【Java寒假打卡】Java基础-并发工具类

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【Java寒假打卡】Java基础-并发工具类

HashMap在多线程下的问题

package com.hfut.edu.test14;

import com.sun.jdi.request.StepRequest;

import java.util.HashMap;

public class test2 {
    public static void main(String[] args) throws InterruptedException {
        HashMap<String, String> hm = new HashMap<>();
        Thread t1 = new Thread(()->{
            for (int i = 0; i < 25; i++) {
                hm.put(i + "",i + "");
            }
        });

        Thread t2 = new Thread(()->{
            for (int i = 25; i < 51; i++) {
                hm.put(i + "", i+ "");
            }
        });

        t1.start();
        t2.start();

        System.out.println("------------------------------");
        Thread.sleep(1000);// 主线程休眠一秒
        for (int i = 0; i < 51; i++) {
            System.out.println(hm.get(i + ""));
        }
    }
}

HashMap是线程不安全的多线程环境下可能会存在问题
为了保证数据的安全性我们可以使用Hashtable,但是Hashtable效率低下

使用Hashtable

package com.hfut.edu.test14;

import com.sun.jdi.request.StepRequest;

import java.util.HashMap;
import java.util.Hashtable;

public class test2 {
    public static void main(String[] args) throws InterruptedException {
        Hashtable<String, String> hm = new Hashtable<>();
        Thread t1 = new Thread(()->{
            for (int i = 0; i < 25; i++) {
                hm.put(i + "",i + "");
            }
        });

        Thread t2 = new Thread(()->{
            for (int i = 25; i < 51; i++) {
                hm.put(i + "", i+ "");
            }
        });

        t1.start();
        t2.start();

        System.out.println("------------------------------");
        Thread.sleep(1000);// 主线程休眠一秒
        for (int i = 0; i < 51; i++) {
            System.out.println(hm.get(i + ""));
        }
    }
}

关于Hashtable

Hashtable采取悲观锁synchronsized的形式保证数据的安全性
只要有线程进行访问会将整张表全部锁起来所以Hashtable的效率低下

在这里插入图片描述

ConcurrentHashMap

HashMap是线程不安全的多线程环境下会有数据安全问题
Hashtable是线程安全的但是会将整张表锁起来效率低下
ConcurrentHashMap也是线程安全的效率较高

package com.hfut.edu.test14;

import com.sun.jdi.request.StepRequest;

import java.util.ConcurrentModificationException;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.concurrent.ConcurrentHashMap;

public class test2 {
    public static void main(String[] args) throws InterruptedException {
        ConcurrentHashMap<String, String> hm = new ConcurrentHashMap<>();
        Thread t1 = new Thread(()->{
            for (int i = 0; i < 25; i++) {
                hm.put(i + "",i + "");
            }
        });

        Thread t2 = new Thread(()->{
            for (int i = 25; i < 51; i++) {
                hm.put(i + "", i+ "");
            }
        });

        t1.start();
        t2.start();

        System.out.println("------------------------------");
        Thread.sleep(1000);// 主线程休眠一秒
        for (int i = 0; i < 51; i++) {
            System.out.println(hm.get(i + ""));
        }
    }
}