Run-Time Metaobject Protocols: the Quest for their Holy Application

Run-time MetaObject Protocols (MOPs) are reflective systems that allow objects to be controlled at run time by one or many metaobjects. These metaobjects can then alter the semantics of the execution for the base objects they control. Jumping to the meta level to alter the semantics is powerful but indeed fairly costly. It is therefore desirable to let users fine-tune their run-time MOP according to their specific needs. This is exactly what we are investigating in this PhD thesis: how to design and build an open run-time MOP. But although our intuition (or faith) never fails, it remains hard to definitely answer the realistic question of “what is this really good for?”. Therefore, we are also investigating applications of run-time MOPs that differentiate them from other program transformation approaches. Since we are worried about the applicability of run-time MOPs, all our work is done with the Java programming language, in a fully portable way. 1 What are run-time MOPs? Run-time MetaObject Protocols (MOPs) are reflective systems [11, 7] that allow objects to be controlled at run time by one or many metaobjects. These metaobjects can then alter the semantics of the execution for the objects they control. A classical example is that of a metaobject that controls method invocations on an object to trace them on a debug output. An object controlled by one or more metaobjects is called a reflective object. A reflective class is a class whose instances are reflective objects. From the point of view of the metaobject, the object it controls is called the base object. Characterizing a run-time MOP starts by specifying which language mechanisms are reified. Reifying means making something implicit explicit, that is to say, accessible as a first-class entity. In object-oriented class-based languages, the language mechanisms that can be reified are many: method invocation, field accesses, types checks, object creation, serialization, finalization, etc. Once reified, these mechanisms can be controlled at the metalevel. The set of mechanisms that a MOP is able to reify partially defines its expressiveness. Some simple run-time MOPs only offer the possibility to control method invocation on the receiver