Java Servelets

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. 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. 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. For such applications, Java Servlet technology defines HTTP-specific servlet classes. The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. 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. 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. 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. The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services.

A servlet is a server side software component, written in java that dynamically extends the functionality of a server. 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. 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. It works behind the scene on server and results of servlet’s processing are returned to the client.

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

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

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 :-). 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. 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. 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.

Powerful. 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. 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. 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

Servlet API It doesn’t run as an application rather it is loaded in memory and as instance is created. 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. When a servlet instance is created its init() is called. Servlets are required to respond to new connections to the server. 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. 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. 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. Servlet class is abstract because service() is defined not implemented; so to implement a servlet, it is necessary to override this method.

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

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

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

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

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

HttpServletResponse addCookie(Cookie) addCookie(Cookie) –Add cookie to response encodeUrl(String) encodeUrl(String) sendRedirect(String) sendRedirect(String) sendError(int) sendError(int)

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

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

Cookie Class Cookie(String,String) Cookie(String,String) setDomain(String) setDomain(String) getDomain() getDomain() setMaxAge(int) setMaxAge(int) –Age in sec; 0 to delete getMaxAge() getMaxAge() setValue(String) setValue(String) getValue() getValue() getName() getName()

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

Survey.html Survey.html Survey.html Survey.java Survey.java Survey.java