How to differentiate when wait(long timeout) exit for notify or timeout?

Question

Having this wait declaration:

public final native void wait(long timeout) throws InterruptedException;

It could exit by InterruptedException, or by timeout, or because Notify/NotifyAll method was called in another thread, Exception is easy to catch but...

There is any way to know if the exits cause was timeout or notify?

EDIT:

This is a tricky way that could work, (although I don't like it)

          long tBefore=System.currentTimeMillis();
          wait(TIMEOUT);
          if ((System.currentTimeMillis() - tBefore) > TIMEOUT) 
            { 
               //timeout
            }

Solution 1

You can't differentiate between the two unless you provide some additional code. For example by adding a ThreadLocal Boolean that is set to true only on notify()

But first you must make sure your logic requires this differentiation.

Solution 2

There is one more reason that notify can return: spurious wakeup. This is an unlikely but possible thing, because preventing spurious wakeups is very expensive on some hardware/OS combinations.

Because of this you always have to call wait() in a loop and re-check the condition that you are waiting for. During this work it's easy to check for timeout at the same time.

For details I recommend the book "Java Concurrency In Practice". And using higher level constructs that will get this all correct for you.

Solution 3

This doesn't exactly answer the question, but it will probably solve your problem: Use higher level concurrency mechanisms. Wait/notify is usually more low-level than you'd want, for this reason among many others.

For example, if you were using BlockingQueue.poll(long, TimeUnit), you could check if the result is null to know if you timed out.

Solution 4

Don't use System.currentTimeMillis(), use System.nanoTime() instead.

The first one meassures absolute time (based on system clock) and might have curious results if the system time is changed. For example: A 5 second wait can have a duration of an hour if the clock is moved backward by an hour, or a 10 minute wait will be done after 0 seconds if the clock is moved foreward.

The second one meassures relative time. It will always run in one direction at constant speed, but it has no origin. That means that the values can only be used to meassure relative time, but can and should not be used to determine a date.

Solution 5

You should use not wait/notify approach.

Will be better to use Lock with Condidions https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/Condition.html#await-long-java.util.concurrent.TimeUnit-

It has await with timeout and will return false if the waiting time detectably elapsed before return from the method, else true

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Comments

[Mark Peters]

The OP already knew an exception wouldn't be thrown and wants to distinguish between a timeout and a notify. This does not provide any useful information to that end.

[YoK]

@Hernán Eche As I have put in my answer its not an absolute guarantee. if you can tell , why you want to identify whether it timedout ? We can look for some solution.

[Per Mildner]

I do not see how it could help to set a thread-local boolean to true when doing notify(). The thread calling notify() will not be the same thread that wakes up from wait().

[SensorSmith]

+1 for encouraging people to quit writing code that breaks on system clock updates. But you failed to mention that one should always check if the reason for waiting is still valid.

[Ruslan Stelmachenko]

You mean simple boolean field, I suppose. Not a ThreadLocal. But simple boolean field could help. It even doesn't need to be volatile, as wait/notify will be called from synchronized block anyway. Just set it to true before notify() and check/reset it after wait().