Byte code instrumentation - implement native or java agent?

Question

If I want to realize a profiler using byte code instrumentation, should I write a native agent using JVMTI or should I write a java agent using the java.lang.instrument package?

If I want to use libraries like ASM - which seems to be mandatory if you want to create a serious profiler - I have to use a java agent. Which confuses me, since I thought a native agent can do everything what a java agent can do and more. But to me, it seems easier writing a java agent.

Are there alternatives? Should one use java agent and native agent combined anyway?

Answer 1

Nearly everyone writes a java agent (with ASM or BCEL) as they don't want to have to write a C/C++ bytecode instrumentor from scratch as there are none publicly available.

What you won't be able to do is instrument and profile/monitor the primordial JVM, and accessing native functions requires JNI calls. There are also several JVMTI calls that may be unavailable to you (if memory serves).

I wrote my own instrumentor in C several years ago, and I'm in the process of writing a new one which I hope to open source ( depending on my evil overlords :-) )

How about a half way house, a separate pre-started JVM that your native agent sends bytecode to. In that JVM your easy-peasy to write ASM based instrumentor does the hard work and sends the resulting bytecode back to the native agent over the wire. Yeah it seems a bit over-complicated but it's easier that writing your own BCI library.