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12 – Java Beans. Session Aims & Objectives. Aims To cover the use of Java Beans Objectives, by end of this week’s sessions, you should be able to: Create and use a Java Bean. PersonList.jsp. <%@page import="Main.*" %> <%@page contentType="text/html" pageEncoding="UTF-8"%> <%!
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Session Aims & Objectives • Aims • To cover the use of Java Beans • Objectives,by end of this week’s sessions, you should be able to: • Create and use a Java Bean
PersonList.jsp <%@page import="Main.*" %> <%@page contentType="text/html" pageEncoding="UTF-8"%> <%! People p = new People(); %> <% String html = ""; p.Open(); p.Select("SELECT * FROM Person;"); while(p.Next()){ html += p.get("Surname") + "<br />"; } p.Close(); %> <!DOCTYPE html> <html> <head><title>People</title></head> <body> <%=html%> </body> </html> • Class complex • Pages simpler Import Package Create Instance Use methods
PersonList.jsp (using Bean) • Class complex • Pages simpler Create Bean <jsp:useBean id="p" scope="session" class="Main.People" /> <%@page contentType="text/html" pageEncoding="UTF-8"%> <% String html = ""; p.Open(); p.Select("SELECT * FROM Person;"); while(p.Next()){ html += p.get("Surname") + "<br />"; } p.Close(); %> <!DOCTYPE html> <html> <head><title>People</title></head> <body> <%=html%> </body> </html> Use methods
JavaBean (Bean) = Java class instance JSP programming style strongly encourages JavaBeans use special tags built-in for JavaBean properties JSP + Bean combination separates page html look from ‘logic’ i.e. the presentation from the code JSP and JavaBeans
Java class meeting specific requirements: Must have a zero-argument constructor: public MyBean() { … } All propertiesprivate (no public properties) data accessed via access methods What is a JavaBean
BANK ACCOUNT BEAN Get and set methods MUST conform to getXxxx() and setXxxx() Beans MUST be in packages 0 Parameter constructor Important Exception is for boolean attributes isXxxx() Can have other methods but method names cannot look like property get / set
An attribute is a variable which belongs to an class/object For objects also known as instance variables For classes also known as class variables Remember final static int COLOUR_ONE Math.PI is a class variable A property is an attribute which has getter and setter methods And that’s it ! REFINING THE TERMINOLOGY
Read-only properties: String getAccountID() returns the accountID property Read/write properties: void setBalance(double bal) double getBalance() Boolean properties: boolean isActive() void setActive(boolean act) JAVABEAN PROPERTIES
It is important to distinguish between a JavaBean as used in a: GUI development tool This is a visual component i.e. will subclass Panel, Button etc. Note there is a visual Bean design tool at: http://java.sun.com/products/javabeans/beanbuilder/index.jsp Server-Side application We are only dealing with the latter MORE THAN ONE BEAN
<jsp: useBean ……… > <jsp: setProperty ……… > <jsp: getProperty ……… > BEAN RELATED TAGS
BEANS WITH JSP A JSP file which makes use of the Class Bank Note: file called Bank.jsp
CREATING AN OBJECT Creates a bean instance called ‘myAccount’ of type ‘BankAccount’ The id attribute is the name of the variable Similar to the following JSP code: <% BankAccountmyAccount = new BankAccount(); %> Or Java: BankAccountmyAccount = new BankAccount(); Note: use of package name Important This / is important
SETTING BEAN PROPERTIES 1 Sets the value of the myAccountpropertybalance to 500 Basically the same operation as: <%= myAccount.setBalance(500) %> Or in Java as: BankAccountmyAccount = newBankAccount(); mybalance = myAccount.setBalance(500);
SETTING BEAN PROPERTIES 2 Also can have a dynamic property which uses an expression tag This example is just setting the balance to some random value between 0 and 100
SETTING BEAN PROPERTIES 3 Although this value is text converted automatically to correct type In this case a double
READING BEAN PROPERTIES Inserts the value of myAccountpropertybalance into the web page Basically the same as: <%= myAccount.getBalance() %> Or in Java as: BankAccountmyAccount = newBankAccount(); double mybalance; mybalance = myAccount.getBalance();
JSP BEANS - REVIEW Note how the value is displayed on the html page This line creates an object called myAccount of class BankAccount This line sets the balance property to 500 This line gets the balance
SETTING BEAN PROPERTIES FROM TEXT BOXES This the same as: String bal = request.getParamter(“openingbalance”); double tempBal = Double.parseDouble(bal); myaccount.setBalance(tempBal); .htmlPage Sets the property ‘balance’ to what ever was typed in the textbox. .jsp Page
USING TEXTBOXES If the textbox name is the same name as the property Then we do not need a ‘param’
SETTING BEAN PROPERTIES … ‘WILDCARDS’ Using wildcards to set properties: • Sets the value of all ‘somebean’ properties to JSP parameters with the same name • If the parameters do not exist, the value of the bean properties do not change
‘WILDCARDS’ EXAMPLE OpenAccount.html NewAccount.jsp
scope= “page” scope= “request” These beans will not last after the request is completed The difference between these 2 scopes is very small Beans such as this do not allow you to share data between servlets and JSPs scope= “application” scope= “session” These beans will last between requests, thus allowing sharing of data between requests Again, the differences between these two requests are mostly cosmetic JAVABEAN SCOPE 1 The default scope
SESSION BEANS As Bank.jsp and Rent.jsp are scoped at session level, the object myAccount is not created in Rent.jsp File: Rent.jsp
SESSION BEANS File: Bank.jsp The file Bank.jsp Creates the object myAccount, which is then used by Rent.jsp Essentially passing information between JSP pages File: Rent.jsp
CONDITIONAL BEANS So far we have used the <jsp: useBean id =“somebean…. > tag jsp:useBean results in new bean being created only if no bean with same id and scope can be found If a bean with same id and scope is found, then that bean is used. This means that any property we initially set will be again be set each time we visit the page This is ok when we visit the a page for the 1st time as we want to set the properties of the bean which will be used across several pages. But what if we wanted to set initial bean properties for a bean which is shared by multiple pages. Since we don’t know which page will be accessed first, we don’t know which page should contain the initialization code.
EXAMPLE: Lets assume we have a ‘back’ link on the PayRent.jsp ??? Balance should be 350.00
Problem is that when we return to the Bank.jsp page the setProperty sets the balance to 500 again
SOLUTION: CONDITIONAL BEAN The <jsp:useBean ... /> replaced by <jsp:useBean ...> statements </jsp:useBean> The statements (i.e. jsp:setProperty elements) are executed only if a new bean is created, not if an existing bean is found. This is subtle but the effects are profound Modified file: Bank.jsp
EXAMPLE: Now we have Balance is correct at 350.00
Apache – http server (html pages) Tomcat – runs JSP + Servlets servlet container (interpreter/compiler) Can run: Standalone Handles simple page requests Handles servlet requests Apache plugin Apache handles HTML pages, CGI, PHP etc Tomcat handles servlets Apache Tomcat
Tomcat Folder Structure Context root Starting html page Netbeans Will create this Structure … Web application deployment descriptor (web.xml) Package name of the HelloServlet class The HelloServlet class
fgfg Tomcat Folder Structure But each need WEB-INF and web.xml Default location is in webapps Can have any number of webapplications in webapps
Tomcat - NetBeans • JRE_HOME = C:\Program Files\Java\jre6 • Control Panel • System • Advanced • Environment Variables • C:\Program Files\Apache Software Foundation\Apache Tomcat 7.0.14\bin • startup.bat (run from command line) • http://localhost:8080/
Hall, M. Servlets and Java Server Pages 2nd Edition Chapter 14: Using Beans with JSP Best coverage Armstrong, E. (2003) The J2EE 1.4 Tutorial chapter 12: Pages 515 - 525 http://java.sun.com/j2ee/1.4/docs/tutorial/doc/index.html REFERENCES - READ AT LEAST ONE OF … 38