java 线程安全单例

1、饿汉，good

public class Singleton {

    private static Singleton instance = new Singleton();
    private Singleton (){}
    public static Singleton getInstance() {
         return instance;
    }

}

2、懒汉，用了synchronized 

public class Singleton {
    private static Singleton instance;
    private Singleton (){}
    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
       }
       return instance;
    }
}

3、懒汉，double check ，一定要用volatile 

public class Singleton {
    private volatile static Singleton singleton;
    private Singleton (){}
    public static Singleton getSingleton() {
        if (singleton == null) {
           synchronized (Singleton.class) {
              if (singleton == null) {
                   singleton = new Singleton();
              }
           }
        }
        return singleton;
    }
}

4、懒汉，good

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

5、Effective Java作者Josh Bloch 推荐

public enum Singleton {
    INSTANCE;
    public void whateverMethod() {
    }
}