For more information on the HTTP protocol, see Appendix A in .NET framework

Painting QR Code in .NET framework For more information on the HTTP protocol, see Appendix A
For more information on the HTTP protocol, see Appendix A
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The Servlet API is currently in version 22, which is the version supported by WebLogic Server 510, and the one we'll use in this book The servlet has the following lifecycle, as defined in the javax servlet Servlet interface, and implemented in GenericServlet and its subclass HttpServlet: Q Q Initialization - the container creates an instance of the servlet and calls init () to initialize the servlet After this method the servlet should be ready to service requests from multiple clients Servicing requests - the servlet usually runs in a multithreaded container, and each client request spawns a new thread (or takes a thread from a pool) to handle the service () method of that servlet You must handle any concurrency issues with resources shared between clients in your code Destruction - the container calls destroy () prior to garbage collecting and finalizing the servlet instance
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One additional feature that the HttpServlet class provides is a series of convenience methods to handle different HTTP requests - doGet (), doPost (), do Put (), and doDelete () The service () method dispatches the request to the appropriate method In developing your own servlet, you typically subclass HttpServlet and override the methods you need
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Creating a Web Front End
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The servlets framework also provides support for application-level and session-level data, through the ServletContext and HttpSession objects respectively
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A Servlet Example
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Here's a very simple example of a servlet, in which the servlet retrieves the username from a request parameter and creates an HTML page with a welcome message for that user The servlet can be invoked by typing the following URL into your browser:
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http://localhost:7001/MyFirstServlet username=Manu
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Alternatively, an HTML form can be used to set the username parameter You'll find the code in c : \wlsbook\src\MyFirst\MyFirstServlet Java First all the necessary packages are imported:
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import import import javaxservlet *; javaxservlethttp*; javaio*;
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Then the servlet itself is defined It extends j avax servlet http HttpServlet, which provides methods to query the container, and the lifecycle methods we saw above Here, we override the service () method (the one that handles the requests and produces the responses):
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public class MyFirstServlet extends HttpServlet {
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public void service(HttpServletRequest request, HttpServletResponse response) throws lOException { PrintWriter out; String title = = "My First Servlet Output";
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String username
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requestgetParameter("username"); responsesetContentType("text/html"); out responsegetWriter( ) ;
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outprintln("<HTML><HEAD><TITLE>"); outprintln(title);
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outprintln( "</TITLEx/HEADxBODY>") ; outprintln("<H1>" + title + "</Hl>"); outprintln("<P>Hello ");
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if ((username != null) && (!usernameequals(""))) {
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outprintln("<B>" + username + "</B>"); } outprintln("!!!"); outprintln("</BODYx/HTML>") ;
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3
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We'll take a look at how to start the Web Logic server later in this chapter In the meantime, this servlet generates the following screenshot:
In the service () method, we are provided with the request object, and we query this for the username parameter The response object is also available; we use it to set the MIME content type for the response, and get a writer object into which we stream the output We'll see more of servlets as we code the examples in this chapter
Introducing JavaServer Pages
JavaServer Pages are an extension to servlets Basically diey are a mirror image of servlets, allowing you to embed Java in HTML pages, whereas servlets embed HTML output in Java code JSP pages provide die ability to combine snippets of Java code with a page template, and have that template filled with data in response to a client request The JSP page is automatically compiled to a servlet class before its first use, converting the HTML information into out print In ("") statements like those used in the servlet above This intermixing of Java code and HTML allows a clearer separation between the design of the page and mechanisms of dynamic content generation This separation can make development and maintenance easier For example, the look of a page can be changed without affecting the code that generates the dynamic content In the WebLogic server, the way aJSP page works is the following: Q Q A web client requests a page with the jsp extension The container has a mapping for j sp to a special servlet, called JSPServlet (in wlsbook\srv\c03-srv\weblogicproperties) This servlet finds the requested JSP file If necessary it will translate the JSP page to a servlet and compile it, before creating an instance of the page to answer the request This only happens the first time it is used or when there has been a change in the JSP This is why the first use of aJSP page is slower than subsequent requests The request is forwarded to the generated servlet