Servlets. An extension of a web server runs inside a servlet container

Transcription

1 Servlets What is a servlet? An extension of a web server runs inside a servlet container A Java class derived from the HttpServlet class A controller in webapplications captures requests can forward requests to JSP s and Servlets can create and use Beans multithreaded by nature. Servlets 21 January

2 A servlet is pure JAVA. Its initial purpose was to generate HTML. Like this package serv; import javax.servlet.*; import javax.servlet.http.*; import java.io.*; public class Serv1 extends HttpServlet { public void doget(httpservletrequest request, HttpServletResponse response) throws IOException, ServletException { response.setcontenttype( text/html ); PrintWriter out = response.getwriter(); out.println( <H1> hello </H1> ); out.println( <a href=\ > klicka här</a> ); public void dopost(httpservletrequest request, HttpServletResponse response) throws IOException, ServletException { doget(request, response); Servlets 21 January

3 Java EE prescribes a certain directory structure that you have to follow. Each application is based in context path. A context path is place in the file system, this acts as a root for my application. Tomcat normally uses a catalogue in its installationhierarchy as the basic context path. The standard location is $TOMCAT-INST/webapps Each application is created in a subdirectory of this root. So if we have a bookshop application we create a catalogue bookshop, this will be our context path. $TOMCAT-INST/webapps/bookshop Each application then should follow a prescribed hierarchy to store its files. Servlet classes should be placed in: context path/web-inf/classes Servlets 21 January

4 Locally on our Solaris system the context path is ~/tomcat/your-username So we put the class file in ~/tomcat/web-inf/classes Since we are using a package here (recommended) the class file should be stored in ~/tomcat/web-inf/classes/serv You can only have one context path in this way. This is due to the local configuration of Tomcat on the Solaris system. Your username will be part Normally you can have as many context paths as you like. Servlets 21 January

5 To compile a servlet, you will need the servlet classes. This is a number of classes distributed as jar-files. They are distributed with tomcat in: $TOMCAT-INST/common/lib/servlet-api.jar. You can also get the library from the course home page. There are several methods to use it: Get the file servlet-api.jar from the tomcat installation or from the course homepage. Store it in ~/tomcat/web-inf/lib/ (on Solaris). Setup your classpath to find them. javac -classpath.:../lib/servlet-api.jar serv/serv1.java You can also set the CLASSPATH environment in the shell if you like. Something like export CLASSPATH=.:/home/olle/tomcat/WEB-INF/lib/servlet-api.jar in.bash_profile Then you can do javac serv/serv1.java Servlets 21 January

6 Another method is to store the jar s you need in your Java installation. You put them in $JAVA-INST/jre/lib/ext The compiler will then pick them up without setting the classpath. Servlets 21 January

7 How do I test my servlet? Normally the servlet is part of a webapplication and is started as part of the application, but you can start a servlet standalone. Use the URL to test this servlet, where app is the name of the application. This is not a recommended way to start a real application but it is OK for test purposes. It is also disabled in Tomcat by default but if you follow the configuration guide in the course homepage you can turn it on. Servlets 21 January

8 (on our Solaris system): where nnn is your user identity. Servlets 21 January

9 The servlet has the following environment: config, request, response, configuration data from the setup request from the browser response to the browser In addition you can request session, ServletContext, the session context the application context These are objects obtained from the container that houses the servlet. Servlets 21 January

10 The structure of a servlet is: An init method that sets up the context for the servlet. Executed once when the servlet is created. It will create a session if needed. The service method do some processing for a HTTP HEAD method and dispatches to the correct method. A destroy method that is executed when the servlet is destroyed. Several doxxx methods, one for each type of HTTP request, eg doget, dopost etc. You don t need to implement any of this, there are default init and destroy methods, and you only need to code the do-methods that you need. Servlets 21 January

11 The servlet lifecycle. client request server servlet not loaded servlet loaded call init call service call destroy call doxxx destroy is called after a specified period of idle time as part of the destruction of the servlet. Servlets 21 January

12 The init method is used to get different context from the webapplication. It can look like this: private static String showpage=null; private static String checkoutpage = null; private static String thankyoupage = null; private static String jdbcurl = null; private static String detailpage=null; private BookListBean booklist = null; /** Initializes the servlet. */ public void init(servletconfig config) throws ServletException { super.init(config); showpage = config.getinitparameter( SHOW_PAGE ); checkoutpage = config.getinitparameter ( CHECKOUT_PAGE ); thankyoupage = config.getinitparameter ( THANKYOU_PAGE ); detailpage = config.getinitparameter( DETAIL_PAGE ); jdbcurl = config.getinitparameter( JDBC_URL );... Servlets 21 January

13 Using the config object, you can ask for different parameters that are setup in the web application with description files. You can have instance variables that you setup in init. It is executed only once. Since servlets are multithreaded, you can only use the instance variables readonly outside the init method. You must not change their value after they have been setup by init, unless you use synchronized code. Servlets 21 January

14 The doxxx methods are declared as: public void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, java.io.ioexception {... The simple principle is: Use the request object to find out what you should do. Do that, either yourself or using Beans or JSP s The response object is returned to the client automagically when the method returns. Servlets 21 January

15 Different things that you can do with the request block: getsession() get your session object getparameter(string) get the value of the requested parameter parameters are sent by the browser getattribute(string) get the value of the requested attribut setattribute(string, Object) set a new value for the attribute getheader() getlocale() get the HTTP header the preferred language getcookies() request all Cookies getrequestdispatcher(url) get a dispatcher that can be used to dispatch work to other servlets or JSP s Servlets 21 January

16 The response block can be used to setcontenttype(string) MIME-type for the output setlocale(locale) set the locality in the answer getwriter() get a PrintWriter to produce HTML-output encodeurl(url) Cookies (name/value pair that are stored in the browser) are used to set a sessionid in the browser. In that way it identifies your requests and connects to the correct session. Some users do not allow the server to store cookies. The encodeurl method will test if it can set a cookie, if not it will rewrite all URL s so that a sessionid is appended to the URL. Otherwise the server can not identify your requests. As a general rule, to avoid problems with cookies, always rewrite all URL s Servlets 21 January

17 You can also set/get attributes in the session or application scope. HttpSession sess = request.getsession(); sess.setattribute( x, y ); or ServletContext sc = getservletcontext(); sc.setattribute( booklist,booklist); Servlets 21 January

18 An example package com.mimer.fredrik; import javax.servlet.*; import javax.servlet.http.*; import java.io.*; public class TestServlet extends HttpServlet { private static final String CONTENT_TYPE = text/html; charset=windows-1252 ; public void init(servletconfig config) throws ServletException { super.init(config); public void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, IOException { if(request.getparameter( action )!= null && request.getparameter( action ).equals( show )) { TestBean tb = new TestBean(); tb.setage(new Integer( 32 )); tb.setname( Fredrik ); RequestDispatcher rd = request.getrequestdispatcher( test.jsp ); request.setattribute( tb, tb); rd.forward(request, response); Servlets 21 January

19 else{ RequestDispatcher rd = request.getrequestdispatcher( error.jsp ); rd.forward(request, response); Servlets 21 January

20 A servlet is pure JAVA. Its purpose is to generate HTML. Like this package serv; import javax.servlet.*; import javax.servlet.http.*; import java.io.*; public class Serv extends HttpServlet { public void doget(httpservletrequest request, HttpServletResponse response) throws IOException, ServletException { response.setcontenttype( text/html ); PrintWriter out = response.getwriter(); out.println( <H1> hello </H1> ); out.println( <a href=\ > klicka här</a> ); public void dopost(httpservletrequest request, HttpServletResponse response) throws IOException, ServletException { doget(request, response); Servlets 21 January

21 As you see it is a bit clumsy to print HTML-code. It very easy get cluttered with a lot of special characters and escape sequences. Therefore we usually avoid this and just use the servlet for dispatching work to JSP s that are better suited to output HTML. Servlets 21 January

22 An example of a real servlet that do something package se.upright.education.uu.pvk.assignmenttwo.servlets; /* * Shop.java * */ import javax.servlet.*; import javax.servlet.http.*; import se.upright.education.uu.pvk.assignmenttwo.beans.*; /** * Fredrik Ålund 1.0 */ public class ShopServlet extends HttpServlet { private static String showpage=null; private static String checkoutpage = null; private static String thankyoupage = null; private static String jdbcurl = null; private static String detailpage=null; private BookListBean booklist = null; /** Initializes the servlet. Get some environment variables the have setup in the web application descrition files. We do this to avoid hardcoded filenames and database connections */ public void init(servletconfig config) throws ServletException { super.init(config); showpage = config.getinitparameter( SHOW_PAGE ); checkoutpage = config.getinitparameter( CHECKOUT_PAGE ); thankyoupage = config.getinitparameter( THANKYOU_PAGE ); Servlets 21 January

23 detailpage = config.getinitparameter( DETAIL_PAGE ); jdbcurl = config.getinitparameter( JDBC_URL ); // try to create a Bean that reads our bookstore into // memory try{ booklist = new BookListBean(jdbcURL); catch(exception e){ throw new ServletException(e); // servletcontext is the same as scope Application // store the Bean as an application attribute ServletContext sc = getservletcontext(); sc.setattribute( booklist,booklist); /** Destroys the servlet. */ public void destroy() { /** Processes requests for both HTTP <code> GET</code> and <code>post</code> methods. request servlet request response servlet response */ protected void processrequest( HttpServletRequest request, HttpServletResponse response) throws ServletException, java.io.ioexception { HttpSession sess = request.getsession(); RequestDispatcher rd = null; Servlets 21 January

24 // create a shopping cart ShoppingBean shoppingcart = getcart(request); // check what to do, ask for the action parameter if(request.getparameter( action ) == null request.getparameter( action ).equals( show )){ // A request dispatcher that s connected to the page. // dispath work to the JSP showpage // we don t get back rd = request.getrequestdispatcher(showpage); rd.forward(request,response); // another action else if(request.getparameter( action ).equals( add )){ // add a book to our shopping cart // check that is it valid first if (request.getparameter( bookid )!= null && request.getparameter( quantity )!=null ){ BookBean bb = null; bb = booklist.getbyid( Integer.parseInt(request.getParameter( bookid ))); if(bb==null){ throw new ServletException( The book is not in stock. ); else { shoppingcart.addbook(bb,integer.parseint( request.getparameter( quantity ))); rd = request.getrequestdispatcher(showpage); rd.forward(request,response); Servlets 21 January

25 // a method that creates and returns a shoppingcart private ShoppingBean getcart(httpservletrequest request){ HttpSession se = null; se=request.getsession(); ShoppingBean sb =null; sb = (ShoppingBean)se.getAttribute( shoppingcart ); if(sb==null){ sb = new ShoppingBean(); se.setattribute( shoppingcart,sb); return sb; /** Handles the HTTP <code>get</code> method. request servlet request response servlet response */ protected void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, java.io.ioexception { processrequest(request, response); /** Handles the HTTP <code>post</code> method. request servlet request response servlet response */ Servlets 21 January

26 protected void dopost(httpservletrequest request, HttpServletResponse response) throws ServletException, java.io.ioexception { processrequest(request, response); /** Returns a short description of the servlet. */ public String getservletinfo() { return The main BookShop ; Servlets 21 January

27 A simple example with servlets and JSP s package com.mimer.fredrik; import javax.servlet.*; import javax.servlet.http.*; import java.io.*; public class TestServlet extends HttpServlet { private static final String CONTENT_TYPE = text/html; charset=windows-1252 ; public void init(servletconfig config) throws ServletException { super.init(config); public void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, IOException { if(request.getparameter( action )!= null && request.getparameter( action ).equals( show )) { TestBean tb = new TestBean(); tb.setage(new Integer( 32 )); tb.setname( Fredrik ); RequestDispatcher rd = request.getrequestdispatcher( test.jsp ); request.setattribute( tb, tb); rd.forward(request, response); Servlets 21 January

28 else{ RequestDispatcher rd = request.getrequestdispatcher( error.jsp ); rd.forward(request, response); Servlets 21 January

29 test.jsp <html><head> <meta http-equiv= Content-Type content= text/html; charset=windows-1252 > <title>hello World</title> </head> <body> <h2>the current time is: </h2> <p> <%= new java.util.date() %></p> <jsp:usebean id= tb scope= request class= com.mimer.fredrik.testbean > Error, the instance is created in the bean </jsp:usebean> <H1> </H1> <jsp:getproperty name= tb property= name /> </body></html> Servlets 21 January

30 error.jsp page contenttype= text/html;charset=windows-1252 %> <html> <head> <meta http-equiv= Content-Type content= text/html; charset=windows-1252 > <title>hello World</title> </head><body> <h2>the current time is: </h2> <p><%= new java.util.date() %></p> Error </body></html> Servlets 21 January

31 TestBean.java package com.mimer.fredrik; public class TestBean { String name; Integer age; public TestBean() { public String getname() { return name; public void setname(string newname) { name = newname; public Integer getage() { return age; public void setage(integer newage) { age = newage; Servlets 21 January