How to pass object from main thread to another thread in java

Question

In my main application class I have an object of JTextArea(say txtArea). I have created two another threads in main_application program. The two threads I have created is for reading and writing in serial port. Now I want to put the serial port read buffer data into an JTextArea object. So I need to pass the JTextArea object created in main_application to read-thread so that read-thread can put the read output to JTextArea.

But after doing like this I am facing null pointer access problem. When I check the JTextArea object in main_application its not null but after passing to thread i checked its null. I dont know how it happens.... if any geek can help me i will be happy....

Thanks , Surjya

Answer 1

Sharing objects between threads can get very messy, you may want to read about the Actor model for a different approach. Specifically in the GUI arena, an Model-View-Controller approach code help out.

But back the the question, you could quickly hack it by making your JTextArea a static variable and just letting all the different threads modify the static instance at will:

public static JTextArea TXTAREA

void main_application(){
    //set up app
    ...
    TXTAREA = new JTextArea()
    ...
}

then in your serial reading thread, set the text for TXTAREA. This solution is not ideal, I think a better approach would be to use an MVC approach and have a controller which accepts the value to put in the text area.

Answer 2

Take a look at SynchronousQueue:

http://java.sun.com/javase/6/docs/api/java/util/concurrent/SynchronousQueue.html

It will allow you to store an object into the queue and will then wait until it is removed.

Answer 3

1. Publishing JTextArea reference to another thread is not correct because Swing compoments are MVC-based (model-ui actually). So, your general way to work with swing components is to update their models and let view (ui) display the change;
2. Another point is that most of swing componenets aren't thread-safe, i.e. you can't be sure that your application works fine if you use swing component from the thread over than EDT;

So, you should do the following: every time you have new data to display submit new task of JTextArea content update to be executed from the EDT. You can achieve that via SwingUtilities.invokeLater()