设计模式-单例模式

类加载到内存后，就实例化一个单例，JVM保证线程安全。类就加载一次。
简单使用，推荐使用。
唯一缺点：不管用到与否，类加载时就完成实例化

package com.yc.designmode.singgleton;

public class Main01 {
    private Main01(){};
    public static final Main01 main01= new Main01();
    public  static Main01 getInstance (){
        return main01;
    }

    public static void main(String[] args) {
        Main01 instance = Main01.getInstance();
        Main01 instance2 = Main01.getInstance();
        System.out.println(instance ==instance2);
    }
}

package com.yc.designmode.singgleton;

import com.yc.designmode.util.ThreadUtil;

import java.util.concurrent.TimeUnit;


public class Main02 {
    private static Main02 instance;
    private Main02 (){};
    public   static Main02 getInstance(){ //线程不安全
   // public  synchronized  static Main02 getInstance(){ //线程安全 但是效率慢
        if (instance ==null){
        ThreadUtil.sleep(1000);
            instance=new Main02();
        }
        return  instance;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
             new Thread(()->{
                 Main02 instance = getInstance();
                 System.out.println(instance.hashCode());
             }).start();
        }
    }
}

package com.yc.designmode.singgleton;

import com.yc.designmode.util.ThreadUtil;

import java.util.concurrent.TimeUnit;


public class Main02 {
    private static Main02 instance;
    private Main02 (){};
     public  synchronized  static Main02 getInstance(){ //线程安全 但是效率慢
        if (instance ==null){
        ThreadUtil.sleep(1000);
            instance=new Main02();
        }
        return  instance;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
             new Thread(()->{
                 Main02 instance = getInstance();
                 System.out.println(instance.hashCode());
             }).start();
        }
    }
}

使用synchronized 虽然线程安全,但是效率不高,我们进行改进
使用双重验证的形式进行判断

package com.yc.designmode.singgleton;

import com.yc.designmode.util.ThreadUtil;

import java.util.concurrent.TimeUnit;


public class Main02 {
    private static Main02 instance;
    private Main02 (){};
    public   static Main02 getInstance(){ //线程不安全
    // 改进版
        if (instance ==null){// 先进行判断,没有锁.效率高,即使多线线程访问到,后面也有锁进行保证安全,
        					//当返回第一个实例的时候,就不满足 == null条件,直接返回
            synchronized (Main02.class) {
                if (instance ==null){
                   
                    instance=new Main02();
                }
            }

        }
        return  instance;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
             new Thread(()->{
                 Main02 instance = getInstance();
                 System.out.println(instance.hashCode());
             }).start();
        }
    }
}