ObjectCleanerThread not be closed

Netty version

4.1.20.Final

JVM version (e.g. java -version)

9.0.1

OS version (e.g. uname -a)

Windows 10

Non-finished threads:

Thread[DestroyJavaVM,5,main]

Thread[ObjectCleanerThread,1,main] at java.base@9.0.1/java.lang.Object.wait(Native Method) at java.base@9.0.1/java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151) at app//io.netty.util.internal.ObjectCleaner$1.run(ObjectCleaner.java:52) at app//io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.base@9.0.1/java.lang.Thread.run(Thread.java:844)

[johnou]

@normanmaurer I would expect something like this to exit gracefully without an explicit System.exit call.

public class Main {

    public static void main(String[] args) {
        ByteBuf byteBuf = PooledByteBufAllocator.DEFAULT.buffer();
        byteBuf.release();
        System.gc();
        System.runFinalization();
        // hangs because ObjectCleanerThread is not a daemon thread / LIVE_SET is not empty..
    }
}

[normanmaurer]

@johnou @hank-whu after reading more up on all of this I think you are right and @Scottmitch and myself were wrong … This must be a deamon thread to ensure we will be able to shutdown in all cases. OpenJDK itself also uses a deamon thread and the GC threads are also deamon threads: https://github.com/dmlloyd/openjdk/blob/342a565a2da8abd69c4ab85e285bb5f03b48b2c9/src/java.base/share/classes/jdk/internal/ref/CleanerImpl.java#L117

Let me come up with a PR