Java多線程系列六——Map實作類別

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參考資料：

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

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

Map的一些實作類別有及其特性

類	安全執行緒	特性
Hashtable	是	Key不能為null
HashMap	否	讀寫效率最高，但在Java6多線程環境下使用不當可能陷入死迴圈，進而導致CPU使用率過高(原理可參見：http://coolshell.cn/articles/9606.html)
Collections.synchronizedMap	是	Collections.SynchronizedMap在Map所有方法基礎上加鎖，效率與HashTable相當
ConcurrentHashMap	是	採用分段鎖，get一般不加鎖，put分段鎖，key/value不能為null，效率僅次於HashMap

以下代碼測試各類的讀寫效率

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: 測試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;    }}

輸出結果如下：

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

 

Java多線程系列六——Map實作類別