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5.10. Networking with

The package defines a number of classes that make writing networked applications surprisingly easy. Various examples follow.

5.10.1. Networking with the URL Class

The easiest networking class to use is URL, which represents a uniform resource locator. Different Java implementations may support different sets of URL protocols, but, at a minimum, you can rely on support for the http://, ftp://, and file:// protocols. As of Java 1.4, secure HTTP is also supported with the https:// protocol. Here are some ways you can use the URL class:


// Create some URL objects 
URL url=null, url2=null, url3=null;
try {
  url = new URL("");        // An absolute URL
  url2 = new URL(url, "catalog/books/javanut4/"); // A relative URL
  url3 = new URL("http:", "", "index.html");
} catch (MalformedURLException e) { /* Ignore this exception */ }

// Read the content of a URL from an input stream
InputStream in = url.openStream();

// For more control over the reading process, get a URLConnection object
URLConnection conn = url.openConnection();

// Now get some information about the URL
String type = conn.getContentType();
String encoding = conn.getContentEncoding();
java.util.Date lastModified = new java.util.Date(conn.getLastModified());
int len = conn.getContentLength();

// If necessary, read the contents of the URL using this stream
InputStream in = conn.getInputStream();

5.10.2. Working with Sockets

Sometimes you need more control over your networked application than is possible with the URL class. In this case, you can use a Socket to communicate directly with a server. For example:


// Here's a simple client program that connects to a web server, 
// requests a document and reads the document from the server.
String hostname = "";  // The server to connect to
int port = 80;                         // Standard port for HTTP 
String filename = "/index.html";       // The file to read from the server
Socket s = new Socket(hostname, port); // Connect to the server

// Get I/O streams we can use to talk to the server
InputStream sin = s.getInputStream();
BufferedReader fromServer = new BufferedReader(new InputStreamReader(sin));
OutputStream sout = s.getOutputStream();
PrintWriter toServer = new PrintWriter(new OutputStreamWriter(sout));

// Request the file from the server, using the HTTP protocol
toServer.print("GET " + filename + " HTTP/1.0\r\n\r\n");

// Now read the server's response, assume it is a text file, and print it out
for(String l = null; (l = fromServer.readLine()) != null; )

// Close everything down when we're done

5.10.3. Secure Sockets with SSL

In Java 1.4, the Java Secure Socket Extension, or JSSE, was added to the core Java platform in the packages and[1] This API enables encrypted network communication over sockets that use the SSL (Secure Sockets Layer, also known as TLS) protocol. SSL is widely used on the Internet: it is the basis for secure web communication using the https:// protocol. In Java 1.4 and later, you can use https:// with the URL class as previously shown to securely download documents from web servers that support SSL.

[1] An earlier version of JSSE using different package names is available as a separate download for use with Java 1.2 and Java 1.3. See

Like all Java security APIs, JSSE is highly configurable and gives low-level control over all details of setting up and communicating over an SSL socket. The and packages are fairly complex, but in practice, you need only a few classes to securely communicate with a server. The following program is a variant on the preceding code that uses HTTPS instead of HTTP to securely transfer the contents of the requested URL:


 * Get a document from a web server using HTTPS. Usage:
 *   java HttpsDownload <hostname> <filename>
public class HttpsDownload {
    public static void main(String[] args) throws IOException {
        // Get a SocketFactory object for creating SSL sockets
        SSLSocketFactory factory =
            (SSLSocketFactory) SSLSocketFactory.getDefault();

        // Use the factory to create a secure socket connected to the 
        // HTTPS port of the specified web server.
        SSLSocket sslsock=(SSLSocket)factory.createSocket(args[0], // Hostname
                                                         443); // HTTPS port

        // Get the certificate presented by the web server
        SSLSession session = sslsock.getSession();
        X509Certificate cert;   
        try { cert = (X509Certificate)session.getPeerCertificates()[0]; }
        catch(SSLPeerUnverifiedException e) { // If no or invalid certificate
            System.err.println(session.getPeerHost() +
                               " did not present a valid certificate.");

        // Display details about the certificate
        System.out.println(session.getPeerHost() + 
                           " has presented a certificate belonging to:");
        System.out.println("\t[" + cert.getSubjectDN().getName() + "]");
        System.out.println("The certificate bears the valid signature of:");
        System.out.println("\t[" + cert.getIssuerDN().getName() + "]");

        // If the user does not trust the certificate, abort
        System.out.print("Do you trust this certificate (y/n)? ");
        BufferedReader console =
            new BufferedReader(new InputStreamReader(;
        if (Character.toLowerCase(console.readLine().charAt(0)) != 'y') return;

        // Now use the secure socket just as you would use a regular socket
        // First, send a regular HTTP request over the SSL socket
        PrintWriter out = new PrintWriter(sslsock.getOutputStream());
        out.print("GET " + args[1] + " HTTP/1.0\r\n\r\n");

        // Next, read the server's response and print it to the console
        BufferedReader in = 
         new BufferedReader(new InputStreamReader(sslsock.getInputStream()));
        String line;
        while((line = in.readLine()) != null) System.out.println(line);
        // Finally, close the socket

5.10.4. Servers

A client application uses a Socket to communicate with a server. The server does the same thing: it uses a Socket object to communicate with each of its clients. However, the server has an additional task in that it must be able to recognize and accept client connection requests. This is done with the ServerSocket class. The following code shows how you might use a ServerSocket. The code implements a simple HTTP server that responds to all requests by sending back (or mirroring) the exact contents of the HTTP request. A dummy server like this is useful when debugging HTTP clients:


public class HttpMirror {
  public static void main(String[] args) {
    try {
      int port = Integer.parseInt(args[0]);       // The port to listen on
      ServerSocket ss = new ServerSocket(port);   // Create a socket to listen
      for(;;) {                                   // Loop forever
        Socket client = ss.accept();              // Wait for a connection
        ClientThread t = new ClientThread(client);// A thread to handle it
        t.start();                                // Start the thread running
      }                                           // Loop again
    catch (Exception e) {
      System.err.println("Usage: java HttpMirror <port>;");

  static class ClientThread extends Thread {
    Socket client;
    ClientThread(Socket client) { this.client = client; }
    public void run() {
      try {
        // Get streams to talk to the client
        BufferedReader in = 
          new BufferedReader(new InputStreamReader(client.getInputStream()));
        PrintWriter out =
          new PrintWriter(new OutputStreamWriter(client.getOutputStream()));
        // Send an HTTP response header to the client
        out.print("HTTP/1.0 200\r\nContent-Type: text/plain\r\n\r\n");
        // Read the HTTP request from the client and send it right back
        // Stop when we read the blank line from the client that marks 
        // the end of the request and its headers. 
        String line;
        while((line = in.readLine()) != null) {
          if (line.length() == 0) break;
      catch (IOException e) { /* Ignore exceptions */ }

This server code could be modified using JSSE to support SSL connections. Making a server secure is more complex than making a client secure, however, because a server must have a certificate it can present to the client. Therefore, server-side JSSE is not demonstrated here.

5.10.5. Datagrams

Both URL and Socket perform networking on top of a stream-based network connection. Setting up and maintaining a stream across a network takes work at the network level, however. Sometimes you need a low-level way to speed a packet of data across a network, but you don't care about maintaining a stream. If, in addition, you don't need a guarantee that your data will get there or that the packets of data will arrive in the order you sent them, you may be interested in the DatagramSocket and DatagramPacket classes:


// Send a message to another computer via a datagram
try {
  String hostname = "";    // The computer to send the data to
  InetAddress address =                    // Convert the DNS hostname
    InetAddress.getByName(hostname);       // to a lower-level IP address. 
  int port = 1234;                         // The port to connect to
  String message = "The eagle has landed.";  // The message to send
  byte[] data = message.getBytes();          // Convert string to bytes
  DatagramSocket s = new DatagramSocket();   // Socket to send message with
  DatagramPacket p =                         // Create the packet to send
    new DatagramPacket(data, data.length, address, port);
  s.send(p);                                // Now send it!
  s.close();                                // Always close sockets when done
catch (UnknownHostException e) {}  // Thrown by InetAddress.getByName()
catch (SocketException e) {}       // Thrown by new DatagramSocket()
catch ( e) {}   // Thrown by DatagramSocket.send()

// Here's how the other computer can receive the datagram
try {
  byte[] buffer = new byte[4096];              // Buffer to hold data
  DatagramSocket s = new DatagramSocket(1234); // Socket that receives it 
                                               // through
  DatagramPacket p = 
    new DatagramPacket(buffer, buffer.length); // The packet that receives it
  s.receive(p);                                // Wait for a packet to arrive
  String msg =                                 // Convert the bytes from the
    new String(buffer, 0, p.getLength());      // packet back to a string. 
  s.close();                                   // Always close the socket
catch (SocketException e) {}       // Thrown by new DatagramSocket()
catch ( e) {}   // Thrown by DatagramSocket.receive()

5.10.6. Testing the Reachability of a Host

In Java 5.0 the InetAddress class has an isReachable( ) method that attempts to determine whether the host is reachable. The following code uses it in a naive Java implementation of the Unix ping utility:


public class Ping {
    public static void main(String[] args) throws IOException {
        try {
            String hostname = args[0];
            int timeout = (args.length > 1)?Integer.parseInt(args[1]):2000;
            InetAddress[] addresses = InetAddress.getAllByName(hostname);
            for(InetAddress address : addresses) {
                if (address.isReachable(timeout))
                    System.out.printf("%s is reachable%n", address);
                    System.out.printf("%s could not be contacted%n", address);
        catch (UnknownHostException e) {
            System.out.printf("Unknown host: %s%n", args[0]);
        catch(IOException e) { System.out.printf("Network error: %s%n", e); }
        catch (Exception e) { 
            // ArrayIndexOutOfBoundsException or NumberFormatException
            System.out.println("Usage: java Ping <hostname> [timeout in ms]");

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