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);
}

@Override
public 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() {

@Override
public void run() {
server.answer(ClientAsync.this, question);
}
}).start();
}

@Override
public 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;
}

@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);
}
}

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.