java 多线程操作map_Java 多线程中ConcurrentHashMap并发读写操作范例

范例1：

package com.contoso;

import java.util.Random;

import java.util.UUID;

import java.util.concurrent.ConcurrentHashMap;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class ConcurrentHashMapExample1 {

/**

* ConcurrentHashMap的并发访问.

* ConcurrentHashMap使用的是区间(或称分段)锁，锁定它里面存储的全部键-值对

* 区间锁通过几个锁来划分被保护的区域

* - 不需要抛ConcurrentModificationException

* - ConcurrentHashMap的size() 和 isEmpty() 可能是不正确的，不要信赖它们

* - ConcurrentHashMap支持原子操作, 不需要客户端锁定

* - 读取数据可以并发访问ConcurrentHashMap, 它的迭代器具有弱一致性

* - 只有少数几个写操作可以同时修改ConcurrentHashMap

public static void useConcurrentHashMap() {

System.out.println("=== ConcurrentHashMap ===");

ExecutorService executor = Executors.newCachedThreadPool();

Random random = new Random();

ConcurrentHashMap map = new ConcurrentHashMap();

// putIfAbsent是原子操作

// 如果key对应的值value不存在就put，且返回null；

// 如果key对应的值value已存在，则返回已存在的值，且value不能为null，否则会报空指针异常。

map.putIfAbsent(UUID.randomUUID(), random.nextInt(50));

// 模拟并发访问 - 插入不重复的键-值对

for (int i = 0; i < 100; i++) {

if (i % 3 == 0) {

// 写操作

executor.execute(() -> {

UUID key = UUID.randomUUID();

Integer value = random.nextInt(100);

System.out.println("Write " + key + " ： " + value);

map.putIfAbsent(key, value);

});

} else {

// 读操作

executor.execute(() -> System.err.println("Read " + map.values().toString()));

}

executor.shutdown();

}

public static void main(String[] args) {

useConcurrentHashMap();

}

run:

=== ConcurrentHashMap ===

Write c93da429-fdd8-4281-a5e2-69834c6af413 ： 41

Read [41, 15]

Write b75e309a-e2e3-4683-a04c-3a90b240ce01 ： 31

Read [41, 31, 15]

Write b35b76b4-1638-418f-a635-d4e3f2fe64c7 ： 19

Read [41, 19, 31, 15]

Write 1742a44d-049b-4d85-9c49-9bf047216d69 ： 48

Read [48, 41, 19, 31, 15]

Write f90e60f0-dddb-4c09-ab0f-c43a1eb98a6e ： 36

Read [48, 41, 19, 31, 15, 36]

Write 2d6a2f8b-2404-4847-8bfb-cb5a7cd0b4ab ： 24

Read [48, 41, 19, 24, 31, 15, 36]

Write 466ddf0c-7860-410c-8ca1-811a66ba4647 ： 10

Read [48, 41, 19, 24, 31, 15, 10, 36]

Write 29008ef0-9ccd-4a1d-9095-a06bdbd88f47 ： 10

Read [48, 10, 41, 19, 24, 31, 15, 10, 36]

Write 3519230b-2481-463d-a8ff-af5ed544e225 ： 30

Read [48, 10, 41, 19, 24, 31, 15, 10, 36]

Write 1f63fefa-bdbb-4a2b-a842-aea424044df0 ： 45

Read [48, 10, 41, 19, 24, 31, 15, 10, 45, 36, 30]

Write c41082e0-135a-4e27-bbb2-36c8f1e80807 ： 13

Write 6eeb2c87-46c8-4d1e-8637-438730c350ef ： 46

Read [48, 10, 46, 10, 36, 30, 41, 19, 24, 13, 31, 15, 45]

Write d310bc76-ad74-48f7-b2d3-11c4cf84743b ： 18

Read [48, 10, 46, 10, 18, 36, 30, 41, 19, 24, 13, 31, 15, 45]

Write 37cc879d-693f-49f0-8120-94a742a94df8 ： 0

Read [48, 10, 46, 0, 10, 18, 36, 30, 41, 19, 24, 13, 31, 15, 45]

Write 898596de-9211-4f5a-bba4-f089eaec5102 ： 47

Read [48, 10, 46, 0, 10, 18, 36, 30, 47, 41, 19, 24, 13, 31, 15, 45]

Write 0e8d4fa4-d242-4bda-9be9-f771c14e6dcd ： 40

Read [48, 10, 46, 0, 10, 18, 36, 30, 47, 41, 19, 24, 13, 31, 15, 45]

Read [48, 10, 46, 0, 40, 10, 18, 36, 30, 47, 41, 19, 24, 13, 31, 15, 45]

Write efa71463-55d1-4668-8847-6085a9eee869 ： 1

Read [48, 10, 46, 0, 40, 10, 18, 36, 30, 47, 41, 19, 24, 13, 31, 15, 45]

Read [48, 10, 46, 0, 40, 10, 18, 36, 30, 47, 41, 19, 24, 13, 1, 31, 15, 45]

Write 420addc9-4e89-4630-b1d6-bc97af487dc6 ： 38

Write 621a6d92-e46a-431a-a66e-59173a97efb4 ： 34

Read [48, 10, 34, 46, 0, 40, 10, 18, 36, 30, 47, 41, 38, 19, 24, 13, 1, 31, 15, 45]

Write eb3dfcbd-2b1b-453c-9213-df871784321f ： 28

Read [48, 10, 34, 46, 0, 40, 10, 18, 36, 30, 47, 41, 38, 19, 24, 13, 1, 31, 15, 45]

Read [48, 10, 34, 46, 0, 28, 40, 10, 18, 36, 30, 47, 41, 38, 19, 24, 13, 1, 31, 15, 45]

Write 7dd73bd6-24b0-46c6-b641-9d1b4ce1de5a ： 46

Write 054deb2d-4806-4a3b-a525-b3076c7a1a4e ： 22

Read [48, 10, 34, 46, 0, 28, 40, 10, 18, 36, 30, 47, 41, 38, 46, 19, 24, 13, 1, 31, 15, 45]

Write 1e3f6c76-952a-421b-ad0d-8785b2ce4cf8 ： 22

Write b221c6cc-a095-483c-b84b-778a40729305 ： 14

Read [10, 22, 0, 10, 22, 18, 36, 47, 41, 46, 13, 45, 48, 34, 46, 28, 40, 30, 38, 19, 24, 1, 31, 15]

Write ae961f26-72f5-456e-8af3-75f6d5c605ac ： 6

Write 7e143d8d-6449-4bc5-883a-a4637014279e ： 5