Building a Web Browser CS1316: Representing Structure and Behavior
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Building a Web Browser
CS1316: Representing
Structure and Behavior
Story
How we access the Web from Java
Creating a Web browser in Java
• URL is an object
• Open a connection, then a stream.
• Basically, treat it like a file.
• 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
Each time we call
reader.nextLine(), we get
the next line from the object
> WebPageReader reader = new
WebPageReader("http://www.yahoo.co
m")
> 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))'>"
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)
**/
We need all of these
// Lots of imports!
for Swing, networking,
import java.awt.*;
I/O (Input/Output
import java.awt.event.*;
exceptions), and
import java.net.*;
HTML processing.
import java.io.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.html.HTMLFrameHyperlinkEvent;
import javax.swing.text.html.HTMLDocument;
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:
Bottom part is the JEditorPane
• Label for entering URL
• Field for entering URL
JEditorPane’s are very flexible
From 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() {
event.getActionCommand()
public void actionPerformed(ActionEvent e) returns the string from the
{
field—the one with the URL
String urlString = e.getActionCommand(); in it.
try {
JEditorPanes can read
webpane.setPage(urlString);
directly from URL! Simply
urlField.setText(urlString);
setPage(String url).
} catch (Exception e2) {
System.out.println("I/O Error -- maybe bad URL?");
System.out.println(e2.getMessage());}
}
});
panel1.add(urlField,BorderLayout.CENTER);
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());
This is copy-pasted from JDK
} catch (Throwable t) {
documentation. Key observation:
t.printStackTrace();
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.
