Conditional Queue

Conditional Queue

• Help build efficient and responsive state-dependent classes
• Conditional Variables are also called conditional queue. Why???
  • every thread which call lock.wait(), will want to wait for thread to notify
• The lock object and the condition queue object must be the same
• Implementation Pattern:

void foo() throws InterruptedException {
    synchronized (lock) {
        while (!conditionPredicate()) {
            lock.wait();
        }
        
        // do your logic
    }
}

• https://stackoverflow.com/questions/12551341/when-is-a-condition-variable-needed-isnt-a-mutex-enough
• https://stackoverflow.com/questions/65117449/why-is-condition-variable-also-called-condition-queue-if-it-is-not-implemented-a

wait(), notify() and notifyAll()

• They all must be called after holding the lock
• All of them throws IllegalMonitorStateException if current thread does not hold the lock on which they are called
• Calling notify()/notifyAll() when no thread is waiting has no impact
• notify() notifies single thread and it is not deterministic which thread will be notified
• Thread States
  • https://stackoverflow.com/questions/15680422/difference-between-wait-and-blocked-thread-states
  • https://stackoverflow.com/questions/28378592/java-thread-state-transition-waiting-to-blocked-or-runnable
  • wait()
    • Current thread goes to WAITING state
    • current lock is released
    • The execution will resume from the same place where wait was called if it re-acquires the lock after getting notify signal
  • notify() / notifyAll()
    • All waiting threads go to BLOCKED state
    • diagrams on internet are wrong which says that they go to RUNNABLE
    • At BLOCKED state, All notified threads are competing equally to the threads who have not entered the synchronization block and waiting for the lock to be acquired
• https://stackoverflow.com/questions/2779484/why-must-wait-always-be-in-synchronized-block
• https://stackoverflow.com/questions/13249835/java-does-wait-release-lock-from-synchronized-block
• https://stackoverflow.com/questions/61562542/wait-and-notify-jmm-semantics

Condition Predicate

• Precondition that makes an operation state-dependent
• Example In Bounded Buffer:
  • take(): “buffer is not empty”
  • put(): “buffer is not full”
• If condition predicate is not true, the operation just waits

ReentrantLock

• Has Condition which has following methods:
  • await()
  • signal()
  • signalAll()

Interview Qs

• https://www.youtube.com/watch?v=zNwFoRQLRZQ

Spurious wakeup

• Sometimes threads can wakeup without explicit notify() or notifyAll() methods
• Happens from OS side
• This is the reason why while loop is used to check the condition again and again
  • This while loop is also known as spin lock?
• https://stackoverflow.com/questions/1050592/do-spurious-wakeups-in-java-actually-happen

Missed signals

• Always use notifyAll() since if you use notify() and you have more than one threads waiting then it will cause missed signal
• See Risk of Threads

wait() vs sleep()

• wait is for unspecified time, sleep for some specified time
• wait releases the lock, sleep does not release the lock
• wait works on condition, sleep works on time elapsed

wait() vs wait(timeout)

• wait(timeout) waits for the specified time and resumes execution
• wait() waits indefinitely until notify signal is received