Java Socket Programming

Java Socket Programming

Elliotte Rusty Harold

Software Development 2003 West

March, 2003

elharo@metalab.unc.edu

http://www.cafeaulait.org/

We will learn how Java handles

I assume you

Applet Network Security Restrictions

Some Background

Hosts

Internet addresses

Domain Name System (DNS)

The InetAddress Class

Creating InetAddresses

The getByName() factory method

Other ways to create InetAddress objects

Getter Methods

Utility Methods

Ports

Protocols

Datagrams

Packets Allow Error Correction

Abstraction

Sockets

Socket Operations

The java.net.Socket class

Constructing a Socket

Opening Sockets

Choosing the Host and the Port

PortScanner

public static void scan(InetAddress remote) {

  String hostname = remote.getHostName();
  for (int port = 1; port < 65536; port++) {
    try {
      Socket s = new Socket(remote, port); 
      System.out.println("A server is listening on port "
       + port + " of " + hostname);
      s.close();
    }
    catch (IOException ex) {
      // The remote host is not listening on this port
    }
  }

}

Picking an IP address

Choosing a Local Port

Sending and Receiving Data

Reading Input from a Socket

Time Client

try {
  Socket s = new Socket("tock.usno.navy.mil", 13);
  InputStream in = s.getInputStream();
  InputStreamReader isr = new InputStreamReader(in);
  BufferedReader br = new BufferedReader(isr, "ASCII");
  int c;
  while ((c = br.read()) != -1) {
    System.out.print((char) c);
  }
  System.out.println();
}
catch (IOException ex) {
  return (new Date()).toString();
}

Writing Output to a Socket

Discard

byte[] b = new byte[128];

try {
  Socket s = new Socket("metalab.unc.edu", 9);
  OutputStream theOutput = s.getOutputStream();
  while (true) {
    int n = theInput.available();
    if (n > b.length) n = b.length;
    int m = theInput.read(b, 0, n);
    if (m == -1) break;
    theOutput.write(b, 0, n);
 }
 s.close();
}
catch (IOException ex) {}

Reading and Writing to a Socket

Synchronous

Request-Response

write
write
write
read
read
read
read

Asynchronous

Very Basic HTTP Client

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

public class SocketCat {

  public static void main(String[] args) {

    for (int i = 0; i < args.length; i++) {  
      int port = 80;
      String file = "/";
      try {
        URL u = new URL(args[i]);
        if (u.getPort() != -1) port = u.getPort();
        if (!(u.getProtocol().equalsIgnoreCase("http"))) {
          System.err.println("I only understand http.");
          continue;
        }
        if (!(u.getFile().equals(""))) file = u.getFile();
        Socket s = new Socket(u.getHost(), port);
        OutputStream theOutput = s.getOutputStream();
        PrintWriter pw = new PrintWriter(theOutput, false);
        pw.println("GET " + file + " HTTP/1.0");
        pw.println("Accept: text/plain, text/html, text/*");
        pw.println();
        pw.flush();
        
        InputStream in = s.getInputStream();
        InputStreamReader isr = new InputStreamReader(in);
        BufferedReader br = new BufferedReader(isr, "ASCII");
        String theLine;
        while ((theLine = br.readLine()) != null) {
          System.out.println(theLine);
        }
      }
      catch (MalformedURLException e) {
        System.err.println(args[i] + " is not a valid URL");
      }
      catch (IOException ex) {
        System.err.println(ex);
      }
      
    }
  
  }

}

Better Example

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

public class BetterCat {

  public static void main(String[] args) {

    for (int i = 0; i < args.length; i++) {  
      int port = 80;
      String file = "/";
      try {
        URL u = new URL(args[i]);
        if (u.getPort() != -1) port = u.getPort();
        if (!(u.getProtocol().equalsIgnoreCase("http"))) {
          System.err.println("I only understand http.");
        }
        if (!(u.getFile().equals(""))) file = u.getFile();
        Socket s = new Socket(u.getHost(), port);
        OutputStream theOutput = s.getOutputStream();
        OutputStreamWriter out = new OutputStreamWriter(theOutput);
        out.write("GET " + file + " HTTP/1.0\r\n");
        out.write("Accept: text/plain, text/html, text/*\r\n");
        out.write("\r\n");
        out.flush();
        theOutput.flush();
        
        InputStream in = s.getInputStream();
        InputStreamReader isr = new InputStreamReader(in);
        BufferedReader br = new BufferedReader(isr);
        int c;
        while ((c = br.read()) != -1) {
          System.out.print((char) c);
        }
      }
      catch (MalformedURLException ex) {
        System.err.println(args[i] + " is not a valid URL");
      }
      catch (IOException ex) {
        System.err.println(ex);
      }
      
    }
  
  }

}

Socket Options

Getter Methods

Servers

Server Sockets

Multiple Clients

Queueing

The java.net.ServerSocket Class

Constructors

Constructing Server Sockets

Binding to Ports

Port Numbers

Expanding the Queue

Choosing an IP address

Example

try {
 InetAddress ia = InetAddress.getByName("199.1.32.90");
 ServerSocket ss = new ServerSocket(80, 50, ia);
}
catch (IOException ex) {
 System.err.println(ex);
}

Checking the Port Number

Accepting Connection

Reading Data with a ServerSocket

A Simple Server

try {
  ServerSocket ss = new ServerSocket(2345);
  Socket s = ss.accept();
  PrintWriter pw = new 
  PrintWriter(s.getOutputStream());
  pw.println("Hello There!");
  pw.println("Goodbye now.");
  s.close();
}
catch (IOException ex) {
  System.err.println(ex);
}

A Better Server

try {
 ServerSocket ss = new ServerSocket(2345);
 Socket s = ss.accept();
 Writer out = new BufferedWriter(
               new OutputStreamWriter(s.getOutputStream(), "UTF-8")
              );
 out.write("Hello There!\r\n");
 out.write("Goodbye now.\r\n");
 out.flush();
 s.close();
}
catch (IOException ex) {
 System.err.println(ex);
}

Writing Data to a Client


try {
  ServerSocket ss = new ServerSocket(port);
  while (true) {
    try {
      Socket s = ss.accept();
      Writer out = new OutputStreamWriter(s.getOutputStream());
      out = new BufferedWriter(out);
      out.write("Hello " + s.getInetAddress() + " on port " 
       + s.getPort() + "\r\n");
      out.write("This is " + s.getLocalAddress() + " on port " 
       + s.getLocalPort() + "\r\n");
      out.flush();
      s.close();
    }
    catch (IOException ex) {
      // skip this connection; continue with the next
    }
  }
}
catch (IOException ex) { 
  System.err.println(ex); 
}

Interacting with a Client

Threading

Adding Threading to a Server

Adding a Thread Pool to a Server

Thread Issues

Thread Pools

New I/O API

Setting Server Socket Options

Utility Methods

UDP

The UDP Classes

java.net.DatagramPacket

java.net.DatagramSocket

UDP ports

Two DatagramPacket Constructors

For example,

String s = "My first UDP Packet"
byte[] b = s.getBytes();
DatagramPacket dp = new DatagramPacket(b, b.length);

With a destination:

try {
  InetAddress metalab = new InetAddess("metalab.unc.edu");
  int chargen = 19;
  String s = "My second UDP Packet"
  byte[] b = s.getBytes();
  DatagramPacket dp = new DatagramPacket(b, b.length, metalab, chargen);
}
catch (UnknownHostException e) {
  System.err.println(ex);
}

DatagramPackets are not immutable. 

public synchronized void setAddress(InetAddress iaddr)
public synchronized void setPort(int iport) 
public synchronized void setData(byte ibuf[])
public synchronized void setLength(int ilength)
public synchronized InetAddress getAddress()
public synchronized int    getPort()
public synchronized byte[] getData()
public synchronized int    getLength()

java.net.DatagramSocket

public DatagramSocket() throws SocketException
public DatagramSocket(int port) throws SocketException
public DatagramSocket(int port, InetAddress laddr) throws SocketException

Sending UDP Datagrams

For example, 

InetAddress metalab = new InetAddess("metalab.unc.edu");
int chargen = 19;
String s = "My second UDP Packet";
byte[] b = s.getBytes();
DatagramPacket dp = new DatagramPacket(b, b.length, ia, chargen);
DatagramSocket sender = new DatagramSocket();
sender.send(dp);

Receiving UDP Datagrams

For example, 

try {
  byte buffer = new byte[65536]; 
  DatagramPacket incoming = new DatagramPacket(buffer, buffer.length);
  DatagramSocket ds = new DatagramSocket(2134);
  ds.receive(incoming);
  byte[] data = incoming.getData();
  String s = new String(data, 0, data.getLength());
  System.out.println("Port" + incoming.getPort() + " on " 
   + incoming.getAddress() + " sent this message:");
  System.out.println(s);
}
catch (IOException ex) {
  System.err.println(ex);
}

Questions?

To Learn More


Index | Cafe con Leche
Copyright 2003 Elliotte Rusty Harold
elharo@metalab.unc.edu
Last Modified February 13, 2003