| Re: [aspectj-dev] RE: How does AspectJ implement load-time weaving? |
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RE: How does AspectJ implement load-time weaving?
The 12-step process explained in your link refers to class
initialization (done after linking) and not loading. From what I know,
ClassLoader.loadClass("C") does not initialize C, it only loads
it - that is, it finds the source bytecode and creates a java.lang.Class.
Initialization of the class only occurs after the class is actually used
in execution code, as explained below.
http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.4
I know that the JVM specification guarantees the initialization of all
superclasses before the initialization of the class. However, by the time
a class is initialized it is already too late to change the bytecode. Hence,
the JVM 1.5 provides a hook at class *load* time, at a time when the superclass
might not have been loaded yet.
In order to uncover the order in which loading and bytecode transformation
is done, I implemented my own Java agent and attached it to a JVM running
the "main" code above. That, along with inspection of the JVM
source code implementation, suggested to me that things happen in the following
order (shown in pseudocode - I used curly braces to indicate a code block
executing within the body of the previous line):
loader.loadClass(C);
{
...
loader.findClass(C);
{
...
loader.defineClass(byte[] C_bytes); //these are the
bytes in class C's bytecode
{
...
transformer.transform(C_bytes); //
transformer is a ClassFileTransformer in my agent
loader.loadClass(B);
{
...
transformer.transform(B_bytes);
loader.loadClass(A)
{
...
transformer.transform(A)
}
}
}
}
}
This code flow indicates that loading C will indeed result in the loading
of B and A. However, the JVM class-load hook into C (that is, the transform
method) occurs before B or A are loaded. Thus, when inspecting C_bytes
to determine whether it extends A, I find that I have neither B_bytes nor
A_bytes available.
What am I missing here? I assume something must be wrong somewhere in my
reasoning, otherwise AspectJ 5 wouldn't be able to provide its LTW capabilities
:-)
Thanks,
-Murali