梦里不知身是客带你一步步实现线程池异步回调 2016-03-29

转载请注明出处

作者:晓渡
文章地址:https://greatestrabit.github.io/2016/03/29/callback/

1.字面意义上的回调

字面意思上理解回调,就是A调用B,B回过头来再调用A,即是回调.既然是这样,当然就要求A中有B,B中有A.如下: 

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class A {
	/**
	 * 提出问题
	 * @author xiaodu.email@gmail.com
	 * @param b
	 * @param question
	 */
	public void ask(final B b, final String question) {
		b.answer(this, question);
	}

	/**
	 * 处理结果
	 * @author xiaodu.email@gmail.com
	 * @param answer
	 */
	public void processResult(final String answer) {
		System.out.println(answer);
	}
}

class B {
	/**
	 * 计算结果
	 * @author xiaodu.email@gmail.com
	 * @param a
	 * @param question
	 */
	public void answer(final A a, final String question) {
		if (question.equals("What is the answer to life, the universe and everything?")) {
			a.processResult("42");
		}
	}
}

/**
 * 相互调用
 * @author xiaodu.email@gmail.com
 *
 */
public class SyncObjectCallback {
	public static void main(final String[] args) {
		B b = new B();
		A a = new A();

		a.ask(b, "What is the answer to life, the universe and everything?");
	}
}

2.面向对象的回调

上面的写法中,B的对象只在方法中被传递了.实际上,这个B对象后来又调用了A中的方法,它的作用应该不止局限在一个方法中,而应该是A的一个部分.也就是,上面的写法不够”面向对象”,让我们来改造一下: 

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class A {
	private final B b;

	public A(final B b) {
		this.b = b;
	}

	public void ask(final String question) {
		this.b.answer(this, question);
	}

	public void processResult(final String answer) {
		System.out.println(answer);
	}
}

class B {
	public void answer(final A a, final String question) {
		if (question.equals("What is the answer to life, the universe and everything?")) {
			a.processResult("42");
		}
	}
}

/**
 * 面向对象的相互调用
 * @author xiaodu.email@gmail.com
 *
 */
public class SyncOOCallback {
	public static void main(final String[] args) {
		B b = new B();
		A a = new A(b);
		a.ask("What is the answer to life, the universe and everything?");
	}
}

3.面向接口的回调

上面的两个例子,估计没人会承认也是回调吧.因为并没什么卵用.不过这个流程对于理解回调是很重要的.其实回调真正有用的地方,在于它的”预测”能力.
我们扩展想象一下.假设上面例子中的B,为A提供了很多服务之后突然觉醒,想为更多的对象提供服务,这样一来,B就变成了Server.而且还要制定规则.规则是什么呢,就是要Server提供服务可以,对方一定要有一个recvAnswer接口供Server调用才行,这样Server才能把结果传回给Client.具体如何制定规则呢?通过Interface.如下: 

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/**
 * 发出请求着需要实现的接口,要实现处理结果的方法
 * @author xiaodu.email@gmail.com
 *
 */
public interface IClient {
	void recvAnswer(String answer);
}
  
/**
 * 响应请求者,即提供服务者
 * @author xiaodu.email@gmail.com
 *
 */
public class Server {
	public void answer(final IClient client, final String question) {
		if (question.equals("What is the answer to life, the universe and everything?")) {
			calclating();
			client.recvAnswer("42");
		}
	}

	private void calclating() {
		try {
			Thread.sleep(new Random().nextInt(5000));
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}
  
/**
 * 发出请求者,同时要处理请求结果
 * @author xiaodu.email@gmail.com
 *
 */
public class ClientSync implements IClient {
	private final Server server;

	public ClientSync(final Server server) {
		this.server = server;
	}

	public void ask(final String question) {
		this.server.answer(this, question);
	}

	@Override
	public void recvAnswer(final String answer) {
		System.out.println(answer);
	}
}
  
/**
 * 面向接口的同步回调
 * @author xiaodu.email@gmail.com
 *
 */
public class SyncInterfaceCallback {
	/**
	 * 使用内部类来实现的方式
	 * @author xiaodu.email@gmail.com
	 */
	private static void innerMain() {
		Server server = new Server();
		server.answer(new IClient() {
			@Override
			public void recvAnswer(final String answer) {
				System.out.println(answer);
			}
		}, "What is the answer to life, the universe and everything?");
	}

	public static void main(final String[] args) {
		Server server = new Server();
		ClientSync client = new ClientSync(server);
		client.ask("What is the answer to life, the universe and everything?");

		innerMain();
	}
}

注意,接口IClient实际上应该是属于Server端的,它是由Server制定的,需要Client来实现的接口,虽然看上去它跟Client很近.
为什么说有”预测”能力呢?想象另一个场景.Server现在是一个底层服务,这个底层服务知道迟早有一天会有高层服务来讨要数据,但是数据如何向上传递呢?底层可以承诺,只有你实现IClient接口,我就会调用其中的recvAnswer方法,把数据传上来.现在底层也可以调用高层的方法,算是有”预测”能力吧?

4.异步回调

上面的调用都是同步的.假设Server计算结果需要较长的时间,你一定希望它能在一个单独的线程中被执行,这是就可以把ask方法的调用用线程包装一下: 

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public class ClientAsync implements IClient {
	private final Server server;

	public ClientAsync(final Server server) {
		this.server = server;
	}

	/**
	 * 在线程中发出请求
	 * @author xiaodu.email@gmail.com
	 * @param question
	 */
	public void ask(final String question) {
		new Thread(new Runnable() {
			@Override
			public void run() {
				server.answer(ClientAsync.this, question);
			}
		}).start();
	}

	@Override
	public void recvAnswer(final String answer) {
		System.out.println(answer);
	}
}
  
/**
 * 基于接口的异步回调,每次建立新的线程
 * @author xiaodu.email@gmail.com
 *
 */
public class AsyncInterfaceCallback {
	/**
	 * 使用内部类的实现方式,此处可见回调地狱
	 * @author xiaodu.email@gmail.com
	 */
	private static void innerMain() {
		Server server = new Server();

		new Thread(new Runnable() {
			@Override
			public void run() {
				server.answer(new IClient() {
					@Override
					public void recvAnswer(final String answer) {
						System.out.println(answer);
					}
				}, "What is the answer to life, the universe and everything?");
			}
		}).start();
		System.out.println("asked ! waiting for the answer...");
	}

	public static void main(final String[] args) {
		Server server = new Server();
		ClientAsync client = new ClientAsync(server);
		client.ask("What is the answer to life, the universe and everything?");
		System.out.println("asked ! waiting for the answer...");

		innerMain();
	}
}

5.线程池异步回调

每次建立新的线程耗费资源巨大,为了重用线程,使用线程池管理异步调用,这时候就要求Client不仅要实现IClient接口,还要同时是一个任务,才能被线程池执行,如下: 

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/**
 * 专门用来执行请求的任务,供线程池调用
 * @author xiaodu.email@gmail.com
 *
 */
public class ClientRunnable implements IClient, Runnable {
	private final Server server;
	private final String question;
	private final int id;

	public ClientRunnable(final Server server, final String question, final int id) {
		this.server = server;
		this.question = question;
		this.id = id;
	}

	@Override
	public void recvAnswer(final String answer) {
		System.out.println("clinet " + this.id + " got answer: " + answer);
	}

	@Override
	public void run() {
		server.answer(ClientRunnable.this, this.question);
	}
}
  
/**
 * 基于线程池的异步回调
 * @author xiaodu.email@gmail.com
 *
 */
public class ThreadpoolCallback {
	public static void main(final String[] args) {
		ExecutorService es = Executors.newCachedThreadPool();

		Server server = new Server();

		for (int i = 0; i < 100; i++) {
			ClientRunnable cr = new ClientRunnable(server, "What is the answer to life, the universe and everything?",
					i);
			es.execute(cr);
			System.out.println("client " + i + " asked !");
		}

		es.shutdown();
	}
}

至此,我们就实现了线程池异步回调.

完整源码请移步:github

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