JAVA-BASED REAL-TIME PROGRAMMING

Real-Time Events – RTEvent
3.4. Message-based communication – Mailboxes
The class se.lth.cs.realtime.event.RTEvent is the EventObject base class
for real-time events. Real-time here basically means that events are timestamped,
whereas true real-time properties have to do with the properties of
the executing threads and their scheduling. Hence, real-time events will be
needed by real-time threads (introduced in a later chapter), but they are also
useful for multi-threaded programming and for the communication between
concurrent and real-time threads, so introducing the RTEvent class here for
message-based concurrency is natural.
Every RTEvent comes from a source object referred to in the EventObject
base class. Additional IDs, such as sequential numbers or the kind of event
within a certain type, are expected to be defined in subclasses together with
other user attributes. The time stamp is, however, fundamental to real-time
systems as a mean to denote the freshness of the data. It is therefore an
attribute here and the motivation for this class.
The source attribute, typically being a thread object, is not supposed
to keep any reference to the event after posting it. Therefore, methods of
this class should not need to be synchronized. To support detection of kept
references, for instance when the event is sent again, there is a field owner
which is set to the same as source when the event is created. When an event
is transferred, for instance via a buffer, the ownership changes to the buffer
and then to the receiving thread. Using available classes, the user should not
need to care about the ownership, it is checked to detect certain programming
errors. Sometimes, however, it is desirable to keep a set of event objects that
are sent several times, for instance every time a certain state transition occurs.
In such cases the specific event subclasses are supposed to set the (protected)
owner field to null, which disables the checking of ownership.
Refer to the online class documentation for further details. In short, neglecting
a few features for later chapters, the se.lth.cs.realtime.RTEvent
class looks as:
public abstract
class RTEvent extends EventObject {
protected long timestamp; // Creation time in ms.
protected volatile transient Object owner; // Responsible thread.
}
public RTEvent(); // Use current Thread & TimeMillis.
public RTEvent(Object source); // Set source , default timestamp.
public RTEvent(long ts); // Set timestamp , default source
public RTEvent(Object source , long ts); // Override both defaults.
public final Object getOwner()
public double getSeconds()
public long getMillis()
public long getNanos()
There is also a setOwner method, but that method is declared final and has
package visibility so it is of no interest for user code. Still, the user can in
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