DestroyJavaVM thread ALWAYS running
When profiling my application I came across a weird behavior - the DestroyJavaVM thread is ALWAYS running - 100% of the time.
After doing a little research on the subject, on which there's hardly any valuable information online, all I understood is that this thread is supposed to unload the JVM upon exit.
If that's the case, why is this thread in RUNNING state 100% of the time from the very first moment I start my application? Doesn't it consume valuable resources and therefore may cause an OutOfMemoryError (like I sometimes get)?
Is there any official reference to what this thread actually does and what triggers its initialization?
Thanks
Answer
This happens because most applications are run in threads.
All POJO apps start by invoking the main method. In its most simple case, this method will do all of the work, creating objects, calling methods etc. Once main completes, the JVM is told to shut down using a DestroyJavaVM thread which waits for all non-daemon threads to complete before doing its work. This is to ensure that any non-daemon threads you create run to completion before the JVM is torn down.
An app with a GUI, however, normally runs as a number of threads. One for watching for system events such as keyboard or mouse events. One for maintaining the windows and display etc. The main method of this kind of app will probably just start up all the required threads and exit. It still creates the DestroyJavaVM thread but now all that it does is wait for all of your created threads to finish before tearing down the VM.
As a result, any app that creates threads and relies solely on their functionality will always have a DestroyJavaVM thread waiting for it to finish. Since all it is doing is joining all other running threads it does not consume any resources.