Is SwingWorker the only way? - java

I have found that in order to keep Java GUIs (using Swing) responsive the only way is to use the SwingWorker class, as opposed to java.lang.Thread. Is SwingWorker truly the only way when it comes multithreaded GUI based desktop apps? Are there any good alternatives? Rarely can I configure the Thread class to do what I want to do, but SwingWorker usually works, if sometimes in a cumbersome way.

SwingWorker is nothing but a thin convenience API around Thread. Therefore it is definitely possible to use Thread without SwingWorker. The main point is that Swing is not thread-safe and any actions you perform on Swing objects must happen on the Event Dispatch Thread. This is the fundamental obstacle, and SwingWorker tries to help you overcome it a bit more conveniently.

The alternative is a continuation using EventQueue.invokeLater(), but SwingWorker has important advantages:
It synchronizes granular access to data shared between itself and the EDT.
It provides property change support for progress indication.
It implements the Future interface.
See also Worker Threads and SwingWorker for details.

Multithreading in GUI application is difficult to implement since there can be so many actions that trigger actions. A good explanation of why is this a "failed dream" can be found here
Multithreaded toolkits: A failed dream.. For solutions to your problem, read this article on concurrency in swing: Concurrency in Swing
Depending on your duration of the action you can go with SwingUtilities.invokeLater() or make a SwingWorker for tasks that take a long time to complete and run in background.
You must use this classes or else you may be in situations where a thread will block your entire application and may seem unresponsive to the user.

Another option, most suitable for repetitive tasks, is javax.swing.Timer (it can be used for one-shot tasks as well).

Related

Java Event-Dispatching Thread explanation

I've recently started learning and exploring the basics of GUI programming in Java.
Having been programming for a while I have only done backend work or work and as a result the closest I've gotten to user interfaces is the command console (embarrassing I know).
I'm using Swing and as far as I can gather that means by extension I am also using AWT.
My question is based on this piece of code:
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new frame.setVisible(true);
}
} );
I have been researching this for a while as I wanted to fully understand this strange piece of code and have come across the term 'Event-Dispatching Thread' multiple times. Correct me if I'm wrong but as I understand it; it has to do with using multiple threads and how Java Swing interprets those threads. I gather as well that the above code is used to make sure all the threads are 'safe' before it creates the window, hence the invokeLater?
I have read that:
"You can only call methods that operate on the frame from the Event-Dispatching Thread"
and that only under certain circumstances can you call methods that operate on the frame from the main method.
Can somebody please clarify to me what exactly the Event-Dispatching Thread is?
How it relates to multiple threads of execution and how those threads are not safe to be called from the main method? Also why do we need this invokeLater?
Can we not just create the window as any other object?
I've hit a bit of a road block in my research as I'm not grasping these relations and ideas.
A side note is that I like to base my knowledge on in-depth understanding as I believe this leads to the best overall outcome and as a result the best programs. If I understand in-depth how something works then you can use the tips and tweaks effectively rather than just parroting them back in to code, so please don't be afraid to give me some extra in-depth explanations and broaden my knowledge.
Thank you.
The event dispatch thread is a special thread that is managed by AWT. Basically, it is a thread that runs in an infinite loop, processing events.
The java.awt.EventQueue.invokeLater and javax.swing.SwingUtilities.invokeLater methods are a way to provide code that will run on the event queue. Writing a UI framework that is safe in a multithreading environment is very difficult so the AWT authors decided that they would only allow operations on GUI objects to occur on a single special thread. All event handlers will execute on this thread and all code that modifies the GUI should also operate on this thread.
Now AWT does not usually check that you are not issuing GUI commands from another thread (The WPF framework for C# does do this), meaning it's possible to write a lot of code and be pretty much agnostic to this and not run into any problems. But this can lead to undefined behavior, so the best thing to do, is to always ensure that GUI code runs on the event dispatch thread. invokeLater provides a mechanism to do this.
A classic example is that you need to run a long running operation like downloading a file. So you launch a thread to perform this action then, when it is completed, you use invokeLater to update the UI. If you didn't use invokeLater and instead you just updated the UI directly, you might have a race condition and undefined behavior could occur.
Wikipedia has more information
Also, if you are curious why the AWT authors don't just make the toolkit multithreaded, here is a good article.
EventDispatchThread (EDT) is special thread reserved only for Swing GUI and *Swing's related events e.g. create/change/update Swing JComponents, more for asked questions here and here
all output to the GUI from BackGround Tasks, Runnable#Thread must be wrapped into invokeLater(), from synchronized Objects into invokeAndWait();

Making Swing components synchronized

I'm reading Java Threads 3rd Ed. by Oaks and Wong (O'Reilly 2004).
They carry an example of a Swing typing game throughout the book.
The classes they define are mostly custom subclasses of javax.swing.JComponent.
What seems quite wrong to me is that they make those JComponents thread safe with various synchronization methods. I was under the impression that Swing components should not be thread safe, but rather that they should always be accessed from the Swing event dispatching thread. (Amusingly, one of the few times where they modify a component through the Swing EDT, it's for a setText, which is one of the very few Swing methods that do not need to be called from the EDT.)
I would like to know from some of you who have a lot of experience writing/reading Swing code:
Is it common for programmers to make Swing components synchronized instead of always modifying them through the EDT? Is it tolerable?
EDIT:
I noticed it is nearly the same question as this thread. However it does not say what programmers actually do in the wild. I'm puzzled that an O'Reilly book would so blatantly violate the Swing threading model.
EDIT:
I discovered that they do briefly explain somewhere in the middle of the book the Swing threading model. Nonetheless I'd like to have an answer to my question. I have the feeling most who read this book will end up violating the Swing threading model since most of their examples do.
EDIT:
If you want to look at the code, you can Download examples code as a zip file. See for example ch03/example1/AnimatedCharacterDisplayCanvas.
EDIT:
I just learned that setText will not be thread-safe in Java7 (release in July 2011).
Trivially, as long as the synchronized methods do not execute on the EventQueue, they won't block the event dispatch thread. Conversely, a method executing on another thread should always use the EventQueue to dispatch code via invokeLater(), invokeAndWait(), or a related mechanism such as javax.swing.Timer or javax.swing.SwingWorker. As a practical matter, these are reliable. The examples may be correct, but they should be examined from this perspective.
The EventQueue API says, "The only requirements are that
events...are dispatched...in the same order as they are enqueued." In my opinion, this is tantamount to the "happens-before" relation of java.util.concurrent
and the JLS. A more detailed discussion may be found here.
You should never have synchronized blocks on Swing components, its going to cause weird problems when its trying to be rendered.
Swing is not thread safe because everything is supposed to be updated on the EDT, even creation of Swing components.
Long running processes should be moved to a background thread or a SwingWorker. When a thread other than the EDT needs to make components or make updates to a component it should be wrapped using SwingUtilities.invokeLater()
"Swing components are not inherently thread safe, and as a general rule, after Swing components have been made visible on the screen, you can only safely modify their data from the event thread. If you modify Swing component data from any thread other than the event dispatching thread, you must take precautions to ensure data integrity. An exception to this rule is the setText method on a JTextComponent or any of its subclasses, or any Swing component method whose documentation explicitly states it is thread safe."
Monica Pawlan
http://java.sun.com/developer/technicalArticles/Threads/swing/

Java Swing Multiple Event Listeners for a Single Event Source in a single-threaded environment

I am currently learning Swing, and I am new to GUI development in general. In my test application, I have multiple event listners for a single event source, and I am wondering which one of these event listeners will be excecuted first.
Also, I am curious to know how Swing event-handling works in a single-threaded environment, especially when you have multiple listeners for a single event source.
Lastly, I would like to know some common cases where I have to use multiple threads in Swing.
Thanks!
I will try to answer all 3 of your questions. First of all, the order that the ActionListeners fire is not specified. One should never assume a specific order that they will fire. If you need actions to take place in a specific order, put them in the same ActionListener.
When programming Swing, you will 'almost' always be in a multi-threaded environment. There is one thread called the Event Dispatch Thread (EDT). This is the thread that handles all events. Any other processing you do should be done on a different thread, otherwise your Swing GUI can become unresponsive.
A common case for multiple threads in Swing is any time you need to do some processing that takes an extended amount of time. (Intense calculations, IO, database connections) You will want to do the hard work on a separate thread from the EDT. That will keep your GUI responsive.
The Oracle network has a great tutorial on concurrency in Swing. I recommend you check it out.
A Swing programmer deals with the following kinds of threads:
Initial threads, the threads that execute initial application code.
The event dispatch thread, where all event-handling code is executed. Most code that interacts with the Swing framework must also execute on this thread.
Worker threads, also known as background threads, where time-consuming background tasks are executed.
The canonical answer to any multi-threading questions in Swing is to use a SwingWorker. It allows you to easily coordinate background work on a separate thread with the EDT. As usual, Oracle has a great tutorial on how to use SwingWorker.

Swing: Passing a value back to the UI from a scheduled thread

I have a system tray UI in Java that requires a schedule database poll. What is the best method for spawning a new thread and notifying the UI?
I'm new to Swing and it's threading model.
SwingWorker is the exact thing designed to do this.
It allows you to run a task that won't block the GUI and then return a value and update the GUI when it is done.
Java has a great tutorial on how to use SwingWorker.
Basically do the database pull in the doInBackground() method. And, in the done() method, update your GUI.
As jinguy mentioned SwingWorker should be the first place you look at.
Wikipedia, of all places, has some interesting examples that may be good to look at before you tackle the JavaDocs.
As jjnguy mentioned, SwingWorker helps abstract away the complexity here, but basically you do the work in a new thread, and when the method comes back, you need to update the GUI in the swing thread. If you aren't using SwingWorker, the underlying method is SwingUtilities (or EventQueue) .invokeLater(Runnable).
Do not update anything Swing related (including models) outside of the swing queue, unpredictable things will happen. And don't attempt to hold a reference to the queue and use that, as queues are suspended and replaced (if for example you open a model dialog box).

Java: Swing Libraries & Thread Safety

I've often heard criticism of the lack of thread safety in the Swing libraries. Yet, I am not sure as to what I would be doing in my own code with could cause issues:
In what situations does the fact Swing is not thread safe come into play ?
What should I actively avoid doing ?
Never do long running tasks in response to a button, event, etc as these are on the event thread. If you block the event thread, the ENTIRE GUI will be completely unresponsive resulting in REALLY pissed off users. This is why Swing seems slow and crusty.
Use Threads, Executors, and SwingWorker to run tasks NOT ON THE EDT ( event dispatch thread).
Do not update or create widgets outside of the EDT. Just about the only call you can do outside of the EDT is Component.repaint(). Use SwingUtilitis.invokeLater to ensure certain code executes on the EDT.
Use EDT Debug Techniques and a smart look and feel (like Substance, which checks for EDT violation)
If you follow these rules, Swing can make some very attractive and RESPONSIVE GUIs
An example of some REALLY awesome Swing UI work: Palantir Technologies. Note: I DO NOT work for them, just an example of awesome swing. Shame no public demo... Their blog is good too, sparse, but good
This is one of those questions that makes me glad I purchased Robinson & Vorobiev's book on Swing.
Anything that accesses the state of a java.awt.Component should be run inside the EDT, with three exceptions: anything specifically documented as thread-safe, such as repaint(), revalidate(), and invalidate(); any Component in a UI that has not yet been realized; and any Component in an Applet before that Applet's start() has been called.
Methods specially made thread-safe are so uncommon that it's often sufficient to simply remember the ones that are; you can also usually get away with assuming there are no such methods (it's perfectly safe to wrap a repaint call in a SwingWorker, for example).
Realized means that the Component is either a top-level container (like JFrame) on which any of setVisible(true), show(), or pack() has been called, or it has been added to a realized Component. This means it's perfectly fine to build your UI in the main() method, as many tutorial examples do, since they don't call setVisible(true) on the top-level container until every Component has been added to it, fonts and borders configured, etc.
For similar reasons, it's perfectly safe to build your applet UI in its init() method, and then call start() after it's all built.
Wrapping subsequent Component changes in Runnables to send to invokeLater() becomes easy to get right after doing it only a few times. The one thing I find annoying is reading the state of a Component (say, someTextField.getText()) from another thread. Technically, this has to be wrapped in invokeLater(), too; in practice, it can make the code ugly fast, and I often don't bother, or I'm careful to grab that information at initial event handling time (typically the right time to do it in most cases anyway).
It's not just that Swing is not thread-safe (not much is), but it's thread-hostile. If you start doing Swing stuff on a single thread (other than the EDT), then when in cases where Swing switches to the EDT (not documented) there may well be thread-safety issues. Even Swing text which aims to be thread-safe, isn't usefully thread-safe (for instance, to append to a document you first need to find the length, which might change before the insert).
So, do all Swing manipulations on the EDT. Note the EDT is not the thread the main is called on, so start your (simple) Swing applications like this boilerplate:
class MyApp {
public static void main(String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() { public void run() {
runEDT();
}});
}
private static void runEDT() {
assert java.awt.EventQueue.isDispatchThread();
...
An alternative to using intelligent skins like substance is to create the following utility method:
public final static void checkOnEventDispatchThread() {
if (!SwingUtilities.isEventDispatchThread()) {
throw new RuntimeException("This method can only be run on the EDT");
}
}
Call it in every method you write that is required to be on the event dispatch thread. An advantage of this would be to disable and enable system wide checks very quickly, eg possibly removing this in production.
Note intelligent skins can of course provide additional coverage as well as just this.
Actively avoid doing any Swing work at all except on the event dispatching thread. Swing was written to be easy to extend and Sun decided a single-threaded model was better for this.
I have had no issues whilst following my advice above. There are some circumstances where you can 'swing' from other threads but I've never found the need.
If you're using Java 6 then SwingWorker is definately the easiest way to deal with this.
Basically you want to make sure that anything that changes a UI is performed on the EventDispatchThread.
This can be found by using the SwingUtilities.isEventDispatchThread() method to tell you if you are in it (generally not a good idea - you should know what thread is active).
If you aren't on the EDT then you use SwingUtilities.invokeLater() and SwingUtilities.invokeAndWait() to invoke a Runnable on the EDT.
If you update UI's not on the EDT you get some incredibly strange behaviour. Personally I don't consider this a flaw of Swing, you get some nice efficiency by not having to synchronize all of the threads to provide a UI update - you just need to remember that caveat.
The phrase 'thread-unsafe' sounds like there is something inherently bad (you know... 'safe' - good; 'unsafe' - bad). The reality is that thread safety comes at a cost - threadsafe objects are often way more complex to implement (and Swing is complex enough even as it is.)
Also, thread-safety is achieved either using locking (slow) or compare-and-swap (complex) strategies. Given that the GUI interfaces with humans, which tend to be unpredictable and difficult to synchronize, many toolkits have decided to channel all events through a single event pump. This is true for Windows, Swing, SWT, GTK and probably others. Actually I don't know a single GUI toolkit which is truly thread-safe (meaning that you can manipulate its objects' internal state from any thread).
What is usually done instead is that the GUIs provide a way to cope with the thread-unsafety. As others noted, Swing has always provided the somewhat simplistic SwingUtilities.invokeLater(). Java 6 includes the excellent SwingWorker (available for previous versions from Swinglabs.org). There are also third party libraries like Foxtrot for managing threads in Swing context.
The notoriety of Swing is because the designers have taken light handed approach of assuming that the developer will do the right thing and not stall the EDT or modify components from outside the EDT. They have stated their threading policy loud and clear and it's up to the developers to follow it.
It's trivial to make each swing API to post a job to the EDT for each property-set, invalidate, etc., which would make it threadsafe, but at the cost of massive slowdowns. You can even do it yourself using AOP. For comparison, SWT throws exceptions when a component is accessed from a wrong thread.
Here's a pattern for makng swing thread-freindly.
Sublass Action (MyAction) and make it's doAction threaded.
Make the constructor take a String NAME.
Give it an abstract actionImpl() method.
Let it look like.. (pseudocode warning!)
doAction(){
new Thread(){
public void run(){
//kick off thread to do actionImpl().
actionImpl();
MyAction.this.interrupt();
}.start(); // use a worker pool if you care about garbage.
try {
sleep(300);
Go to a busy cursor
sleep(600);
Show a busy dialog(Name) // name comes in handy here
} catch( interrupted exception){
show normal cursor
}
You can record the time taken for the task, and next time, your dialog can show a decent estimate.
If you want to be really nice, do the sleeping in another worker thread too.
Note that not even the model interfaces are thread safe. The size and the content are queried with separate get methods and so there is no way of synchronizing those.
Updating the state of the model from another thread allows for it to at least paint a situation where size is still bigger (table row is still in place), but the content is no longer there.
Updating state of the model always in EDT avoids these.
invokeLater() and invokeAndWait() really MUST be used when you are doing any interaction with GUI components from any thread that is NOT the EDT.
It may work during development, but like most concurrent bugs, you'll start to see weird exceptions come up that seem completely unrelated, and occur non-deterministly - usually spotted AFTER you've shipped by real users. Not good.
Also, you've got no confidence that your app will continue to work on future CPUs with more and more cores - which are more prone to encountering weird threading issues due to them being truely concurrent rather than just simulated by the OS.
Yes, it gets ugly wrapping every method call back into the EDT in a Runnable instance, but that's Java for you. Until we get closures, you just have to live with it.
For more details about threading, Taming Java Threads by Allen Holub is an older book but a great read.
Holub, really promotes responsive UI and details examples and how to alleviate problems.
http://www.amazon.com/Taming-Java-Threads-Allen-Holub/dp/1893115100
http://www.holub.com/software/taming.java.threads.html
Love the "If i was king" section in the end there.

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