Web applications and JSP. Carl Nettelblad

Transcription

1 Web applications and JSP Carl Nettelblad

2 Outline Review and assignment Jara Server Pages Web application structure

3 Review We send repeated requests using HTTP Each request asks for a specific file/resource Request contains a path, possibly including a query string, headers and (possibly) a request body A request gives a response with headers and body (content) Important verbs: GET and POST Your browser will switch between them Client HTTP Request Server HTTP Response

4 URLs Protocol Domain Port number (optional) Path Query string (optional) Anchor (only on the client, optional)

5 HTML and CSS HTML is a text-based format Plain text enclosed in tags with a specific structure Each tag <h1> </h1> can have attributes <h1 class="myclass"> Formatting can be controlled by CSS Separate files with rules on the form selector {declaration;} h1 {font-size: 30px;}.myclass {text-decoration: overline; color: blue;} #sometagid {background-color: #00ff00;} h1.myclass {visibility:hidden;}

6 Servlets Single instance serving requests to a part of the web application Configured using annotations or web.xml What paths are served by this servlet init, service, destroy service calls doget, dopost init similar to constructor, but full configuration ready Contexts Request, session, config, servlet context Parameters and attributes

7 Forwarding and filters RequestDispatcher can be used to find the handler for another URL Process a complex action Render a simple view Filters can be used to provide common processing for all parts of an application Security Logging Establish standard objects in contexts

8 Assignment details Memos for both assignments will be online on or before April 5 Assignment 1 A web application containing Purpose: At least one servlet At least one JSP page Some interaction between the servlet and the JSP Some interaction between the components and the user(s) Responses changing depending on user actions Getting familiar with the environment and the technologies The hard part is understanding the architecture, not the programming

9 Execution Done in groups 25 slots in the Student Portal ~60 people taking the course Groups of 2-3 people Make sure that you have a group Enroll in the Student Portal, Project group division

10 Groups Identical group for all parts of the course Groups will be fully locked after lecture April 13 Groups with fewer than 3 people might be asked to take on an extra member Don t join a group yourself without contacting the existing members first

11 Assignment reporting Upload the code in the Student Portal after April 13 (when groups are finalized), before the lab on April 23 All group members should be present during the lab Can be asked questions about the assignment or the structure around it An approximate schedule for reporting during the lab will be posted (which groups presents to whom, when)

12 Don t forget the forums Student portal forums Feedback or comments on lectures Questions regarding the assignments/software issues Forum to find project partners

13 Java Server Pages Presentation framework Native part of the Java EE standard Presentation based on HTML

14 Different paradigms Servlet-like model Give the request to a piece of code Render the full response Also the model for Common Gateway Interface (CGI) many early interactive web pages were Perl scripts using CGI Template/inline model Static and dynamic content handled in a similar way Seamless transition between a fully static page and a page with some interaction Microsoft ASP, JSP, PHP

15 The basic conflict Java is an imperative language How do I want something to happen HTML is a declarative language What do I want to happen Java code generating HTML can easily get ugly Mixing the two is kind of tricky Simple as what is the basis for indentation

16 What is a JSP, really? A normal Java class is compiled from source to a Java.class file A JSP page is compiled from JSP source to a Java class file Specifically a servlet extending HttpServlet A rather tedious service method that emits the full page The container is responsible to recompile on the fly Netbeans and Glassfish accomplished this together in our demo for servlets

17 JSP building blocks HTML JSP-specific tags and tag libraries Expression Language expressions Scriptlets JSP pages and JSP documents A more standardized XML syntax in JSP documents We are showing the JSP page syntax here

18 Scriptlets <html> <body> <% out.print("welcome to jsp"); %> </body> </html> Any Java code can go into <% %> tags Ends up inside the service method, in the proper place If all logic is done like this, it s just as hard to read as a separate servlet

19 Expression tags <html> <body> <%= "welcome to jsp" %> </body> </html> <%= request.getattribute("tb") %> Any Java expression that can evaluate to a string Note, no semicolon

20 Declaration tags <html> <body> <%! int data = 50; int square(int n){ return n*n; } %> <%= "Square of 50 is:"+square(data) %> </body> </html> <%! %> is used for class-level declarations of methods or variables Remember what we said about instance variables in servlets Shared among all requests If you find that you need to use separate methods in your JSP, your JSP is probably getting too complex But it s still better than cut n paste spaghetti of the same code over and over

21 Special objects request response config application ServletContext pagecontext wrapper to access all contexts with common methods page holds a reference to the hidden servlet, but of Object type out output writer exception only used in special error pages, exception that caused the error page to be shown

22 Page directive Common control of all code in a single JSP page Several options not used that frequently Some are more important

23 Page import Equivalent to import in Java Needed to avoid fully qualified names for types page import="java.util.date" %>

24 Page errorpage Any uncaught exception occurring on the page results in the specified error page being loaded instead page errorpage="showerror.jsp" %> Showing good errors is important for the user experience Showing too much information in errors to the user can constitute a security risk

25 Page contenttype The content type sent in the HTTP response headers page contenttype="text/html" %> Showing good errors is important for the user experience Showing too much information in errors to the user can constitute a security risk

26 Include directive You can include another source file Useful to always have a similar menu/header <%@include file="somefile.ext"> Also the taglib directive, more about that later

27 Action elements Replacement for the native JSP syntax <% in JSP documents Some specific actions <jsp:usebean> <jsp:setproperty> <jsp:getproperty> <jsp:forward> <jsp:include>

28 Use a bean Get or create a bean with a specific name from a specific context (page, request, session, application) Made available with just that name as a variable in the rest of the code Code inside tag only included if bean not already found, but created anew <jsp:usebean id="tb" scope="request" class="testbean"> Bean was created, should have been found in request </jsp:usebean>

29 Set a property Change a single value in a variable <jsp:setproperty name="tb" property="name" value="carl" /> Change all fields that match parameter values in the request <jsp:setproperty name="tb" property="*" /> Be careful, the user can add any parameters Imagine having a user bean with a method setpassword, not too good

30 Get a property Get a property from a bean and write that in the response <jsp:getproperty name="tb" property="name" /> Simpler (?) equivalent <%= tb.name %>

31 Forward a request Similar to servlets, a request can be forwarded to another JSP or servlet With additional parameters, if you want to <jsp:forward page="anotherpage.jsp"> <jsp:param name="name" value="carl" /> </jsp:forward>

32 More on forwarding No part of the existing page is sent to the client If data has already been transmitted, forwarding is not allowed Settings like buffer sizes and flushing can determine this The destination can be controlled by JSP code like <%= someurlvariable %>

33 Inclusion JSP files can be included <jsp:include page="/another/page.html" /> Parameters allowed here as well Different from > The include directive is done at compile time, file source must be fixed All variables are shared between original and included file The include action is done at execution, path can be determined dynamically

34 Expression language Java code is cumbersome Get attributes with.getattribute("attributename"), or.getattributename(), etc Type casts to the correct class Page author needs to know what kind of Java object is this Java code is too flexible Logic structure can get complex Calling methods causing changes, rather than just looking

35 Structure of an expression ${1 + 3} Equivalent to <%= %> ${sessionscope.cart.nitems} or even ${cart.nitems} Equivalent to <%= ((SomeCartClass) session.getattribute("cart").getnitems() %> or: <jsp:usebean id="cart" scope="session" class="somecartclass" /> <%= cart.getnitems() %> If and only if you have the right page imports Syntax for deferred evaluation #{} instead More about this in JSF and JSTL lectures

36 Implicit objects pagecontext param all page parameter paramvalues all page parameters as arrays (a query string can containg param1=value1&param1=value2 ) header headervalues cookie initparam

37 And the scopes pagescope requestscope sessionscope applicationscope In Java, you need to access the contexts explicitly In EL, the contexts work as real scopes ${somevar} will look for the somevar attribute in the page scope, the request scope, the session scope, and the application scope, in order ${applicationscope.somevar} only necessary if the name is used in multiple places

38 Note about the page scope Attributes in the page context are not the same thing as variables in JSP <% int x = 42; %> ${x} Does not work! Instead: <% pagecontext.setattribute("x", 42); %> ${x}

39 The lifetime of the scopes Request all the processing of this request (remains during forwarding) Page the processing of this page (remains with >, not <jsp:include>) Session the processing of all requests coming from the same user session Application as long as the application is loaded (not redeployed), common to all servlets and JSPs in it Identical for servlets, JSPs and EL

40 EL is not Java No loops No if statements Trinary operator works: condition? value-if-true : value-if-false a < b? a : b gives the smallest value of a or b Different scopes Maps and get methods are accessed with a field-like syntax Special names in maps can be accessed with ['field-name'] brackets String literals can be written using " " or '

41 Back to our servlet example // check if we have a parameter called action // if so, check its value if(request.getparameter("action")!= null && request.getparameter("action").equals("show")) { // create a bean object and populate it TestBean tb = new TestBean(); tb.setage(new Integer("32")); tb.setname("fredrik"); // get a dispatcher and forward the request // store the bean object in the session context first RequestDispatcher rd = request.getrequestdispatcher("test.jsp"); request.setattribute("tb", tb); request.setattribute("date", new java.util.date()); rd.forward(request, response); } else { // if the condition above failed, forward the // request to another JSP } RequestDispatcher rd = request.getrequestdispatcher("error.jsp"); rd.forward(request, response);

42 Plain JSP version of test.jsp <html><head> </head> <meta http-equiv="content-type" content="text/html; charset=windows-1252"> <title>hello World</title> <body> <h2>the current time is: </h2> <p> <%= new java.util.date() %></p> <jsp:usebean id="tb" scope="request" class="testbean"> Error, the instance is created in the bean </jsp:usebean> <H1> <jsp:getproperty name="tb" property="name" /> </H1> <%= tb.getname() %> <br> <jsp:usebean id="date" scope="request" class="java.util.date"/> <%= date %> </body></html>

43 EL version of test.jsp <html><head> </head> <meta http-equiv="content-type" content="text/html; charset=windows-1252"> <title>hello World</title> <body> <h2>the current time is: </h2> <p> EL CAN T CREATE NEW OBJECTS </p> <h1> ${tb.name} </h1> ${tb.name} <br> ${date} </body></html>

44 A comment about comments HTML comments <!-- --!> Part of the HTML standard, transmitted to the client Don t put secrets there A true JSP comment <%-- --%> Only on the server Inside scriptlets (Java code), normal Java comments work /* */, //

45 Rule of thumb If you are using JSP Try to express what you can using tags and EL JSTL adds more tags You can write your own specific tag libraries More complex logic should go into servlets, populating the contexts Most serious drawbacks of JSP Hard to modularize rendering of complex UI components Yet, different enough from HTML to just open and test layout in a browser

46 Alternatives to JSP Java Server Faces Java EE technology for rendering user interfaces and complex applications A step further away from directly doing HTML, but still declarative Other examples Apache Wicket, Apache Tapestry More modern templating systems tries to respect HTML more Examples have been Freemarker, Apache Velocity, Thymeleaf Tends to come and go Templates which render well without the server are practical for client-heavy applications

47 The web application archive An extended JAR (a zip file with a specific structure) Static content and JSP files in the root Open section Special directories META-INF, for any JAR file (including a web application WAR) WEB-INF Servlets, configuration, property files Closed section

48 Inside of WEB-INF Files directly web.xml Other configuration files Directory classes Java class files for servlets, beans etc Directory lib Archives with dependencies (jar files used by our code) Other property and configuration files in other directories as well

49 Example ls -R error.jsp test.jsp test.xsl WEB-INF./WEB-INF: classes c.tld lib src web.xml x.tld./web-inf/classes: mypackage./web-inf/classes/mypackage: TestBean.class TestServlet.class./WEB-INF/lib: jstl.jar standard.jar./web-inf/src: mypackage./web-inf/src/mypackage: TestBean.java TestServlet.java

50 The role of the IDE An IDE or a build system will create the full WAR It s very unreliable to copy the right files to the subdirectories manually

51 web.xml Used to be crucial for configuration Example content: Initialization and global context parameters Servlet mappings Filter mappings Tag lib definitions Authorization declarations Authentication details Most of this can now be controlled by annotations All configuration in one place vs. configuration close to the code

52 web.xml example <?xml version = '1.0' encoding = 'UTF-8'?> <web-app xmlns=" xmlns:xsi=" xsi:schemalocation = " version="3.0" > <context-param> <param-name>dbname</param-name> <param-value>itdb</param-value> </context-param>

53 web.xml example <servlet> <servlet-name> 6. servlet </servlet-name> <servlet-class> 6. servlet </servlet-class> <init-param> <param-name> filename </param-name> <param-value>../webapps/murach/user .txt </param-value> </init-param> </servlet> </web-app>

54 Servlet mapping Adding specific URL patterns for a servlet <servlet-mapping> <servlet-name> 6. servlet </servlet-name> <url-pattern> /myservlet </url-pattern> </servlet-mapping>

55 Defining init parameters Context parameters go into ServletContext, common to all the application Init parameters specific to the servlet <servlet> </servlet> <servlet-name>a Servlet</servlet-name> <servlet-class>com.controller.testservlet</servlet-class> <init-param> </init-param> <description>this is an init parameter example</description> <param-name>initparam</param-name> <param-value>init param value</param-value> Accessible in init method in servlet (hence the name)

56 Lifetime of sessions <session-config> <session-timeout> 30 </session-timeout> </session-config> Sessions time out Security: don t let another user use an unattended computer Server resources: saving tens of thousands of stale sessions is expensive

57 Mappings for static content The HTTP client is supposed to look at Content-Type header, not file extension The WAR doesn t contain content type information for static files So, the server needs to know what type to state for different extensions (defaults might be sensible) <mime-mapping> <extension> html </extension> <mime-type> text/html </mime-type> </mime-mapping>

58 Start page Index page, welcome page What page to show if the user just puts /application/ as the URL <welcome-file-list> <welcome-file> index.jsp </welcome-file> <welcome-file> index.html </welcome-file> </welcome-file-list>

59 Common error handling <error-page> <exception-type> java.lang.throwable </exception-type> <location> / 6/error.html </location> </error-page>

60 Specific errors <error-page> <exception-type> java.lang.ioexception </exception-type> <location> / 6/ioerror.html </location> </error-page>

61 HTTP errors <error-page> <error-code> 404 </error-code> <location> / 6/filenotfound.html </location> </error-page>

62 Summarizing JSP is powerful Transformed into a servlet behind the scenes The full power of Java Try to avoid using that power Much can be accomplished with just Expression Language (EL), HTML and (later) custom tags This will be easier to maintain More similar to other modern templating engines