On 14/11/2015 16:07, Rafael Winterhalter wrote:
Hi Alan,
As for your suggestion to rather retransform/redefine classes, this
would unfortunately not be a feasible alternative for most
applications that I have seen. The HotSwap functionaltiy does not
allow to add fields or to override methods of super types as the class
layout cannot be changed. Also, many code generation frameworks
function by renaming existing methods and by reimplementing the same
methods to call a user method. The renamed methods can then be invoked
by injecting a proxy class into the user code that makes the original
code available using a standard interface. If the user method is
non-static, this dispatcher also needs to be stored within a new field
of the class being instrumented.
Yes, I'm aware of the limitations to redefine/retransformClasses, I was
pointing out that refransformClasses will invoke the
ClassFileTransformer with the class bytes and may be the most reliable
way to get them.
It is correct, of course that a class file does not always exist so
the above approach might fail but in practice this is not a problem.
Users do normally only want to instrument specific classes of their
application via Java agents that are available on the class path and
not runtime generated code.
As for the necessity to locate class file, cglib can be named as
another example (used by e.g. Spring, Hibernate, Mockito) which needs
to parse class files to determine the target of a bridge methods. For
this, cglib calls:
Class<?> clazz = ...
clazz.getClassLoader().getResourceAsStream(clazz.getName().replace('.', '/')
+ '.class');
If this would return null as it does with the current build of Jigsaw,
most real-world applications would stop to function. Therefore my
concern with the current implementation where the above call to the
class loader returns null.
The current proposal is that resources in named modules be encapsulated.
There is discussion on EG list for JSR 376 on this topic. The
ClassLoader.getResourcesXXX do of course work exactly as before for
resources on the class path or loaded by custom other class loaders.
A general point on frameworks is that we expect they will need need at
least some changes to work with modules.
-Alan
