Java多线程16 Guarded Suspension设计模式

Java多线程目录

Guarded Suspension意为保护暂停，其核心思想是仅当服务进程准备好时，才提供服务。设想一种场景，服务器可能会在很短时间内承受大量的客户端请求，客户端请求的数量可能超过服务器本身的即时处理能力，而服务端程序又不能丢弃任何一个客户请求。此时，最佳的处理方案莫过于让客户端要求进行排队，由服务端程序一个接一个处理。这样，既保证了所有的客户端请求均不丢失，同时也避免了服务器由于同时处理太多的请求而崩溃

1.Guarded Suspension模式的结构

Guarded Suspension模式的主要成员有：Request、RequestQueue、ClientThread、 ServerThread

Request：表示客户端请求
RequestQueue：用于保存客户端请求队列
ClientThread：客户端进程
ServerThread：服务器进程
其中，ClientThread负责不断发起请求，并将请求对象放入请求队列。ServerThread则根据其自身的状态，在有能力处理请求时，从RequestQueue中提取请求对象加以处理。

从流程图中可以看到，客户端的请求数量超过了服务线程的能力。在频繁的客户端请求中，RequestQueue充当了中间缓存，存放未处理的请求，保证了客户请求不丢失，同时也保护了服务线程不会受到大量并发的请求，而导致计算机资源不足

2. Guarded Suspension模式的简单实现

public class ClientThread extends Thread {

    private final RequestQueue queue;

    private final Random random;

    private final String sendValue;

    public ClientThread(RequestQueue queue, String sendValue) {
        this.queue = queue;
        this.sendValue = sendValue;
        this.random = new Random(System.currentTimeMillis());
    }

    @Override
    public void run() {

        for (int i = 0; i < 10; i++) {
            System.out.println("Client -> request " + sendValue);
            queue.putRequest(new Request(sendValue));

            try {
                Thread.sleep(random.nextInt(1000));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class Request {

    private final String value;

    public Request(String value) {
        this.value = value;
    }

    public String getValue() {
        return value;
    }
}

public class RequestQueue {

    private final LinkedList queue = new LinkedList<>();

    public Request getRequest() {
        synchronized (queue) {
            while (queue.size() <= 0) {
                try {
                    queue.wait();
                } catch (InterruptedException e) {
                    return null;
                }
            }
            return queue.removeFirst();
        }
    }

    public void putRequest(Request request) {

        synchronized (queue) {
            queue.addLast(request);
            queue.notifyAll();
        }
    }
}

public class ServerThread extends Thread {

    private final RequestQueue queue;

    private final Random random;

    private volatile boolean closed = false;

    public ServerThread(RequestQueue queue) {
        this.queue = queue;
        random = new Random(System.currentTimeMillis());
    }

    @Override
    public void run() {

        while (!closed) {
            Request request = queue.getRequest();
            if (null == request) {
                System.out.println("Received the empty request.");
                continue;
            }
            System.out.println("Server ->" + request.getValue());
            try {
                Thread.sleep(random.nextInt(1000));
            } catch (InterruptedException e) {
                return;
            }
        }
    }

    public void close() {
        this.closed = true;
        this.interrupt();
    }
}

public class SuspensionClient {
    public static void main(String[] args) throws InterruptedException {

        final RequestQueue queue = new RequestQueue();
        new ClientThread(queue,"Jack").start();
        ServerThread serverThread =  new ServerThread(queue);
        serverThread.start();

        Thread.sleep(10000);
        serverThread.close();
    }
}

运行

Client -> request Jack
Server ->Jack
Client -> request Jack
Server ->Jack
Client -> request Jack
Server ->Jack
Client -> request Jack
Server ->Jack
Client -> request Jack
Client -> request Jack
Client -> request Jack
Server ->Jack
Client -> request Jack
Client -> request Jack
Server ->Jack
Client -> request Jack
Server ->Jack
Server ->Jack
Server ->Jack
Server ->Jack
Received the empty request.