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I know what "thread" means and if I understand the event dispatching thread (EDT) as "just a thread", it explains a lot but, apparently, it does not explain everything.
I do not understand what is special about this thread. For example I do not understand why we should start a GUI in a the EDT? Why the "main" thread is bed for GUI? Well, if we just do not want to occupy the main thread why we cannot start GUI just in "another thread" why it should be some "special" thread called EDT?
Then I do not understand why we cannot start the EDT like any other thread? Why we should use some special tool (called invokeLater). And why GUI, unlike any other thread, does not start immediately. We should wait until it is ready to accept our job. Is it because EDT can, potentially execute several task simultaneously?
If you decide to answer this question, could you pleas use a really simple terminology because otherwise, I am afraid, I will not be able to understand the answer.
ADDED:
I always thought that we have one "task" per thread. So, in every thread we execute a predefined sequence of commands. But it seems to me that in the event dispatching thread we can have sever task. Well, they are not executed simultaneously (thread switches between different task but there are still several task in one thread). Is it right? For example there is one thread in the EDT which display the main window, and then additionally to that we sent to the EDT another task which should update one of the window components and EDT will execute this new task whenever it is ready. Is EDT differ from other threads in this way?
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The Event Dispatch ThreadMost code that invokes Swing methods also runs on this thread. This is necessary because most Swing object methods are not "thread safe": invoking them from multiple threads risks thread interference or memory consistency errors.
An invokeLater() method is a static method of the SwingUtilities class and it can be used to perform a task asynchronously in the AWT Event dispatcher thread. The SwingUtilities. invokeLater() method works like SwingUtilities. invokeAndWait() except that it puts the request on the event queue and returns immediately.
Their only difference is indicated by their names: invokeLater simply schedules the task and returns; invokeAndWait waits for the task to finish before returning. You can see examples of this throughout the Swing tutorial: SwingUtilities. invokeLater(new Runnable() { public void run() { createAndShowGUI(); } });
What is an Event? Change in the state of an object is known as Event, i.e., event describes the change in the state of the source. Events are generated as a result of user interaction with the graphical user interface components.
The event dispatching thread is the thread that handles all GUI events and manages your Swing GUI. It is started somewhere in the Swing code if you have any GUI in your program. The reason it is done behind the scenes is because of simplicity - you do not have to bother with starting and managing an extra thread by yourself.
Regarding the fact that you have to update your GUI with invokeLater() it is because of concurrency issues. The GUI can be modified only from one thread because Swing is not thread safe(it is worth to note that most of toolkits are not thread safe, there is a nice article that gives some ideas why). This is why you have to submit all GUI updates to run on EDT.
You can read more on concurrency in Swing and event dispatching thread in Sun tutorial on concurrency in Swing. Also, if you would like to see how this could be done in a different way you might like to check out SWT toolkit. In SWT you have to manage EDT by yourself.
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I always thought that we have one "task" per thread. So, in every thread we execute a predefined sequence of commands. But it seems to me that in the event dispatching thread we can have sever task. Well, they are not executed simultaneously (thread switches between different task but there are still several task in one thread). Is it right? For example there is one thread in the EDT which display the main window, and then additionally to that we sent to the EDT another task which should update one of the window components and EDT will execute this new task whenever it is ready. Is EDT differ from other threads in this way?
No, the EDT is not fundamentally different from other threads. And "task" is not a good word to use, because it could be confused with OS-level processes (which are also often called task). Better use Runnable, the interface used to give code to the EDT to execute via invokeLater().
The EDT is basically connected to a queue of things it has to do. When the user clicks a button on the GUI, a Runnable that notifies all listeners attached to the button goes into the queue. When a window is resized, a Runnable doing revalidate&repaint goes into the queue. And when you use invokeLater(), your Runnable goes into the queue.
The EDT simply runs an endless loop that says "take a Runnable from the queue (and if it's empty sleep until you're notified that it's not) and execute it.
Thus, it executes all those little Runnable pieces of code one after another, so that each of them basically has the GUI all to itself while it runs, and doesn't have to worry about synchronizing anything. When you manipulate the GUI from another thread, this assumption is broken, and you can end up with the GUI in a corrupted state.
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