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DoubleCheckedLocking

DoubleCheckedLocking

Partially created objects can be returned by the Double Checked Locking pattern when used in Java. An optimizing JRE may assign a reference to the baz variable before it calls the constructor of the object the reference points to.
Note: With Java 5, you can make Double checked locking work, if you declare the variable to be volatile.
For more details refer to: http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-double.html or http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
This rule is defined by the following Java class: net.sourceforge.pmd.lang.java.rule.basic.DoubleCheckedLockingRule
Example(s):


  
public class Foo {
	/*volatile */ Object baz = null; // fix for Java5 and later: volatile
	Object bar() {
		if (baz == null) { // baz may be non-null yet not fully created
			synchronized(this) {
				if (baz == null) {
					baz = new Object();
        		}
      		}
    	}
		return baz;
	}
}