ThreadLocal和ConcurrentHashMap的不同，扫盲篇

Posted 2022-12-09 yvan1115

本文主要是针对ThreadLocal和ConcurrentHashMap的使用说一个简单的说明，其中示例说明了他们两个从结果上看的明显不同，扫盲篇

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什么是ThreadLocal

ThreadLocal并不是一个Thread，而是Thread的局部变量。将类变量放到ThreadLocal类型的对象中，使变量在每个线程中都有独立拷贝，不会出现一个线程读取变量时而被另一个线程修改的现象。

ThreadLocal实现原理

ThreadLocal是如何做到为每一个线程维护变量的副本的呢？其实实现的思路很简单：在ThreadLocal类中定义了一个ThreadLocalMap，每一个Thread中都有一个该类型的变量——threadLocals——用于存储每一个线程的变量副本，Map中元素的键为线程对象，而值对应线程的变量副本。[来自百度百科]

示例

package com.yvan.concurrentMap;
/**
 * ThreadLocal
 * @author yvan
 *
 */
public class AppMainTest 
    private static ThreadLocal<Object> threadLocal = new ThreadLocal<Object>()

        @Override
        protected Object initialValue() 
            return "初始化值";
        

    ;
    public static void main(String[] args)   
        new Thread("Thread1")  
            @Override  
            public void run()   
                threadLocal.set(Thread.currentThread().getName());
              
        .start();  

        new Thread("Thread2")  
            @Override  
            public void run()   
                threadLocal.set(Thread.currentThread().getName());
                System.out.println(Thread.currentThread().getName()+"==="+threadLocal.get()); 
              
        .start();  

        new Thread("Thread3")  
            @Override  
            public void run()   
                threadLocal.set(Thread.currentThread().getName());
              
        .start();  
        System.out.println(Thread.currentThread().getName()+"==="+threadLocal.get());  
      

结果

Thread2===Thread2
main===初始化值

可以明显的看到两个线程之前互不干扰。

ThreadLocal和ConcurrentHashMap的不同用法

稍微修改一下示例

package com.yvan.concurrentMap;

import java.util.Map.Entry;
import java.util.concurrent.ConcurrentHashMap;

/**
 * ThreadLocal
 * @author yvan
 *
 */
public class AppMainTest 
    private static ConcurrentHashMap<String,String> map = new ConcurrentHashMap<String, String>(); 
    private static ThreadLocal<Object> threadLocal = new ThreadLocal<Object>()

        @Override
        protected Object initialValue() 
            return "初始化值";
        

    ;
    public static void main(String[] args)   
        new Thread("Thread1")  
            @Override  
            public void run()   
                threadLocal.set(Thread.currentThread().getName());
                map.put(Thread.currentThread().getName(), Thread.currentThread().getName());
              
        .start();  

        new Thread("Thread2")  
            @Override  
            public void run()   
                threadLocal.set(Thread.currentThread().getName());
                System.out.println(Thread.currentThread().getName()+"==="+threadLocal.get()); 
                map.put(Thread.currentThread().getName(), Thread.currentThread().getName());
              
        .start();  

        new Thread("Thread3")  
            @Override  
            public void run()   
                threadLocal.set(Thread.currentThread().getName());
                map.put(Thread.currentThread().getName(), Thread.currentThread().getName());
              
        .start();  
        System.out.println(Thread.currentThread().getName()+"==="+threadLocal.get());
        for (Entry<String, String> item : map.entrySet()) 
             System.out.println(item.getKey()+"==="+item.getValue());
        
      

结果

Thread2===Thread2
main===初始化值
Thread3===Thread3
Thread2===Thread2
Thread1===Thread1