java 線程池 Executors，ExecutorService

1，線程池對象

1，Executors 工廠類

• 方法 newFixedThreadPool(int num)，創建固定個數線程池，num 代表線程個數

• 方法 newCachedThreadPool()，創建彈性個數的線程池，當忙碌時，會不斷創建線程，當空閑時，會回收線程

2，ExecutorService 線程池

3，線程計數器 CountDownLatch

• new CountDownLatch(10)，一般實例化會傳入具體等待幾個線程
• countDownLatch.countDown()，會把傳入的值 減一，當數字是 0 的時候會繼續往下執行
• countDownLatch.await(60, TimeUnit.HOURS) ，等待countDownLatch.countDown()將傳入的值減到0，當超時的時候也會繼續往下執行

2，簡單的使用

package com.hwq.admin.back.service;

import com.hwq.common.exception.ServerException;
import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

@Service
public class ThreadPoolService {

    private final ExecutorService executorService = Executors.newFixedThreadPool(5);

    public List<String> listPoolName() {
        CountDownLatch countDownLatch = new CountDownLatch(10);
        List<String> list = new ArrayList<>();
        List<String> synList = Collections.synchronizedList(list);  // 注意線程安全
        for (int i = 0; i < 10; i++) {
            executorService.execute(() -> {
                try {
                    Thread.sleep(1000);
                    Thread t = Thread.currentThread();
                    synList.add(t.getId() + " : " + t.getName());
                    countDownLatch.countDown();           // 通知計數器完成任務
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }

        try {
            countDownLatch.await(60, TimeUnit.SECONDS);   // 線程等待
            return list;
        } catch (InterruptedException ex) {
            throw new ServerException(ex);
        }
    }
}