How do I get a byte array from an object passed into the JNI?

Question

This is what my java code looks like:
public class streamedData
protected byte[] m_value = null;
public byte[] getByteArray ()
return m_value;
/* code not pertaining to this question snipped. */
jclass streamedDataClass = env->GetObjectClass(jstreamedData);
// Get methodIDs for the various NPKI_Extension methods
value = env->GetMethodID(streamedDataClass, "getByteArray", "()[B");
lstreamedData->value = (unsigned char *)malloc (lstreamedData->length);
if (lstreamedData->value == NULL)
return INSUFFICIENT_MEMORY;
memset (lstreamedData->value, 0, lstreamedData->length);
memcpy (lstreamedData->value, env->CallByteMethodA(streamedDataClass, value, NULL), lstreamedData->length);
This is not working.
My question is, how do I get a copy of or read m_value in the JNI layer? Do I have to make the byte array public and access it as a field ID?
Any help would be appreciated. Why oh why can't there be a CallByteArrayMethod????

Answer 1

My question is, how do I get a copy of or read m_value in the JNI layer? Do I have to make the byte array public and access it as a field ID?
You can retrieve a handle to the byte array, through a method call, as you're doing now, or by directly accessing it through GetFieldID.
In the case of the method call, you'll want to do:jmethodID getByteArrayMethod = env->GetMethodID(streamedDataClass, "getByteArray", "()[B");
jbyteArray byteArray = static_cast<jbyteArray>( env->CallObjectMethod( jstreamedData, getByteArrayMethod ) );In the case of field access, you can do:jfieldID m_valueFieldID = env->GetFieldID( streamedDataClass, "m_value", "[B" );
jbyteArray byteArray = static_cast<jbyteArray>( env->GetObjectField( jstreamedData, m_valueFieldID ) );Once you have byteArray, you need to access it using the array operations:lstreamedData->value = env->GetByteArrayElements( byteArray, 0 );
// Do what you want to do, then release the array
env->ReleaseByteArrayElements( byteArray, lstreamedData->value, 0 );As always, Jace, http://jace.reyelts.com/jace, makes this easier. For example,void printByteArray( jobject jstreamedData ) {
  streamedData data( jstreamedData );
  JArray<JByte> byteArray = streamedData.m_value();
  for ( int i = 0; i < byteArray.length(); ++i ) {
    cout << byteArray[ i ];
}God bless,
-Toby Reyelts