The tactic of initializing a static field the first time it is used, known as "lazy initialization", can be problematic in a multi-threaded context when used without proper synchronization. If a separate thread starts executing before the field is initialized, the thread may see an incompletely initialized object.

If lazy initialization is desirable for performance reasons, the best solution is usually to declare the enclosing method synchronized. Otherwise, avoid lazy initialization and initialize static fields using static initializers. A third possibility is to declare the field volatile and use the double-checked locking idiom as explained in the article referenced below. As the article points out, it is crucial to declare the field volatile: double-checked locking by itself is not correct under the Java memory model.

In the following example, the static field resource is initialized without synchronization.

In the following modification of the above example, Singleton uses the recommended approach of using a static initializer to initialize resource.

University of Maryland Department of Computer Science: The "Double-Checked Locking is Broken" Declaration.