Appendix C. Implementation Notes
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# Appendix C. Implementation Notes

## Compiler Notes

The initial implementations of AspectJ have all been compiler-based implementations. Certain elements of AspectJ's semantics are difficult to implement without making modifications to the virtual machine, which a compiler-based implementation cannot do. One way to deal with this problem would be to specify only the behavior that is easiest to implement. We have chosen a somewhat different approach, which is to specify an ideal language semantics, as well as a clearly defined way in which implementations are allowed to deviate from that semantics. This makes it possible to develop conforming AspectJ implementations today, while still making it clear what later, and presumably better, implementations should do tomorrow.

According to the AspectJ language semantics, the declaration

      before(): get(int Point.x) { System.out.println("got x"); }

should advise all accesses of a field of type int and name x from instances of type (or subtype of) Point. It should do this regardless of whether all the source code performing the access was available at the time the aspect containing this advice was compiled, whether changes were made later, etc.

But AspectJ implementations are permitted to deviate from this in a well-defined way -- they are permitted to advise only accesses in code the implementation controls. Each implementation is free within certain bounds to provide its own definition of what it means to control code.

In the current AspectJ compiler, ajc, control of the code means having bytecode for any aspects and all the code they should affect available during the compile. This means that if some class Client contains code with the expression `new Point().x` (which results in a field get join point at runtime), the current AspectJ compiler will fail to advise that access unless Client.java or Client.class is compiled as well. It also means that join points associated with code in native methods (including their execution join points) cannot be advised.

Different join points have different requirements. Method and constructor call join points can be advised only if ajc controls the bytecode for the caller. Field reference or assignment join points can be advised only if ajc controls the bytecode for the "caller", the code actually making the reference or assignment. Initialization join points can be advised only if ajc controls the bytecode of the type being initialized, and execution join points can be advised only if ajc controls the bytecode for the method or constructor body in question. The end of an exception handler is underdetermined in bytecode, so ajc will not implement after or around advice on handler join points. Similarly, ajc cannot implement around advice on initialization or preinitialization join points. In cases where ajc cannot implement advice, it will emit a compile-time error noting this as a compiler limitation.

Aspects that are defined `perthis` or `pertarget` also have restrictions based on control of the code. In particular, at a join point where the bytecode for the currently executing object is not available, an aspect defined `perthis` of that join point will not be associated. So aspects defined `perthis(Object)` will not create aspect instances for every object unless `Object`is part of the compile. Similar restrictions apply to `pertarget` aspects.

Inter-type declarations such as `declare parents` also have restrictions based on control of the code. If the bytecode for the target of an inter-type declaration is not available, then the inter-type declaration is not made on that target. So, `declare parents : String implements MyInterface` will not work for `java.lang.String` unless `java.lang.String` is part of the compile.

When declaring members on interfaces, the implementation must control both the interface and the top-level implementors of that interface (the classes that implement the interface but do not have a superclass that implements the interface). You may weave these separately, but be aware that you will get runtime exceptions if you run the affected top-level classes without the interface as produced by the same ajc implementation. Any intertype declaration of an abstract method on an interface must be specified as public, you will get a compile time error message indicating this is a compiler limitation if you do not specify public. A non-abstract method declared on an interface can use any access modifier except protected. Note that this is different to normal Java rules where all members declared in an interface are implicitly public. Finally, note that one cannot define static fields or methods on interfaces.

When declaring methods on target types, only methods declared public are recognizable in the bytecode, so methods must be declared public to be overridden in any subtype or to be called from code in a later compile using the target type as a library.

Other AspectJ implementations, indeed, future versions of ajc, may define code the implementation controls more liberally or restrictively, so long as they comport with the Java language. For example, the `call` pointcut does not pick out reflective calls to a method implemented in `java.lang.reflect.Method.invoke(Object, Object[])`. Some suggest that the call "happens" and the call pointcut should pick it out, but the AspectJ language shouldn't anticipate what happens in code outside the control of the implementation, even when it is a a well-defined API in a Java standard library.

The important thing to remember is that core concepts of AspectJ, such as the join point, are unchanged, regardless of which implementation is used. During your development, you will have to be aware of the limitations of the ajc compiler you're using, but these limitations should not drive the design of your aspects.

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