Question

I have a main class which spawns a thread, let’s call them MainClass and MyThread.

public class MainClass extends javax.swing.JFrame {
   int sharedVariable;
   MyThread threadInstance;
   public MainClass (){
      sharedVariable = 2;
      threadInstance = new MyThread(this);
      threadInstance.run();
   }

  public int getSharedVariable(){ return sharedVariable; }

  public static void main(String[] args){
    //begin main class
 }
} 


public class MyThread implements Runnable {

     MainClass class;
     public MyThread(MainClass main_class){
         this.main_class= main_class;
      }

     @Override
     public run(){

     while(this.main_class is still active){

      //grab status of sharedVariable and wait for x amount of time.
      }
     }
    } 

The problem is I do not know how to implement the while condition which checks if the MainClass instance is still alive and if it is, it has to use the this.main_class.getSharedVariable() to get the value of sharedVariable, then wait for x amount of time. MainClass has the main method .

Answer 1

I would recommend holding onto the Thread instance and then calling threadInstance.interrupt() right before the main(...) method exits.

Something like:

 public static void main(String[] args){
    MainClass mainClass = new MainClass();
    try {
        ...
        // do main stuff here
        ...
    } finally {
        mainClass.threadInstance.interrupt();
    }
 }

Then in your thread you’d do:

 while(!Thread.currentThread().isInterrupted()){
     ...
 }

You’d also want to handle InterruptedException correctly:

 try {
     Thread.sleep(1000);
 } catch (InterruptedException e) {
     // always a good pattern to re-interrupt the thread here
     Thread.currentThread().interrupt();
     // if we are interrupted quit
     return;
 }

Btw, it is very bad form to leak the instance of an object during construction to another thread:

 new MyThread(this);

See here: Why shouldn't I use Thread.start() in the constructor of my class?

Also, you aren’t starting a thread when you call threadInstance.run();. You are just running it in the current thread. You should use threadInstance.start() but not inside of the constructor like that.