Islamic University of Gaza Faculty of Engineering Department of Computer Engineering ECOM Advanced Internet Technology Lab.

Transcription

1 Islamic University of Gaza Faculty of Engineering Department of Computer Engineering ECOM 5049 Advanced Internet Technology Lab Lab # 10 JAVABEANS IN JSP El-masry May, 2014

2 Objectives Understanding JavaBeans. Understanding the benefits of beans. Creating beans. Accessing bean properties. Explicitly setting bean properties. Automatically setting bean properties from request parameters. Sharing beans among multiple servlets and JSP pages. What are JavaBeans? Beans are regular Java classes that follow some simple conventions defined by the JavaBeans specification: A bean class must have a zero-argument (default) constructor. A bean class should have no public instance variables (fields). Persistent values should be accessed through methods called getxxx and setxxx. Why You Should Use Accessors, Not Public Fields? You can put constraints on values. You can change your internal representation without changing the interface. You can perform arbitrary side effects. Using Beans in JSP jsp:usebean: It can either build a new bean or access a preexisting one. <jsp:usebean id="beanname" class="package.class" /> It can be thought of as equivalent to the scriptlet: <% package.class beanname = new package.class(); %> But jsp:usebean has two additional advantages: It is easier to derive object values from request parameters. It is easier to share objects among pages or servlets. 2

3 jsp:getproperty: Allow access to bean properties. <jsp:getproperty name="beanname" property="prop" /> It is equivalent to the JSP expression: <%= beanname.getprop () %> jsp:setproperty: Allow setting of bean properties. <jsp:setproperty name="beanname" property="prop" value="val" /> It is equivalent to the scriptlet: Example - JavaBean <% beanname.setprop ("val"); %> 3

4 - JSP Page Output Beans with Forms To assign bean properties with request parameters, there are three cases: 1- Explicit Conversion & Assignment. 2- Associating Individual Properties with Input Parameters. 3- Associating All Properties with Input Parameters. 4

5 Example - JSP Form: - Java Bean: 5

6 - SaleEntry1.jsp: Explicit Conversion 6

7 - The Output: 7

8 - SaleEntry2.jsp: Individual Parameters Associating: The Output 8

9 - SaleEntry3.jsp: All Properties Association The Output 9

10 JavaBeans Sharing You can specify a scope attribute that associates the bean with more than just the current page. If beans can be shared, it is useful to obtain references to existing beans, rather than always building a new object. So, the jsp:usebean action specifies that a new object is instantiated only if there is no existing one with the same id and scope. Values of the scope attribute <jsp:usebean id=" " class=" " scope=" " /> 1- page (<jsp:usebean scope="page"/> or <jsp:usebean >) It is a default value. Bean object should be placed in the PageContext object for the duration of the current request. 2- request (<jsp:usebean scope="request"/>) Bean object should be placed in the HttpServletRequest object for the duration of the current request. 3- session (<jsp:usebean scope="session"/>) Bean will be stored in the HttpSession object associated with the current request. 4- application (<jsp:usebean scope="application"/>) Bean will be stored in ServletContext that is shared by all servlets and JSP pages in the Web application. Conditional Bean Operations To execute some statements only if a new bean is created, not if an existing bean is found, use this form of the action: Instead of the form: <jsp:usebean...>statements</jsp:usebean> <jsp:usebean.../> 10

11 Examples - JSP Form - JavaBean 11

12 - Page Scope The Output 1 st Request 12

13 2 nd request Request Scope - BakedBeanDisplay.jsp 13

14 - BakedBeanDisplay2.jsp The Output 14

15 Session Scope The Output 1 st Request. 2 nd request that is in the same session. 15

16 3 rd request from another session. Application Scope The Output 1 st Request 16

17 2 nd Request in the same session 3 rd request from different session. 17