Overview
The aim of JEgg is to simplify the construction of
multithreaded applications in Java by providing a
development paradigm that enables the application
code to be written without using java.lang.Thread
or use of the synchronized keyword.
To that end, JEgg active objects, called "eggs" here,
don't communicate using synchronous method invocations.
Instead, they communicate using messages,
ordinary Java objects with no special pedigree
(but likely to be application specific), which are both sent
and received asynchronously. Moreover, the sender doesn't
need a reference to the receiver in order to send a message
since each egg can publish its "message port" in a built-in
registry from which other eggs can retrieve it.
Message delivery is treated like an event in the receiver,
which must implement specific handlers for the types of
messages that it expects to receive. The JEgg framework
uses reflection to polymorphically deliver a message to
the receiver's most specific handler (based on the message's
concrete type).
Messages are dispatched to each egg independently of any
other egg, and the framework ensures that only one message
at a time is delivered to the egg.
The assignment of physical threads to eggs is independent
of any egg's implementation. By default, all eggs execute
on the same thread, but if a particular egg blocks for
long periods while handling its messages, it is trivial for
the application to assign that egg to a different physical
thread without requiring any changes to the egg's implementation.
Because of the extremely low coupling between eggs,
each egg is naturally cohesive, and the message-based
communication with "one-message-at-a-time" semantics
eliminates thread synchronization issues (deadlocks,
race conditions, etc). The end result is that a JEgg-based
applications can be written with less, more focused, code and
achieve a higher level of robustness.
Some developers may initially find the message-based inter-object
communication disconcerting, but it is easy to adjust to, and
the JEgg framework provides facilities to make it easy. Developers
with finite state machine development experience may even find
the paradigm natural from the start.
In JEgg, active objects are referred to as "eggs" to highlight
the featureless aspect of each egg's interface. From the outside,
all eggs look alike!