Meta-AspectJ Web Page

Overview

Meta-AspectJ, MAJ, is a language tool for generating syntactically correct AspectJ (and Java) programs. MAJ can be used to:

implement generators using AspectJ
extend the power of AspectJ using generation

MAJ extends Java with 3 keywords:

` : pronounced "quote", this keyword allows the user to generate the abstract syntax tree for a piece of AspectJ code.
# : pronounced "unquote", allows the user to splice a previously defined piece of code into a piece of quoted code.
infer : allows the user to declare a variable whose type, initiated with a quoted piece of code, should be inferred by MAJ.

MAJ guarantees the syntactic correctness of all generated code by type-checking the generator. A research paper ("Generating AspectJ Programs with Meta-AspectJ"--GPCE'04 Best Paper award) describes MAJ in detail.

Why Meta-AspectJ?

Meta-AspectJ combines AspectJ with code-generation. The value of MAJ is thus found in the reasons why neither AspectJ nor code-generation alone is sufficient.

AspectJ is a great tool for addressing cross-cutting concerns such as security and persistence. While it provides some syntax for dealing with the dynamic creation of aspects, such as wildcard (*) characters in specifying names, it does not provide any support for adapting an application based on characteristics not evident in the names. For example, you cannot create an aspect that only applies to classes that do not already implement a particular interface. Using MAJ, a user can leverage the full power of Java to create complex joinpoints that AspectJ does not support.

Why then, can we not use code-generation alone? We could, but AspectJ provides a convenient and mature vocabulary for talking about many commonly used program transformations. It is a sufficient abstraction that code generation can work on top of to provide the extra flexibility.

MAJ as Meta-Java

MAJ can also be used to generate plain Java programs. In fact, MAJ is possibly the most advanced meta-programming tool for Java. MAJ's inference capabilities make it very user-friendly because the user does not need to remember the types of syntax tree entities.

MAJ Applications

We have used MAJ to reimplement GOTECH, a framework used for turning existing Java classes into EJB's with minimum user guidance. The original GOTECH framework uses XDoclet, which is a text-based tool, to generate AspectJ. A complete GOTECH package implemented with MAJ will be posted here.