JVMTI Class load callback not being called on Windows, but works on Linux

[Mikael Vidstedt]

Just a control question: Are you *sure* asserts are actually enabled? Having actual application logic in an assert is normally a bad idea because if/when you compile the release/production version with asserts disabled the application behavior will change.

Cheers,
Mikael

> On Jan 10, 2021, at 8:33 AM, Charlie <cvdcamilleri at gmail.com> wrote:
> 
> Hi,
> 
> I have a working jvmti agent on Linux (Java(TM) SE Runtime Environment (build 1.8.0_271-b09)) that hooks the class load event.
> 
> This same agent compiled for windows claims to work (all asserts pass), but my hook never gets called, is anyone able to tell me why?
> 
> I register my callback like so:
> 
> jint res = vm->GetEnv(reinterpret_cast<void **>(&jvmti), JVMTI_VERSION_1);
>     if (res != JNI_OK || jvmti==nullptr) {
>         fprintf(stderr,"Unable to get JVMTI!");
>     }
>     assert(res == JVMTI_ERROR_NONE);
> 
>     jvmtiCapabilities capabilities = {0};
>     capabilities.can_retransform_classes=1;
>     capabilities.can_generate_all_class_hook_events  = 1;
> 
>     assert(jvmti->AddCapabilities(&capabilities) == JVMTI_ERROR_NONE);
> 
>     jvmtiEventCallbacks callbacks;
>     memset(&callbacks,0,sizeof(callbacks));
>     callbacks.ClassFileLoadHook = &class_file_load_hook_handler;
>     assert(jvmti->SetEventCallbacks(&callbacks, sizeof(callbacks)) == JVMTI_ERROR_NONE);
>     assert(jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_CLASS_FILE_LOAD_HOOK, nullptr) == JVMTI_ERROR_NONE);
> 
> The callback itself is as follows:
> 
> static void JNICALL class_file_load_hook_handler(
>         jvmtiEnv* jvmti,
>         JNIEnv* env,
>         jclass class_being_redefined,
>         jobject loader,
>         const char* name,
>         jobject protection_domain,
>         jint class_data_len,
>         const unsigned char* class_data,
>         jint* new_class_data_len,
>         unsigned char** new_class_data
> ) {
>     if (name == nullptr) {
>         return;
>     }
> 
>     printf("loaded %s\n",name);
> 
> }
> 
> (to clarify, all JNI functions in the lib work fine)
> 
> The Windows machine's java version string is this:
> 
> java version "1.8.0_271"
> Java(TM) SE Runtime Environment (build 1.8.0_271-b09)
> Java HotSpot(TM) 64-Bit Server VM (build 25.271-b09, mixed mode)
>