How to implement thread-safe lazy initialization?

Question

What are some recommended approaches to achieving thread-safe lazy initialization? For instance,

// Not thread-safe public Foo getInstance(){     if(INSTANCE == null){         INSTANCE = new Foo();     }      return INSTANCE; } 

Answer 1

If you're using Apache Commons Lang, then you can use one of the variations of ConcurrentInitializer like LazyInitializer.

Example:

ConcurrentInitializer<Foo> lazyInitializer = new LazyInitializer<Foo>() {          @Override         protected Foo initialize() throws ConcurrentException {             return new Foo();         }     }; 

You can now safely get Foo (gets initialized only once):

Foo instance = lazyInitializer.get(); 

If you're using Google's Guava:

Supplier<Foo> fooSupplier = Suppliers.memoize(new Supplier<Foo>() {     public Foo get() {         return new Foo();     } }); 

Then call it by Foo f = fooSupplier.get();

From Suppliers.memoize javadoc:

Returns a supplier which caches the instance retrieved during the first call to get() and returns that value on subsequent calls to get(). The returned supplier is thread-safe. The delegate's get() method will be invoked at most once. If delegate is an instance created by an earlier call to memoize, it is returned directly.