Merge branch 'bug-1461-doc'

pmd-core/src/main/java/net/sourceforge/pmd/Rule.java

@@ -13,6 +13,12 @@ import net.sourceforge.pmd.lang.rule.properties.StringProperty;
 
 /**
  * This is the basic Rule interface for PMD rules.
+ *
+ * <p><strong>Thread safety:</strong>
+ * PMD will create one instance of a rule per thread. The instances are
+ * not shared across different threads. However, a single rule instance is
+ * reused for analyzing multiple files.
+ * </p>
  */
 // FUTURE Implement Cloneable and clone()
 public interface Rule extends PropertySource {

pmd-core/src/main/java/net/sourceforge/pmd/processor/AbstractPMDProcessor.java

@@ -50,6 +50,15 @@ public abstract class AbstractPMDProcessor {
 			 );
 	}
 
+	/**
+	 * Create instances for each rule defined in the ruleset(s) in
+	 * the configuration.
+	 * Please note, that the returned instances <strong>must not</strong>
+	 * be used by different threads. Each thread must create its own
+	 * copy of the rules (see {@link PmdRunnable.PmdThread#getRuleSets(String)}).
+	 * @param factory
+	 * @return the rules within a rulesets
+	 */
 	protected RuleSets createRuleSets(RuleSetFactory factory) {
 		return RulesetsFactoryUtils.getRuleSets(configuration.getRuleSets(), factory);
 	}

src/site/markdown/customizing/howtowritearule.md

@@ -264,10 +264,10 @@ finds all private fields.  You can see the code that determines all the attribut
 
 Thanks to Miguel Griffa for writing a longer [XPath tutorial](xpathruletutorial.html).
 
-## I want to implement a rule that analyse more than the class!
+## I want to implement a rule that analyze more than the class!
 
 An obvious limitation of the previous mechanism is the "class-centric" focus of the rule. How can you implement a
-rule that checks stuff accross the all source code? Let's take a dummy example. Let's say you want to implement a
+rule that checks stuff across the all source code? Let's take a dummy example. Let's say you want to implement a
 rule that count how many Expression Node you have in your source code (told you, it was a dummy example :) ).
 
 You realize quite simply. You just have to add static field to the RulesContext, as an attribute, and uses
@@ -324,7 +324,7 @@ classname to the violation report.
 [sonar]: http://www.sonarsource.com/
 
 
-## I need somekind of Type Resolution for my rule!
+## I need some kind of Type Resolution for my rule!
 
 ### Inside an XPath query
 
@@ -367,6 +367,27 @@ you get the actual Class instance by calling getType().
 Otherwise, you'll have to string-compare the image, e.g.
 `"com.forbidden.class".equals(node.getImage())`
 
+## Thread safety, concurrency issues and reuse of rule instances
+
+When executing the rule, PMD will instantiate a new instance of your rule.
+If PMD is executed in multiple threads, then each thread is using its own
+instance of the rule. This means, that the rule implementation
+**does not need to care about threading issues**,
+as PMD makes sure, that a single instance is not used
+concurrently by multiple threads.
+
+However, for performance reasons, the rule instances are used for multiple files. This means,
+that the constructor of the rule is only executed once (per thread) and the rule instance is reused.
+If you rely on a proper initialization of instance properties, you can do the initialization
+e.g. in the visit-method of the `ASTCompilationUnit` AST node - which is visited as first node
+and only once per file. However, this solution would only work for rules written for the Java language.
+A language independent way is to override the method `apply` of the rule (and call super). The
+apply method is called exactly once per file.
+
+If you want to share data across multiple files, see the above section
+"I want to implement a rule that analyze more than the class".
+
+
 ## Bundle it up
 
 To use your rules as part of a nightly build or whatever, it's helpful to bundle up both