Return list<unsigned char*> from C++ to Java using JNI

Question

I have the following JNI method,

JNIEXPORT jobject JNICALL Java_projlib_DeserializeBuffer
(JNIEnv *env, jobject obj, jbyteArray inBufferData)

I have created a list of unsigned char* and filling it using some data extracted from the inBufferData in my C++ code

list<unsigned char*> returnBuffer

I want to return the returnBuffer to my Java code, where it will be a List of byte array, List<byte[]>.

Please tell me how to pass the list of unsigned char* via a jobject through JNI and then get it in Java for further processing.

Rudolfs Bundulis · Accepted Answer · 2015-10-12 08:48:18Z

1

You will not be able to do that directly. You will have to instantiate an instance of the needed Java list implementation (since List is an interface) in C++, put it in a jobject and then add jbytearray items to it from your list and then return the list.

EXAMPLE

Since I do not have a working JNI environment, this snippet is only illustrational (feel free to edit it when you get it working), but what you need could be achieved by this:

jclass arrayListClass = env->FindClass("java/util/ArrayList"); // Find ArrayList class
jmethodID constructor = env->GetMethodID(arrayListClass, "<init>", "()V"); // Find ArrayList constructor
jobject arrayList = env->NewObject(arrayListClass, constructor); // Create new ArrayList instance
jmethodID add = env->GetMethodID(arrayListClass, "add", "(Ljava/lang/Object;)Z"); // Find the ArrayList::add method
jbyteArray item =env->NewByteArray(10); // Instantiate a new byte[]
env->CallBooleanMethod(arrayList, add, item); // Add the byte[] to the ArrayList

edited Oct 12, 2015 at 8:48

answered Oct 12, 2015 at 7:02

Rudolfs Bundulis

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5 Comments

AnkitG Over a year ago

I am very new to C++, can you share some snippets?

Rudolfs Bundulis Over a year ago

@AnkitG I added a snippet but since I do not have a working JNI environment to test it on it is only illustrational, feel free to provide feedback or edit it.

Rudolfs Bundulis Over a year ago

@AnkitG I updated the snippet, the signature for the add method was wrong and it caused Java exception.

AnkitG Over a year ago

Thanks! Had one clarification. the data I want to return is not a ByteArray, it is a list<unsigned char*> returnBuffer in C++ and in the Java world it will be a ByteArray. From your code snippet, I will simply remove the line, jbyteArray item =env->NewByteArray(10); and in place of item, put my list/vector object. Let me know if that is a correct approach

Rudolfs Bundulis Over a year ago

@AnkitG In your question you mentioned List<byte[]> which seems correct. list<unsigned char*> is a list of pointers to arrays - so unless you have chosen the wrong container it is not the same as byte[]. How many items are in your list? If multiple then how do you hope to represent multiple arrays with one byte[]?

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