CODEWITHSUNDEEP
javac++multithreadingwinapijava-native-interface
Parker Kemp
Parker Kemp

Reputation: 765

JNI - Java exits before native threads finish executing

I'm in the early stages of developing an API in C++, which I'm wrapping in Java using JNI. The native code creates a socket listener thread using WinAPI which should run indefinitely, thereby keeping the program open indefinitely (tested and works fine).

However, when I try to invoke this code in Java, the JVM still terminates when it reaches the end of main, ignoring the running thread. A little research has hinted that Java might think the thread is a daemon rather than a "user thread". But if that's the case, then I can't quite figure out how to convince Java that it actually is a user thread.

Does anyone have a clue about this?

Upvotes: 0

Views: 641

Answers (2)

user207421
user207421

Reputation: 311018

You need to call AttachCurrentThread() for all your native threads, to ensure Java knows about them, so it will wait for them to finish.

Upvotes: 2

Harry Johnston
Harry Johnston

Reputation: 36348

Windows doesn't have daemon threads. The process exits when ExitProcess() is called or when the initial thread returns from the application's main function. (In principle, it will also exit if the last thread exits, but that can't be relied upon because Windows may create threads in your process that you don't know about.)

The Java runtime presumably waits for all of its own threads to exit (except for those that it considers daemon threads) before exiting the process. But your threads were created directly via the Win32 API, so Java doesn't know about them and therefore won't wait for them.

If your API wants to continue performing some task beyond the natural lifetime of the calling process, it should probably create a child process rather than a thread. (Or, if the API is Java-specific, it can presumably make use of JNI to ask that Java create the thread on its behalf, or to register the thread with Java.)

Upvotes: 1

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