Sharing data between threads in Java

Question

I'm having trouble accessing data in my main thread from my new thread. I can do this just fine without threading by just using the main class getters/setters. But when I try to launch a new thread I can no longer do so.

Main Class:

public class Driver extends Application{

//create socket handlers
Runnable myNewThread = new workerThread();

//variables
private String lastMessage = "";

//getters and setters
public String setMyVariable() {
    this.MyVariable = MyVariable;
}

//main
public static void main(String[] args) {
    //launch threads
    new Thread(myNewThread).start();
}

NewThread Class:

public class workerThread implements Runnable{

public void run() {
    Driver.setMyVariable("test");
}

I get the error "Cannot resolve symbol 'setMyVariable'" in my workerThread class. As far as I can tell it's because my workerThread thread doesn't know what instance of Driver to refer to (there is only one instance, but it doesn't know that). Could someone help me understand what I'm missing? I've seen examples of declaring the new class within a function of the main class, but I'm trying to avoid this in the interest of code organization as my worker thread is going to be a bit large.

Answer 1

You are calling setMyVariable("test") as if it is a static method. You need to pass an instance of Driver class to instance of workerThread.

public class workerThread implements Runnable {
  private Driver driver;

  public workerThread(Driver d, Class c) {
    this.driver = d;
    //do whatever you are doing with existing Classs parameter
  }

  public void run() {
    driver.setMyVariable("test");
  }

And also make changes to Driver class

public class Driver extends Application{

  //create socket handlers
  Runnable myNewThread = new workerThread(this, this.getClass());

 //variables
 private String lastMessage = "";

 //getters and setters
 public String setMyVariable() {
   this.MyVariable = MyVariable;
 }

 //main
 public static void main(String[] args) {
   //launch threads
   new Thread(new Driver().myNewThread).start();
 }
}

UPDATE:

And because myNewThread variable is also non-static you have to do the following in Driver.main() to be able to compile:

new Thread(new Driver().myNewThread).start();

Answer 2

When you call a function from a class directly the system expects it to be a static one (Like you do here: Driver.setMyVariable("test");) But your function is not static and I think it shouldn't be as you variable isn't static as well. If you wish to work with separate objects you need to instantiate them first with it's constructor

Answer 3

You're attempting to call a static method without defining your method as static. Change your method to:

public static String setMyVariable() {
    this.MyVariable = MyVariable;
}

Then, you'll have to change the definition of MyVariable to be static as well.

I'm not sure, though, you understand the consequences of what you're doing. Creating a static variable will allow you to share your data between threads as there will be one instance of the variable across all instances of your class, but this is a potentially dangerous thing to do if you don't really understand static variables.

Make sure you do your homework on this before making this decision.

Answer 4

Don't make it static. Pass an instance of your object.

//Runnable myNewThread = new workerThread(this.getClass());
Runnable myNewThread = new workerThread(this);

Then your workerThread should have a constructor.

public class workerThread implements Runnable{
    Driver driver;
    public workerThread(Driver driver){
        this.driver=driver;
    }
    public void run() {
        driver.setMyVariable("test");
    }
}

Answer 5

public class Driver extends Application{

   public static Driver instance = new Driver();

   //create socket handlers
   public static Runnable myNewThread = new workerThread();

   //variables
   private String lastMessage = "";
   private static String MyVariable = "";

   //getters and setters
   public static String setMyVariable(String MyVariable) {
      instance.MyVariable = MyVariable;
      return MyVariable;
  }

  //main
  public static void main(String[] args) {
  //launch threads
      new Thread(myNewThread).start();
  }
}