Networks Programming Engr. Abdul-Rahman Mahmood MS, PMP, MCP, QMR(ISO9001:2000)

Transcription

1 Networks Programming Engr. Abdul-Rahman Mahmood MS, PMP, MCP, QMR(ISO9001:2000) alphapeeler.sf.net/pubkeys/pkey.htm pk.linkedin.com/in/armahmood abdulmahmood-sss alphasecure mahmood_cubix VC++, VB, ASP

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3 Communicating with Server- Side Programs Through GET The URL class makes it easy for Java applets and applications to communicate with serverside programs such as CGIs, servlets, PHP pages, and others that use the GET method. Server-side programs that use the POST method require the URLConnection class (will be discussed later)

4 HTML form input The method the form uses should be the value of the METHOD attribute of the FORM element. URL that precedes the query string is given by the value of the ACTION attribute of the FORM element. Finally, the names in the name-value pairs are simply the values of the NAME attributes of the INPUT elements. Example: The program that processes the form is accessed via the URL It has four separate name-value pairs, three of which have default values.

5 <form name="search" action=" method="get"> <input name="q" /> <input type="hidden" value="cafeconleche.org" name="domains" /> <input type="hidden" name="sitesearch" value="cafeconleche.org" /> <input type="hidden" name="sitesearch2" value="cafeconleche.org" /> <br /> <input type="image" height="22" width="55 src="images/search_blue.gif" alt="search" border="0 name="search-image" /> </form>

6 The UI for the Open Directory

7 The UI for the Open Directory <form class="center mb1em" action="search" method="get"> <input style="*vertical-align:middle;" size="45" name="q" value="" class="qn"> <input style="*vertical-align:middle; *padding-top:1px;" value="search" class="btn" type="submit"> <a href="search?type=advanced"><span class="advn">advanced</span></a> </form> There are only two input fields in this form: the Submit button and a text field named q. Thus, to submit a search request to the Open Directory, you just need to append q=searchterm to

8 Open Directory search program

9 Accessing Password-Protected Sites The Authenticator Class The java.net package : Authenticator class you can use to provide a username and password for sites that protect themselves using HTTP authentication: public abstract class Authenticator extends Object Since Authenticator is an abstract class, you must subclass it. To make the URL class use the subclass, install it as the default authenticator by passing it to the static Authenticator.setDefault() method: public static void setdefault(authenticator a) For example, if you ve written an Authenticator subclass named DialogAuthenticator, you d install it like this: Authenticator.setDefault(new DialogAuthenticator());

10 The Authenticator Class The HTTP server provides the prompt. It s typically the name of the realm for which authentication is required. (Some large web servers such as have multiple realms, each of which requires different usernames and passwords.) Untrusted applets are not allowed to ask the user for a name and password. Trusted applets can do so, but only if they possess requestpasswordauthentication getpasswordauthentication(), collect username /password from the user or some other source protected PasswordAuthentication getpasswordauthentication()

11 The Authenticator Class Usernames and passwords are cached within the same virtual machine session. Once you set the correct password for a realm, you shouldn t be asked for it again unless you ve explicitly deleted the password by zeroing out the char array that contains it. You can get more details about the request by invoking any of these methods inherited from the Authenticator superclass: protected final InetAddress getrequestingsite() protected final int getrequestingport() protected final String getrequestingprotocol() protected final String getrequestingprompt() protected final String getrequestingscheme() protected final String getrequestinghost() protected final String getrequestingurl() protected Authenticator.RequestorType getrequestortype()

12 The Authenticator Class requestpasswordauthentication() or return null if that information is not available. (If the port isn t available, getrequestingport() returns -1.)

13 The PasswordAuthentication Class public PasswordAuthentication(String username, char[] password) Each is accessed via a getter method: public String getusername() public char[] getpassword()

14 JPasswordField Class : Swing public class JPasswordField extends JTextField JPasswordField also stores the passwords as a char array so that when you re done with the password you can overwrite it with zeros. It provides the getpassword() method to return this: public char[] getpassword() Otherwise, you mostly use the methods it inherits from the JTextField superclass.

15 download password-protected web pages

16 HTTP HTTP) is a standard that defines how a web client talks to a server and how data is transferred from the server back to the client. HTTP is application-level protocol & uses (TCP) as a transport mechanism. Most prolific network application protocol in the short history of the Internet.

17 HTTP request/response cycle For each request (client - server), there is a sequence of 4 steps: Client opens a TCP connection to server on port 80 (default). Client sends a message to server requesting the resource at a specified path. Request includes a header, and optionally a blank line followed by data for the request. Server sends a response to client which begins with a response code, a header full of metadata, a blank line, and the requested document or an error message. Server closes the connection.

18 HTTP request/response cycle

19 request / response format Each request and response has the same basic form: a header line, an HTTP header containing metadata, a blank line, and then a message body. A typical client request looks something like this: GET /index.html HTTP/1.1 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:20.0) Gecko/ Firefox/20.0 Host: en.wikipedia.org Connection: keep-alive Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip, deflate Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 GET requests like this one do not contain a message body, so the request ends with a blank line.

20 HTTP request The first line is called the request line, and includes a method, a path to a resource, and the version of HTTP. Lines in the header are terminated by a carriage-return linefeed pair. User-Agent, tells server what browser is being used. The following line says that the request comes from version 2.4 of the Lynx browser: User-Agent: Lynx/2.4 libwww/2.1.4 oldest first-generation browsers also include a Host field specifying the server s name, which allows web servers to distinguish between different named hosts served from the same IP address: Host:

21 HTTP request Keyword Accept, tells the server the types of data the client can handle (though servers often ignore this). For example, the following line says that the client can handle four MIME media types, corresponding to HTML documents, plain text, and JPEG and GIF images: Accept: text/html, text/plain, image/gif, image/jpeg MIME (Multi-Purpose Internet Mail Extensions) MIME types are classified at two levels: a type and a subtype. Finally, the request is terminated with a blank line that is, two carriage return/linefeed pairs, \r\n\r\n.

22 MIME types Eight top-level types have been defined: text/* for human-readable words image/* for pictures model/* for 3D models such as VRML files audio/* for sound video/* for moving pictures, possibly including sound application/* for binary data message/* for protocol-specific envelopes such as messages and HTTP responses multipart/* for containers of multiple documents and resources

23 MIME types RFC 1341 The Content-Type field for multipart entities requires one parameter, "boundary", which is used to specify the encapsulation boundary. Content-Type: multipart/mixed; boundary=gc0p4jq0m2yt08ju534c0p This indicates that the entity consists of several parts, and that the parts are each preceded by the line --gc0p4jq0m2yt08ju534c0p Encapsulation boundaries must > 70 characters. Current list of registered MIME types is available from assignments/media-types/ In addition, nonstandard custom types and subtypes can be freely defined as long as they begin with x-. For example, Flash files are commonly assigned the type application/x-shockwave-flash.

24 MIME multipart example From: Nathaniel Borenstein To: Ned Freed Subject: Sample message MIME-Version: 1.0 Content-type: multipart/mixed; boundary="simple boundary" This is the preamble. It is to be ignored, though it is a handy place for mail composers to include an explanatory note to non-mime compliant readers. --simple boundary This is implicitly typed plain ASCII text. It does NOT end with a linebreak. --simple boundary Content-type: text/plain; charset=us-ascii This is explicitly typed plain ASCII text. It DOES end with a linebreak. --simple boundary-- This is the epilogue. It is also to be ignored.

25 HTTP response Server sees blank line; it sends response over the same connection. Response: a status line, a header (name: value syntax), a blank line, and the requested resource: HTTP/ OK Date: Sun, 21 Apr :12:46 GMT Server: Apache Connection: close Content-Type: text/html; charset=iso Content-length: 115 <html> <head> <title> A Sample HTML file </title> </head> <body> The rest of the document goes here </body> </html>

26 HTTP response The first line indicates the protocol the server is using (HTTP/1.1), followed by a response code. 200 OK is the most common response code. 1XX Informational 100 continue 101 Switching Protocols 2XX Successful Request succeeded. 200 OK 201 Created 202 Accepted 3XX Redirection Relocation 301 Moved Permanently 301 Moved Temporarily 305 Use Proxy 4XX Client error. 400 Bad Request 401 Unauthorized 402 Payment Required 403 Forbidden 404 Not Found 405 Method Not Allowed 406 Not Acceptable 407 Proxy Authentication Required 5XX Server error. 500 Internal Server Error 502 Bad Gateway 503 Service Unavailable

27 4 main HTTP Methods GET retrieves a representation of a resource. side effect free can be repeated without concern if it fails. (idempotent) its output is often cached, (though it can be controlled with the right headers) GET requests can be bookmarked. PUT uploads representation of a resource to server at a known URL. side effect free can be repeated without concern if it fails. (idempotent)

28 4 main HTTP Methods DELETE removes a resource from a specified URL. side effect free can be repeated without concern if it fails. (idempotent) POST POST is the most general method. It too uploads a representation of a resource to a server at a known URL. POST should be used for unsafe operations that should not be repeated, such as making a purchase.

29 When to use GET or POST GET is intended for noncommital actions, like browsing a static web page, adding an item to a shopping cart POST : for actions that commit to something, like placing the order. Misconception: reason for preferring POST over GET is when forms require large amounts of input. Today all major browsers are good up to URL lengths of at least 2,000 characters. If you have more form data to submit than that, you may indeed need to support POST;

30 Other HTTP methods HEAD: which acts like a GET except it only returns the header for the resource, not the actual data. Used to check modification date of a file To see that a copy stored in local cache is still valid.

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32 Opening URLConnections A program that uses the URLConnection class directly follows this basic sequence of steps: 1. Construct a URL object. 2. Invoke the URL object s openconnection() method to retrieve a URLConnection object for that URL. 3. Configure the URLConnection. 4. Read the header fields. 5. Get an input stream and read data. 6. Get an output stream and write data. 7. Close the connection.

33 Reading Data from a Server 1. Construct a URL object. 2. Invoke the URL object s openconnection() method to retrieve a URLConnection object for that URL. 3. Invoke the URLConnection s getinputstream() method. 4. Read from the input stream using the usual stream API. The differences between URL and URLConnection : URLConnection provides access to the HTTP header. URLConnection can configure the request parameters sent to the server. URLConnection can write data to the server as well as read data from the server.

34 Download web page with a URLConnection

35 Reading the Header Retrieving Specific Header Fields The first six methods request specific, particularly common fields from the header. These are: Content-type Content-length Content-encoding Date Last-modified Expires

36 public String getcontenttype() The getcontenttype() method returns the MIME media type of the response body. It relies on the web server to send a valid content type. It throws no exceptions and returns null if the content type isn t available. Examples: text/html, text/plain, image/gif, application/xml, image/jpeg. If the content type is some form of text, this header may also contain a character set part: Content-type: text/html; charset=utf-8 Content-Type: application/xml; charset=iso jp

37 Download a web page with the correct character set

38 public int getcontentlength() Java 7 adds a getcontentlengthlong() method that works just like getcontentlength() except that it returns a long instead of an int and thus can handle much larger resources: public int getcontentlengthlong() // Java 7

39 Downloading a binary file from a website and saving it to disk

40 public String getcontentencoding() returns a String that tells you how the content is encoded. With HTTP servers, content is unencoded, and method returns null It throws no exceptions. commonly used content encoding is x-gzip, which can be straightforwardly decoded using a java.util.zip.gzipinputstream Character encoding vs Content encoding : Character encoding is determined by the Content-type header; specifies how characters are encoded in bytes. Content encoding specifies how the bytes are encoded in other bytes.

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42 public long getdate() returns a long that tells when document was sent, in milliseconds since 12 AM, January 1, [Linux epoch] You can convert it to a java.util.date. For example: Date documentsent = new Date(uc.getDate());

43 Ex: Return the header

44 Retrieving Arbitrary Header Fields Use these methods to get header fields that Java s designers did not plan for. If the requested header is found, it is returned. Otherwise, the method returns null. public String getheaderfield(string name) The getheaderfield() method returns the value of a named header field. The name of the header is not case sensitive String contenttype = uc.getheaderfield("content-type"); String contentencoding = uc.getheaderfield("content-encoding")); String data = uc.getheaderfield("date"); String expires = uc.getheaderfield("expires"); String contentlength = uc.getheaderfield("content-length"); public String getheaderfieldkey(int n) returns the key (i.e., the field name) of the nth header field public String getheaderfield(int n) returns the value of the nth header field. public int getheaderfieldint(string name, int default) This method retrieves the value of the header field name int contentlength = uc.getheaderfieldint("content-length", -1); returns 1 if the Content-length header isn t present.

45 Print the entire HTTP header

46 Authenticator class