众所周知hashMap　是线程不安全的，在多线程访问的情况下，要做同步的处理

ConcurrentHashMap get()　总能拿到最新的值，类似于关键字　volatile保证100%读取到最新的数据

如下用代码说明：

 

package com.iteye.javaso.demo;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class CopyOfConCurrentHashMapThreadTest2 {

    ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();

    public static void main(String args[]) {
        CopyOfConCurrentHashMapThreadTest2 test = new CopyOfConCurrentHashMapThreadTest2();
        Runnable sleep = new ThreadSleep2(test, "第一条线程");

        ThreadSecond2 charge2 = new ThreadSecond2(test, "改变值的第2线程");
        ThreadSecond2 charge3 = new ThreadSecond2(test, "改变值的第3线程");
        ThreadSecond2 charge4 = new ThreadSecond2(test, "改变值的第4线程");

        ThreadSecond23 chargeXX = new ThreadSecond23(test, "改变值的XXXX线程");

        ExecutorService exc = Executors.newFixedThreadPool(5);

        exc.execute(sleep);

        exc.execute(charge3);
        exc.execute(charge2);
        exc.execute(chargeXX);
        exc.execute(charge4);
        exc.shutdown();
        System.out.println("洗洗睡吧－－－－－－－－－－－－－－－－");
        try {
            Thread.sleep(16000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("多个线程后，最终运行结果：" + test.map.get("firstKey"));
    }

    public  void put(String value, int state) {
        map.put("firstKey", value);

        // Thread thread= Thread.currentThread();
        if (state == 0) {
            System.out.println("开始睡觉了--------------");
            try {
                Thread.sleep(4000);
                System.out.println("睡醒了－－－－－－－");
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        }

    }

}

class ThreadSleep2 extends Thread {

    ConcurrentHashMap<String, String> map;

    public ThreadSleep2(ConcurrentHashMap<String, String> map, String threadName) {

        this.map = map;
        this.name = threadName;
    }

    CopyOfConCurrentHashMapThreadTest2 test;

    public ThreadSleep2(CopyOfConCurrentHashMapThreadTest2 test,
            String threadName) {

        super(threadName);
        this.test = test;
        this.name = threadName;
    }

    @Override
    public void run() {
        Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
        System.out.println("---------------进入第一条线程-----睡十秒先--------");

        System.out.println("第一条线程未设置前值为：***　" + test.map.get("firstKey"));

        test.put(name, 0);

        System.out.println("第一条线程执行完毕  Map中值为：" +test.map.get("firstKey"));

    }

    String name = "";

    public String toString() {

        return "当前线程的名字是：" + name;
    }

}

class ThreadSecond2 extends Thread {

    ConcurrentHashMap<String, String> map;

    public ThreadSecond2(ConcurrentHashMap<String, String> map,
            String threadName) {
        super(threadName);
        this.map = map;
        this.name = threadName;
    }

    CopyOfConCurrentHashMapThreadTest2 test;

    public ThreadSecond2(CopyOfConCurrentHashMapThreadTest2 test,
            String threadName) {

        super(threadName);
        this.test = test;
        this.name = threadName;
    }

    @Override
    public void run() {
        System.out.println("-----------进入其它线程---------");
        System.out.println("当前线程是：" + this.name + " 未设置map前值为："
                + test.map.get("firstKey"));

        test.put(name, 2);
        System.out.println("hashMap中 firstKey值为：" + name);
    }

    String name = "";

    public String toString() {

        return "当前线程的名字是：" + name;
    }

}

class ThreadSecond23 extends Thread {

    ConcurrentHashMap<String, String> map;

    public ThreadSecond23(ConcurrentHashMap<String, String> map,
            String threadName) {
        super(threadName);
        this.map = map;
        this.name = threadName;
    }

    CopyOfConCurrentHashMapThreadTest2 test;

    public ThreadSecond23(CopyOfConCurrentHashMapThreadTest2 test,
            String threadName) {

        super(threadName);
        this.test = test;
        this.name = threadName;
    }

    @Override
    public void run() {
        System.out.println("-----------进入XXXXXXX线程---------");
        System.out.println("当前线程是：" + Thread.currentThread().getName());
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        test.put(name, 2);
        System.out.println("hashMap中 firstKey值为：" + name);
    }

    String name = "";

    public String toString() {

        return "当前线程的名字是：" + name;
    }

}