How to use JNI_CreateJavaVm in a multi threaded environment (C++)

Question

I'm Working in JNI with C++ as my native .I'm able to create the (c++) shared library and i'm able to call the java functions with the help of shared library.

Steps involved in my process:

1) Creating VM by using JNI_CreateJavaVm.[IN C++]

2) Process with the created VM.

3) Exit from the Thread

If i will again doing the same process the JNI_CreateJavaVm is not creating any VM and it is returning the JNI error code as -1 (Unknown error).Then I check for the getCreatedJavaVMs returns which returns 0, while I try to get the env with GetEnv it crashed.

I also tried the function GetJavaVM but it is getting crashed with the error message,

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00c1b3ed, pid=8645, tid=2961177456
#
# JRE version: 7.0_25-b15
# Java VM: Java HotSpot(TM) Server VM (23.25-b01 mixed mode linux-x86 )
# Problematic frame:
# C  [libVsphere.so+0x6a3ed]  _Jv_JNIEnv::GetJavaVM(_Jv_JavaVM**)+0xb

Why it is happening and how to solve the issue?

How to use the JNI_CreateJavaVm,JNI_GetCreatedVMs and GetJavaVM in a multithreaded environment.

Answer 1

You should have only one global instance of the JavaVM created at the begin of the program in one thread only :

/* Global instance */
JavaVM *jvm;

int main() {
/* ...call to JNI_CreateJavaVm ...*/
}

Then on each thread if you want to grab the Java environment, you should do it using the global pointer to the java machine ( jvm ):

JNIEnv *env;
(*jvm)->AttachCurrentThread(jvm, (void **)&env, NULL);

Finall using that environment you can access methods / classes, etc :

jclass ex = (*env)->FindClass(env, "java/lang/NullPointerException");

Answer 2

You can use AttachCurrentThread() to attach the current thread to the existing Java VM. There is no need to create JavaVM for every thread.