JVMTI - how to get the value of a method parameter

I am recording all method entries from my Java app thanks to a JVMTI Agent. For now, I am able to get the name of each method, but I'd want to be able to get the value of the parameters that method received.

This problem has already been discussed in an older topic (see How to get parameter values in a MethodEntry callback); it fits perfectly what I'm looking for, so I know I have to use GetLocalObject function, but I can't figure out how to (the example given in the topic is broken).

Can anyone help me finding out how to do this? Thanks.

I think you want to access arbitrary method parameters without foreknowledge of their content, if not could you clarify your question?

See the JVMTI docs on local variables.

First, you need to ensure you have enabled local variable access in your capabilities list. Then, find out what parameters are available using GetLocalVariableTable. The returned table will contain a description of each local variable in the method, including the parameters. Don't forget to Deallocate it when you're done.

You'll need to work out which variables are parameters. You can do that by finding the current jlocation and eliminating local variables which are not yet available. This won't tell you the parameter order, but it'll tell you which locals are parameters. You can probably assume that the slot number is the correct order.

Find the current jlocation using GetFrameLocation, iterate over the local variable table, and for each local variable whose start_location is less than or equal to your current location, add the slot number and type to your list of parameters.

For each parameter, call the appropriate GetLocal{X} method based on its type. You'll need the depth of your current frame, which you already have from GetFrameLocation.

That should get you your parameters, but it'll be slow and tricky to implement. You'd be far better off following the guide's recommendation of avoiding MethodEntry callbacks and use bytecode instrumentation (BCI) instead.