how to copy byte array from c to java byte[] attribute directly with jni

Question

I am writing a C module with jni for android.
my java class is

public class Payment {
    private static Payment payment = null;
    private long nativeObj;
    private byte[] sendBuffer;
    private byte[] recvBuffer;
    private byte[] msg;

    private Payment() {
        this.sendBuffer = new byte[1024];
        this.recvBuffer = new byte[1024];
        this.msg = new byte[1024];
    }

    public native void setArray();

 }

i want to fill byte arrays of Payment instance in c and i can not do it.
what is the procedure of jni call for this purpose?

i get the field id of sendBuffer with

jclass thisClass = (*env)->GetObjectClass(env, obj);
jfieldID sendId = (*env)->GetFieldID(env, thisClass, "sendBuffer", "[B");

but i can not figure out how to copy data from char[1024] to sendBuffer.

cleblanc · Accepted Answer · 2017-09-29 15:10:37Z

3

You can use something like this to copy data from C to java bytes array;

jint_Java_com_stack_overflow_copyBytes(JNIEnv *e, jclass obj, jlong p)
{
    jclass thisClass = (*env)->GetObjectClass(e, obj);
    jfieldID sendId = (*env)->GetFieldID(e, thisClass, "sendBuffer", "[B");        
    jbyteArray bytes = (*e)->GetObjectField(e, obj, sendId)

    jbyte* b = (*e)->GetByteArrayElements(e, bytes, NULL);
    memcpy(myCBytes, b, len);
    (*e)->ReleaseByteArrayElements(e, bytes, b, 0);
}

edited Sep 29, 2017 at 15:10

answered Sep 29, 2017 at 14:59

cleblanc

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3 Comments

H00man Over a year ago

i don't want to pass my instance attribute (byte[]) to native function!

cleblanc Over a year ago

@COP you can use jbyteArray bytes = (*e)->GetObjectField(e, obj, sendId) to access the jbyteArray if you want.

Tom Blodget Over a year ago

@cleblanc It is sometimes best to refactor data copying and conversion operations from JNI code to Java code. Often, in such cases, the native methods would become private (and possibly static), being bridges to JNI rather than part of the class's public interface.

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