1. 定义
要点
-
与Java并发(二十二)----同步模式之保护性暂停中的保护性暂停中的 GuardObject 不同，不需要产生结果和消费结果的线程一一对应
-
这样的好处是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
2. 实现
// 线程安全，只有get方法，且没有子类
final 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;
}
}
// 消息队列类 Java 线程之间通信
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();
}
}
// 从队列头部获取消息并返回
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();
}
}
// 将新的消息加在尾部
queue.addLast(message);
queue.notifyAll();
}
}
}3. 应用
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(); // 模拟生产消息，三秒后返回response
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();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