public class LengthFramer implements Framer {
public static final int MAXMESSAGELENGTH = 65535;
public static final int BYTEMASK = 0xff;
public static final int SHORTMASK = 0xffff;
public static final int BYTESHIFT = 8;
private DataInputStream in; // wrapper for data I/O
public LengthFramer(InputStream in) throws IOException {
this.in = new DataInputStream(in);
}
public void frameMsg(byte[] message, OutputStream out) throws IOException {
if (message.length > MAXMESSAGELENGTH) {
throw new IOException("message too long");
}
// write length prefix
out.write((message.length >> BYTESHIFT) & BYTEMASK);
out.write(message.length & BYTEMASK);
// write message
out.write(message);
out.flush();
}
public byte[] nextMsg() throws IOException {
int length;
try {
length = in.readUnsignedShort(); // read 2 bytes
} catch (EOFException e) { // no (or 1 byte) message
return null;
}
// 0 <= length <= 65535
byte[] msg = new byte[length];
in.readFully(msg); // if exception, it's a framing error.
return msg;
}
}
