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Tip of the Day
Language: Java Language
Expertise: Beginner
May 5, 2000

Interplatform Networking

Question:
I want to connect a Java client program to a C/C++ server application using sockets. What is the easiest way of doing this?

Answer:
Java provides the ability to write platform independent programs that will run on any machine. TCP/IP provides the ability for programs to communicate with each other over a network without regard to their native platforms. Java provides the ability to communicate with other machines via TCP/IP through the java.net package. The Socket class is used to establish TCP connections from a Java program to another program on the network. Communication is treated as a full-duplex stream of bytes from one end to the other. Every byte you write to the stream is read from the other end in the same order that it was written. Likewise, every byte you read from the stream is returned in the same order that it was written by the other end. Use getInputStream() and getOutputStream() to read and write to a Socket.

Different hardware platforms represent primitive data types using different byte orderings and bit lengths. For example, current Intel processors store integers in a 32-bit little-endian format while some of the latest SPARC processors use a 64-bit big-endian format. Therefore, in order for different hardware platforms to communicate over a network, they need to agree upon a data representation format. Homogeneous distributed systems may opt to use native representation formats to avoid converting to and from a network byte-ordering.

However, the Internet is a heterogeneous system, and the generally accepted format for transmitting integers over the network is in a 32-bit big-endian format. More detailed standards exist, such as XDR (external data representation), which is used primarily to exchange parameters and results via remote procedure calls. The end result, is that when you interface a Java client with a non-Java server, you need to understand the data representation convention used by the server and how to convert that to Java data types. The hangups involved in doing this are what make CORBA, XML, and RMI so attractive for building distributed systems. They remove data representation issues from the programmer.

I've provided a simple client program example that connects to a Time protocol server (IETF RFC 868), reads the protocol-defined data, converts it to a Java data type, and prints out the result.

import java.io.*;
import java.net.*;
import java.util.*;

public class rdate {
  public static final int TIME_PORT            = 37;
  public static final String DEFAULT_TIME_HOST = "time.nist.gov";

  /***
   * The number of seconds between 00:00 1 January 1900 and
   * 00:00 1 January 1970.  This value can be useful for converting
   * time values to other formats.
   */
  public static final long SECONDS_1900_TO_1970 = 2208988800L;

  public static void main(String[] args) {
    String timehost = DEFAULT_TIME_HOST;
    long seconds;
    DataInputStream input;
    Socket socket;
    Date date;

    if(args.length > 0)
       timehost = args[0];

    try {
      socket = new Socket(timehost, TIME_PORT);
      input   = new DataInputStream(socket.getInputStream());

      // Retrieve the time from the server.  The time
      // is the number of seconds since 00:00 (midnight)
      // January 1900 GMT, as specified by RFC 868.  This reads
      // the raw 32-bit big-endian unsigned integer from the server and
      // converts it to a Java long.
      seconds = (((long)input.readInt()) & 0xffffffffL);

      input.close();
      date = new Date((seconds - SECONDS_1900_TO_1970)*1000L);
      System.out.println(date.toString());
    } catch(IOException e) {
      e.printStackTrace();
      return;
    }
  }
}
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