On 01/04/2018 22:02, Rafael Winterhalter wrote:
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1. Java agents cannot define auxiliary classes.
:
The reason for
using Unsafe is that many instrumentations need to define auxiliary classes
to aid an instrumentation similar to javac which sometimes needs to define
anonymous classes or even synthetic classes. For example, if a Java agent
wants to register an event listener to some framework, such listeners often
declare multiple methods what makes it impossible to fullfil the listener
contract using a lambda expression. Instead, one typically injects an
additional class into the same package as the instrumented class.
This seems a reasonable requirement. As you know, JSR-163 created this
API (and JVM TI) for tools to instrument code in mostly benign ways
where any additional agent provided helper classes are made visibility
via the appendToXXXClassLoaderSearch methods. I don't think the use-case
of dynamically generated helper classes came up, I don't recall it
coming up on serviceability-dev in the intervening years either. In any
case, I think there should be a way to support this scenario, it amounts
to a ClassFileTransformer providing the class bytes of additional
classes to be defined in the same runtime package as the class being
loaded or transformed. There are a number of API choices and it's
probably best if we bring proposals to serviceability-dev as that is
where this API is maintained.
:
2. Java proxies cannot invoke default methods of proxied interfaces
The Java proxy API does not currently allow the invocation of an overridden
default method since
the InvocationHandler API only supplies an instance of
java.lang.reflection.Method.
The issue of Proxies and default methods has come up on core-libs-dev a
few times. In the mean-time, JEP 274 added support for MethodHandles
that bind to non-abstract methods in interfaces. I just double checked
and I can create a proxy where the invocation handler creates a method
handle to the default method, binds it to proxy, and invokes it. I also
checked the case where the interface is public in an exported package of
a named module. Can you say a bit more, or provide an example, where you
run into issues? I'm wondering if you are running into an accessibility
issue or something else.
-Alan
