Lock and Condition in Java

In Java, interface Lock defines a method newCondition().

This method returns an object of type Condition.

Condition is a device to partition waiting threads into multiple groups such that threads belonging to one group can be notified separately from other sets’ threads.

Just as Lock is a fine tuned replacement for synchronized, Condition is a fine tuned replacement for monitor.

Lock.lock() helps all threads to go into waiting state and only one of them can enter the critical section (like synchronized(monitor) block which allows only one thread to go into the criitcal section).

Condition.await() helps a thread in the critical section to give up the lock and go into waiting state till its notified (like monitor.wait() which does exactly the same thing).

Condition offers a finer control by allowing several conditions per lock so that threads can wait/notified in groups (rather than just notify/notifyAll in normal monitors)

Example:

The 2 methods put() and take() are both guarded by lock() and unlock() calls on the same Lock object. This means that if a producer is putting an object, a consumer cannot take the object at the same time and vice versa. We cannot allow both producer and consumer to be active at the same time because both attempt to change the count property. So both are doing a write operation on count property and so cannot do that simultaneously.

This locking mechanism is equivalent to synchronized(lock) functionality.

If buffer is full, multiple writers may be waiting.

To notify one of the writers, we use notFull.signal()

Note that use of two different Condition variables helps us to put writers in one waiting set and readers in other waiting set.

If we were using plain monitors, then this separation would not have been possible. We would had to use notify() or notifyAll() which would awake any thread among the readers or writers.

To summarize, Condition helps to partition waiting threads into groups such that each group can be woken up indepently of the other groups.