按需初始化持有者 Idiom 如何确保只有“单个”对象？实例被创建过吗？

Question

我浏览了维基 https://en.wikipedia.org/wiki/Initialization-on -demand_holder_idiom 来理解 Singleton 模式的按需持有者初始化习惯用法，尽管 wiki 提到了它如何是线程安全的，但它并没有说明这实际上如何确保只有一个实例创建过，即如果在两个不同的类中调用 Something.getInstance() 他们如何拥有相同的对象？

我是 Java 新手，所以任何帮助将不胜感激。

Answer 1

单例的基本模式：

//final so it can't be extended, not needed, but I like it for security
public final class MySingleton {
    //Private so no other class can access it directly
    //static so it stays resident in memory
    //final so it can't be nulled out or reassigned;
    //The constructor is called when the class is loaded.
    private static final MySingleton instance = new MySingleton();

    //The constructor is private so no more instances can be created
    private MySingleton() {
    }

    public static final MySingleton getInstance() {
       return instance;
    }

    //Alternatively, you could have the instance initialized to null
    //and have creation when getInstance() is called
    public static final MySingleton getIntance() {
        synchronized(instance) {
           if (instance == null) {
               instance = new MySingleton();
           }
           return instance
        }
}