单例模式大家都知道有两种方式，一种是饿汉式，一种是懒汉式

饿汉式：一开始就初始化实例

public class Singleton {

    private final static Singleton singleton = new Singleton();

    private Singleton(){}

    public static Singleton getInstance(){
        return singleton;
    }
}

懒汉式，记住这种双重检查的方式，保证线程的安全:

public class Singleton {

    private static volatile Singleton singleton;

    private Singleton() {}

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

这种静态内部类的方式，也很可靠：调用的时候才初始化

public class Singleton {

private Singleton() {}

private static class SingletonInstance {

private static final Singleton singleton = new Singleton();

}

public static Singleton getInstance() {

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    return SingletonInstance.singleton;

    }

}