/** * The applet URLExampleApplet demonstrates how to read data from * a url over the Internet using classes from the packages java.net * and java.io. If the data from the url is of type text, then * the data is displayed. * The user can specify the url to load by typing the url * into an input box. An initial URL to load can also be specified * in an applet parameter with the name "URL". * * (Note that this program uses a thread, although threads are not * covered until after this example has been given.) */ import java.awt.*; // make graphical user interface classes available import java.awt.event.*; // make event-handling classes available import javax.swing.*; // make Swing GUI classes available import java.io.*; // make I/O classes such as InputStream available import java.net.*; // make networking classes such as URL available public class ReadURLApplet extends JApplet implements Runnable, ActionListener { private JTextArea textDisplay; // data loaded from url is displayed here private JTextField inputBox; // user enters name of url to be loaded here private JButton loadButton; // user clicks on this button to load the url private String urlName; // the name of the url to be loaded by the run() method. // Note: this url can be specified relative to the location // of the HTML document that contains the applet. private Thread loader; // thread for reading data from the url /** * Lays out the applet when it is first created. If there is an applet parameter * named "URL", then that parameter is read and loaded into the input box. * the user can then load the url by clicking on load. */ public void init() { JPanel content = new JPanel(); setContentPane(content); content.setBackground(Color.GRAY); textDisplay = new JTextArea(); textDisplay.setEditable(false); loadButton = new JButton("Load"); loadButton.addActionListener(this); String url = getParameter("URL"); if (url == null) inputBox = new JTextField(); else inputBox = new JTextField(url); inputBox.addActionListener(this); JPanel bottom = new JPanel(); bottom.setBackground(Color.GRAY); bottom.setLayout(new BorderLayout(3,3)); bottom.add(inputBox, BorderLayout.CENTER); bottom.add(loadButton, BorderLayout.EAST); bottom.add(new JLabel("Enter URL:"), BorderLayout.WEST); content.setLayout(new BorderLayout(3,3)); content.add(new JScrollPane(textDisplay), BorderLayout.CENTER); content.add(bottom, BorderLayout.SOUTH); content.setBorder(BorderFactory.createLineBorder(Color.GRAY, 3)); inputBox.requestFocus(); } /** * This method is called by actionPerformed() to load a url. * The interface elements are "turned off" so they can't be used while the * url is loading. Then a separate thread is started to do the loading. * The interface elements will be turned back on when that thread finishes * its execution. *

NOTE: I use a thread to do the loading so that the loading can take * place asynchronously, while other things are going on in the applet. * (For this simple example, there really isn't anything else to do, but * the technique is useful to know. Furthermore, it can take some time * to load the data -- or to discover an error -- and it's not a good idea * to block the graphical user interface completely while waiting.) * @param urlToLoad the url to be loaded, in text form. */ private void doLoad(String urlToLoad) { inputBox.setEditable(false); loadButton.setEnabled(false); textDisplay.setText("Connecting..."); urlName = urlToLoad; // set the urlName which will be used by the thread loader = new Thread(this); loader.start(); } /** * Responds when the user clicks on the load button or presses return in the input box. */ public void actionPerformed(ActionEvent evt) { String urlToLoad = inputBox.getText().trim(); if (urlToLoad.equals("")) { textDisplay.setText("No URL found in text-input box"); textDisplay.requestFocus(); } else doLoad(urlToLoad); } /** * Loads the data in the url specified by an instance variable named * urlName. The data is displayed in a TextArea named textDisplay. * Exception handling is used to detect and respond to errors that might * occur. This method is run in a separate thread to avoid blocking the * graphical user interface. */ public void run() { try { /* Create a URL object. This can throw a MalformedURLException. */ URL url = new URL(getDocumentBase(), urlName); /* Open a connection to the URL, and get an input stream for reading data from the URL. */ URLConnection connection = url.openConnection(); InputStream urlData = connection.getInputStream(); /* Check that the content is some type of text. */ String contentType = connection.getContentType(); if (contentType == null || contentType.startsWith("text") == false) throw new IOException("URL does not refer to a text file."); /* Copy lines from the input stream until 10000 characters have been copied or end-of-file is encountered. */ textDisplay.setText("Receiving Data..."); BufferedReader in; // For reading from the connection's input stream. in = new BufferedReader(new InputStreamReader(urlData)); StringBuffer chars; // Store chars here before adding to text area. chars = new StringBuffer(10000); while (true) { String line = in.readLine(); if (line == null) break; chars.append(line); chars.append('\n'); if ( chars.length() >= 10000) { chars.append("\n\nINPUT TRUNCATED TO " + chars.length() + " CHARACTERS\n"); break; } } in.close(); /* Finally, put the text that has been received into the text area. */ textDisplay.setText(chars.toString()); } catch (MalformedURLException e) { // Can be thrown when URL is created. textDisplay.setText( "\nERROR! Improper syntax given for the URL to be loaded."); } catch (SecurityException e) { // Can be thrown when the connection is created. textDisplay.setText("\nSECURITY ERROR! Can't access that URL."); } catch (IOException e) { // Can be thrown while data is being read. textDisplay.setText( "\nINPUT ERROR! Problem reading data from that URL:\n" + e.toString()); } finally { // This part is done, no matter what, before the thread ends. // Set up the user interface of the applet so the user can // enter another URL. loadButton.setEnabled(true); inputBox.setEditable(true); inputBox.selectAll(); inputBox.requestFocus(); } } // end of run() method } // end class ReadURLApplet