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I would like to debug java applications that are using a jvmti agent I have written. This seems to only work if the jvmti agent is NOT trying to enable "can_generate_breakpoint_events" capabilities.

Trying to enable the breakpoint capabilities and starting the app in debug mode results in the following error: 

ERROR: JVMTI: 98(Unknown): Unable to get necessary JVMTI capabilities. [..\src\agent.cpp:437]

Is there any way to debug an application that is using a jvmti agent that sets breakpoints as well?

I checked Java JVMTI doesn't work alongside -Xdebug -Xrunjdwp but the accepted answer does not seem to be correct as it really seems to depend on the enabled capabilities as well.

I know that profilers (like YourKit, etc) are using jvmti agents as well and still allow you to run your app in debug mode (well, maybe they are just not using the capabilities that lead to these conflicts).

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Haasip Satang
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  • Which JVMTI capability are you failing to get? What is the agent.cpp:437 ? – Vladimir Kondratyev Nov 07 '16 at 22:35
  • The capability to set breakpoints (can_generate_breakpoint_events as mentioned above). agent.cpp is my own agent the needs the capabilites. I guess I understand what the problem is though, see my answer below. a – Haasip Satang Nov 24 '16 at 23:07

2 Answers2

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TL;DR

To debug JVM agent use native code debugger, e.g. gdb.

Slightly more details

There are two kinds of code that live in a Java application - Java code (the application itself and java.lang.instrument-based agents) and native code (native methods of Java classes, JVMTI agents and the VM itself). To debug the former you need to use a Java debugger (that uses JDWP). To debug the latter you need to use a regular native code debugger like you would do for a C/C++ program. If you want to debug both Java and native code at the same time then you need to use both debuggers.

About JVMTI agents, JDWP and what requires what

A JVM may be started with multiple agents running together, each agent has its own JVMTI environment (capabilities, event listeners, etc). However, if multiple agents (or multiple instances of a single agent), use the same resource, e.g. the same global variable, then they can affect each other (this is the case with JDK's JDWP agents - that's why you can have at most one -agentlib:jdwp).

JDWP is a protocol for the Java debuggers. JDWP agent is a JVMTI agent shipped with the JDK that enables support of the JDWP protocol in a JVM. It is a normal JVMTI agent and can be started alongside other agents.

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Stanislav Lukyanov
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Based on my current understaing the answer is actually the following. From the JVMTI doc:

The potentially available capabilities of each JVM TI implementation are different. Depending on the implementation, a capability:

  • ...
  • may be possessed by only one environment at a time
  • ...

Setting breakpoints and watchpoints seem to be the type of capability that can be possessed only by one environement.

So the problem is that my agent is trying to claim can_generate_breakpoint capabilities but the debugger agent needs them as well. Hence there are two agents requesting these capabilities which leads to the error above.

As a result to me this means that you can not debug a Java application that is started with a JVM TI agent that needs to be able to set breakpoints.

The only theoretical solution I can think of is that my JVM TI agent additionally implements support for JDWP and allows a debugger to connect to it (which would be quite a project by itself I suppose)

Haasip Satang
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