Re: [rvm-research] Adding Runtime Extension

On 6/29/2014 11:29 PM, Matthew Ma wrote:
> Hi,
> If I want to add a field to any class (in user code), where in the RVM compiler should I start instrumenting the byte code.
> Is there any existing examples that I can take a look at?
> I am using asm tool and java assist.

You can't add fields to arrays, and probably not to some
built-in objects.

With Jikes RVM you CAN add header fields -- this would be
more toward the GC oriented code, but in the JVM code
related to headers and header layout.

However, as I have tried to tell you before, a JVMTI agent
allows you to associate an arbitrary 64-bit value with any
object.  You could (for example) instrument bytecode to add,
after the constructor invocation on a new object, a call to
a C/C++ routine in your agent saying "please associate this
information with this object".  In fact, the C/C++ code could
do it any way it wants, if you use the 64-bit field to assign
unique ids to objects, and then use the unique ids as keys
into tables managed with the C/C++ code.

My point is that maintaining information via a map keyed on
the object is as good as storing information IN the object.

Regards -- EM

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