Question

JOptionPane.showMessageDialog is supposed to be a useful utility for getting user feedback as it blocks your current thread while you wait.

I would expect therefore that it would be thread-safe and that you wouldn’t need to wrap the call in an invokeLater or an invokeAndWait.

Is this the case?

Answer 1

Taken from the javax.swing package description:

Swing’s Threading Policy

In general Swing is not thread safe. All Swing components and related classes, unless otherwise documented, must be accessed on the event dispatching thread. Typical Swing applications do processing in response to an event generated from a user gesture. For example, clicking on a JButton notifies all ActionListeners added to the JButton. As all events generated from a user gesture are dispatched on the event dispatching thread, most developers are not impacted by the restriction.

Where the impact lies, however, is in constructing and showing a Swing application. Calls to an application’s main method, or methods in Applet, are not invoked on the event dispatching thread. As such, care must be taken to transfer control to the event dispatching thread when constructing and showing an application or applet. The preferred way to transfer control and begin working with Swing is to use invokeLater. The invokeLater method schedules a Runnable to be processed on the event dispatching thread.

JOptionPane does not document that it is thread safe, so you have to use invokeLater().