Threads and Swing Multithreading

Contents I. Simulation on Inserting and Removing Items in a Combo Box II. Event Dispatch Thread III. Rules for Running Time-Consuming Tasks IV. The Reading Text File Problem

I. Simulation on Inserting and Removing Items in a Combo Box Develop a GUI program to insert/remove an item (a positive integer) to/from a combo box continuously and randomly in a separate thread A sample GUI is below  As long as the button is clicked, the simulation runs

Solution 1. Developing a Non-Thread-Safe Solution 2. Observing the Problem 3. Correcting the Problem

1. Developing a Non-Thread-Safe Solution 1.1. The Main Class 1.2. Developing the View 1.3. Adding Listener to the Button

1.1. The Main Class

1.2. Developing the View

1.3. Adding Listener to the Button

2. Observing the Problem Click the Remove/Insert button > Click the combo box a few times > Move the scrollbar > Move the window > Click the Remove/Insert button again > Keep clicking the combo box > Eventually, you should see an exception report What is going on?  When an element is inserted into the combo box, the combo box fires an event to update the display  Then, the display code springs into action, reading the current size of the combo box and preparing to display the values  But the worker thread keeps going - occasionally resulting in a reduction of the count of the values in the combo box. The display code then thinks that there are more values in the model than there actually are, asks for nonexistent values, and triggers an ArrayIndexOutOfBounds exception

3. Correcting the Problem: Put the Updating GUI Code to the Event Dispatch Thread

II. Event Dispatch Thread The thread of control that passes events such as mouse clicks and keystrokes to the user interface components  Every Java application starts with a main method that runs in the main thread  In a Swing program, the main thread is short-lived. It schedules the construction of the user interface in the event dispatch thread and then exits  The event dispatch thread keeps the program alive until it is terminated, either by closing the frame or by calling the System.exit method

III. Rules for Running Time-Consuming Tasks If an action takes a long time, do it in a separate worker thread and never in the event dispatch thread. Do not touch Swing components in any thread other than the event dispatch thread by using EventQueue.invokeLater() or EventQueue.invokeAndWait()  The invokeLater method returns immediately when the event is posted to the event queue  The invokeAndWait method waits until the run method has actually been executed

IV. The Reading Text File Problem Develop a program for loading a text file and for canceling the file loading process Since we want to be able to load big files, a separate thread should be used for loading While the file is read, the Open menu item is disabled and the Cancel item is enabled (see the next slide) After each line is read, a line counter in the status bar is updated After the reading process is complete, the Open menu item is reenabled, the Cancel item is disabled, and the status line text is set to Done

Solution 1. Developing the View 2. Adding Listeners to the Menu Items

1. Developing the View

2. Adding Listeners to the Menu Items 2.1. Adding Listener to the Open Menu Item 2.2. Adding Listener to the Cancel Menu Item

2.1 Adding Listener to the Open Menu Item Getting the Selected File Showing Content in the Selected File

2.1.1 Getting the Selected File

2.1.2 Showing Content in the Selected File A Single-Threading Version A Multiple-Threading Version

A Single-Threading Version

A Multiple-Threading Version Using SwingWorker Defining Result Type and Progress Data Type Developing the Text Reading Thread

Using SwingWorker The typical UI activities of a background task:  After each work unit, update the UI to show progress.  After the work is finished, make a final change to the UI. The SwingWorker class makes it easy to implement such a task.

Using SwingWorker Since the reading process can take a long time, we use the SwingWorker class to fire up a new thread to do the work SwingWorker(T, V)  T: Final result of type T  V: Intermediate progress data of type V

Using SwingWorker Override the doInBackground method to do the time-consuming work  The doInBackground method is executed in a worker thread.  In the doInBackground method, occasionally call publish method to communicate work progress. The publish method causes a process method to execute in the event dispatch thread to deal with the progress data.

When the work is complete, the done method is called in the event dispatch thread so that we can finish updating the UI. To cancel the work in progress, use the cancel method.  When the work is canceled, the get method throws a CancellationException.

Event Dispatch ThreadWorker Thread swObj.execute()swObj.doInBackground() Do the job swObj.publish() swObj.process() swObj.done() The get() does not throw a CancellationException

Defining Final Result Type and Progress Data Type Final Result Type  String, but StringBuilder would be more efficient for big files Progress Data Type: The Current Line

Developing the Text Reading Thread

Developing doInBackground() for Carrying out the Task

Developing process() for Processing Progress Data in the Event Dispatch Thread

Developing done()

2.2. Adding Listener to the Cancel Menu Item

Run the program and try to click Cancel

Ignore the exception message

References 1. Core Java, Volume I - Fundamentals, Eighth Edition, Chapter 14. Cay S. Horstmann and Gary Cornell. Prentice Hall, 2008