Java Servelets

1. Java Servelets

2. A servlet is a server side software component, written in java that dynamically extends the functionality of a server. • Unlike applets servlets don’t display GUI. • It works behind the scene on server and results of servlet’s processing are returned to the client.

3. What Is a Servlet? • A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed via a request-response programming model. • Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by Web servers. • For such applications, Java Servlet technology defines HTTP-specific servlet classes.

4. A servlet is an instance of a class that implements the java.servlet.Servlet interface. • The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. • All servlets must implement the Servlet interface, which defines life-cycle methods. • When implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API. • The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services.

5. javax.servlet.GenericServletSignature: public abstract class GenericServlet extends java.lang.Object implements Servlet, ServletConfig, java.io.Serializable • GenericServlet defines a generic, protocol-independent servlet. • GenericServlet gives a blueprint and makes writing servlet easier. • GenericServlet provides simple versions of the lifecycle methods init and destroy and of the methods in the ServletConfig interface. • GenericServlet implements the log method, declared in the ServletContext interface. • To write a generic servlet, it is sufficient to override the abstract service method.

6. javax.servlet.http.HttpServletSignature: public abstract class HttpServlet extends GenericServlet implements java.io.Serializable • HttpServlet defines a HTTP protocol specific servlet. • HttpServlet gives a blueprint for Http servlet and makes writing them easier. • HttpServlet extends the GenericServlet and hence inherits the properties GenericServlet.

7. Advantages • Capable of running in same process space a the web server. • Compiled. • Crash resistant • Cross platform • Durable • Dynamically loaded across the network • Extensible • Multithreaded • Protocol independent • Written in java

8. Functions • Dynamically build and return an HTML file based on client request. • Process user input of HTML Form and return appropriate response. • Facilitate communication among many clients. • Interact with server resources like database, other application. • Multiplayer games.

9. HTTP Request Structure • It is a stateless protocol. • Server does not have the overhead of tracking client connections. • HTTP transactions are either a request or response. • Servlet can overcome the stateless nature of HTTP by tracking client state using session information stored in URL, hidden fields or cookies.

10. 3 parts of HTTP transactions • A single request or response line • <HTTP Method>/<document address>HTTP/ <Version No> e.g. • GET /index.html HTTP/1.1 • Response line contains an HTTP status code. HTTP/1.1 200 OK Date:……………………….GMT Server: Last-modified: Content-type: text/html Content-length:….bytes

11. 501 error is generated by the server when a servlet is sent an HTTP request that it does not handle. E.g. if you write your servlet to handle only GET requests, but it receives a POST request, a 501 status will return.

12. HTTP headers • A set of fields used to exchange information between client and server. • Client uses the headers to tell the server about its configuration and the document types it’ll accept. • Server uses the header to return information about the requested document. • HTTP Body • Optionally used by client to send any additional information. • The server uses the body to return the requested document.

13. HTTP Methods • GET is used to request information from server. • POST is used to send data to the server. • GET can also be used to pass information in form of query string in URL and POST can be used for request. • Information sent as GET is visible to the client and append at URL. • POST sends data directly after the header, in body of message and doesn’t append at URL.

14. HEAD • If client wants information about a document but does not want the document to be returned. • PUT • Requests to server to store the body at a specified URL • DELETE • Requests the removal of data at URL • TRACE • Used for debugging. The HTTP body is simply returned by the server.

15. Servlet Environement • The web server provides support for servlets with extensions called servlet containers. • Servlet Containers Functions: • Network services over which request / response are sent. • Registers the servlet against one or more URL • Manage servlet life cycle • Decodes MIME (Multipurpose Internet Mail Extensions) requests • Constructs MIME response • Support HTTP protocol • Enforce security restrictions on environment.

16. Advantage of Servlets Over CGI • Java servlets are more efficient, easier to use, more powerful, more portable, and cheaper than traditional CGI and than many alternative CGI-like technologies. (More importantly, servlet developers get paid more than Perl programmers :-). • Efficient. • With traditional CGI, a new process is started for each HTTP request. • If the CGI program does a relatively fast operation, the overhead of starting the process can dominate the execution time. • With servlets, the Java Virtual Machine stays up, and each request is handled by a lightweight Java thread, not a heavyweight operating system process. • Similarly, in traditional CGI, if there are N simultaneous request to the same CGI program, then the code for the CGI program is loaded into memory N times. • With servlets, however, there are N threads but only a single copy of the servlet class. Servlets also have more alternatives than do regular CGI programs for optimizations such as caching previous computations, keeping database connections open, and the like. • Convenient. • You already know Java. • Why learn Perl too? Besides the convenience of being able to use a familiar language, servlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting HTTP headers, handling cookies, tracking sessions, and many other such utilities.

17. Powerful. • Java servlets let you easily do several things that are difficult or impossible with regular CGI. • For one thing, servlets can talk directly to the Web server (regular CGI programs can't). • This simplifies operations that need to look up images and other data stored in standard places. • Servlets can also share data among each other, making useful things like database connection pools easy to implement. • They can also maintain information from request to request, simplifying things like session tracking and caching of previous computations. • Portable. • Servlets are written in Java and follow a well-standardized API. • Consequently, servlets written for, say I-Planet Enterprise Server can run virtually unchanged on Apache, Microsoft IIS, or WebStar. • Servlets are supported directly or via a plugin on almost every major Web server. • Inexpensive. • There are a number of free or very inexpensive Web servers available that are good for "personal" use or low-volume Web sites. • However, with the major exception of Apache, which is free, most commercial-quality Web servers are relatively expensive. • Nevertheless, once you have a Web server, no matter the cost of that server, adding servlet support to it (if it doesn't come preconfigured to support servlets) is generally free or cheap

18. Servlet API • It doesn’t run as an application rather it is loaded in memory and as instance is created. • When a servlet instance is created its init() is called. • Servlets are required to respond to new connections to the server. • When a new connection is detected, a call is made to the service() of the servlet. • service() takes two parameters defined by interface type called ServletRequest and ServletResponse. • Servlet class is abstract because service() is defined not implemented; so to implement a servlet, it is necessary to override this method.

19. Servlet class is not protocol specific. • A subclass of Servlet class, the HttpServlet class is provided to handle http protocol. • Two interfaces are defined for use with HttpServlet class • HttpServletRequest • HttpServletResponse • These extend the ServletRequest & ServletResponse.

20. Life Cycle of Servlet • Servlet life cycle is defined by javax.servlet.Servlet inteface. • All servlets must implement javax.servlet.Servlet interface to run in a servlet engine.

21. Servlet Class Garbage Collection Server no longer has a reference to the object Instantiation & loading Servlet engine can instantiate more than one servlet instance Destruction destroy() Initialization init(ServletConfig conf) Ready service() A service() executes for each servlet instance

22. Servlet • init() • destroy() • getServletConfig() • getServletInfo() • service()

23. GenericServlet • init() • distroy() • log() • getServletConfig() • getServletContext() • getInitParameter() • getInitParameterNames() • getServletInfo() • getServletName() • service()

24. HTTPServlet • doGet() • doPost() • doHead() • doDelete() • doOptions() • doPut() • doTrace() • getLastModified() • service()

25. Creating servlets • protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException • Standard HTTP requests like Get and Post are supported by doGet() and doPost().

26. HttpServletRequest • getMethod() • Returns get/post with which request was made. • getQueryString() • getRemoteUser() • getRequestSessionId() • getSession(boolean) • If FALSE returns current valid session otherwise creates a new session. • isRequestedSessionIdValid() • getCookies()

27. HttpServletResponse • addCookie(Cookie) • Add cookie to response • encodeUrl(String) • sendRedirect(String) • sendError(int)

28. Servlet Context • The javax.servlet.ServletContext interface provides a set of methods that the servlet can use to communicate with the web server. • The ServletContext object is contained within javax.servlet.ServletConfig Object which is provided to the servlet when it is initialized.

29. Functions of ServletContext Object • Set and store attributes that other servlets in the context can access. • Log events • Obtain URL references to resources. • Get values assigned to initialization parameters. • Get the MIME types of files. • Obtain information about the servlet container such as its name and version. • A servlet context is associated with a web application and shared by all the servlets within that application.

30. Web Application Files and Directory Structure • Each web application has a root called the context path. • No two applications, in the same Web Server, can have the same context path because this would cause URL conflicts. • Document root is a special directory called WEB-INF. • Contents of WEB-INF are- • Web.xml file web application deployment descriptor • /classes directory • /lib directory storage area for JAR file • /tags directory used to store tag libraries

31. There may be META-INF directory that contains implementation-specific deployment descriptor and WSDL (Web Service Definition Language) directory. • /META-INF/MANIFEST.MF • /WEB-INF/classes/HTMLPage.class • /WEB-INF/classes/VerifyData.class • /WEB-INF/Web.xml • /WEB-INF/sun-Web.xml • /index.html

32. Handling Errors • HTTP Errors • XML error-page tag can be used <error-page> <error-code>404</error-code> <location>/examples/error404.html</location> </error-page> • On error, the servlet can use either of the following method to set HTTP status code: public void HttpServletResponse.sendError(int sc) public void HttpServletResponse.sendError(int sc,String msg) • On error servlet can be redirected to another URL • HttpServletResponse.sendRedirect(String location)

33. Servlet Exceptions • Catch all servlet generated exceptions in servlet and take actions accordingly. • When a fatal error is generated an error message can be displayed Catch(RemoteException e){ Res.sendError(503,”Internal Error”); Res.sendRedirect(“/example/ReportErrorPage”); } • During initialization or while handling requests the servlet instance can throw an UnavailableException or ServletException.

34. Cookie Class • Cookie used for session management with HTTP and HTTPS. • Used to get browsers to hold small data associated with user’s browsing. • Cookies are named and have single value. • Assigned by the servers using fields added to HTTP response headers. • Cookies are saved one at a time into HTTP response headers using javax.servlet.http.HttpServletResponse.addCookie().

35. Web browsers are expected to support 20 cookies per host of at least 4KB each. • HTTP request fields are retrieved using javax.servlet.http.HttpServletRequest.getCookie(). • This returns all cookies found in request.

36. Cookie Class • Cookie(String name,String value) • setDomain(String) • getDomain() • setMaxAge(int) • Age in sec; 0 to delete • getMaxAge() • setValue(String) • getValue() • getName()

37. Example • FirstServlet.html • FirstServlet.java • STEPS • Set classpath=c:/jsdk2.0/lib/jsdk.jar;C:/jsdk2.0/src • Compile .java file • .class file should be in example directory • Run servletrunner • Run FirstServlet.html in browser • C:\JSDK2.0\examples\FirstServlet.html

38. Survey.html • Survey.java

39. Questions • What are two methods of HTTP to send requests to web server? Which should you use to send large amounts of information to the server? • What are the main uses of ServletContext Object? • What are the differences between Generic Servlet and Servlet? • What is pre initialization and lazy initialization of servlet? • Write short notes on Error handling of Servlet.