Java ConcurrentHashMap在多线程应用程序中未提供正确的输出
我创建了一个示例来理解并发HashMap,但它似乎在7-8次运行后给出了不确定的输出 在我的示例中,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),它们更新了一个concurent HashMap,该HashMap只有3个键值对(键值是三个名称a、B、C,运行结束时的值应该是三个主题的累积分数) 我正在张贴下面的代码,请告知 下面显示了错误的输出(正确的输出应该以全部完成main:{C=27,A=57,B=42}结尾) ConcurrentHashMapExample类:Java ConcurrentHashMap在多线程应用程序中未提供正确的输出,java,concurrency,Java,Concurrency,我创建了一个示例来理解并发HashMap,但它似乎在7-8次运行后给出了不确定的输出 在我的示例中,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),它们更新了一个concurent HashMap,该HashMap只有3个键值对(键值是三个名称a、B、C,运行结束时的值应该是三个主题的累积分数) 我正在张贴下面的代码,请告知 下面显示了错误的输出(正确的输出应该以全部完成main:{C=27,A=57,B=42}结尾) 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;
}
}
package com.ll.thread.concurrency;
导入java.util.Iterator;
导入java.util.concurrent.ConcurrentHashMap;
导入java.util.concurrent.CountDownLatch;
公共类ConcurrentHashMapExample{
公共静态void main(字符串[]args){
//TODO自动生成的方法存根
ConcurrentHashMap ConcurrentHashMap=新的ConcurrentHashMap();
concurrentHashMap.put(“A”,0);
concurrentHashMap.put(“B”,0);
concurrentHashMap.put(“C”,0);
CountDownLatch CountDownLatch=新的CountDownLatch(3);
Runnable runnableScience=新工人(concurrentHashMap,“科学”,倒计时锁存);
Runnable runnableMath=新工作者(concurrentHashMap,“数学”,countDownLatch);
Runnable runnableEnglish=新工人(concurrentHashMap,“英语”,倒计时);
新线程(runnableScience,“Science”).start();
新线程(runnableMath,“Math”).start();
新线程(runnableEnglish,“English”).start();
试一试{
倒计时闩锁。等待();
}捕捉(中断异常e){
//TODO自动生成的捕捉块
e、 printStackTrace();
}
System.out.println(“全部完成”+Thread.currentThread().getName()+”:“+concurrentHashMap);
concurrentHashMap=null;
}
}
工人阶级:
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();
}
}
}
package com.ll.thread.concurrency;
导入java.util.Iterator;
导入java.util.concurrent.ConcurrentHashMap;
导入java.util.concurrent.CountDownLatch;
公共类Worker实现了Runnable{
ConcurrentHashMap ConcurrentHashMap;
字符串threadName=“”;
倒计时闩锁倒计时闩锁;
公共工作者(ConcurrentHashMap ConcurrentHashMap,
字符串主题,CountDownLatch(CountDownLatch){
this.concurrentHashMap=concurrentHashMap;
this.threadName=主题;
this.countDownLatch=countDownLatch;
//TODO自动生成的构造函数存根
}
@凌驾
公开募捐{
//TODO自动生成的方法存根
System.out.println(“运行内”+Thread.currentThread().getName()+”:“+concurrentHashMap);
试一试{
for(Iterator Iterator=concurrentHashMap.keySet().Iterator();Iterator.hasNext();){
String key=iterator.next();
//已同步(此){
if(“Math”.equals(Thread.currentThread().getName())){
如果(“A”。等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+20);
否则,如果(“B”。等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+15);
否则,如果(“C”等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+10);
}否则
if(“Science”.equals(Thread.currentThread().getName())){
如果(“A”。等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+19);
否则,如果(“B”。等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+14);
否则,如果(“C”等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+9);
}
其他的
if(“English”.equals(Thread.currentThread().getName())){
如果(“A”。等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+18);
否则,如果(“B”。等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+13);
否则,如果(“C”等于(键))
concurrentHashMap.put(key,concurrentHashMap.get(key)+8);
}
}
// }
}
最后{
System.out.println(“已完成”+Thread.currentThread().getName()+”:“+concurrentHashMap);
countdownlock.countDown();
}
}
}
我认为您在测试中不需要ConcurrentHashMap
。在执行线程之前初始化映射。相反,您可以使用由String
键和AtomicInteger
值参数化的常规HashMap
:
HashMap<String, AtomicInteger> concurrentHashMap = ...
concurrentHashMap.put("A", new AtomicInteger(0));
...
你期待什么?
ConcurrentHashMap
(在ConcurrentHashMap
中)意味着,为了保护其内部结构,map尽可能地负责同步。这并不意味着使用它会自动修复使用map的代码中的竞争条件。concurrentHashMap.put(key,concurrentHashMap.get(key)+19)代码>不是原子的:映射可能会在get和put之间更新…从显示的代码来看,您没有正确使用它!我建议您首先阅读接口ConcurrentMap的javadocs(ConcurrentHashMap是一个实现),特别是理解为什么需要新方法Hanks..查看了文档..似乎我的用例不适合测试ConcurrentHashMap。
HashMap<String, AtomicInteger> concurrentHashMap = ...
concurrentHashMap.put("A", new AtomicInteger(0));
...
concurrentHashMap.get(key).addAndGet(20);