03_异步模式之生产者消费者

一、定义

要点：

• 与前面的保护性暂停中的 GuardObject 不同，不需要产生结果和消费结果的线程一一对应
• 消费队列可以用来平衡生产和消费的线程资源
• 生产者仅负责产生结果数据，不关心数据该如何处理，而消费者专心处理结果数据
• 消息队列是有容量限制的，满时不会再加入数据，空时不会再消耗数据
• JDK 中各种阻塞队列，采用的就是这种模式

---

二、实现

package com.gyz.interview.concurrent;

import lombok.extern.slf4j.Slf4j;

import java.io.IOException;
import java.util.LinkedList;
import java.util.List;

/**
 * @author: gongyuzhuo
 * @since: 2024-02-20 12:25
 * @description: 异步模式之生产者/消费者
 */
@Slf4j
public class Message {
    private int id;
    private Object message;

    public Message(int id, Object message) {
        this.id = id;
        this.message = message;
    }

    public int getId() {
        return id;
    }

    public Object getMessage() {
        return message;
    }
}

@Slf4j
class MessageQueue {
    private LinkedList<Message> queue;
    private int capacity;

    public MessageQueue(int capacity) {
        this.capacity = capacity;
        queue = new LinkedList<>();
    }

    public Message take() {
        synchronized (queue) {
            while (queue.isEmpty()) {
                log.debug("没货了, wait");
                try {
                    queue.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            Message message = queue.removeFirst();
            queue.notifyAll();
            return message;
        }
    }

    public void put(Message message) {
        synchronized (queue) {
            while (queue.size() == capacity) {
                log.debug("库存已达上限, wait");
                try {
                    queue.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            queue.addLast(message);
            queue.notifyAll();
        }
    }
}

---

三、* 应用

public static void main(String[] args) {
    MessageQueue messageQueue = new MessageQueue(2);
    // 4 个生产者线程, 下载任务
    for (int i = 0; i < 4; i++) {
        int id = i;
        new Thread(() -> {
            try {
                log.debug("download...");
                List<String> response = Downloader.download();
                log.debug("try put message({})", id);
                messageQueue.put(new Message(id, response));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }, "生产者" + i).start();
    }
    // 1 个消费者线程, 处理结果
    new Thread(() -> {
        while (true) {
            Message message = messageQueue.take();
            List<String> response = (List<String>) message.getMessage();
            log.debug("take message({}): [{}] lines", message.getId(), response.size());
        }
    }, "消费者").start();
}

package com.gyz.interview.concurrent;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;

public class Downloader {
    public static List<String> download() throws IOException {
        HttpURLConnection conn = (HttpURLConnection) new URL("https://www.baidu.com/").openConnection();
        List<String> lines = new ArrayList<>();
        try (BufferedReader reader =
                     new BufferedReader(new InputStreamReader(conn.getInputStream(), StandardCharsets.UTF_8))) {
            String line;
            while ((line = reader.readLine()) != null) {
                lines.add(line);
            }
        }
        return lines;
    }
}

某次运行结果

10:48:38.070 [生产者3] c.TestProducerConsumer - download...
10:48:38.070 [生产者0] c.TestProducerConsumer - download...
10:48:38.070 [消费者] c.MessageQueue - 没货了, wait
10:48:38.070 [生产者1] c.TestProducerConsumer - download...
10:48:38.070 [生产者2] c.TestProducerConsumer - download...
10:48:41.236 [生产者1] c.TestProducerConsumer - try put message(1)
10:48:41.237 [生产者2] c.TestProducerConsumer - try put message(2)
10:48:41.236 [生产者0] c.TestProducerConsumer - try put message(0)
10:48:41.237 [生产者3] c.TestProducerConsumer - try put message(3)
10:48:41.239 [生产者2] c.MessageQueue - 库存已达上限, wait
10:48:41.240 [生产者1] c.MessageQueue - 库存已达上限, wait
10:48:41.240 [消费者] c.TestProducerConsumer - take message(0): [3] lines
10:48:41.240 [生产者2] c.MessageQueue - 库存已达上限, wait
10:48:41.240 [消费者] c.TestProducerConsumer - take message(3): [3] lines
10:48:41.240 [消费者] c.TestProducerConsumer - take message(1): [3] lines
10:48:41.240 [消费者] c.TestProducerConsumer - take message(2): [3] lines
10:48:41.240 [消费者] c.MessageQueue - 没货了, wait