Re: Thread-safe Singleton Design Implementation

Eric Sosman <>
Mon, 30 Jul 2007 15:00:41 -0400
yaru22 wrote On 07/30/07 13:31,:

I was reading Wikipedia about Singleton Design pattern and it had some
sample codes for the implementation.

Under Java example implementation, there was "A thread-safe Java
programming language lazy-loaded solution." But I don't understand why
it is thread-safe & lazy-loaded solution. Could anyone explain it?

This is the source code from the Wikipedia (
wiki/Singleton_pattern) :
public class Singleton
  // Private constructor suppresses generation of a (public) default
  private Singleton() {}

  private static class SingletonHolder
    private final static Singleton INSTANCE = new Singleton();

  public static Singleton getInstance()
    return SingletonHolder.INSTANCE;

    Bloch explains this technique in "Effective Java."

    It is lazy-loaded because the SingletonHolder class
is not loaded until it is needed, and it won't be needed
until the first time someone calls getInstance.

    It is thread-safe because class loading is itself
thread-safe. Only one thread will load SingletonHolder.
If twenty-seven threads all call getInstance at the same
time, SingletonHolder is still loaded and initialized only
once, and all those threads see the same INSTANCE value.


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