Introduction
It is important to note that write access is allowed from the event dispatch thread only. Any code that needs to update the UI must be run on the event dispatch thread. Otherwise, you will see the following error message:
Exception in thread “main” java.lang.IllegalStateException: write access is allowed from the event dispatch thread only
at javax.swing.text.AbstractDocument.writeLock(AbstractDocument.java:1308)
at javax.swing.text.AbstractDocument.insertString(AbstractDocument.java:758)
at Main$1$1$2$1run(Main.java:172)
at java_util_concurrent_Executors$RunnableAdaptercall(Executors.java:511)
at java_util_concurrent_FutureTasktryFire(FutureTask.java:264)
at java_util_concurrent_FutureTaskfireIncrementRunState(FutureTaskjava:331)
at java_util_concurrentTrilean ComputationException TimedOutException FutureTaskget(FutureTask):380)
As the error message states, write access is only allowed from the event dispatch thread. This means that any code that needs to update the UI must be run on the event dispatch thread
What is the Event Dispatch Thread?
The Event Dispatch Thread is a specialized thread that is responsible for handling events in Java. This includes everything from mouse clicks to keystrokes, and all other events that are generated by the various components in a Java application. One of the main benefits of using the Event Dispatch Thread is that it helps to ensure that events are processed in a timely fashion, and that they are handled in the correct order.
Why is write access allowed from the Event Dispatch Thread only?
Swing components are not thread-safe. All Swing events are processed on the Event Dispatch Thread (EDT). In order to prevent concurrent access to a component from two different threads, all component accesses must be done on the EDT.
The EDT is a single thread that processes all events and repaints Swing components. All Swing component methods are either “safe” or “unsafe”. Safe methods can be called from any thread without invoking special code. Unsafe methods can only be called from the EDT.
Calling an unsafe method from any thread other than the EDT will result in an IllegalStateException being thrown.
What are the consequences of not following this rule?
If you don’t follow the rule that write access is allowed from event dispatch thread only, your program may experience unexpected behavior or even crash. This is because the Event Dispatch Thread is responsible for handling all events and updating the GUI components accordingly. If you attempt to modify a GUI component from a different thread, the Event Dispatch Thread will not be able to process the event properly, which can lead to unforeseen consequences.
How to ensure that write access is allowed from the Event Dispatch Thread only?
It is recommended that all code that deals with Swing components is executed on the Event Dispatch Thread (EDT). The EDT is a single thread that is responsible for all events and GUI updates in Swing. In order to ensure that code is executed on the EDT, we can use the SwingUtilities.invokeLater() or SwingUtilities.invokeAndWait() methods.
The invokeLater() method will execute the code on the EDT as soon as possible, but it doesn’t wait for the code to finish executing before continuing. The invokeAndWait() method will execute the code on the EDT and wait for it to finish before continuing.
It is important to note that both of these methods accept a Runnable object as an argument. A Runnable object is simply an object that contains a run() method. The code that we want to execute on the EDT should be placed in this run() method.
Conclusion
To sum it up, it is not possible to write to a file from the event dispatch thread only. Doing so would result in an IllegalStateException. The only time that this would not be the case is if the file being written to is on the same thread as the event dispatch thread.