Condition Variables

Condition Variables

Semaphore as particular example of condition variable Is number of Permits > 0

• if condition is not met, thread-1 goes to sleep until another thread changes the semaphore’s state.

Condition variable is always associated with a lock. Lock ensures atomic check and modification of the shared variables involved in the condition.

Lock lock  = new ReentrantLock();
Condition condition = lock.newCondition();

Example signalling

Lock lock  = new ReentrantLock();
Condition condition = lock.newCondition();
//Shared resources
String username = null, password = null;

await() unlocks the lock, wait until signal or some time

lock.lock();
try{
    while(username == null || password == null)
        //condition.await();
        condition.await(1, TimeUnit.SECONDS);
} finally {
    lock.unlock();
}
performTask();

signal() wakes up a single thread, waiting on the condition variable

lock.lock();
try{
    username = getUserFromUiTextBox();
    password = getPasswordFromUiTextBox();
    condition.signal();
}finally(){
    lock.unlock();
}