Question

I’ve found plenty of tutorials / questions on Stackoverflow that deal with copying char arrays from C/JNI side into something like a byte[] in Java, but not the other way around.

I am using a native C library which expects a byte array. I simply want to get data from a byte[] in java, into preferably an unsigned char[] in C.

Long story short: What is the best way of copying data from a jBytearray in JNI? Is there any way to detect it’s size?

Answer 1

Here’s a working example I just lifted from my AS/400 JNI library to resolve a native user-queue pointer to test the queue’s existence – it copies the queue library and name from a Java byte array (already translated to job’s CCSID) into native code and uses it. Take note of the release function calls; these can be changed to copy the native array contents back into the Java byte arrays to move data the other way:

JNIEXPORT jboolean JNICALL Java_com_mycompany_jni400_UserQueue_jniResolve(JNIEnv *jep,jobject thsObj,                
jbyteArray queueLibrary,jbyteArray queueName) {                                                                             
    jbyte            *lib,*nam;                                                                                             
    bool             rtn;                                                                                                   

    thsObj=thsObj;                                                                                                          
    lib=(*jep)->GetByteArrayElements(jep,queueLibrary,0);                                                                   
    nam=(*jep)->GetByteArrayElements(jep,queueName,0);                                                                      
    rtn=(usrq_resolve((byte*)lib,(byte*)nam)!=NULL);                                                                        
    (*jep)->ReleaseByteArrayElements(jep,queueLibrary,lib,JNI_ABORT); /* abort to not copy back contents */                 
    (*jep)->ReleaseByteArrayElements(jep,queueName   ,nam,JNI_ABORT); /* abort to not copy back contents */                 
    if(rtn) { return JNI_TRUE;  }                                                                                           
    else    { return JNI_FALSE; }                                                                                           
    }