本文主要是針對ThreadLocal和ConcurrentHashMap的使用說一個簡單的說明，其中示例說明了他們兩個從結果上看的明顯不同，掃盲篇

什麼是ThreadLocal

ThreadLocal並不是一個Thread，而是Thread的區域性變數。將類變數放到ThreadLocal型別的物件中，使變數在每個執行緒中都有獨立拷貝，不會出現一個執行緒讀取變數時而被另一個執行緒修改的現象。

ThreadLocal實現原理

ThreadLocal是如何做到為每一個執行緒維護變數的副本的呢？其實實現的思路很簡單：在ThreadLocal類中定義了一個ThreadLocalMap，每一個Thread中都有一個該型別的變數——threadLocals——用於儲存每一個執行緒的變數副本，Map中元素的鍵為執行緒物件，而值對應執行緒的變數副本。[來自百度百科]

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());  
    }  
}

結果

Thread2===Thread2
main===初始化值

可以明顯的看到兩個執行緒之前互不干擾。

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());
        }
    }  
}

結果

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