【Java多线程系列六】Map实现类

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：

package com.concurrent.test;

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;

/**

 *  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<Integer, Integer>());

         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(final 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 < 50000; 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 104

Test class java.util.HashMap average time 31

Test class java.util.Collections$SynchronizedMap average time 79

Test class java.util.concurrent.ConcurrentHashMap average time 66

巴特西

【Java多线程系列六】Map实现类

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