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Building a Web Browser

Building a Web Browser. CS1316: Representing Structure and Behavior. Story. How we access the Web from Java URL is an object Open a connection, then a stream. Basically, treat it like a file. Creating a Web browser in Java JEditorPane understands HTML (and Text and RTF)

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Building a Web Browser

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  1. Building a Web Browser CS1316: Representing Structure and Behavior

  2. Story • How we access the Web from Java • URL is an object • Open a connection, then a stream. • Basically, treat it like a file. • Creating a Web browser in Java • JEditorPane understands HTML (and Text and RTF) • Have to deal with hyperlinks as an event

  3. URL is an object • URL objects represent (surprise!) URLs (Uniform Resource Locators). • They can be queried to get all kinds of information about the URL, including a connection to the object at the URL.

  4. Getting the Content from a URL • To get the content from a URL: • You first create a connection which allows you to access the network. • You then create the stream access for that URL—the same (hard) way we have before. URL url = new URL(“http://www.cnn.com”); URLConnection con = url.openConnection(); BufferedReader stream = new BufferedReader(new InputStreamReader(con.getInputStream()));

  5. But there are exceptions • Accesses to the network can (of course!) lead to network errors, so we have to deal with that possibility. /** * Open with a URL **/ public WebPageReader(String s){ // Create the URL and the connection to it try { url = new URL(s); con = url.openConnection(); stream = new BufferedReader(new InputStreamReader(con.getInputStream())); } catch (Exception e) { System.out.println("An error opening the URL occurred."); System.out.println(e.getMessage());} }

  6. Using the WebPageReader > WebPageReader reader = new WebPageReader("http://www.yahoo.com") > reader.getType() "text/html" > reader.readyToRead() true > reader.nextLine() "<html><head>" > reader.nextLine() "<script language=javascript>" > reader.nextLine() "var now=new Date,t1=0,t2=0,t3=0,t4=0,t5=0,t6=0,hp=0,cc='',ylp='';t1=now.getTime();" > reader.nextLine() "</script>" > reader.nextLine() "<title>Yahoo!</title>" > reader.nextLine() "<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l r (cz 1 lz 1 nz 1 oz 1 vz 1) gen true for "http://www.yahoo.com" r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l r (n 0 s 0 v 0 l 0) gen true for "http://www.yahoo.com" r (n 0 s 0 v 0 l 0))'>" Each time we call reader.nextLine(), we get the next line from the object

  7. Creating the WebPageReader /** * WebPageReader class * Given a URL, can return information about that page. **/ import java.net.*; import java.io.*; import java.util.*; public class WebPageReader { //// Fields private URL url; private URLConnection con; private BufferedReader stream;

  8. Constructor /** * Open with a URL **/ public WebPageReader(String s){ // Create the URL and the connection to it try { url = new URL(s); con = url.openConnection(); stream = new BufferedReader(new InputStreamReader(con.getInputStream())); } catch (Exception e) { System.out.println("An error opening the URL occurred."); System.out.println(e.getMessage());} }

  9. Checking if the connection is working /** * A WebPageReader is ready to read if the stream is ready **/ public boolean readyToRead(){ try {return stream.ready();} catch (Exception e) {System.out.println("I/O error occurred."); System.out.println(e.getMessage()); return false;} }

  10. What’s out there? /** * The type of the material at the other end of the URL is * the contentType from the URLConnection **/ public String getType(){return con.getContentType();} // “text/html” is the MIME type for normal Web pages

  11. Reading from that content /** * Next line is the next line from the material at the * other end of the URL. We read it like a file. * There is more material there as long as readyToRead() returns * true. We may also read a null when it's done. **/ public String nextLine(){ try {return stream.readLine();} catch (Exception e) {System.out.println("I/O error occurred."); System.out.println(e.getMessage()); return null;} }

  12. Building a Web Browser • Building a web browser in Java is very easy. • Swing component JEditorPane understands HTML. • And plain text • And RTF (Rich Text Format—format that Word and other word processors can generate) • Does not understand JavaScript, CSS, etc. • Just plain HTML

  13. > SimpleBrowser sb = new SimpleBrowser()

  14. SimpleBrowser /** * A Simple Web Browser * Uses a JEditorPane() which knows how to interpret HTML (and RTF and Text) **/ // Lots of imports! import java.awt.*; import java.awt.event.*; import java.net.*; import java.io.*; import javax.swing.*; import javax.swing.event.*; import javax.swing.text.html.HTMLFrameHyperlinkEvent; import javax.swing.text.html.HTMLDocument; We need all of these for Swing, networking, I/O (Input/Output exceptions), and HTML processing.

  15. SimpleBrowser public class SimpleBrowser extends JFrame { /// Fields /** A field for the URL to be entered **/ private JTextField urlField; private JEditorPane webpane;

  16. Describing our UI: Assembled in Constructor • Top pane deals with URL specification: • Label for entering URL • Field for entering URL • Bottom part is the JEditorPane

  17. JEditorPane’s are very flexible From JDK JavaDoc

  18. Constructor: Building the UI /*** * Most of the action is in the constructor. **/ public SimpleBrowser(){ super("Simple Browser"); // Make a panel with a label and the URL field JPanel panel1=new JPanel(); this.getContentPane().add(panel1,BorderLayout.NORTH); JLabel label1= new JLabel("URL:"); panel1.add(label1,BorderLayout.EAST); urlField = new JTextField("http://www.cnn.com");

  19. How we load URLs (upon enter key) urlField.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { String urlString = e.getActionCommand(); try { webpane.setPage(urlString); urlField.setText(urlString); } catch (Exception e2) { System.out.println("I/O Error -- maybe bad URL?"); System.out.println(e2.getMessage());} } }); panel1.add(urlField,BorderLayout.CENTER); event.getActionCommand() returns the string from the field—the one with the URL in it. JEditorPanes can read directly from URL! Simply setPage(String url).

  20. Setting up the JEditorPane // Second part of the browser is the viewable pane webpane = new JEditorPane(); webpane.setEditable(false);

  21. Dealing with HyperLink // Make hyperlinks work (from 1.4JDK docs) webpane.addHyperlinkListener( new HyperlinkListener() { public void hyperlinkUpdate(HyperlinkEvent e) { if (e.getEventType() == HyperlinkEvent.EventType.ACTIVATED) { JEditorPane pane = (JEditorPane) e.getSource(); if (e instanceof HTMLFrameHyperlinkEvent) { HTMLFrameHyperlinkEvent evt = (HTMLFrameHyperlinkEvent)e; HTMLDocument doc = (HTMLDocument)pane.getDocument(); doc.processHTMLFrameHyperlinkEvent(evt); } else { try { pane.setPage(e.getURL()); } catch (Throwable t) { t.printStackTrace(); } } } } }); This is copy-pasted from JDK documentation. Key observation: Dealing with a new kind of event and listener!

  22. JEditorPane gets a scrollpane this.getContentPane().add(new JScrollPane(webpane), BorderLayout.CENTER); this.pack(); this.setVisible(true); } JScrollPanes contain something that is scrolled—here, a JEditorPane. We put the JScrollPane in the Center so that it gets emphasized in the BorderLayout renderer.

  23. How we load pages public void loadPage(String urlString){ try { webpane.setPage(urlString); urlField.setText(urlString); } catch (Exception e) { System.out.println("I/O Error -- maybe bad URL?"); System.out.println(e.getMessage());} } Dealing with exceptions in all these cases is required. The compiler flags these as errors.

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