单身形态 [英] Singleton pattern

问题描述

以下代码片段很简单，

public MyClass getInstance() {
    if(uniqueInstance == null) {
        uniqueInstance = new MyClass();
    }
    return uniqueInstance;
}

以下是做什么？

public MyClass getInstance() {
    if(uniqueInstance == null) {
        synchronized(MyClass.class) {
            uniqueInstance = new MyClass();
        }
    }
    return uniqueInstance;
}

推荐答案

它可以安全地阻止由至少两个线程引起的竞争条件，这两个线程同时进入，如果阻止。

It's a poor attempt to make it threadsafe to prevent a race condition caused by at least two threads which have simultaneously entered the if block.

更安全的方法是添加一个额外的空值，也称为双重锁定。

A bit more safe approach is adding an extra nullcheck, also known as double-checked locking.

public MyClass getInstance() {
    if (uniqueInstance == null) {
        synchronized(MyClass.class) {
            if (uniqueInstance == null) {
                uniqueInstance = new MyClass();
            }
        }
    }
    return uniqueInstance;
}

然而，我通常更喜欢 只需在Singleton上创建一个模式。

However, I usually prefer Just Create One pattern over Singleton.

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