JVMTI GetLocalVariableTable（）始终提供JVMTI_ERROR_ABSENT_INFORMATION

Question

First of all, I have tried to find the solution to my problem for a while, but got nothing useful. Therefore, I want to have some suggestions from the experts.

I'm using JVMTI to write an agent, in which I need to get local variable info of some method. Here's the summary of what I did:

(1) Turn on the corresponding capability.

JNIEXPORT jint JNICALL
Agent_OnLoad(JavaVM *jvm, char *options, void *reserved)
{
  ...
  jvmtiCapabilities capa = {0};
  ...
  capa.can_access_local_variables = 1;
  ...
}

(2) During the live phase (in my case, the handler of VMInit event), get the jclass through JNI FindClass() , and then get the jmethodID through JNI GetMethodID() (of course, if it's static method, I use GetStaticMethodID() ). Each step is checked, and jclass and jmethodID are all good.

(3) Then, I try to get the local variable table.

...
jvmtiLocalVariableEntry *entTab = NULL;
jint entCnt = 0;
jvmtiError errNum = jvmti->GetLocalVariableTable(mthID, &entCnt, &entTab);
...

I checked the errNum, and find it is JVMTI_ERROR_ABSENT_INFORMATION every time. I also tried this in other event handlers, which behave the same.

Any ideas or suggestions?

Thanks!

Answer 1

The JVMTI_ERROR_ABSENT_INFORMATION error indicates that the method you're trying to load the local variable table for doesn't have a proper LocalVariableTable attribute in the corresponding file.

If you have access to the original source code, you can compile it with javac using the -g option to generate it.

If you just want to get the method signature information, you can try the GetMethodName function. Otherwise, you need to obtain a stack frame containing the method in question and you can retrieve the values of all the local variables using GetLocal* by passing in the slot number.