/* field IDs for commonly used object fields */
static jfieldID td_fdID;
static jfieldID td_eofID;
static jfieldID td_eomID;
static jfieldID IO_fd_fdID;

/* Class:     TapeDevice
 * Method:    initFields
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_BasicTapeDevice_initFields
  (JNIEnv *env, jclass cls) 
{
    /* retrieve field IDs for the fd, eof, and eom member variables */
    td_fdID = (*env)->GetFieldID(env, cls, "fd",
        "Ljava/io/FileDescriptor;");
    td_eofID = (*env)->GetFieldID(env, cls, "eof", "Z");
    td_eomID = (*env)->GetFieldID(env, cls, "eom", "Z");

    /* retrieve the field ID for the private fd member of FileDescriptor */
    cls = (*env)->FindClass(env, "java/io/FileDescriptor");
    IO_fd_fdID = (*env)->GetFieldID(env, cls, "fd", "I");
}

Example 2: Initialization of the native library.