Instantiate Java class from JNI

Question

I need to create an instance of a Java class in my native code. To do it, I am using the following C code:

jobject Java_com_mypackage__myClass_myMethod(JNIEnv* env, jobject thiz, jint index){
    int fd = pDevs[index].ufds.fd; // fd = open(....); it's a input/eventX file.
    jclass class = (*env)->FindClass(env,"com/mypackage/ClassName");
    jmethodID mid = (*env)->GetMethodID(env,class,"<init>","(Ljava/lang/String;)V");
    return (*env)->NewObject(env,class,mid,(*env)->NewStringUTF(env, pDevs[index].device_path));
}

But when I invoke myMethod, I keep getting fatal signal 11 (SIGSEGV). Is the code wrong?

Your system log probably contains important information, e.g. in which function the signal was issued. Also, you should check the return values fron JNI calls you make: isn't class NULL? Isn't mid NULL? — Alex Cohn
– Alex Cohn, Commented Jul 13, 2013 at 11:02

Community · Accepted Answer · 2017-05-23 12:21:04Z

1

You should use logging/debbuger to find place where segmentation fault happenned. The easiest way is to use android logging system as described here

jclass class = (*env)->FindClass(env,"com/mypackage/ClassName");
if(class == null)
{
  __android_log_print(ANDROID_LOG_VERBOSE, "TAG", "class is null");
}

For example if ClassName is an inner class of some activity you should use com/mypackage/ActivityName#ClassName instead of com/mypackage/ClassName. But I can only guess before you provide your logs.

edited May 23, 2017 at 12:21

CommunityBot

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answered Jul 13, 2013 at 19:26

Dmitry Stukalov

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1 Comment

Vektor88 Over a year ago

I wasn't aware of how to print logs in native code, this was really helpful.

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