带你一步步实现线程池异步回调

        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:

class A {public void ask(final B b, final String question) {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");}}}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:

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");}}}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:

public interface IClient {void recvAnswer(String answer);}

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(5000);} catch (InterruptedException e) {e.printStackTrace();}}}

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);}@Overridepublic void recvAnswer(final String answer) {System.out.println(answer);}}

public class SyncInterfaceCallback {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?");}}

        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:

public class ClientAsync implements IClient {private final Server server;public ClientAsync(final Server server) {this.server = server;}public void ask(final String question) {new Thread(new Runnable() {@Overridepublic void run() {server.answer(ClientAsync.this, question);}}).start();}@Overridepublic void recvAnswer(final String answer) {System.out.println(answer);}}

public class AsyncInterfaceCallback {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...");}}

        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:

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;}@Overridepublic void recvAnswer(final String answer) {System.out.println("clinet " + this.id + " got answer: " + answer);}@Overridepublic void run() {server.answer(ClientRunnable.this, this.question);}}

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.