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

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.