How to implement thread-safe lazy initialization?

Question

What are some recommended approaches to achieve thread-safe lazy initialization?

Answer 1

For singletons, you can delegate the task to the JVM code for static initialization.

public class Something {

    private Something() {

    }

    private static class LazyHolder {

            public static final Something INSTANCE = new Something();

    }

    public static Something getInstance() {

            return LazyHolder.INSTANCE;

    }

}

If you are using Apache Commons Lang, then you can use the variations o ConcurrentInitializer like LazyInitializer.

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();

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