29.3. Popup Menus

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25.7. Retrieving Files from Web Servers

You developed client/server applications in the preceding sections. Java allows you to develop clients that retrieve files on a remote host through a Web server. In this case, you don't have to create a custom server program. The Web server can be used to send the files, as shown in Figure 25.12.

Figure 25.12. The applet client or the application client retrieves files from a Web server.

To retrieve a file, first create a URL object for the file. The java.net.URL class was introduced in §16.9, "Locating Resource Using the URL Class." You can create a URL object using the following constructor:

   public   URL(String spec)   throws   MalformedURLException 

For example, the statement given below creates a URL object for http://www.cs.armstrong.edu/liang/index.html:

   try   { URL url =   new   URL(   "http://www.cs.armstrong.edu/liang/index.html"   ); }   catch   (MalformedURLException ex) { } 

A MalformedURLException is thrown if the URL string has a syntax error. For example, the URL string "http://www.cs.armstrong.edu/liang/index.html" would cause a Malformed-URLException runtime error because two slashes ( // ) are required after the colon ( : ).

You can then use the openStream() method defined in the URL class to open an input stream to the file's URL.

 InputStream inputStream = url.openStream(); 

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Now you can read the data from the input stream. Listing 25.10 gives an example that demonstrates how to retrieve a file from a Web server. The program can run as an application or an applet. The user interface includes a text field in which to enter the URL of the filename, a text area in which to show the file, and a button that can be used to submit an action. A label is added at the bottom of the applet to indicate the status, such as File loaded successfully or Network connection problem . A sample run of the program is shown in Figure 25.13.

Figure 25.13. The program displays the contents of a specified file on the Web server.

Listing 25.10. ViewRemoteFile.java

(This item is displayed on pages 835 - 836 in the print version)

 1   import   java.awt.*; 2   import   java.awt.event.*; 3   import   java.io.*; 4   import   java.net.*; 5   import   javax.swing.*; 6 7   public class   ViewRemoteFile   extends   JApplet { 8  // Button to view the file  9   private   JButton jbtView =   new   JButton(   "View"   ); 10 11  // Text field to receive file name  12   private   JTextField jtfURL =   new   JTextField(   12   ); 13 14  // Text area to store file  15   private   JTextArea jtaFile =   new   JTextArea(); 16 17  // Label to display status  18   private   JLabel jlblStatus =   new   JLabel(); 19 20  /** Initialize the applet */  21   public void   init() { 22  // Create a panel to hold a label, a text field, and a button  23 JPanel p1 =   new   JPanel(); 24 p1.setLayout(   new   BorderLayout()); 25 p1.add(   new   JLabel(   "Filename"   ), BorderLayout.WEST); 26 p1.add(jtfURL, BorderLayout.CENTER); 27 p1.add(jbtView, BorderLayout.EAST); 28 29  // Place text area and panel p to the applet  30 setLayout(   new   BorderLayout()); 31 add(   new   JScrollPane(jtaFile), BorderLayout.CENTER); 32 add(p1, BorderLayout.NORTH); 33 add(jlblStatus, BorderLayout.SOUTH); 34 

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 35  // Register listener to handle the "View" button  36  jbtView.addActionListener(   new   ActionListener() {  37   public void   actionPerformed(ActionEvent e) { 38 showFile(); 39 } 40 }); 41 } 42 43   private void   showFile() { 44  // Declare buffered stream for reading text for the URL  45 BufferedReader infile =   null   ; 46 URL url =   null   ; 47 48   try   { 49  // Obtain URL from the text field  50  url =   new   URL(jtfURL.getText().trim());  51 52  // Create a buffered stream  53  InputStream is = url.openStream();  54 infile =   new   BufferedReader(    new   InputStreamReader(is)  ); 55 56  // Get file name from the text field  57 String inLine; 58 59  // Read a line and append the line to the text area  60   while   ((inLine = infile.readLine()) !=   null   ) { 61 jtaFile.append(inLine +   '\n'   ); 62 } 63 64 jlblStatus.setText(   "File loaded successfully"   ); 65 } 66   catch   (FileNotFoundException e) { 67 jlblStatus.setText(   "URL "   + url +   " not found."   ); 68 } 69   catch   (IOException e) { 70 jlblStatus.setText(e.getMessage()); 71 } 72   finally   { 73   try   { 74   if   (infile !=   null   ) infile.close(); 75 } 76   catch   (IOException ex) {} 77 } 78 } 79 } 

Line 54 new URL(jtfURL.getText().trim()) creates a URL for the filename entered from the text field. Line 57 url.openStream() creates an InputStream from the URL. After the input stream is established, reading data from the remote file is just like reading data locally. A BufferedReader object is created from the input stream (line 58). The text from the file is displayed in the text area (line 65).