Embedded Java - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 656

pass through the memory or start over from the beginning if they are interrupted,
an incremental GC works in short steps and thus allows for interruptions in
between the steps. The JVMs from NewMonics and Aicas, for example, use
incremental GC.
The JVM specifications do not preclude enhancements needed to make a real-
time implementation, and in 1998 work began on the Real-Time Specification for
Java (RTSJ) by a group in the Java Community Process [11,12]. Their job was to
detail a set of standard extensions for a real-time JVM. The RTSJ was released in
2003 and the company TimeSys provides the reference implementation [13]. The
extension classes come via the javax.realtime API. Standard Java programs
can run without modification in a real-time JVM.
The RTSJ does not specify a particular GC algorithm such as incremental
collection. It does define new types of memory areas that allow for avoiding the
GC altogether. There is immortal memory in which objects are never destroyed
except when the program ends. Scoped memory is used only while a process works
within a particular section, or scope, of the program such as a method. Objects
there are automatically destroyed when the process leaves the scope. Neither
immortal nor scoped memories are garbage collected, so using them avoids the
problems of GC interference. Note, however, that the programmer must watch
out for overflow of the immortal memory.
Another important aspect of the RTSJ was the addition of real-time threads ,
which provide for more precise scheduling than with standard threads. They have
28 levels of priority and their priority is strictly enforced. They are not subject to
so-called priority inversion situations where a lower priority thread has a block
on a resource needed by the higher priority thread and thus prevents the higher
priority thread from running. In addition, the RTSJ includes “non-heap real-
time threads” that cannot be interrupted by the GC.
The RTSJ also provides for asynchronous event handlers that deal with exter-
nal events (or happenings ,asthey are called, to distinguish them from the events
in the AWT). Asynchronous transfer of control allows one thread to interrupt
another thread in a safe manner, unlike the deprecated suspend() and stop()
methods for standard threads.
Timing is obviously important for real-time programming so the javax.
realtime API includes the abstract class HighResolutionTime and its sub-
class AbsoluteTime ,which represents a point in time, and RelativeTime ,
which represents a duration. The base class uses a long value for milliseconds
and an int value for nanoseconds. Unlike standard Java, RTSJ requires that an
implementation provide sub-millisecond precision. (Of course, the accuracy will
vary according to the capability of the clock on a particular system.)
While still maintaining security protections, the RTSJ allows direct access
to physical memory. This means that device drivers can be created with Java.
Previously, Java had to link to native code to communicate directly with
hardware.