Order of thread execution in Java

问题

I have doubts about order of Thread execution in java. This example

  public class Thr implements Runnable {
    String string = "Yes ";
    public void run(){
      this.string = "No ";
    }
    public static void main (String args[]){
      Thr t = new Thr();
      new Thread(t).start();
      for(int i = 0; i < 10; i++)
          System.out.println(t.string);
    } 
  }

gives output :

Yes 
No 
No 
No 
No 
No 
No 
No 
No 
No

I have red this topic Java. The order of threads execution and I am confused why string is changed to no after first pass through for loop. I tought that , as main thread doesn't have large job to do, main thread will execute first, and that output will be

Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes
No

When I put some print in run method instead this.string = "No" then, print from run method prints last.

回答1:

I think that I figured out what is happening. Thing is, that this threads , main thread and new Thread(t) are both executing parallely. And it is kind of lottery, because they both make request for processor time, so, as main thread begin to execute, with all code that it needs to execute, main thread spend processor time, and it only prints first yes, and then new Thread(t) gets his processor time, changes "yes" to no, and finish execution, and than main thread again continue to execute. I've made a little test, and in

public class Thr implements Runnable {
  String string = "Yes ";
  public void run(){
    this.string = "No ";
  }
  public static void main (String args[]){
  Thr t = new Thr();
  Thread tr = new Thread(t);
  tr.start();
  for(int i = 0; i < 10; i++)
      System.out.println(t.string + tr.isAlive());
  } 
}

and the output is

Yes true
No false
No false
No false
No false
No false
No false
No false
No false
No false

回答2:

When working with threads be sure of one thing, you can't predict when a thread will start even if you call this:

new Thread(t).start();

When you call it the thread enter in a state that is "Ready to start running or Runnable state" and then the thread scheduler can take the thread and put it in a running state IF HE WANTS, it's a little bit complex but you can't predict what will happen when you have a bunch of them, sometimes if you start two threads the last would run first. For more understanding of the topic see this

来源：https://stackoverflow.com/questions/38234138/order-of-thread-execution-in-java

标签

java

multithreading