Example analysis of synchronization and asynchronousness of objects in Java threads

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1. Synchronization and asynchronousness in a multi-threaded environment

Synchronization: Thread A wants to request a resource, but this resource is being used by thread B. Because of the synchronization mechanism, thread A cannot request it. What should I do? Thread A can only wait.

package com.jalja.org.thread.demo01;

public class Thread02 {
 public synchronized void method1(){
  System.out.println("method1:"+Thread.currentThread().getName());
  try {
   Thread.sleep(3000);
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
 public synchronized void method2(){
  System.out.println("method2:"+Thread.currentThread().getName());
 }
 public static void main(String[] args) {
  final Thread02 th=new Thread02();
  Thread thread1=new Thread(new Runnable() {
   public void run() {
    th.method1();
   }
  },"th1");
  
  Thread thread2=new Thread(new Runnable() {
   public void run() {
    th.method2();
   }
  },"th2");
  
  thread1.start();
  thread2.start();
 }
}

Observe the output: method1:th1 outputs method2:th2 after 3 seconds. This is because method2() and method1() are both synchronous methods, and thread thread1 It operates on the same object th as thread2, so when thread2 executes the method2() method, it needs to obtain the lock of the th object first.

Asynchronous: A thread wants to request a resource, but this resource is being used by B thread. Because there is no synchronization mechanism, A thread still requests it. , A thread does not need to wait.

package com.jalja.org.thread.demo01;

public class Thread02 {
 public synchronized void method1(){
  System.out.println("method1:"+Thread.currentThread().getName());
  try {
   Thread.sleep(3000);
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
 public void method2(){
  System.out.println("method2:"+Thread.currentThread().getName());
 }
 public static void main(String[] args) {
  final Thread02 th=new Thread02();
  Thread thread1=new Thread(new Runnable() {
   public void run() {
    th.method1();
   }
  },"th1");
  
  Thread thread2=new Thread(new Runnable() {
   public void run() {
    th.method2();
   }
  },"th2");
  
  thread1.start();
  thread2.start();
 }
}

Observe the output results: method1:th1 and method2:th2 are output at the same time. This is because method2 does not add synchronization control, so thread thread2 does not use it when executing the method2() method. To obtain execution permission (object lock).

2. Dirty reading of data

We must consider the integrity of the business when designing the business, otherwise data consistency will occur Sexual issues.

package com.jalja.org.thread.demo01;

public class Thread03 {
 private String name="zs";
 private String passWorrd="123";
 public synchronized void setValue(String name,String passWord){
  this.name=name;
  try {
   Thread.sleep(2000);
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
  this.passWorrd=passWord;
  System.out.println("set:name="+this.name +" passWorrd="+this.passWorrd);
 }
 public void getValue(){
  System.out.println("get:name="+this.name +" passWorrd="+this.passWorrd);
 }
 public static void main(String[] args) throws InterruptedException {
  final Thread03 th=new Thread03();
  Thread thread=new Thread(new Runnable() {
   public void run() {
    th.setValue("LS", "456");
   }
  });
  thread.start();
  Thread.sleep(100);
  th.getValue();
 }
}

Result: get:name=LS passWorrd=123 set:name=LS passWorrd=456 It can be seen from the result get There is obviously a problem with the data. This is because when the thread thread is setting, the main thread is executing the get method. To avoid this situation, we must ensure that when a thread is operating the data of the same object, otherwise other threads will also operate the data of the object at the same time. In this case, we can just add the synchronized keyword to the get method.

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