问Java的本机方法是在哪里定义的？

EN

/** 
 * Reads into an array of bytes at the specified offset using
 * the received socket primitive. 
 * @param fd the FileDescriptor
 * @param b the buffer into which the data is read
 * @param off the start offset of the data
 * @param len the maximum number of bytes read
 * @param timeout the read timeout in ms
 * @return the actual number of bytes read, -1 is
 *          returned when the end of the stream is reached. 
 * @exception IOException If an I/O error has occurred.
*/
private native int socketRead0(FileDescriptor fd, 
        byte b[], int off, int len, int timeout)
        throws IOException;