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Building a Web Browser CS1316: Representing Structure and Behavior
| Date post: | 24-Dec-2015 |
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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)
• Have to deal with hyperlinks as an event
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.
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()));
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());} }
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
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;
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());} }
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;} }
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
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;} }
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
> SimpleBrowser sb = new SimpleBrowser()
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.
SimpleBrowser
public class SimpleBrowser extends JFrame {
/// Fields
/** A field for the URL to be entered **/
private JTextField urlField;
private JEditorPane webpane;
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
JEditorPane’s are very flexibleFrom JDK JavaDoc
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");
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).
Setting up the JEditorPane
// Second part of the browser is the viewable pane
webpane = new JEditorPane();
webpane.setEditable(false);
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!
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.
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.
