Java单例模式的9种实现方式

• Java单例模式的9种实现方式
• • • • 一.饿汉式
      • 二.懒汉式，线程不安全
      • 三.懒汉式，线程安全
      • 四.双检锁/双重校验锁（DCL，即 double-checked locking）
      • 五.静态内部类
      • 六.枚举
      • 七.双检锁/双重校验锁（DCL，即 double-checked locking）(加volatile)
      • 八.使用ThreadLocal实现单例
      • 九.使用CAS锁实现单例
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public class Singleton {  
    private static Singleton instance = new Singleton();  
    private Singleton (){}  
    public static Singleton getInstance() {  
    return instance;  
    }  
}

JDK中，java.lang.Runtime就是典型的使用饿汉式的单例模式

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

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

JDK 版本：JDK1.5 起

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;  
    }  
}

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

JDK 版本：JDK1.5 起

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

JDK 版本：JDK1.5 起

public class Singleton {  
	# 使用volatile设置内存屏障，确保线程访问资源可见性。
    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;  
    }  
}

public class Singleton {  
	// 用空间换时间
	private static final ThreadLocal tl = new ThreadLocal();
    @Override
    protected Singleton initialValue() {
        return new Singleton();
    }

    private Singleton() {}

    public static Singleton getInstance() {
        return tl.get();
    }
}

private static final AtomicReference INSTANCE = new AtomicReference();

    private Singleton() {
    }

    public static final Singleton getInstance() {
        for (; ; ) {
            Singleton current = INSTANCE.get();
            if (current != null) {
                return current;
            }

            current = new Singleton();
            if (INSTANCE.compareAndSet(null, current)) {
                return current;
            }
        }
    }

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