Monitors

In concurrent programming, a monitor is an object intended to be used safely by more than one thread. The defining characteristic of a monitor is that its methods are executed with mutual exclusion. That is, at each point in time, at most one thread may be executing any of its methods. This mutual exclusion greatly simplifies reasoning about the implementation of monitors compared with code that may be executed in parallel.
Monitors also provide a mechanism for threads to temporarily give up exclusive access, in order to wait for some condition to be met, before regaining exclusive access and resuming their task. Monitors also have a mechanism for signaling other threads that such conditions have been met.
Monitors were invented by C.A.R. Hoare and Per Brinch Hansen, and were first implemented in Brinch Hansen's Concurrent Pascal language.
Upon calling one of the methods, a thread must wait until no thread is executing any of the monitor's methods before starting execution of its method. Note that without this mutual exclusion, in the present example, two threads could cause money to be lost or gained for no reason; for example two threads withdrawing 1000 from the account could both return without error while causing the balance to drop by only 1000.