Re: SingletonHolder reinvented.

"Daniel Pitts" <>
12 Feb 2007 13:03:46 -0800
On Feb 12, 7:55 am, "Andrey Ryabov" <> wrote:

Just simplified version:

public class SingletonHolder<T> implements Callable<T> {
        private AtomicReference<T> _instance = new AtomicReference<T>();
        private AtomicReference<FutureTask<T>> _future = new

        public T get() {
                try {
                        T result = _instance.get();
                        if (result != null) {
                                return result; // Return value if it has already been initialized.
                        if (_future.compareAndSet(null, new FutureTask<T>(this))) { //
create and try to set to _future
                                _future.get().run(); // run the task if previous operation
succeed, executed only once!
                        result = _future.get().get(); // get result of execution..
                        if (result == null) {
                                throw new IllegalStateException(...); // It must not be null
                        _instance.compareAndSet(null, result); // set only if it was not
set already
                        return result;
                } catch (InterruptedException e) {
                        throw new RuntimeException(...);
                } catch (ExecutionException e) {
                        throw new RuntimeException(...);

        // This method is to be overridden by subclasses.
        public T call() throws Exception {
                throw new IllegalStateException("call is not implemented");


What's wrong with this approach:

import java.util.concurrent.Future;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;

public class SafeSingletonFactory<T> {
    private T instance;
    final Future<T> factory;

    public SafeSingletonFactory(Callable<T> factory) {
        this.factory = new FutureTask<T>(factory);

    public T get() throws InterruptedException {
        try {
            return instance == null ? instance = factory.get() :
        } catch (InterruptedException e) {
            throw e;
        } catch (ExecutionException e) {
            if (e.getCause() instanceof Error) {
                throw (Error)e.getCause();
            if (e.getCause() instanceof RuntimeException) {
                throw (RuntimeException)e.getCause();
            throw new RuntimeException("Exception while initializing
singleton.", e.getCause());

This way, you don't have to worry about:
1. Thread safety and synchronization (delegated to FutureTask.get())
2. Extending your singleton class. One class can have many
SafeSingletonFactory<T> instances.
3. atomic operations (again, delgated to FutureTask.get());

So, to use in a Static Singleton Factory Method (which IMHO is a bad
idea to use, but seems to be the common idiom used for singletons:

public class MySingleton {
    public final static SafeSingletonFactory<MySingleton> instance =
             new SafeSingletonFactory<MySingleton>(new
Callable<MySingleton>() {
        public MySingleton call() {
            return new MySingleton();
    private MySingleton() {

Thus, you can call MySingleton.instance.get() safely from anywhere,
without too much overhead.

Actually, you could probably just use a Future<MySingleton> instance;
but it can be prematurely cancled by client code, and client code has
more exception handling to do.

I have to say though, in my experience, most Singleton patterns can be
refactored into Dependency Injection with a lot of benefit to the
overal program design.

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