我们一起聊聊Java极简设计模式：单例模式（Singleton）

package io.binghe.concurrency.example.singleton;

public class SingletonExample1 {

    private SingletonExample1(){}

    private static SingletonExample1 instance = null;

    public static SingletonExample1 getInstance(){
        //多个线程同时调用，可能会创建多个对象
        if (instance == null){
            instance = new SingletonExample1();
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;

public class SingletonExample2 {

    private SingletonExample2(){}

    private static SingletonExample2 instance = new SingletonExample2();

    public static SingletonExample2 getInstance(){
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;

public class SingletonExample3 {

    private SingletonExample3(){}

    private static SingletonExample3 instance = null;

    public static synchronized SingletonExample3 getInstance(){
        if (instance == null){
            instance = new SingletonExample3();
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;

public class SingletonExample4 {

    private SingletonExample4(){}

    private static SingletonExample4 instance = null;

    //线程不安全
    //当执行instance = new SingletonExample4();这行代码时，CPU会执行如下指令：
    //1.memory = allocate() 分配对象的内存空间
    //2.ctorInstance() 初始化对象
    //3.instance = memory 设置instance指向刚分配的内存
    //单纯执行以上三步没啥问题，但是在多线程情况下，可能会发生指令重排序。
    // 指令重排序对单线程没有影响，单线程下CPU可以按照顺序执行以上三个步骤，但是在多线程下，如果发生了指令重排序，则会打乱上面的三个步骤。

    //如果发生了JVM和CPU优化，发生重排序时，可能会按照下面的顺序执行：
    //1.memory = allocate() 分配对象的内存空间
    //3.instance = memory 设置instance指向刚分配的内存
    //2.ctorInstance() 初始化对象


    //假设目前有两个线程A和B同时执行getInstance()方法，A线程执行到instance = new SingletonExample4(); B线程刚执行到第一个 if (instance == null){处，
    //如果按照1.3.2的顺序，假设线程A执行到3.instance = memory 设置instance指向刚分配的内存，此时，线程B判断instance已经有值，就会直接return instance;
    //而实际上，线程A还未执行2.ctorInstance() 初始化对象，也就是说线程B拿到的instance对象还未进行初始化，这个未初始化的instance对象一旦被线程B使用，就会出现问题。


    public static SingletonExample4 getInstance(){
        if (instance == null){
            synchronized (SingletonExample4.class){
                if(instance == null){
                    instance = new SingletonExample4();
                }
            }
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;

public class SingletonExample5 {

    private SingletonExample5(){}

    //单例对象  volatile + 双重检测机制来禁止指令重排
    private volatile static SingletonExample5 instance = null;

    public static SingletonExample5 getInstance(){
        if (instance == null){
            synchronized (SingletonExample5.class){
                if(instance == null){
                    instance = new SingletonExample5();
                }
            }
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;

public class SingletonExample6 {

    private SingletonExample6(){}

    private static SingletonExample6 instance = null;

    static {
        instance = new SingletonExample6();
    }

    public static SingletonExample6 getInstance(){
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;

public class SingletonExample7 {

    private SingletonExample7(){}

    public static SingletonExample7 getInstance(){
        return Singleton.INSTANCE.getInstance();
    }

    private enum Singleton{
        INSTANCE;
        private SingletonExample7 singleton;

        //JVM保证这个方法绝对只调用一次
        Singleton(){
            singleton = new SingletonExample7();
        }
        public SingletonExample7 getInstance(){
            return singleton;
        }
    }
}