本文阐述ConcurrentHashMap线程安全问题,ConcurrentHashMap可以保证多线程读写操作时的安全,实际代码使用时,可能会有以下误区,从下面的实例代码中进行演示。
两个线程分别进行++操作,总共加2000次,核对输出结果是否是2000;
有误区的实例代码
实例代码如下所示。
package com.yang.concurrent; import java.util.concurrent.ConcurrentHashMap; /** * 错误使用ConcurrentHashMap */ public class ErrorUseConcurrentHashMap implements Runnable { private static ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>(); /** * 这种写法会导致多线程情况下value是线程不安全的,和ConcurrentHashMap无关 */ @Override public void run() { for (int i = 0; i < 1000; i++) { int value = map.get("tom"); value++; map.put("tom", value); } } public static void main(String[] args) throws InterruptedException { map.put("tom", 0); ErrorUseConcurrentHashMap errorUseConcurrentHashMap = new ErrorUseConcurrentHashMap(); Thread thread1 = new Thread(errorUseConcurrentHashMap); Thread thread2 = new Thread(errorUseConcurrentHashMap); thread1.start(); thread2.start(); thread1.join(); thread2.join(); System.out.println("最终输出值:" + map.get("tom")); } }
输出结果如下图:
正确的实例代码
实例代码如下图,我们将上述的代码优化下。使用CAS的操作方式对其进行优化。
package com.yang.concurrent;
import java.util.concurrent.ConcurrentHashMap;
/**
* 正确使用ConcurrentHashMap
*/
public class CorrectUseConcurrentHashMap implements Runnable {
private static ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
/**
* 此处我们使用CAS操作方式,来替换Synchronized,提供效率
*/
@Override
public void run() {
for (int i = 0; i < 1000; i++) {
while (true){
int oldValue = map.get("tom");
int newValue=oldValue+1;
boolean flag=map.replace("tom",oldValue,newValue);
if (flag){
break;
}
}
}
}
public static void main(String[] args) throws InterruptedException {
map.put("tom", 0);
CorrectUseConcurrentHashMap errorUseConcurrentHashMap = new CorrectUseConcurrentHashMap();
Thread thread1 = new Thread(errorUseConcurrentHashMap);
Thread thread2 = new Thread(errorUseConcurrentHashMap);
thread1.start();
thread2.start();
thread1.join();
thread2.join();
System.out.println("最终输出值:" + map.get("tom"));
}
}
最终输出结果: