写一个线程安全的单例模式

写一个线程安全的单例模式

线程安全的单例模式有很多种写法：

1、饿汉式

我们以jdk源码为例，Runtime类的源码：
类加载为静态变量赋值。

public class Runtime {
    private static Runtime currentRuntime = new Runtime();

    public static Runtime getRuntime() {
        return currentRuntime;
    }

    private Runtime() {}
}

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2、懒汉式

synchronized修饰静态方法。

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

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

        return instance;
    }
    
}

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3、使用双重校验 + volatile

synchronized修饰代码块，volatile可见性。

public class Singleton {
	private static volatile Singleton instance = null;

	private Singleton() {}

	public static Singleton getInstance() {
	    if (instance == null) { // 先判断实例是否存在，不存在再加锁
	        synchronized (Singleton.class) {
	            if (instance == null) { // 多线程情况下，只有一个能进入，避免创建多个对象。
	                return instance = new Singleton();
	            }
	        }
	    }
	    return instance;
	}
	
}

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4、枚举单例

一个枚举就是一个单例。

enum SingletonEnum {
    INSTANCE;
}

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5、Lock

public class Singleton {
    private static Singleton instance;
    private static final Lock lock = new ReentrantLock();

    private Singleton() { }

    public static Singleton getInstance() {
        lock.lock();
        try {
            if (instance == null) {
                instance = new Singleton();
            }
            return instance;
        } finally {
            lock.unlock();
        }
    }
    
}

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6、AtomicReference + CAS

public class Singleton {
    private static final AtomicReference<Singleton> instance = new AtomicReference<Singleton>();

    private Singleton() { }

    public static Singleton getInstance() {
        for (;;) {
            Singleton singleton = instance.get();
            if (null != singleton) {
                return singleton;
            }

            singleton = new Singleton();
            if (instance.compareAndSet(null, singleton)) {
                return singleton;
            }
        }
    }
    
}

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