How to solve the multi-threading problem in java for loop

java uses multi-threading to speed up loop efficiency

Thread pool with locking

Involved knowledge: Executors (thread pool), CountDownLatch ( Locking)

Advantages: The code is concise, easy to read, and the performance is stable;

Disadvantages: The thread pool created by Executors is public. If this cyclic multi-threading method is used in multiple places, It will rob the thread pool resources, so the running speed will also be reduced;

import java.util.*;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
 
public class test{
 
    /** 固定的线程池（当前线程池大小为5） */
    private static final ExecutorService executor = Executors.newFixedThreadPool(5);
    
    public static void main(String[] args) throws Exception {
        /**
         *  两个要点：
         *  1.用Executors实现固定大小的线程池，从而达到控制硬件资源消耗的目的。
         *  2.用CountDownLatch（闭锁），来确保循环内的多线程都执行完成后，再执行后续代码
         */
        
        // 初始化数据
        List<Map<String,Object>> list = new ArrayList<>();
        for(int i=0;i<50;i++){
            Map<String,Object> object = new HashMap<>();
            object.put("index",i);
            list.add(object);
        }
 
        // 初始化计时器
        CountDownLatch cdl = new CountDownLatch(list.size());
        System.out.println("====== 线程开始 =====");
 
        // 遍历
        for(Map<String,Object> object:list){
            // 开启线程
            executor.submit(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread t = Thread.currentThread();
                        String name = t.getName();
                        // 模拟运行耗时
                        Thread.sleep(500);
                        System.out.println(name+"：执行到"+object.get("index"));
                        object.put("status","已经执行过");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    // 闭锁-1
                    cdl.countDown();
                }
            });
        }
        
        // 调用闭锁的await()方法，该线程会被挂起，它会等待直到count值为0才继续执行
        // 这样我们就能确保上面多线程都执行完了才走后续代码
        cdl.await();
        
        //关闭线程池
        executor.shutdown();
        System.out.println("====== 线程结束 =====");
 
        // 校验多线程正确性
        for(Map<String,Object> object:list){
            System.out.println(object.get("index") + ":" + object.get("status"));
        }
 
    }
}

Use multi-threading in the for loop

Each service is responsible for one business. If you repeat the business multiple times, you need to use the for loop. , such as traversing a collection of stored ids and creating something for each id.

But using a single thread to execute tasks will waste a lot of time waiting for the last task to be executed, and once an error is reported during an execution, the task will stop executing, which obviously does not meet our requirements.

In this case, use multi-threading in the for loop.

import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
 
public class Test {
 
    /**
     * ThreadPool 自定义一个线程池
     */
    private static final ThreadPoolExecutor EXECUTOR = new ThreadPoolExecutor(2, 4, 60L, TimeUnit.SECONDS, new LinkedBlockingQueue<>(60000));
 
    public static void main(String[] args) {
        //自己的数组或集合,这里不再进行填充
        String[] ids = new String[10]
        
        for (String id : ids) {
            EXECUTOR.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        //需要执行的业务逻辑
                        System.out.println("业务逻辑正在执行");
                    } catch (Exception e) {
                        //todo
                    }
                }
            });
        }
    }
 
}

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