Murali, the Java “language”
specification guarantees that the VM will always load the superclass of a class
before it loads the class itself. So this situation can never arise.
See step 7 at http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.4.2
for the details of how this is guaranteed.
From:
aspectj-dev-bounces@xxxxxxxxxxx [mailto:aspectj-dev-bounces@xxxxxxxxxxx] On Behalf Of murvaja (sent by Nabble.com)
Sent: Thursday, January 05, 2006
9:31 PM
To: aspectj-dev@xxxxxxxxxxx
Subject: [aspectj-dev] How does
AspectJ implement load-time weaving?
I've been trying to
understand how AspectJ manages to match the '+' pointcut construct at runtime
for load-time weaving. The example I have in mind is for the Hotspot JVM 1.5,
with load-time weaving enabled through the -javaagent option, and goes as follows.
Say I have a class hierarchy like this:
public class A
{
public void foo(){}
}
public class B extends A
{
}
public class C extends B
{
public void foo(){}
}
And I write a pointcut and advice as follows:
pointcut pc() : execution (* A+.foo() )
before() : pc() {
//weaving code here
...
}
According to the pointcut definition, all subclasses of A will match the
pointcut, so the load-time weaving should insert the weaving code at the
beginning of both A.foo and C.foo. This will happen at their respective
class-load times, that is, A.foo will be weaved when A is loaded and C.foo will
be weaved when C is loaded.
My question is, how does the weaver do this if C is loaded before A and B, like
in the following code (assume none of A, B or C can be loaded by the system
class loader) ?
public static void main (String args[])
{
ClassLoader loader = new MyClassLoader();
loader.loadClass("C");
}
At the time C is loaded, the weaver will have C's bytes at hand and will have
to decide whether to weave them or not. According to the pointcut definition,
the bytecodes should be weaved because C is of type A. However, the weaver
cannot know that at this time, because the C bytecode only contains a reference
to its direct superclass B (not A), and the bytes for B are not yet available
because B is not yet loaded.
Any leads? I've been trying to find documentation on this but to no avail.
Thanks in advance,
-Murali
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