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Re: Thread and synchonized question

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On 9/30/2019 3:32 AM, mike wrote:
> Hi,
> 
> We have class A with method synchronized connect(int) that calls
> class B that has synchronized connect() (main thread)
> 
> I guess the lock will be on class A?

     The thread will hold two locks, but exactly which two depends
on whether the two connect() methods are `static' or not:

     - The thread first obtains the lock on the A.class object if
       A's connect() is static, or it obtains the lock on a particular
       instance of A if connect() is an instance method.  This lock
       (whichever it is) remains held until A's connect() finishes.

     - When A's connect() calls B's a similar thing happens: The thread
       takes a lock on B.class or on a particular B instance, and holds
       that lock until B's connect() finishes.

     When B's connect() finishes, whether by returning or by throwing
an uncaught exception, the thread releases its B lock.  And then when
A's connect() finishes (either way), the thread releases its A lock.
"First on, last off."

> What if class B and synchronized connect()is called by another thread ( reconnect thread) I guess lock is on class B.

     Let's assume the first thread is still executing B's connect()
method, so it still holds whichever kind of B lock is appropriate.
Then there are three cases:

     - If B's connect() is static, the second thread tries to obtain
       the lock on the B.class object.  The first thread still holds
       that lock, so the second thread pauses until the first thread
       lets go.

     - If B's connect() is an instance method and the second thread
       uses the same B instance as the first, the second thread tries
       to obtain the lock on that B instance.  Again, the first thread
       still holds it, so the second thread waits.

     - If B's connect() is an instance method and the second thread
       uses a different B instance from the first, the second thread
       tries to obtain the lock -- and succeeds, because it's not the
       same lock the first thread is holding.  The second thread sails
       merrily along, and both threads are executing B's connect() at
       the same time -- albeit using different B instances.

> So does this mean that both main and reconnect threads can access the synchronized connect() on class B?

     If B's connect() is an instance method and the two threads use
different B instances, yes.  If it's a static method or if both threads
use the same B instance, no.

     There's a fairly readable tutorial on this stuff at

https://docs.oracle.com/javase/tutorial/essential/concurrency/sync.html

and I think you might be particularly interested in

https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html

-- 
esosman@comcast-dot-net.invalid
Four hundred seventy-eight days to go.

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Thread

Thread and synchonized question mike <mikaelpetterson@hotmail.com> - 2019-09-30 00:32 -0700
  Re: Thread and synchonized question Andreas Leitgeb <avl@logic.at> - 2019-09-30 10:10 +0000
    Re: Thread and synchonized question mike <mikaelpetterson@hotmail.com> - 2019-09-30 04:38 -0700
      Re: Thread and synchonized question Andreas Leitgeb <avl@logic.at> - 2019-09-30 13:48 +0000
  Re: Thread and synchonized question Eric Sosman <esosman@comcast-dot-net.invalid> - 2019-09-30 17:07 -0400
  Re: Thread and synchonized question Arne Vajhøj <arne@vajhoej.dk> - 2019-09-30 19:48 -0400

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