AspectJ Compatibility and Implementations
AspectJ has been designed to be implemented in many ways, including source- or bytecode-weaving and directly in the VM (virtual machine). In all cases, the AspectJ program is transformed into a valid Java program run in a Java VM. Any classes affected by aspects are binary-compatible with the unaffected classes (in order to maintain compatibility with any classes that were compiled with the unaffected originals). Supporting multiple implementations allows the language to grow as technology changes, and being Java-compatible ensures platform availability.
Key to its success has been engineering and language decisions designed to make the language usable and programs deployable. The original Xerox AspectJ implementation used source weaving, which required access to source code. When Xerox contributed the code to Eclipse, AspectJ was reimplemented using the Eclipse Java compiler and a bytecode weaver based on BCEL, so developers could write aspects for code in binary (.class) form. At this time the AspectJ language was restricted to support a per-class model essential for incremental compilation and load-time weaving. This made IDE integrations as responsive as their Java counterparts, and it enabled developers to deploy aspects without altering the build process. This led to increased adoption, as AspectJ became usable for impatient Java programmers and enterprise-level deployments. Since then, the Eclipse team has increased performance and correctness, upgraded the AspectJ language to support Java 5 language features like generics and annotations, and integrated annotation-style pure-java aspects from AspectWerkz.
The Eclipse project supports both command-line and Ant interfaces. A related Eclipse project has steadily improved the Eclipse IDE support (AJDT) for AspectJ and other providers of crosscutting structure. IDE support for emacs, NetBeans, and JBuilder foundered when Xerox put them into open source, but support for Oracle's JDeveloper did appear. IDE support has been key to Java programmers using AspectJ and understanding crosscutting concerns.
BEA has offered limited support in a VM for aspect-oriented extensions, but for extensions to be supported in all Java VM's would require agreement through Sun's Java Community Process (see also the java.lang.instrument package available since Java SE 5 which is some kind of common ground for JVM load-time instrumentation).
Academic interest in both the semantics and implementation of aspect-oriented languages has surrounded AspectJ since its release. The leading research implementation of AspectJ is the AspectBench Compiler, or abc; it supports extensions for changing the syntax and semantics of the language and forms the basis for many AOP experiments that the AspectJ team can no longer support, given its broad user base.
Many programmers discover AspectJ as an enabling technology for other projects they use, most notably Spring AOP. A sister Spring project, Spring Roo, automatically maintains AspectJ inter-type declarations as its principal code generation output.
Read more about this topic: AspectJ