Do Java threads appear to be Runnable even if they are waiting/blocked on system call?

Question

As per my understanding, making a select/epoll/kqueue system call blocks a thread or process making the call, till a socket is ready for IO or the timeout event occurs. When doing a Thread Dump analysis on a netty based program I am seeing the parent group thread is in runnable state after making select call,

   "Parent Group1" #11 prio=5 os_prio=0 tid=0x000000001e575000 nid=0x3f98 runnable [0x000000001f2ce000]
   java.lang.Thread.State: RUNNABLE
    at sun.nio.ch.WindowsSelectorImpl$SubSelector.poll0(Native Method)
    at sun.nio.ch.WindowsSelectorImpl$SubSelector.poll(WindowsSelectorImpl.java:296)
    at sun.nio.ch.WindowsSelectorImpl$SubSelector.access$400(WindowsSelectorImpl.java:278)
    at sun.nio.ch.WindowsSelectorImpl.doSelect(WindowsSelectorImpl.java:159)
    at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
    - locked <0x000000076bb59088> (a io.netty.channel.nio.SelectedSelectionKeySet)
    - locked <0x000000076b9706b8> (a java.util.Collections$UnmodifiableSet)
    - locked <0x000000076b970308> (a sun.nio.ch.WindowsSelectorImpl)
    at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
    at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:101)
    at io.netty.channel.nio.SelectedSelectionKeySetSelector.select(SelectedSelectionKeySetSelector.java:68)
    at io.netty.channel.nio.NioEventLoop.select(NioEventLoop.java:803)
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:457)
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
    at java.lang.Thread.run(Thread.java:745)

Do Java threads appear Runnable even if it is waiting on system call or am I missing something.

Answer 1

As you can see from the dump, yes they are.

Java threads are not "runnable" only if blocked using a Java built-in synchronization mechanism, like waiting to enter a monitor (eg Object.wait , Thread.sleep , Thread.join ).

Native calls to select / epoll / kqueue and JNI do not affect thread state.