Different type of threads?

Question

Dear,
Is it we should use different methods to invoke the threads for different purposes? eg. use swingworker class to start time comsuming task...; use invokelater() to start a thread which used to alert the GUI... is it true?

Answer 1

hi,
Swing components are not Thread-safe, what this means is that the components can be accessed only from a single thread at a time (Generally the event-dispatching thread) after they have been realized, there are certain methods though that are gauranteed to be thread-safe (like paint, thread-safe methods are marked out in the API ref).
herez the golden rule:
Once a Swing component has been realized, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread.
A component becomes realized if itz paint method is going to be called or has been called.
Event-Dispatching thread is the thread which handles the event-handling code..
Now if the task at hand is going to take up a lot of time u should not do it in the event-dispatching thread, you should in turn do it in a separate thread.. but letz say the results of the operation require u to update a gui, u cannot update the gui from that thread (non event-dispatching thread) , for updating the gui you would use the SwingUtilities.invokeLater or SwingUtilities.invokeAndWait methods. SwingWorker is a separate Class (not part of jdk) that can be downloaded from the sun site at
http://java.sun.com/products/jfc/tsc/articles/threads/src/SwingWorker.java
the idea of this class is that it eases the task of the programmer who has to create new threads that will take up time to execute, this class provides a mechanism to start a thread to perform a task that will take time, also after the task is over it automatically calls a method (finished()) in which you can then do whatever is required with the result (for eg update the gui using the result in a event-dispatching thread using the SwingUtilities.invokeAndWait())..
for eg If you are doing IO operations that are going to take time you could use the SwingWorker like
SwingWorker worker = new SwingWorker()
   public Object construct()  //method defined in swingwoker which is to be overridden
          return myMethod(); //myMethod returns a object after taking lotz of time letz say..     
   public void finished()   //method is automatically invoked by swingwoker after myMethod finishes.
        Object obj = get();  // get() returns the value returned by myMethod();
worker.start();now letz say you are doing some task in a non-event dispatching thread and you want to update the gui (you should not directly do it from that thread as it can lead to problems) you would do it using the SwingUtilities.invokeLater or SwingUtilities.invokeAndWait
eg
  Runnable myRunnable = new  Runnable()
        public void run()
             //update the gui here
    SwingUtilities.invokeAndWait(myRunnable);the invokeAndWait method blocks whereas the invokeLater does not (so itz preferable to use this !!)..
hope that helpzz
cheerz
ynkrish