1. Callback function classification

Callback function distinction: synchronous callback and asynchronous callback

Synchronous callback: the meaning is only to complete the method call;

Asynchronous call: concurrency can be realized, and the main business thread can be released in time; asynchronous thread completes the work, executes callback function, and completes the aftermath work; the execution efficiency is improved.

2. Code examples

1. Note testing

package com.callback2;

public class AsyncCallBack {

    public static void main(String[] args) {
        System.out.println("Start of the main business thread ID：" + Thread.currentThread().getId());
        System.out.println("------------------");

        Son son = new Son();
        Mother mother = new Mother(son);

        mother.notice();
        son.writeHomeWork();

        System.out.println("End of the main business thread ID：" + Thread.currentThread().getId()+"\n");

    }

}

2. Mother

package com.callback2;

public class Mother {
    private Son son;
    public Mother(Son son) {
        this.son = son;
    }

    public void notice() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("Notify the mother of the thread ID." + Thread.currentThread().getId());
                cookFood("Bread");
            }
        }).start();
    }

    public void cookFood(String bread) {
        System.out.println("Current cooking thread ID." + Thread.currentThread().getId());
        try {
            System.out.println("Mother Baking" + bread + "...");
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Mother has baked the bread");
        String message = "Ming, come and eat!";

        son.callback(message);

    }

}

3. My son Xiao Ming

package com.callback2;

public class Son {
    private String status = "";

    public void writeHomeWork() {
        System.out.println("Xiaoming writing homework thread ID: " + Thread.currentThread().getId());
        System.err.println("Ming is writing his homework...") ;
        setStatus("Writing homework");
    }


    public void callback(String message) {
        System.out.println("Callback Xiaoming meal thread ID: " + Thread.currentThread().getId());
        System.err.println(message);
        System.err.println("Okay, coming right up!") ;
        System.out.println("Ming started eating!") ;
        setStatus("Eating in progress");
        System.out.println("Xiaoming executing meal thread ID." + Thread.currentThread().getId());
    }

    public String getStatus() {
        return status;
    }
    public void setStatus(String status) {
        this.status = status;
    }
}

4. Implementation results

Business Main Thread Start ID: 1
------------------
Xiaoming writing homework thread ID: 1
Xiaoming writing homework in progress...
Business main thread end ID：1

Notify mother thread ID：12
Currently cooking thread ID：12
Mother baking bread...
Mother baked the bread
Callback to Xiaoming to eat Thread ID: 12
Ming, come and eat!
Okay, coming right up!
Ming starts eating!
Xiaoming executes meal thread ID: 12

5. Result analysis

1) Xiao Ming uses the main thread to do his homework (if he can’t finish it, he doesn’t need to care);

2) Mother uses the new thread to cook [concurrent], mother finishes the meal, executes the callback function, and notifies Xiao Ming to eat;

3) Xiao Ming uses a new thread and starts to eat.