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Java Beans: Solving Scripting Issues with Beans in Web Development

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  1. CS320 Web and Internet ProgrammingJava Beans Chengyu Sun California State University, Los Angeles

  2. Problems with Scripting Elements • Mixing presentation and processing • hard to debug • hard to maintain • No clean and easy way to reuse code • Solution – separate out Java code • Bean • Custom taglib

  3. Java Beans • Initially designed for GUI builders • Beans support • Introspection • Customization • Events • Properties • Persistence

  4. Java Beans for Dummies • A zero-argument constructor • No public class variables • Property foo • read property foo: getFoo() or isFoo() • set property foo: setFoo()

  5. About Bean Properties • Property naming conventions • 1st letter is always in lower case • 1st letter must be capitalized in accessor (getter) and/or mutator (setter) • A property does not have to be a class variable • Property types • read-only property: only getter • write-only property: only setter • read/write property: both

  6. Bean Example • BGBean.java • package!

  7. jsp:useBean class id scope page (default) request session application jsp:getProperty name property jsp:setProperty name property value param Bean Tags and Attributes

  8. Bean in JSP Example • BGColor2.jsp <jsp:useBean id=“bg" class=“cs320.stu31.BGBean" /> <jsp:setProperty name=“bg” property=“r” value=“255” /> <jsp:setProperty name=“bg” property=“r” param=“r” /> <jsp:setProperty name=“bg” property=“r” /> <jsp:getProperty name=“bg” property=“r” />

  9. More About <jsp:setProperty> • Note the auto-conversion from parameter string to property type • e.g. “255”  255 • What if auto-conversion fails?? • What if a parameter is not specified??

  10. A Few More Ways to setProperty <%-- Use JSP expression for the value attribute --%> <jsp:setProperty name="fb" property="fname" value='<%= request.getParameter("fname") %>'/> <%-- Use <jsp:attribute> for value attribute --%> <jsp:setProperty name="fb" property="age"> <jsp:attribute name="value" trim="true"> <%= request.getParameter("age") %> </jsp:attribute> </jsp:setProperty> <%-- Set all properties --%> <jsp:setProperty name="fb" property="*"/>

  11. Example: Form Bean • Set all properties • And an easier way to “get property”: Expression Language First name: ${ fb.fname } Last name: ${ fb.lname }

  12. Scopes and Data Sharing • page scope – data is valid within current page • include • request scope – data is valid throughout the processing of the request • forward • session scope – data is valid throughout the session • redirect, multiple separate requests • application scope – data is valid throughout the life cycle of the web application

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