Java多线程系列六——Map实现类

参考资料：

https://crunchify.com/hashmap-vs-concurrenthashmap-vs-synchronizedmap-how-a-hashmap-can-be-synchronized-in-java/

https://*.com/questions/35534906/java-hashmap-getobject-infinite-loop

Map的一些实现类有及其特性

类	线程安全	特性
Hashtable	是	Key不能为null
HashMap	否	读写效率最高，但在Java6多线程环境下使用不当可能陷入死循环，进而导致CPU使用率过高(原理可参见：http://coolshell.cn/articles/9606.html)
Collections.synchronizedMap	是	Collections.SynchronizedMap在Map所有方法基础上加锁，效率与HashTable相当
ConcurrentHashMap	是	采用分段锁，get一般不加锁，put分段锁，key/value不能为null，效率仅次于HashMap

以下代码测试各类的读写效率

import java.util.Collections;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

/**
 * @Description: 测试map
*/
public class ThreadMapTest {
public static void main(String[] args) throws InterruptedException {
        Map<Integer, Integer> hashtable = new Hashtable<>();
        Map<Integer, Integer> hashmap = new HashMap<>();
        Map<Integer, Integer> synchronizedHashMap = Collections.synchronizedMap(new HashMap<>());
        Map<Integer, Integer> concurrentHashMap = new ConcurrentHashMap<>();

        test(hashtable);
        test(hashmap);
        test(synchronizedHashMap);
        test(concurrentHashMap);
    }

private static void test(Map<Integer, Integer> map) throws InterruptedException {
int testTimes = 5;
long totalTimeMillis = 0;
for (int k = 0; k < testTimes; k++) {
            totalTimeMillis += costTimeMillis(map);
        }
        System.out.println("Test " + map.getClass() + " average time " + (totalTimeMillis / testTimes));
    }

private static long costTimeMillis(Map<Integer, Integer> map) throws InterruptedException {
int count = 5;
        ExecutorService executorService = Executors.newFixedThreadPool(count);
long startMillis = System.currentTimeMillis();
for (int i = 0; i < count; i++) {
            executorService.execute(new Runnable() {
                @Override
public void run() {
for (int j = 0; j < 500000; j++) {
                        map.put(0, 0);
                        map.get(0);
                    }
                }
            });
        }
        executorService.shutdown();
        executorService.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
return System.currentTimeMillis() - startMillis;
    }
}

输出结果如下：

Test class java.util.Hashtable average time 267
Test class java.util.HashMap average time 67
Test class java.util.Collections$SynchronizedMap average time 262
Test class java.util.concurrent.ConcurrentHashMap average time 167