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我创建了一个例子来理解并发HashMap,但它似乎在7-8次运行后给出了不确定的输出。ConcurrentHashMap在多线程应用中没有给出正确的输出
在我的例子中,我创建了三个线程(它们模仿三个服务来获取数学,科学,英语的分数),它更新了一个只有3个键值对的共享HashMap(键是三个名字A,B,C并在运行结束时的值应该是三个科目的累积分数)
我发布了下面的代码,请注意。
错误的输出如下所示(正确的应该结束等来全部完成主:{C = 27,A = 57,B = 42})
Within run Math : {C=0, A=0, B=0}
Within run Science : {C=0, A=0, B=0}
Completed Science : {C=10, A=39, B=29}
Completed Math : {C=10, A=39, B=29}
Within run English : {C=0, A=0, B=0}
Completed English : {C=18, A=57, B=42}
All Done main : {C=18, A=57, B=42}
ConcurrentHashMapExample类:
package com.ll.thread.concurrency;
import java.util.Iterator;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
public class ConcurrentHashMapExample {
public static void main(String[] args) {
// TODO Auto-generated method stub
ConcurrentHashMap<String, Integer> concurrentHashMap = new ConcurrentHashMap<String, Integer>();
concurrentHashMap.put("A", 0);
concurrentHashMap.put("B", 0);
concurrentHashMap.put("C", 0);
CountDownLatch countDownLatch = new CountDownLatch(3);
Runnable runnableScience = new Worker(concurrentHashMap , "Science" , countDownLatch);
Runnable runnableMath = new Worker(concurrentHashMap , "Math" , countDownLatch);
Runnable runnableEnglish = new Worker(concurrentHashMap , "English" , countDownLatch);
new Thread(runnableScience , "Science").start();
new Thread(runnableMath ,"Math").start();
new Thread(runnableEnglish ,"English").start();
try {
countDownLatch.await();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("All Done " + Thread.currentThread().getName() + " : "+concurrentHashMap);
concurrentHashMap = null;
}
}
Worker类:
package com.ll.thread.concurrency;
import java.util.Iterator;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
public class Worker implements Runnable{
ConcurrentHashMap<String, Integer> concurrentHashMap ;
String threadName = "";
CountDownLatch countDownLatch ;
public Worker(ConcurrentHashMap<String, Integer> concurrentHashMap,
String subject, CountDownLatch countDownLatch) {
this.concurrentHashMap = concurrentHashMap;
this.threadName = subject ;
this.countDownLatch = countDownLatch;
// TODO Auto-generated constructor stub
}
@Override
public void run() {
// TODO Auto-generated method stub
System.out.println("Within run " +Thread.currentThread().getName() + " : "+concurrentHashMap);
try{
for(Iterator<String> iterator = concurrentHashMap.keySet().iterator() ; iterator.hasNext();){
String key = iterator.next();
//synchronized (this) {
if("Math".equals(Thread.currentThread().getName())) {
if("A".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +20);
else if("B".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) + 15);
else if("C".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +10);
}else
if("Science".equals(Thread.currentThread().getName())) {
if("A".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +19);
else if("B".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +14);
else if("C".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +9);
}
else
if("English".equals(Thread.currentThread().getName())) {
if("A".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +18);
else if("B".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +13);
else if("C".equals(key))
concurrentHashMap.put(key, concurrentHashMap.get(key) +8);
}
}
// }
}
finally{
System.out.println("Completed " + Thread.currentThread().getName() + " : " + concurrentHashMap);
countDownLatch.countDown();
}
}
}
什么办法? 'Concurrent'(在'ConcurrentHashMap'中)意味着,为了保护其内部结构,映射尽可能地保持同步。这并不意味着,使用它会自动修复使用地图的代码中的竞争条件。 – Dirk
'concurrentHashMap.put(key,concurrentHashMap.get(key)+19);'不是原子的:地图可以在get和put之间更新... – assylias
你没有正确使用它显示的代码!我建议你从阅读接口ConcurrentMap(ConcurrentHashMap是一个实现)的javadocs开始,特别是理解为什么需要新方法 – Scorpion