JAVA-BASED REAL-TIME PROGRAMMING

2. Fundamentals
been possible to develop despite unsafe languages. Still, any program
may contain fatal bugs, but a safe language such as Java ensures that
at least the errors are kept local and exception handling works.
7. Libraries, subsystems, or legacy software in general may not be thread
safe. That is, variables (or attributes or fields) for storage of local data
within a function (or method or procedure) may be declared static. That
used to be a way to speed up programs and to save stack space, but the
result is that the function can only be called by one thread at a time. By
obtaining concurrency only via OS processes, we do not have to require
the software to be thread safe since each process has its own memory
allocated by the OS.
Sometimes you may want to develop classes and active objects which can be
run either as threads or as a processes, without changing the code. Such issues
and the principles of how to design and implement multi-process applications
are topics of Chapter 7.
Note that from a concurrent design point of view, neglecting implementation
issues such as if concurrency should be accomplished by threads or
processes, we usually use the term process to refer to a (concurrently executing)
activity. This is in line with non-technical vocabulary where a process
refers to some ongoing activity that cannot be completed immediately. (Pay
attention to how the word is used in the news, in economy, and in management
principles.)
2.8 Object interaction and encapsulation
The interaction (information exchange) between objects may be synchronous
or asynchronous. It can be built into the programming language, and/or it
can be programmed. Synchronous communication can be implemented on top
of asynchronous communication, or vice versa.
Here, we consider synchronous communication as more fundamental. With
the execution properties of Java this simply means a method invocation, which
is based on ordinary function calls (works exactly as in C/C++ and most other
languages). Thus, it is built into the language. This is the most computingefficient
way to achieve object interaction in compiled languages.
The asynchronous communication means passing some kind of message
without requiring the sender/caller to wait until the receiver/callee replies;
the caller performs its communication asynchronously with the callee. The
messages are often referred to as events. But note, however, most of the
event processing done for instance in Java’s AWT or Microsoft’s MFC actually
constitutes synchronous communication because there are no buffers providing
the asynchronous behaviour. It depends on the application demands if we want
buffering or not. We will take a closer look at this issue in later chapters.
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