通俗地解释回调函数

在以下场景中，假设A要给B发送消息，并且B收到消息后，需要告诉A我事情做完了。如果A不知道B到底有没有做完，就不能进行下一步操作，这时候我们就需要用到回调函数。

/**
 * java的回调函数
 *
 * @author lewic
 * @since 2020/4/29 20:29
 */
public class CallbackDemo {
    public static void main(String[] args) {
        // 测试
        ASend aSend = new ASend();
        aSend.sendB();
    }
}

/**
 * 发送方A
 */
class ASend {
    /**
     * 发送给B消息
     */
    public void sendB() {
        System.out.println("A send to new B, contain A object");
        // 注意!!此处this为回调函数的精髓
        new BReceived().receivedA(this);
    }

    /**
     * 告诉我结果
     */
    public boolean tellMe(boolean res) {
        System.out.println("received result tell A by B");
        return res;
    }
}

/**
 * 接收方B
 */
class BReceived {

    private ASend aSend;

    /**
     * 接收A的请求
     */
    public void receivedA(ASend aSend) {
        System.out.println("B received A, set aSend");
        this.aSend = aSend;
        executeMy();
    }

    /**
     * 接收到A请求后,B需要做的事
     */
    private void executeMy() {
        try {
            System.out.println("exec my program");
            finallyMy(true);
        } catch (Exception e) {
            finallyMy(false);
        }
    }

    /**
     * 事做完了,回调,通知A做的成功与否
     */
    private void finallyMy(boolean res) {
        System.out.println("B exec program, tell A result");
        aSend.tellMe(res);
    }

}

返回结果:
A send to new B, contain A object
B received A, set aSend
exec my program
B exec program, tell A result
received result tell A by B

Process finished with exit code 0

此处，有个不足之处就是，当有多方C,D,E,F等等，都发送给B消息时，总不能在B中设置多个C,D,E,F吧。

此时，我们可以用接口或者抽象类完美解决这个问题。

/**
 * java的回调函数
 *
 * @author lewic
 * @since 2020/4/29 20:29
 */
public class CallbackDemo2 {
    public static void main(String[] args) {
        // 测试
        ASend aSend = new ASend();
        aSend.sendB();
        System.out.println();
        CSend cSend = new CSend();
        cSend.sendB();
    }
}

interface SendCallback {
    boolean tellMe(boolean res);
}

/**
 * 发送方A
 */
class ASend implements SendCallback{
    /**
     * 发送给B消息
     */
    public void sendB() {
        System.out.println("A send to new B, contain A object");
        new BReceived().receivedA(this);
    }

    /**
     * 告诉A结果
     */
    @Override
    public boolean tellMe(boolean res) {
        System.out.println("received result tell A by B");
        return res;
    }

}

/**
 * 发送方C
 */
class CSend implements SendCallback{
    /**
     * 发送给B消息
     */
    public void sendB() {
        System.out.println("C send to new B, contain C object");
        new BReceived().receivedA(this);
    }

    /**
     * 告诉C结果
     */
    @Override
    public boolean tellMe(boolean res) {
        System.out.println("received result tell C by B");
        return res;
    }

}

/**
 * 接收方B
 */
class BReceived {

    private SendCallback send;

    /**
     * 接收A的请求
     */
    public void receivedA(SendCallback send) {
        System.out.println("B received A, set aSend");
        this.send = send;
        executeMy();
    }

    /**
     * 接收到A请求后,B需要做的事
     */
    private void executeMy() {
        try {
            System.out.println("exec my program");
            finallyMy(true);
        } catch (Exception e) {
            finallyMy(false);
        }
    }

    /**
     * 事做完了,回调,通知Send做的成功与否
     */
    private void finallyMy(boolean res) {
        System.out.println("B exec program, tell Send result");
        send.tellMe(res);
    }

}

结果如下:

A send to new B, contain A object
B received A, set aSend
exec my program
B exec program, tell Send result
received result tell A by B

C send to new B, contain C object
B received A, set aSend
exec my program
B exec program, tell Send result
received result tell C by B

Process finished with exit code 0