19.12 A Debugging Web Server

This section presents a mini "Web server" that is useful when you are trying to understand the behavior of HTML forms. We used it for many of the examples earlier in the chapter. The server simply reads all the HTTP data sent to it by the browser, then returns a Web page with those lines embedded within a PRE element.

This server is also useful for debugging servlets. When something goes wrong, the first task is to determine if the problem lies in the way in which you collect data or the way in which you process it. The WebClient program of Section 3.6 lets you see the raw data resulting from the server-side program; EchoServer lets you see the raw data transmitted by the client form.

Starting the EchoServer on, say, port 8088 of your local machine, then changing your forms to specify http://localhost:8088/ lets you see if the data being collected is in the format you expect. In addition to seeing the sent form data, EchoServer also shows the HTTP request headers sent from the browser.

EchoServer

Listing 19.16 presents the top-level server code. You typically run EchoServer from the command line, specifying a port to listen on, or accepting the default port of 8088. EchoServer then accepts repeated HTTP requests from clients , packaging all HTTP data sent to it inside a Web page that is returned to the client. In most cases, the server reads until a blank line is received, indicating the end of GET , HEAD , or most other types of HTTP requests. In the case of POST , however, the server checks the Content-Length request header and reads that many bytes beyond the blank line.

Listings 19.17 and 19.18 present some utility classes that simplify networking. The EchoServer is built on top of them.

Listing 19.16 EchoServer.java

 import java.net.*; import java.io.*; import java.util.*; /** A simple HTTP server that generates a Web page showing all  *  the data that it received from the Web client (usually  *  a browser). To use this server, start it on the system of  *  your choice, supplying a port number if you want something  *  other than port 8088. Call this system server.com. Next,  *  start a Web browser on the same or a different system, and  *  connect to http://server.com:8088/whatever. The resultant  *  Web page will show the data that your browser sent. For  *  debugging in a servlet or other server-side program, specify  *  http://server.com:8088/whatever as the ACTION of your HTML  *  form. You can send GET or POST data; either way, the  *  resultant page will show what your browser sent.  */ public class EchoServer extends NetworkServer {   protected int maxRequestLines = 50;   protected String serverName = "EchoServer";   /** Supply a port number as a command-line    *  argument. Otherwise, use port 8088.    */   public static void main(String[] args) {     int port = 8088;     if (args.length > 0) {       try {         port = Integer.parseInt(args[0]);       } catch(NumberFormatException nfe) {}     }     new EchoServer(port, 0);   }   public EchoServer(int port, int maxConnections) {     super(port, maxConnections);     listen();   }   /** Overrides the NetworkServer handleConnection method to    *  read each line of data received, save it into an array    *  of strings, then send it back embedded inside a PRE    *  element in an HTML page.    */   public void handleConnection(Socket server)       throws IOException{     System.out.println         (serverName + ": got connection from " +          server.getInetAddress().getHostName());     BufferedReader in = SocketUtil.getReader(server);     PrintWriter out = SocketUtil.getWriter(server);     String[] inputLines = new String[maxRequestLines];     int i;     for (i=0; i<maxRequestLines; i++) {       inputLines[i] = in.readLine();       if (inputLines[i] == null) // Client closed connection.         break;       if (inputLines[i].length() == 0) { // Blank line.         if (usingPost(inputLines)) {           readPostData(inputLines, i, in);           i = i + 2;         }         break;       }     }     printHeader(out);     for (int j=0; j<i; j++) {       out.println(inputLines[j]);     }     printTrailer(out);     server.close();   }   // Send standard HTTP response and top of a standard Web page.   // Use HTTP 1.0 for compatibility with all clients.   private void printHeader(PrintWriter out) {     out.println       ("HTTP/1.0 200 OK\r\n" +        "Server: " + serverName + "\r\n" +        "Content-Type: text/html\r\n" +        "\r\n" +        "<!DOCTYPE HTML PUBLIC " +          "\"-//W3C//DTD HTML 4.0 Transitional//EN\">\n" +        "<HTML>\n" +        "<HEAD>\n" +        "  <TITLE>" + serverName + " Results</TITLE>\n" +        "</HEAD>\n" +        "\n" +        "<BODY BGCOLOR=\"#FDF5E6\">\n" +        "<H1 ALIGN=\"CENTER\">" + serverName +          " Results</H1>\n" +        "Here is the request line and request headers\n" +        "sent by your browser:\n" +        "<PRE>");   }   // Print bottom of a standard Web page.   private void printTrailer(PrintWriter out) {     out.println       ("</PRE>\n" +        "</BODY>\n" +        "</HTML>\n");   }   // Normal Web page requests use GET, so this server can simply   // read a line at a time. However, HTML forms can also use   // POST, in which case we have to determine the number of POST   // bytes that are sent so we know how much extra data to read   // after the standard HTTP headers.   private boolean usingPost(String[] inputs) {     return(inputs[0].toUpperCase().startsWith("POST"));   }   private void readPostData(String[] inputs, int i,                             BufferedReader in)       throws IOException {     int contentLength = contentLength(inputs);     char[] postData = new char[contentLength];     in.read(postData, 0, contentLength);     inputs[++i] = new String(postData, 0, contentLength);   }   // Given a line that starts with Content-Length,   // this returns the integer value specified.   private int contentLength(String[] inputs) {     String input;     for (int i=0; i<inputs.length; i++) {       if (inputs[i].length() == 0)         break;       input = inputs[i].toUpperCase();       if (input.startsWith("CONTENT-LENGTH"))         return(getLength(input));     }     return(0);   }   private int getLength(String length) {     StringTokenizer tok = new StringTokenizer(length);     tok.nextToken();     return(Integer.parseInt(tok.nextToken()));   } } 

Listing 19.17 NetworkServer.java

 import java.net.*; import java.io.*; /** A starting point for network servers. You'll need to  *  override handleConnection, but in many cases listen can  *  remain unchanged. NetworkServer uses SocketUtil to simplify  *  the creation of the PrintWriter and BufferedReader.  */ public class NetworkServer {   private int port, maxConnections;   /** Build a server on specified port. It will continue to    *  accept connections, passing each to handleConnection until    *  an explicit exit command is sent (e.g., System.exit) or    *  the maximum number of connections is reached. Specify    *  0 for maxConnections if you want the server to run    *  indefinitely.    */   public NetworkServer(int port, int maxConnections) {     setPort(port);     setMaxConnections(maxConnections);   }   /** Monitor a port for connections. Each time one is    *  established, pass resulting Socket to handleConnection.    */   public void listen() {     int i=0;     try {       ServerSocket listener = new ServerSocket(port);       Socket server;       while((i++ < maxConnections)  (maxConnections == 0)) {         server = listener.accept();         handleConnection(server);       }     } catch (IOException ioe) {       System.out.println("IOException: " + ioe);       ioe.printStackTrace();     }   }   /** This is the method that provides the behavior to the    *  server, since it determines what is done with the    *  resulting socket. <B>Override this method in servers    *  you write.</B>    *  <P>    *  This generic version simply reports the host that made    *  the connection, shows the first line the client sent,    *  and sends a single line in response.    */   protected void handleConnection(Socket server)       throws IOException{     BufferedReader in = SocketUtil.getReader(server);     PrintWriter out = SocketUtil.getWriter(server);     System.out.println       ("Generic Network Server: got connection from " +        server.getInetAddress().getHostName() + "\n" +        "with first line '" + in.readLine() + "'");     out.println("Generic Network Server");     server.close();   }   /** Gets the max connections server will handle before    *  exiting. A value of 0 indicates that server should run    *  until explicitly killed.    */   public int getMaxConnections() {     return(maxConnections);   }   /** Sets max connections. A value of 0 indicates that server    *  should run indefinitely (until explicitly killed).    */   public void setMaxConnections(int maxConnections) {     this.maxConnections = maxConnections;   }   /** Gets port on which server is listening. */   public int getPort() {     return(port);   }   /** Sets port. <B>You can only do before "connect" is    *  called.</B> That usually happens in the constructor.    */   protected void setPort(int port) {     this.port = port;   } } 

Listing 19.18 SocketUtil.java

 import java.net.*; import java.io.*; /** A shorthand way to create BufferedReaders and  *  PrintWriters associated with a Socket.  */ public class SocketUtil {   /** Make a BufferedReader to get incoming data. */   public static BufferedReader getReader(Socket s)       throws IOException {     return(new BufferedReader(        new InputStreamReader(s.getInputStream())));   }   /** Make a PrintWriter to send outgoing data.    *  This PrintWriter will automatically flush stream    *  when println is called.    */   public static PrintWriter getWriter(Socket s)       throws IOException {     // Second argument of true means autoflush.     return(new PrintWriter(s.getOutputStream(), true));   } }