Java单例模式——线程安全的懒汉模式

写法一：同步方法

class Single{
    private Single() {
    }

    private static Single instance = null;

    public static synchronized Single getInstance(){
        if (instance == null){
            instance = new Single();
        }
        return instance;
    }
}
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写法二：同步代码块

class Single{
    private Single(){}

    private static Single instance = null;
    public static Single getInstance(){
        synchronized(Single.class){
            if (instance == null){
                instance = new Single();
            }
        }
        return instance;
    }
}
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写法三：

class Single{
    private Single(){}

    private static volatile Single instance = null;

    public static Single getInstance(){
        if (instance == null) {
            synchronized (Single.class){
                if (instance == null){
                    instance = new Single();
                }
            }
        }
        return instance;
    }
}
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此方法会导致指令重排，所以要加上 volatile关键字