java单例模式

1.双重校验锁

public class SSingleton {
    private static SSingleton INSTANCE;
    public static SSingleton getInstance() {
        if (INSTANCE == null) {
            synchronized (SSingleton.class) {
                if (INSTANCE == null)
                    INSTANCE = new SSingleton();
            }
        }
        return INSTANCE;
    }
    private SSingleton() {
    }
}

2.枚举

public enum ESingleton {
    INSTANCE;
    private ESingleton() {
        System.out.println("ESingleton");
    }
    public void getI(){
        System.out.println("asdf");
    }
}

3.静态内部类

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

4.一种新的单例模式

public class RxSingleton {
    private static final AtomicReference<RxSingleton> INSTANCE = new AtomicReference<RxSingleton>();
    private RxSingleton() {
    }
    public static RxSingleton getInstance() {
        for (; ; ) {
            RxSingleton current = INSTANCE.get();
            if (current != null) {
                return current;
            }
            current = new RxSingleton();
            if (INSTANCE.compareAndSet(null, current)) {
                return current;
            }
        }
    }
}

5.饿汉

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

6.懒汉

线程不安全

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

线程安全,效率很低

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