit
     TypeDeclaration
      ClassDeclaration:(package private)
       UnmodifiedClassDeclaration(Example)
        ClassBody
         ClassBodyDeclaration
          MethodDeclaration:(package private)
           ResultType
           MethodDeclarator(bar)
            FormalParameters
           Block
            BlockStatement
             Statement
              WhileStatement
               Expression
                PrimaryExpression
                 PrimaryPrefix
                  Name:baz
               Statement
                StatementExpression:null
                 PrimaryExpression
                  PrimaryPrefix
                   Name:buz.doSomething
                  PrimarySuffix
                   Arguments

You can generate this yourself by:

- Run the batch file bin/designer.bat
- Paste the code into the left text area and click the "Go" button
- Note that there's another panel and a textfield to test out XPath expressions; more on that later.
- Here's a screenshot:

So you can see in the example above that the AST for a `WhileStatement` looks kind of like this (excluding that expression gibberish for clarity):

    WhileStatement
     Expression
     Statement
      StatementExpression

If you were to add curly braces around the call to `buz.doSomething()`and click "Go" again, you'd see that the AST would change a bit. It'd look like this:

    WhileStatement
     Expression
     Statement
      Block
       BlockStatement
        Statement
         StatementExpression

Ah ha! We see that the curly braces add a couple more AST nodes - a `Block` and a `BlockStatement`. So all we have to do is write a rule to detect a `WhileStatement` that has a `Statement` that's not followed by a `Block`, and we've got a rule violation.

By the way, all this structural information - i.e., the fact that a Statement may be followed a Block - is concisely defined in the EBNF grammar. So, for example, the Statement definition looks like this:

    void Statement() :
    {}
    {
      LOOKAHEAD( { isNextTokenAnAssert() } ) AssertStatement()
    | LOOKAHEAD(2) LabeledStatement()
    | Block()
    | EmptyStatement()
    | StatementExpression() ";"
    | SwitchStatement()
    | IfStatement()
    | WhileStatement()
    | DoStatement()
    | ForStatement()
    | BreakStatement()
    | ContinueStatement()
    | ReturnStatement()
    | ThrowStatement()
    | SynchronizedStatement()
    | TryStatement()
    }

showing that a Statement may be followed by all sorts of stuff.

### Write a rule class

Create a new Java class that extends `net.sourceforge.pmd.AbstractRule`:

    import net.sourceforge.pmd.*;
    public class WhileLoopsMustUseBracesRule extends AbstractRule {
    }

That was easy. PMD works by creating the AST and then traverses it recursively so a rule can get a callback for any type it's interested in. So let's make sure our rule gets called whenever the AST traversal finds a `WhileStatement`:

    import net.sourceforge.pmd.*;
    import net.sourceforge.pmd.ast.*;
    public class WhileLoopsMustUseBracesRule extends AbstractRule {
        public Object visit(ASTWhileStatement node, Object data) {
            System.out.println("hello world");
            return data;
        }
    }

We stuck a `println()` in there for now so we can see when our rule gets hit.

### Put the WhileLoopsMustUseBracesRule rule in a ruleset file

Now our rule is written - at least, the shell of it is - and now we need to tell PMD about it. We need to add it to a ruleset XML file. Look at `rulesets/basic.xml`; it's got lots of rule definitions in it. Copy and paste one of these rules into a new ruleset - call it `mycustomrules.xml` or something. Then fill in the elements and attributes:

- name - WhileLoopsMustUseBracesRule
- message - Use braces for while loops
- class - Wherever you put the rule. Note this doesn't have to be in `net.sourceforge.pmd`; it can be in `com.yourcompany.util.pmd` or whereever you want
- description - Use braces for while loops
- example - A little code snippet in CDATA tags that shows a rule violation

The whole ruleset file should look something like this:

            <?xml version="1.0"?>
            <ruleset name="My custom rules"
                xmlns="http://pmd.sf.net/ruleset/1.0.0"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://pmd.sf.net/ruleset/1.0.0 http://pmd.sf.net/ruleset_xml_schema.xsd"
                xsi:noNamespaceSchemaLocation="http://pmd.sf.net/ruleset_xml_schema.xsd">
                <rule name="WhileLoopsMustUseBracesRule"
                      message="Avoid using 'while' statements without curly braces"
                      class="WhileLoopsMustUseBracesRule">
                  <description>
                  Avoid using 'while' statements without using curly braces
                  </description>
                    <priority>3</priority>

                  <example>
            <![CDATA[
                public void doSomething() {
                  while (true)
                      x++;
                }
            ]]>
                  </example>
                </rule>
            </ruleset>

### Run PMD using your new ruleset

OK, let's run the new rule so we can see something work. Like this:

                pmd.bat c:\path\to\my\src xml c:\path\to\mycustomrules.xml

This time your "hello world" will show up right after the AST gets printed out. If it doesn't, post a message to the forum so we can improve this document :-)

### Write code to add rule violations where appropriate

Now that we've identified our problem, recognized the AST pattern that illustrates the problem, written a new rule, and plugged it into a ruleset, we need to actually make our rule find the problem, create a `RuleViolation`, and put it in the `Report`, which is attached to the `RuleContext`. Like this:

    public class WhileLoopsMustUseBracesRule extends AbstractRule {
        public Object visit(ASTWhileStatement node, Object data) {
            SimpleNode firstStmt = (SimpleNode)node.jjtGetChild(1);
            if (!hasBlockAsFirstChild(firstStmt)) {
                addViolation(data, node);
            }
            return super.visit(node,data);
        }
        private boolean hasBlockAsFirstChild(SimpleNode node) {
            return (node.jjtGetNumChildren() != 0 && (node.jjtGetChild(0) instanceof ASTBlock));
        }
    }

TODO - if you don't understand the code for the rule, post a message to the forum so we can improve this document :-)

### Writing a rule as an XPath expression

Daniel Sheppard integrated an XPath engine into PMD, so now you can write rules as XPath expressions. For example, the XPath expression for our WhileLoopsMustUseBracesRule looks like this:

                //WhileStatement[not(Statement/Block)]

Concise, eh? Here's an article with a lot more detail.

Note that for XPath rules you'll need to set the `class` attribute in the rule definition to `net.sourceforge.pmd.rules.XPathRule.` Like this:

                    <rule name="EmptyCatchBlock"
                          message="Avoid empty catch blocks"
                          class="net.sourceforge.pmd.rules.XPathRule">
                      <description>
                      etc., etc.

Note that access modifiers are held as attributes, so, for example,

    //FieldDeclaration[@Private='true']

finds all private fields. You can see the code that determines all the attributes here

Thanks to Miguel Griffa for writing a longer XPath tutorial.

### Bundle it up

To use your rules as part of a nightly build or whatever, it's helpful to bundle up both the rule and the ruleset.xml file in a jar file. Then you can put that jar file on the CLASSPATH of your build. Setting up a script or an Ant task to do this can save you some tedious typing.

### Repeat as necessary

I've found that my rules usually don't work the first time, and so I have to go back and tweak them a couple times. That's OK, if we were perfect programmers PMD would be useless anyhow :-).

As an acceptance test of sorts, I usually run a rule on the JDK 1.4 source code and make sure that a random sampling of the problems found are in fact legitimate rule violations. This also ensures that the rule doesn't get confused by nested inner classes or any of the other oddities that appear at various points in the JDK source.

You're rolling now. If you think a rule would benefit the Java development community as a whole, post a message to the forum so we can get the rule moved into one of the core rulesets.

Or, if you can improve one of the existing rules, that'd be great too! Thanks!

Finally, for many more details on writing rules, pick up PMD Applied!