java - 使用 AtomicReference 的单例

Question

使用 AtomicReference 的惰性初始化单例的实现是否正确？如果不是 - 可能的问题是什么？

import java.io.ObjectStreamException;
import java.io.Serializable;
import java.util.concurrent.atomic.AtomicReference;

public class Singleton implements Serializable {

    private static final Singleton _instance = new Singleton();

    private static AtomicReference<Singleton> instance = new AtomicReference<Singleton>();

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance.compareAndSet(null, _instance)) {
            synchronized (_instance) {
                _instance.init();
                instance.set(_instance);
            }
        }
        return instance.get();
    }

    private void init() {
        // do initialization
    }

    private Object readResolve() throws ObjectStreamException {
        return getInstance();
    }

}

Answer 1

不，这很糟糕:

public static Singleton getInstance() {
    // new "singleton" for every method call
    Singleton s = new Singleton();
                   ^^^^^^^^^^^^^^
    if (instance.compareAndSet(null, s)) {
        synchronized (s) {
            s.init();
        }
    }
    return instance.get();
}

使用 AtomicReference 是个好主意，但它行不通，因为 Java 没有惰性求值。

---

经典的 post 1.5 单例方法是:

渴望单例:

public final class Singleton{
    private Singleton(){}
    private static final Singleton INSTANCE = new Singleton();
    public Singleton getInstance(){return INSTANCE;}
}

带有内部 holder 类的惰性单例:

public final class Singleton{
    private Singleton(){}
    private static class Holder{
        private static final Singleton INSTANCE = new Singleton();
    }
    public static Singleton getInstance(){return Holder.INSTANCE;}
}

枚举单例:

public enum Singleton{
    INSTANCE;
}

你可能应该坚持其中之一