CS 4773 Object Oriented Systems Review of Swing And Threads

Threads and Java

We already know:
- Every Java application starts with a main method that runs a main thread.
- We saw this in the PingPong example.
- When the first window is shown, a second thread, the AWT event dispatch thread is created.
- All event notifications are run in the event dispatch thread.
- In response to an action, the event dispatch thread should only execute for a short time.
- If you need to run something that will take a long time, create a new thread to do it.
All of this is fine as long as the created thread(s) do not have to interact with the Swing widgets.
Most Swing methods are not thread-safe.
The consequence is that the user interface can become corrupted if a Swing method is called from within a user thread.

Using threads with Swing

If an action takes a long time, fire up a thread to handle it
If an action can block on input or output, fire up a new thread to handle it.
If you need to wait for a specific amount of time, don't sleep in the event dispatch thread, use a timer event.
The work done in the worker threads cannot touch the user interface.
- Read any information you need from the user interface before starting the thread.
- Launch the thread
- update the user interface from the event dispatch thread.

The last of these rules is called the single thread rule for Swing programming.

Exception to the single thread rule:

A few of the Swing methods are thread-safe.
They are specifically marked as being thread safe in the documentation.
Some of the thread-safe methods are:
JTextComponent.setText JTextArea.insert JTextArea.append JTextArea.replaceRange
Note that JLabel.setText is not thread-safe.
A JLabel is not a JTextComponent.
The following methods are thread-safe:
repaint revalidate
You can add ir remove event listeners in any thread.
The corresponding methods will be called by the event dispatch thread.
You can construct components, set their properties, and add them to containers, as long as they have not be realized.
- A component has been realized if it can receive repaint or validation events.
- This happens when setVisible(true) or pack has been called.
- It also happens when it is added to a container that has been realized.
- You can create the GUI in the main method before calling setVisible(true).

How to update from the event dispatch queue: Use one of the static methods in the EventQueue class:
invokeLater(Runnable runnable) or
invokeAndWait(Runnbale runnable) throws InterruptedException, InvocationTargetException
Example:

EventQueue.invokeLater(new Runnable()
                      {
                         public void run() {
                            label.setText("This is a label");
                         }
                      });

See the SwingThreadTest example.
Some modifications made form the Core Java code:

Keep a list of the threads created
Display the total number of threads when a new one is created.
Add a button to stop all threads
Insert informative string instead of random number in combo box.