Url programming
- 1. 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
- 2. 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
- 3. 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());
- 4. 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();
- 5. 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);
- 6. 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.
- 7. 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
- 8. 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 String content = "action=“ + URLEncoder.encode("100”); content += "&zipcode=“ + URLEncoder.encode(“49008"); content += "&name=“ + URLEncoder.encode(“Bill Somebody"); content += "&passwd=“ + URLEncoder.encode(“cisco"); Step 2: Make an HttpURLConnection object in the usual way: URL url = new URL("http://www.cs.wmich.edu/servlet/converter“ + “?” + contents); Step 3: HttpURLConnection conn = (HttpURLConnection)url.openConnection(); Step 4: conn.connect(); Step 5: You can read the reply as explained before.
- 9. Making POST Form Submissions Step 1: Encode POST parameters String content = "action=“ + URLEncoder.encode("100”); content += "&zipcode=“ + URLEncoder.encode(“49008"); content += "&name=“ + URLEncoder.encode(“Bill Somebody"); content += "&passwd=“ + URLEncoder.encode(“cisco"); Step 2: Make an HttpURLConnection object in the usual way: URL url = new URL("http://www.cs.wmich.edu/servlet/converter“); Step 3: HttpURLConnection conn = (HttpURLConnection)url.openConnection(); 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.