Java multithreading details my first contact with multithreading

Posted by karq on Fri, 26 Nov 2021 12:01:57 +0100

Detailed explanation of Java multithreading

Thread introduction

Thread implementation (key)

Thread state

Thread synchronization (key)

Thread communication problem

Advanced topics

Multitasking

In reality, there are too many examples of doing common things at the same time. It seems that more than a dozen tasks are being done. In fact, our brain still does one thing at the same time.

Multithreading

It turned out to be a road. Slowly, because there are too many cars, the road is blocked and the efficiency is very low.

In order to make full use of the efficiency of question number use, multiple lanes are added.

From then on, my mother never used it again, but the road was blocked.
Tell me about your multithreading examples (life, games, programming)

Common method call multithreading

Program. Process. Thread

Process and Thread

Speaking of the process, I have to say the procedure. Program is an ordered collection of instructions and data. It has no running meaning. It is a static concept.
The process is an execution process of the execution program. It is a dynamic concept and the unit of system resource allocation.
Usually, several threads can be replaced in a process. Of course, there is at least one thread in a process, otherwise it has no meaning. Thread is the unit of CPU scheduling and execution.
Note: many multithreads are simulated. Real multithreading refers to having multiple CPUs, i.e. multiple cores, such as servers. If it is a simulated multithreading, that is, in the case of a CPU, at the same point in time, the CPU can only execute one code. Because the switching is fast, there is the illusion of simultaneous execution.

Core concepts of this chapter

Threads are independent execution paths;
When the program is running, even if it does not create its own thread, there will be multiple threads in the background, such as main thread and gc thread;
main() is called the main thread, which is the entry of the bit system and is used to execute the whole program;
In a process, if multiple threads are opened up, the running scheduler of threads arranges scheduling. The scheduler is closely related to the operating system, and the sequence can not be interfered by human beings.
When operating on the same resource, there will be a problem of resource grabbing, and concurrency control needs to be added;
Threads will bring additional overhead, such as CPU scheduling time and concurrency control overhead.
Each thread interacts in its own working memory. Improper memory control will cause data inconsistency.

Thread creation

Thread,Runnable,Callable

Three creation methods

Thread

(learning tips: View JDK help documents)

The custom Thread class inherits the Thread class
Rewrite the run() method to write the thread executor
Create a thread object and call the start() method to start the thread

package com.xiancheng.demo01;

//Common threading method 1: inherit the Thread class, rewrite the run() method, and call start() to start the Thread
public class TestThread1 extends Thread{
    @Override
    public void run() {
        //run method thread body
        for (int i = 0; i < 20; i++) {
            System.out.println("I'm looking at the code---"+i);
        }
    }

    public static void main(String[] args) {
        //main thread

        //Create a thread object
        TestThread1 testThread1 = new TestThread1();

        //Call the start() method to start the current thread
        testThread1.start();

        for (int i = 0; i < 20; i++) {
            System.out.println("I'm learning multithreading--"+i);
        }
    }

}

Summary: note that thread clearing is not necessarily executed immediately, but is scheduled by the CPU

Case: download pictures

package com.xiancheng.demo01;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.io.IOException;
import java.net.URL;

//Contact TestThread to realize multi-threaded synchronous downloading of pictures
public class TestThread2 extends Thread{

    private String url;//Network picture address
    private String name;//Save file name

    public TestThread2(String url,String name){
        this.url = url;
        this.name = name;

    }


    @Override
    public void run() {

        WebDownloader webDownloader = new WebDownloader();
        webDownloader.downloader(url,name);
        System.out.println("Downloaded a file named:"+name);

    }

    public static void main(String[] args){
        TestThread2 testThread1 = new TestThread2("https://imgoss.douyucdn.cn/bj/yuba/default/2019/12/20/201912200221045779841615885.jpg","1.jpg");
        TestThread2 testThread2 = new TestThread2("https://imgoss.douyucdn.cn/bj/yuba/default/2021/03/26/202103262200428765268485985.jpg","2.jpg");
        TestThread2 testThread3 = new TestThread2("https://img.douyucdn.cn/data/yuba/default/2019/08/18/201908182138163129577786480.200x0.jpg","3.jpg");

        testThread1.start();
        testThread2.start();
        testThread3.start();


    }
}


//Downloader
class WebDownloader{
    //Download method
    public  void downloader(String url, String name){
        try {
            FileUtils.copyURLToFile(new URL(url),new File(name));
        } catch (IOException e) {
            e.printStackTrace();
            System.out.println("IO Abnormal, downloader There is a problem with the method");
        }
    }
}

Threads execute simultaneously

Implement Runnable

Define the MyRunable class to implement the Runable interface
Implement the run() method and write the thread execution body
Create a thread object and call the start() method to start the thread

package com.xiancheng.demo01;

//Thread creation mode 2: implement the runnable method, rewrite the run() method, and call start to start the thread

//Summary: note that thread startup does not specify immediate execution, but is scheduled by the CPU
public class TestThread3 implements Runnable{
    @Override
    public void run() {
        //run method thread body
        for (int i = 0; i < 1000; i++) {
            System.out.println("I'm looking at the code---"+i);
        }
    }

    public static void main(String[] args) {
        //Create an implementation class object for the runnable interface
        TestThread3 testThread3 = new TestThread3();
        //Create a thread object and start our thread agent through the thread object
        //Thread thread = new Thread(testThread3);
        //thread.start();

        //Abbreviation
        new Thread(testThread3).start();

        for (int i = 0; i < 1000; i++) {
            System.out.println("I'm learning multithreading--"+i);
        }
    }
}

Using Runna interface to download pictures

package com.xiancheng.demo01;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.io.IOException;
import java.net.URL;

//Practice TestThread to download pictures synchronously through multiple threads
public class TestThread2 implements Runnable{

    private String url;//Network picture address
    private String name;//Save file name

    public TestThread2(String url,String name){
        this.url = url;
        this.name = name;

    }


    @Override
    public void run() {

        WebDownloader webDownloader = new WebDownloader();
        webDownloader.downloader(url,name);
        System.out.println("Downloaded a file named:"+name);

    }

    public static void main(String[] args){
        TestThread2 testThread1 = new TestThread2("https://imgoss.douyucdn.cn/bj/yuba/default/2019/12/20/201912200221045779841615885.jpg","1.jpg");
        TestThread2 testThread2 = new TestThread2("https://imgoss.douyucdn.cn/bj/yuba/default/2021/03/26/202103262200428765268485985.jpg","2.jpg");
        TestThread2 testThread3 = new TestThread2("https://img.douyucdn.cn/data/yuba/default/2019/08/18/201908182138163129577786480.200x0.jpg","3.jpg");

        new Thread(testThread1).start();
        new Thread(testThread2).start();
        new Thread(testThread3).start();


    }
}


//Downloader
class WebDownloader{
    //Download method
    public  void downloader(String url, String name){
        try {
            FileUtils.copyURLToFile(new URL(url),new File(name));
        } catch (IOException e) {
            e.printStackTrace();
            System.out.println("IO Abnormal, downloader There is a problem with the method");
        }
    }
}

Summary

Inherit Thread class
- The subclass inherits the Thread class and has multithreading capability
- start thread: subclass object. start
- Not recommended: avoid the limitations of OPP single thread inheritance
Implement Runnable interface
- The implementation interface Runnale has multithreading capability
- Delivery Thread: pass in target object + Thread object. start()
- Recommended: it avoids the limitation of single inheritance, is flexible and convenient, and is convenient for the same object to be used by multiple threads
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