单例模式之饿汉单例与懒汉单例

public class HungrySingleton {
    private static HungrySingleton instance = new HungrySingleton();

    private HungrySingleton() {
    }

    public static HungrySingleton getInstance() {
        return instance;
    }
}

懒汉

public class LazySingleton {
    // volailte出现在这个位置 是禁止指令重排序
    private volatile static LazySingleton instance;

    private LazySingleton() {
    }

    
    public static LazySingleton getInstance() {
        if (instance == null) {
            synchronized (LazySingleton.class) {
                if (instance == null) {
                    // 1, 分配内存
                    // 2,new 对象 (10  5)
                    // 3,给引用赋值 把内存地址  132
                    instance = new LazySingleton();
                }
            }
        }
        return instance;
    }
}