Sending and Receiving Messages - TCP/IP Sockets in Java Practical Guide for Programmers - page 42

DataOutputStream BufferedOutputStream OutputStream

3.14 (8 bytes) 3.14

writeDouble(3.14)

writeInt(343) 343 (4 bytes) 343 14 bytes

800 (2 bytes) 800

writeShort(800)

Network

DataInputStream BufferedInputStream InputStream

readDouble() 3.14 (8 bytes) 3.14

343 (4 bytes) 343 14 bytes

readInt()

800 (2 bytes) 800

readShort()

Figure 3.1: Stream composition.

3.2 Composing I/O Streams

Java's stream classes can be composed to provide powerful encoding and decoding facilities.
For example, we can wrap the OutputStream of a Socket instance in a BufferedOutputStream
instance to improve performance by buffering bytes temporarily and flushing them to the
underlying channel all at once. We can then wrap that instance in a DataOutputStream to send
primitive data types. We would code this composition as follows:

Socket socket = new Socket(server, port);
DataOutputStream out = new DataOutputStream(
new BufferedOutputStream(socket.getOutputStream()));

Figure 3.1 demonstrates this composition. Here, we write our primitive data values, one by
one, to DataOutputStream , which writes the binary data to BufferedOutputStream , which buffers
the data from the three writes and then writes once to the socket OutputStream , which controls
writing to the network. We create a parallel composition for the InputStream on the other
endpoint to eciently receive primitive data types.
A complete description of the Java I/O API is beyond the scope of this text; however, Table
3.1 provides a list of some of the relevant Java I/O classes as a starting point for exploiting its
capabilities.

3.3 Framing and Parsing

Converting data to wire format is of course only half the story; the original information must
be recovered at the receiver from the transmitted sequence of bytes. Application protocols
typically deal with discrete messages, which are viewed as collections of fields. Framing refers
to the problem of enabling the receiver to locate the beginning and end of the message in