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单例模式的三种写法

程序员文章站 2022-07-13 23:34:28
...

1、懒汉式

public class SingleDemo {
    private static SingleDemo instance = null; //懒汉式
    private SingleDemo(){
        System.out.println(Thread.currentThread().getName()+"\t我是构造方法singleDemo");
    }

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

    public static void main(String[] args) {
        System.out.println(SingleDemo.getInstance() == SingleDemo.getInstance());
        System.out.println(SingleDemo.getInstance() == SingleDemo.getInstance());

//        for(int i = 1 ; i <= 100 ; i++){
//            new Thread(() -> {
//                SingleDemo.getInstance();
//            },String.valueOf(i)).start();
//        }

    }
}

2、饿汉式

public class EHanDemo {

    private static EHanDemo instance =new EHanDemo(); //饿汉式

    private EHanDemo(){}

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

3、双重检测锁模式

双重检测锁模式不一定线程安全,有指令重排的存在,加入volatile可以禁止指令重排

public class DoubleCheck { //双重检测锁

    private static volatile DoubleCheck instance = null;

    private DoubleCheck(){
        System.out.println(Thread.currentThread().getName() + "\t我是构造方法SingletonDemo()");
    }

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

    public static void main(String[] args) {
        for (int i = 0; i < 10 ; i++) {
            new Thread(() -> {
                DoubleCheck.getInstance();
            },String.valueOf(i)).start();
        }
    }
}
相关标签: java多线程 java