JAVA-BASED REAL-TIME PROGRAMMING

3.4. Message-based communication – Mailboxes
as a shared resource in a monitor, while the input can be directly available
for posting events (from several threads since the RTEventBuffer by itself is a
monitor). Within such a monitor holding an RTEventBufferInput, a potentially
blocking call such as doFetch should only be used with great care since the
blocked thread will lock also the outer monitor.
In general for producer-consumer situations, the input and output interfaces
to the buffer simply provide a convenient way of passing one end of the
buffer to the producer or the consumer. Of course an object (a monitor or an
ordinary object) given an RTEventBufferInput can check if it is an instance of
RTEventBuffer (which it does not need to be since any object can implement
the same interface), and if so, cast to RTEventBuffer and use also the post
methods. Hence, these interfaces are provided for convenience for certain programming
situations involving cooperating objects, but as usually, misuse can
be made cumbersome but not completely prevented. Recall, however, that
multi-threading mainly is about cooperating concurrency.
3.4.3 Threads and event-based communication
A mailbox, here accomplished by an instance of RTEventBuffer 7 , can be declared
and referred to in different ways:
• The mailbox(es) can be made globally available, for example created
in the main program and passed as argument to the constructors of
the different sending/receiving objects( typically threads), or statically
defined attributes of some class known to other classes at compile time.
With such globally defined mailboxes, they are visible in the software
design, as entities of their own.
• We can also view the provided buffering as a property of some object,
and have the mailbox as an attribute internal to the class that provides
the methods that make use of the buffering. Typically the enclosing class
will be of type Thread and act as the receiver of the events (although the
sender can be unaware of that if sending is accomplished by some other
method of the receiving thread. With such object-internal mailboxes,
the buffering is part of the internal design of that class.
Thus, the the main difference here is a design choice: should mailboxes be
exposed-to or part-of the objects that make use of them? Following common
object-oriented design rules, this depends on what we see as entities within
our design. If the buffers form the core of the application it may be preferable
to have them exposed. However, in most cases the buffers are introduced to
7 The upcoming new release of the LJRT classes will include both the RTEventBuffer and
other buffer classes with different properties and different implementations. The Java-doc
pages provide the valid description for programming using the available buffers, while the
(not complete) description here covers the general ideas of events and buffers.
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