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[multithreading] how to implement timer by yourself

2022-06-29 22:12:00 【51CTO】

Catalog

What is a timer
Timer usage of standard library
How to implement a timer by yourself

1. Describe task
2. Organizational tasks
3. Perform the task when the time is up
Code

What is a timer

Timer is an important component in multithreaded programming
Like an alarm clock Set a time for a thread to execute
Timers are particularly common in network programming

such as ：

Before we open the browser , Open any web page
If all goes well , You can access the website right away
however ！ Sometimes because the network is unstable , Or the server of the website crashed .
that , At this point, it will be loaded in circles .
But the browser will not always be here waiting for feedback from the visited website .
There is actually a waiting in it Of “ Timeout time ”, After this time , It won't wait , Just tell you “ The visit to fail / The website is missing ”
At this point, we can soon think of join, There is a usage , Add the specified... In parentheses “ Timeout time ”.
sleep It can also achieve this effect ,sleep( Specify sleep time ) .
join and sleep Are based on the timer inside the system , To achieve

that , We can use Timer in code , To implement similar functions .

First introduce the timer usage of the standard library , Then we'll see how to implement a timer by ourselves

Timer usage of standard library

Timer <-----> java.util.Timer
The core approach ：schedule
schedule, Its function is just like it The Chinese meaning is the same .
Every call schedule‘, Will give the timer Arrange a task .
Through this method , You can put the Mission Sign up to Timer internal .
The internal counter supports register More than one task .

schedule Method , Yes Two parameters ：

Mission
How long will it take to execute

【 Multithreading 】 How to implement timer by yourself _ Timer

How to implement a timer by yourself

A problem ：Timer class What do you need inside ？

from Timer Of Start with the work content

Manage many tasks
Perform the task when the time is up

Management tasks And then you can subdivide it into 2 individual ：
1、 Describe task （ Create a special class to represent the task in a timer 【Timer Task】）
2、 Organizational tasks （ Use certain data and structure to organize data , Put some tasks together .）

The specific task sequence is
1、 Describe task （ Create a special class to represent the task in a timer 【Timer Task】）
2、 Organizational tasks （ Use certain data and structure to organize data , Put some tasks together .）
3、 Perform the task when the time is up

1. Describe task

//  Create a class , To describe a specific task 
class MyTask{
    //  What is the specific task to do 
    private Runnable runnable;
    //  The specific execution time of the task : Save the millisecond timestamp of the task to be executed 
    private long time;
    // after  It's a time gap , Not an absolute timestamp value 
    public MyTask(Runnable runnable,long after){
        this.runnable = runnable;
        //  It's simple , It means starting from the current time , after  Seconds later , This task is performed .
        this.time = System.currentTimeMillis()+after;
    }
    //  By calling here  run Method , What specific tasks do we have to do 
    public void run(){
        runnable.run();
    }
}

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2. Organizational tasks

【 Multithreading 】 How to implement timer by yourself _java_02

3. Perform the task when the time is up

You need to perform the task with the highest time first
such as ： Ten minutes later , Let's go and have a rest .
however , At present, we can't judge the time , So we need to use a thread to constantly check the first element of the current priority queue , Let's see if it's time for the current top task
adopt Self constructed Mytimer Timer class Of Construction method .
Create a thread , Help us with a check

【 Multithreading 】 How to implement timer by yourself _ Timer _03

There are two very serious problems in the above code ！！！

【 Multithreading 】 How to implement timer by yourself _java_04

Code

import java.util.concurrent.PriorityBlockingQueue;

//  Create a class , To describe a specific task 
class MyTask implements Comparable<MyTask>{
    //  What is the specific task to do 
    private Runnable runnable;
    //  The specific execution time of the task : Save the millisecond timestamp of the task to be executed 
    private long time;
    // after  It's a time gap , Not an absolute timestamp value 
    public MyTask(Runnable runnable,long after){
        this.runnable = runnable;
        //  It's simple , It means starting from the current time , after  Seconds later , This task is performed .
        this.time = System.currentTimeMillis()+after;
    }
    //  By calling here  run Method , What specific tasks do we have to do 
    public void run(){
        runnable.run();
    }

    public long getTime() {
        return time;
    }

    @Override
    public int compareTo(MyTask o) {
        return (int) (this.time - o.time);
    }
}

class MyTimer{
    //  The timer should be able to store multiple tasks 
    private PriorityBlockingQueue<MyTask> queue = new PriorityBlockingQueue<>();
    public void schedule(Runnable runnable,long after){
        MyTask task = new MyTask(runnable,after);
        queue.put(task);
        synchronized(locker){
            locker.notify();
        }
    }
    private Object locker = new Object();

    public MyTimer(){
        Thread t = new Thread(()->{
           while(true){
               try {
                   //  Take out the team leader element 
                   MyTask task =queue.take();
                   // Compare again to see if the current task time is up 
                   long curTime = System.currentTimeMillis();
                   //  Take the current time   and   Compare task execution time 
                   if(curTime < task.getTime()){
                       // It's not time yet , Put the task back in the queue 
                       queue.put(task);
                       //  Specify a waiting time 
                       synchronized(locker){
                          locker.wait(task.getTime() - curTime);
                       }
                   }else{
                       //  time out , Perform this task 
                       task.run();
                   }
               } catch (InterruptedException e) {
                   e.printStackTrace();
               }
           }
        });
        t.start();
    }
}

public class Test25 {
    public static void main(String[] args) {
        MyTimer myTimer = new MyTimer();
        myTimer.schedule(new Runnable() {
            @Override
            public void run() {
                System.out.println("hello timer");
            }
        },3000);
        System.out.println("main");
    }
}



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