TIOBE - Java Coding Standard

Rule: Design36

Synopsis:	Singleton is not thread safe
Language:	Java
Severity Level:	1
Category:	Design

Description:

Non-thread safe singletons can result in bad state changes. Eliminate static singletons if possible by instantiating the object directly. Static singletons are usually not needed as only a single instance exists anyway. Other possible fixes are to synchronize the entire method or to use an initialize-on-demand holder class (do not use the double-check idiom). See Effective Java, item 48.

private static Foo foo = null;

//multiple simultaneous callers may see partially initialized objects
public static Foo getFoo() {
    if (foo==null)
        foo = new Foo();
    return foo;
}