JAVA-BASED REAL-TIME PROGRAMMING

3. Multi-Threaded Programming
public void take() Causes the calling thread to block until the counter that
represents this semaphore obtains a positive value. On return the counter,
named count below, is decremented by one. Basically this means executing
the following code atomically (as for give):
while (count{\textless}1) wait(); // wait() suspends until notified.
--count; // Or set to false for a binary sem.
public boolean tryTake(long timeout) Causes the calling thread to block
until the counter that represents this semaphore obtains a positive value
(just like the ordinary take method) or until the timeout time has passed.
If, due to other threads calling give, the semaphore is possible to take,
but the caller of take is not rescheduled for execution until after the
timeout time after its call of take, it is considered as a timeout and
hence false is returned (and the internal state of the semaphore is left
unchanged).
Note that the semaphore operations are not ordinary methods; they have to
utilize some type of system call to suspend and resume execution. That is,
when take results in suspended execution as mentioned in the comment above,
the thread is no longer ready for execution and the scheduler of the operating
system (or JVM if so called green threads are used) puts the thread (in terms
of data including the context) in the waiting queue of the semaphore. Hence,
the thread consumes no CPU time while being blocked. The mentioned system
call can be a native semaphore operation (when semaphores are supported by
the OS). When using a JVM, the system call is accomplished by the use of
synchronized (explained below).
Semaphore classes
The classes implementing the Semaphore interface, and their motivation, are
as follows:
BinarySem A Semaphore containing a boolean flag instead of a counter, to
support signaling in cases when waiting threads should not catch up
with successive missed calls of give. In the default Java implementation
a boolean is used to keep the state of the semaphore. Native mappings
can use that, or a bit, or a counter with a maximum value of one as in
win32.
CountingSem Basic type of semaphore containing a counter which is incremented
by one for each call to give, and decremented by one for each
returning call of take. A typical signaling example using this type of
semaphore was shown in Figure 3.2.
64 2012-08-29 16:05