jni.java

https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/design.html

jdouble Java_pkg_Cls_f__ILjava_lang_String_2 (
     JNIEnv *env,        /* interface pointer */
     jobject obj,        /* "this" pointer */
     jint i,             /* argument #1 */
     jstring s)          /* argument #2 */
{
     /* Obtain a C-copy of the Java string */
     const char *str = (*env)->GetStringUTFChars(env, s, 0);

     /* process the string */
     ...

     /* Now we are done with str */
     (*env)->ReleaseStringUTFChars(env, s, str);

     return ...
}

Global and Local References
The JNI divides object references used by the native code into two categories: 
local and global references. Local references are valid for the duration of a native method call, and are automatically freed after the native method returns. Global references remain valid until they are explicitly freed.
 
     
     
The JNI functions that invoke a Java method return the result of the Java method. The programmer must call ExceptionOccurred() to check for possible exceptions that occurred during the execution of the Java method.