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How to Use Progress Bars
Sometimes a task running within a program
might take a while to complete.
A user-friendly program provides some
indication to the user
that the task is occurring,
how long the task might take, and
how much work has already been done.
One way of indicating work,
and perhaps the amount of progress,
is to use an animated image.
Another way of indicating work
is to set the wait cursor,
using the
Cursor class
and the Component-defined
setCursor method. For example, the following code
makes the wait cursor be displayed
when the cursor is over container
(including any components it contains
that have no cursor specified):
container.setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));
To convey how complete a task is,
you can use a progress bar
like this one:
-->
Sometimes you can't immediately determine
the length of a long-running task,
or the task might stay stuck at the same state of completion
for a long time. You can show work without measurable progress
by putting the progress bar in
indeterminate mode.
A progress bar in indeterminate mode displays animation
to indicate that work is occurring.
As soon as the progress bar can display more meaningful information,
you should switch it back into its default, determinate mode.
In the Java look and feel,
indeterminate progress bars
look like this:
Swing provides three classes to help you
use progress bars:
-
JProgressBar - A visible component to graphically display
how much of a total task has completed.
See Using Determinate Progress Bars
for information and an example of using a typical progress bar.
The section Using Indeterminate Mode
tells you how to animate a progress bar to show activity
before the task's scope is known.
-
ProgressMonitor - Not a visible component.
Instead, an instance of this class
monitors the progress of a task and
pops up a dialog if necessary.
See How to Use Progress Monitors
for details and an example of using a progress monitor.
-
ProgressMonitorInputStream - An input stream with an attached progress monitor,
which monitors reading from the stream.
You use an instance of this stream like any of the other
input streams described in
Basic I/O.
You can get the stream's progress monitor
with a call to
getProgressMonitor
and configure it as described in
How to Use Progress Monitors.
After you see a progress bar and a progress monitor in action,
Deciding Whether to Use a Progress Bar or a Progress Monitor
can help you figure out which is appropriate for your application.
Using Determinate Progress Bars
Here's a picture of a small demo application that
uses a progress bar to measure the progress of a task
that runs in its own thread:
Try this:
Below is the code from
ProgressBarDemo.java
that creates and sets up the progress bar:
//Where member variables are declared:
JProgressBar progressBar;
...
//Where the GUI is constructed:
progressBar = new JProgressBar(0, task.getLengthOfTask());
progressBar.setValue(0);
progressBar.setStringPainted(true);
The constructor that creates the progress bar
sets the progress bar's minimum and maximum values.
You can also set these values with
setMinimum
and
setMaximum.
The minimum and maximum values used in this program are 0 and the
length of the task, which is typical of many programs and tasks.
However, a progress bar's minimum and maximum
values can be any value, even negative.
The code snippet also sets the progress bar's current value to 0.
The call to setStringPainted
causes the progress bar to display, within its bounds,
a textual indication of the percentage of the task that has completed.
By default, the progress bar displays the value
returned by its getPercentComplete method
formatted as a percent, such as 33%.
Alternatively, you can replace the default with a different
string by calling setString. For example,
if (/*...half way done...*/)
progressBar.setString("Half way there!");
When the user clicks Start, an instance of the inner class
Task is created and executed.
public void actionPerformed(ActionEvent evt) {
startButton.setEnabled(false);
setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));
done = false;
task = new Task();
task.addPropertyChangeListener(this);
task.execute();
}
Task is a
subclass of
javax.swing.SwingWorker. The
Task instance does three important things for
ProgressBarDemo:
- The instance invokes the
doInBackground in a
separate thread. This is where the long-running task is actually
executed. Using a background thread instead of the
event-dispatching thread prevents the user interface from freezing
while the task is running.
- When the background task is complete, the instance invokes the
done method in the event-dispatching thread.
- The instance maintains a bound property,
progress, that is updated to indicate the progress of
the task. The propertyChange method is invoked each
time progress changes.
See
Worker Threads and SwingWorker
in
Concurrency in Swing
for more information about
SwingWorker.
The background task in ProgressBarDemo simulates a real
task by reporting random amounts of progress at random intervals. The
propertyChange method responds to changes in the the
task's progress property by updating the progress bar:
public void propertyChange(PropertyChangeEvent evt) {
if (!done) {
int progress = task.getProgress();
progressBar.setValue(progress);
taskOutput.append(String.format(
"Completed %d%% of task.\n", progress));
}
When the background task is complete, the task's done
method resets the progress bar:
public void done() {
//Tell progress listener to stop updating progress bar.
done = true;
Toolkit.getDefaultToolkit().beep();
startButton.setEnabled(true);
setCursor(null); //turn off the wait cursor
progressBar.setValue(progressBar.getMinimum());
taskOutput.append("Done!\n");
}
Note that the
done method sets the
done
field to
true, preventing
propertyChange
from making further updates to the progress bar. This is necessary
because the final updates to the
progress property may
occcur after
done is invoked.
Using Indeterminate Mode
In ProgressBarDemo2 indeterminate mode is set until
actual progress begins:
public void propertyChange(PropertyChangeEvent evt) {
if (!done) {
int progress = task.getProgress();
if (progress == 0) {
progressBar.setIndeterminate(true);
taskOutput.append("No progress yet\n");
} else {
progressBar.setIndeterminate(false);
progressBar.setString(null);
progressBar.setValue(progress);
taskOutput.append(String.format(
"Completed %d%% of task.\n", progress));
}
}
}
The other changes in the code are related to string display.
A progress bar that displays a string is likely to be taller
than one that doesn't,
and, as the demo designers, we've arbitarily decided that this progress bar
should display a string only when it's in the default, determinate mode.
However, we want to avoid the layout ugliness
that might result if the progress bar changed height
when it changed modes.
Thus, the code leaves in the call to
setStringPainted(true)
but adds a call to setString("")
so that no text will be displayed.
Later, when the progress bar switches from indeterminate to determinate mode,
invoking setString(null)
makes the progress bar display its default string.
One change we did not make
was removing the call to progressBar.setValue
from the progress event handler.
The call doesn't do any harm because an indeterminate progress bar
doesn't use its value property,
except perhaps to display it in the status string.
In fact, keeping the progress bar's data
as up-to-date as possible is a good practice,
since some look and feels might not support indeterminate mode.
Try this:
- Click the Launch button to run the ProgressBar2 Demo using
Java™ Web Start (download JDK 6).
Alternatively, to compile and run the example yourself,
consult the
example index.
- Push the Start button.
Note that the progress bar starts animating as soon as
the button is pressed,
and then switches back into
determinate mode (like ProgressBarDemo).
How to Use Progress Monitors
Now let's rewrite ProgressBarDemo to use a progress
monitor instead of a progress bar.
Here's a picture of the new demo program,
ProgressMonitorDemo:
Try this:
- Click the Launch button to run the ProgressMonitor Demo using
Java™ Web Start (download JDK 6).
Alternatively, to compile and run the example yourself,
consult the
example index.
- Push the Start button.
After a certain amount of time, the program displays a progress dialog.
- Click the OK button.
Note that the task continues even though the dialog is gone.
- Start another task. After the dialog pops up,
click the Cancel button.
The dialog goes away and the task stops.
A progress monitor cannot be used again, so a new
one must be created each time a new task is started.
This program creates a progress monitor each time the
user starts a new task with the
Start button.
Here's the statement that creates the progress monitor:
progressMonitor = new ProgressMonitor(ProgressMonitorDemo.this,
"Running a Long Task",
"", 0, task.getLengthOfTask());
This code uses
ProgressMonitor's only constructor
to create the monitor and
initialize several arguments:
- The first argument provides the parent component to
the dialog popped up by the progress monitor.
- The second argument is a string that describes
the nature of the task being monitored.
This string is displayed on the dialog.
see The Progress Monitoring API
for details about this argument.
- The third argument is another string
that provides a changeable status note.
The example uses an empty string to indicate that
the dialog should make space for a changeable status note,
but that the note is initially empty.
If you provide
null for this argument,
the note is omitted from the dialog.
The example updates the note
each time the progress property changes.
It updates the monitor's current value at the same time:
int progress = task.getProgress();
String message = String.format("Completed %d%%.\n", progress);
progressMonitor.setNote(message);
progressMonitor.setProgress(progress);
taskOutput.append(message);
- The last two arguments provide the minimum and maximum
values, respectively, for the progress bar displayed in the dialog.
By default, a progress monitor waits a minium of 500 milliseconds
before deciding whether to pop up the dialog. It also waits for the
progress to become more than the minimum value. If it calculates that
the task will take more than 2000 milliseconds to complete, the
progress dialog appears. To adjust the minimum waiting period, invoke
setMillisToDecidedToPopup. To adjust the minimum progress
time required for a dialog to appear, invoke
setMillisToPopup.
By the simple fact that this example uses a progress monitor,
it adds a feature that wasn't present in the version of the
program that uses a progress bar:
The user can cancel the task by clicking the Cancel
button on the dialog.
Here's the code in the example that checks
to see if the user canceled the task or if the task exited normally:
if (progressMonitor.isCanceled() || task.isDone()) {
progressMonitor.close();
Toolkit.getDefaultToolkit().beep();
if (progressMonitor.isCanceled()) {
task.cancel(true);
taskOutput.append("Task canceled.\n");
} else {
taskOutput.append("Task completed.\n");
}
startButton.setEnabled(true);
}
Note that the progress monitor doesn't itself cancel the task.
It provides the GUI and API to allow the program to do so easily.
Deciding Whether to Use a Progress Bar or a Progress Monitor
Use a progress bar if:
- You want more control over the configuration of the progress bar.
If you are working directly with a progress bar,
you can
set it to be indeterminate,
make it display vertically,
provide a string for it to display,
register change listeners on it, and
provide it with a bounded range model to control
the progress bar's minimum, maximum, and current values.
- The program needs to display other components
along with the progress bar.
- You need more than one progress bar.
With some tasks, you need to monitor more than one parameter.
For example, an installation program might monitor
disk space usage in addition to how many files
have been successfully installed.
- You need to reuse the progress bar.
A progress bar can be reused; a progress monitor cannot.
Once the progress monitor has decided to display a dialog (or not),
the progress monitor cannot do it again.
Use a progress monitor if:
- You want an easy way to display progress
in a dialog.
- The running task is secondary and
the user might not be interested in the progress of the task.
Progress monitor provides a way for the user to
dismiss the dialog while the task is still running.
- You want an easy way for the task to be cancelled.
Progress monitor provides a GUI for the user to cancel the task.
All you have to do is
call progress monitor's
isCanceled method to find out
if the user pressed the Cancel button.
- Your task displays a short message periodically while running.
The progress monitor dialog provides the
setNote
method so that the task can provide further information about what
it's doing. For example, an installation task might report the
name of each file as it's installed.
- The task might not take a long time to complete.
You decide at what point a running task is taking
long enough to warrant letting the user know about it.
Progress monitor won't pop up a dialog if the task completes
within the timeframe you set.
If you decide to use a progress monitor and
the task you are monitoring is reading from an input stream,
use the
ProgressMonitorInputStream class.
The following tables list the commonly used
API for using progress bars and progress monitors.
Because JProgressBar is a subclass of
JComponent,
other methods you are likely to call on a
JProgressBar
are listed in
The JComponent Class.
Note that
ProgressMonitor is a subclass of Object
and is not a visual component.
The API for monitoring progress
falls into these categories:
This following examples use JProgressBar or
ProgressMonitor.
| Example
|
Where Described
|
Notes
|
ProgressBarDemo
|
This section
|
Uses a basic progress bar to show progress
on a task running in a separate thread.
|
ProgressBarDemo2
|
This section
|
Uses a basic progress bar to show progress
on a task running in a separate thread.
|
ProgressMonitorDemo
|
This section
|
Modification of the previous example that uses
a progress monitor instead of a progress bar.
|
