In-depth understanding of the communication methods between threads in JAVA multi-threading

1. Introduction

This summary summarizes my understanding of the communication methods between threads in JAVA multi-threading. I mainly discuss the communication between threads using code combined with text, so I excerpted some of the passages from the book. Sample code.

2. Communication methods between threads

①Synchronization

The synchronization mentioned here refers to the communication between multiple threads through the synchronized keyword.

Reference example:

public class MyObject {
 
  synchronized public void methodA() {
    //do something....
  }
 
  synchronized public void methodB() {
    //do some other thing
  }
}
 
public class ThreadA extends Thread {
 
  private MyObject object;
//省略构造方法
  @Override
  public void run() {
    super.run();
    object.methodA();
  }
}
 
public class ThreadB extends Thread {
 
  private MyObject object;
//省略构造方法
  @Override
  public void run() {
    super.run();
    object.methodB();
  }
}
 
public class Run {
  public static void main(String[] args) {
    MyObject object = new MyObject();
 
    //线程A与线程B 持有的是同一个对象:object
    ThreadA a = new ThreadA(object);
    ThreadB b = new ThreadB(object);
    a.start();
    b.start();
  }
}

Since thread A and thread B hold the same object object of the MyObject class, although the two threads need to call different methods, they are executed synchronously. For example: Thread B needs to wait for thread A to finish executing the methodA() method before it can execute the methodB() method. In this way, thread A and thread B achieve communication.

This method is essentially "shared memory" communication. Multiple threads need to access the same shared variable. Whoever gets the lock (obtains access permission) can execute it.

②while polling method

The code is as follows:

import java.util.ArrayList;
import java.util.List;
 
public class MyList {
 
  private List<String> list = new ArrayList<String>();
  public void add() {
    list.add("elements");
  }
  public int size() {
    return list.size();
  }
}
 
import mylist.MyList;
 
public class ThreadA extends Thread {
 
  private MyList list;
 
  public ThreadA(MyList list) {
    super();
    this.list = list;
  }
 
  @Override
  public void run() {
    try {
      for (int i = 0; i < 10; i++) {
        list.add();
        System.out.println("添加了" + (i + 1) + "个元素");
        Thread.sleep(1000);
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}
 
import mylist.MyList;
 
public class ThreadB extends Thread {
 
  private MyList list;
 
  public ThreadB(MyList list) {
    super();
    this.list = list;
  }
 
  @Override
  public void run() {
    try {
      while (true) {
        if (list.size() == 5) {
          System.out.println("==5, 线程b准备退出了");
          throw new InterruptedException();
        }
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}
 
import mylist.MyList;
import extthread.ThreadA;
import extthread.ThreadB;
 
public class Test {
 
  public static void main(String[] args) {
    MyList service = new MyList();
 
    ThreadA a = new ThreadA(service);
    a.setName("A");
    a.start();
 
    ThreadB b = new ThreadB(service);
    b.setName("B");
    b.start();
  }
}

In this way, thread A constantly changes the conditions, and thread ThreadB constantly detects this through the while statement Whether the condition (list.size()==5) is established, thereby realizing communication between threads. But this method will waste CPU resources. The reason why it is said to be a waste of resources is because when the JVM scheduler hands the CPU to thread B for execution, it does not do any "useful" work. It is just constantly testing whether a certain condition is true. It's similar to how in real life, a person keeps looking at the phone screen to see if a call is coming, instead of: doing other things. When a call comes, the phone rings to notify him/her that the call is coming.

③wait/notify mechanism

The code is as follows:

import java.util.ArrayList;
import java.util.List;
 
public class MyList {
 
  private static List<String> list = new ArrayList<String>();
 
  public static void add() {
    list.add("anyString");
  }
 
  public static int size() {
    return list.size();
  }
}
 
 
public class ThreadA extends Thread {
 
  private Object lock;
 
  public ThreadA(Object lock) {
    super();
    this.lock = lock;
  }
 
  @Override
  public void run() {
    try {
      synchronized (lock) {
        if (MyList.size() != 5) {
          System.out.println("wait begin "
              + System.currentTimeMillis());
          lock.wait();
          System.out.println("wait end "
              + System.currentTimeMillis());
        }
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}
 
 
public class ThreadB extends Thread {
  private Object lock;
 
  public ThreadB(Object lock) {
    super();
    this.lock = lock;
  }
 
  @Override
  public void run() {
    try {
      synchronized (lock) {
        for (int i = 0; i < 10; i++) {
          MyList.add();
          if (MyList.size() == 5) {
            lock.notify();
            System.out.println("已经发出了通知");
          }
          System.out.println("添加了" + (i + 1) + "个元素!");
          Thread.sleep(1000);
        }
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}
 
public class Run {
 
  public static void main(String[] args) {
 
    try {
      Object lock = new Object();
 
      ThreadA a = new ThreadA(lock);
      a.start();
 
      Thread.sleep(50);
 
      ThreadB b = new ThreadB(lock);
      b.start();
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}

Thread A needs to wait until a certain condition is met (list.size()==5) before executing the operation. Thread B adds elements to the list and changes the size of the list.

How do A and B communicate? In other words, how does thread A know that list.size() is already 5?

The wait() and notify() methods of the Object class are used here.

When the conditions are not met (list.size() !=5), thread A calls wait() to give up the CPU and enter the blocking state. ---It does not occupy the CPU like ②while polling

When the conditions are met, thread B calls notify() to notify thread A. The so-called notification of thread A is to wake up thread A and let it enter the runnable state.

One benefit of this method is that the CPU utilization rate is improved.

But there are also some shortcomings: for example, thread B executes first, adds 5 elements at once and calls notify() to send a notification, while thread A is still executing at this time; when thread A executes and calls wait (), then it can never be awakened. Because thread B has already sent a notification and will no longer send notifications in the future. This shows that notifying too early will disrupt the execution logic of the program.

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