Url programming

URL Programming

Introduction
How to write a program that fetches
URL content

How to write a program that submits
GET or POST requests to a web server
or a servlet

Start simply

No programming involved!
Connect to server (telnet)
Send request (type in an http request)
Get input

Using TCP Sockets
Connect to port 80
Send HTTP request (GET, POST, HEAD)
Read HTTP response

URL Programming In Java

A URL is a name for a web resource (page, applet)
http://www.cs.wmich.edu/index.html
Java URL programming allows Java programs to access
web resources by sending protocol requests and
receiving protocol responses.
Related Classes:
URL class
URLConnection class
HttpURLConnection class
URLEncoder class

URL Class
Represents a Uniform Resource Locator
scheme (protocol)
hostname
port
path
query string

Parsing a URL

You can use a URL object as a parser:

URL u = new URL(“http://www.cs.wmich.edu/index.html”);

System.out.println(“Proto:” + u.getProtocol());

System.out.println(“File:” + u.getFile());

Retrieving URL contents

There are a number of ways to do this:

Object getContent();

InputStream openStream();

URLConnection openConnection();

Getting Header Information

There are methods that return information
extracted from response headers:
String getContentType();
String getContentLength();
long getLastModified();

URLConnection Class

Represents the connection (not the URL
itself).
More control than URL
can write to the connection (send POST data).
can set request headers.
HttpURLConnection is a subclass of
URLConnection that is specific to HTTP

Example: Reading from a URL
// -- open connection
URL url = new URL(“http://www.google.com/index.html”);

HttpURLConnection conn = (HttpURLConnection)url.openConnection();

conn.connect();

// -- read in html ----------------
BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));

String line;
String page = new String("");
while (null != ((line = in.readLine()))) {
page += line;
}

in.close();
System.out.println(page);

Points to note

An HttpURLConnection represents a
connection to a particular page on a
server, not just to the server.

An HttpURLConnection object is NOT
made by the usual use of new(). Instead it
is created by the use of the Url’s
openConnection() method

More points to note
the connect method (of
HttpURLConnection) not only opens the
connection but also makes the GET
request!

the code for reading in the HTML from the
connection is identical to that for reading in
a file.

Faking GET Form Submission

Not all webpages are produced in
response to a simple GET request
Some are the output of a program
which may require parameters
passed using the GET method (via the
URL), e.g.
http://www.cs.wmich.edu/servlet/convert?id=57&amount=10&units=pint

Faking POST Form Submission

The POST request sends parameters
(data) in the HTTP request body
Any data to be sent must be encoded as a
string of key-value pairs:
id=57&amount=10&units=pint

URLEncoder Class

Used to encode GET and POST parameters.

Example:

String content = "action=“ + URLEncoder.encode("100”);

content += "&zipcode=“ + URLEncoder.encode(“49008");

content += "&name=“ + URLEncoder.encode(“Bill Somebody");

content += "&passwd=“ + URLEncoder.encode(“cisco");

Making GET Form Submissions

Step 1: Encode GET parameters
Step 2: Make an HttpURLConnection object in the usual way:
URL url = new URL("http://www.cs.wmich.edu/servlet/converter“ + “?” + contents);
Step 3:
Step 4:
conn.connect();
Step 5: You can read the reply as explained before.

Making POST Form Submissions

Step 1: Encode POST parameters

Step 2: Make an HttpURLConnection object in the usual way:
URL url = new URL("http://www.cs.wmich.edu/servlet/converter“);

Step 3:

Making POST Form Submissions (Contd.)

Step 4: Tell it that you want to use the POST method
conn.setRequestMethod("POST");
conn.setDoOutput(true);

Step 5: Build a Printer writer to send things out via the connection
and send the data.
PrintWriter out = new PrintWriter(conn.getOutputStream());
out.println(content);
out.close();

Step 6: You can read the reply as explained before.

Url programming

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