# 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. Finally, note that one cannot define static fields or methods on interfaces.

Other AspectJ implementations, indeed, future versions of ajc, may define code the implementation controls more liberally or restrictively.

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.

## Bytecode Notes

### The .class expression and String +

The java language form `Foo.class` is implemented in bytecode with a call to `Class.forName` guarded by an exception handler catching a `ClassNotFoundException`.

The java language + operator, when applied to String arguments, is implemented in bytecode by calls to `StringBuffer.append`.

In both of these cases, the current AspectJ compiler operates on the bytecode implementation of these language features; in short, it operates on what is really happening rather than what was written in source code. This means that there may be call join points to `Class.forName` or `StringBuffer.append` from programs that do not, at first glance, appear to contain such calls:

      class Test {
          void main(String[] args) {
              System.out.println(Test.class);        // calls Class.forName
              System.out.println(args[0] + args[1]); // calls StringBuffer.append
          }
      }

In short, the join point model of the current AspectJ compiler considers these as valid join points.

### The Handler join point

The end of exception handlers cannot reliably be found in Java bytecode. Instead of removing the handler join point entirely, the current AspectJ compiler restricts what can be done with the handler join point:

- After and around advice cannot apply to handler join points.
- The control flow of a handler join point cannot be detected.

The first of these is relatively straightforward. If any piece of after advice (returning, throwing, or "finally") would normally apply to a handler join point, it will not in code output by the current AspectJ compiler. A compiler warning is generated whenever this is detected to be the case. Before advice is allowed.

The second is that the control flow of a handler join point is not picked out. For example, the following pointcut

      cflow(call(void foo()) || handler(java.io.IOException))

will capture all join points in the control flow of a call to `void foo()`, but it will not capture those in the control flow of an `IOException` handler. It is equivalent to `cflow(call(void foo()))`. In general, `cflow(handler(Type))` will pick out no join points.

This does not restrict programs from placing before advice on handlers inside other control flows. This advice, for example, is perfectly fine:

      before(): handler(java.io.IOException) && cflow(void parse()) {
          System.out.println("about to handle an exception while parsing");
      }

A source-code implementation of AspectJ (such as AspectJ 1.0.6) is able to detect the endpoint of a handler join point, and as such will likely have fewer such restrictions.

### Initializers and Inter-type Constructors

The code for Java initializers, such as the assignment to the field d in

      class C {
          double d = Math.sqrt(2);
      }

are considered part of constructors by the time AspectJ gets ahold of bytecode. That is, the assignment of d to the square root of two happens inside the default constructor of C.

Thus inter-type constructors will not necessarily run a target type's initialization code. In particular, if the inter-type constructor calls a super-constructor (as opposed to a `this` constructor), the target type's initialization code will not be run when that inter-type constructor is called.

      aspect A {
          C.new(Object o) {} // implicitly calls super()

          public static void main(String[] args) {
             System.out.println((new C()    ).d);    // prints 1.414...
             System.out.println((new C(null)).d);    // prints 0.0
      }

It is the job of an inter-type constructor to do all the required initialization, or to delegate to a `this` constructor if necessary.