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NeuronRobotics / nrjavaserial

A Java Serial Port system. This is a fork of the RXTX project that uses in jar loading of the native code.
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Internal thread is not a daemon thread #125

Closed michab66 closed 5 years ago

michab66 commented 6 years ago

The thread started by nrjavaserial is not declared as a daemon thread.

This results in a hanging application if the #main() operation simply returns, since non-daemon threads prevent the Java-VM from termination.

Better is to declare these threads as daemon threads.

A workaround is to call System.exit().

The thread in question is started in RXTXPort.java

dougmeredith commented 5 years ago

I understand that it's appealing to not have background threads block JVM exit, but I'm not sure this is a correct use of daemon threads. My understanding is that they are intended to do low-priority background tasks like garbage collection.

See this article for some information. This article talks about a potential pitfall of using daemon threads.

michab66 commented 5 years ago

Hi dougmeredith,

your second reference is in my eyes a misunderstanding of the API and in particular of daemon threads. So I won't talk about this.

Your first reference explains that a daemon thread 'serves a user thread'. This is actually the case with nrjavaserial. The thread in nrjavaserial serves user threads processing the incoming data from a serial port.

If all user threads have terminated, then there is and will never be again a user thread that can cooperate with the nrjavaserial thread. So in this case, the VM could simply go down, vs. the current situation, where the user thread has to call System.exit(); explicitly.

The good case with nrjavaserial is that when the user properly closes the connection the thread is also stopped. So a hanging application is the consequence of a missing close on a serial stream, and thus an application error.

Nevertheless, because of the above argument, I think it would be a step forward for nrjavaserial if the internal thread would be a daemon.

Best wishes, Michael.

Am Di., 22. Jan. 2019, 20:07 hat dougmeredith notifications@github.com geschrieben:

I understand that it's appealing to not have background threads block JVM exit, but I'm not sure this is a correct use of daemon threads. My understanding is that they are intended to do low-priority background tasks like garbage collection.

See this article https://www.baeldung.com/java-daemon-thread for some information. This article https://meteatamel.wordpress.com/2012/05/22/when-a-daemon-thread-is-not-so-daemon/ talks about a potential pitfall of using daemon threads.

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