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 getI         


        

Answer 1

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

Example:

ConcurrentInitializer lazyInitializer = new LazyInitializer() {

        @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 fooSupplier = Suppliers.memoize(new Supplier() {
    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.