1 / 38

12 – Java Beans

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"%> <%!

inara
Download Presentation

12 – Java Beans

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 12 – Java Beans

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. <jsp: useBean ……… > <jsp: setProperty ……… > <jsp: getProperty ……… > BEAN RELATED TAGS

  12. BEANS WITH JSP A JSP file which makes use of the Class Bank Note: file called Bank.jsp

  13. 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

  14. 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);

  15. 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

  16. SETTING BEAN PROPERTIES 3 Although this value is text converted automatically to correct type In this case a double

  17. 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();

  18. 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

  19. 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

  20. USING TEXTBOXES If the textbox name is the same name as the property Then we do not need a ‘param’

  21. 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

  22. ‘WILDCARDS’ EXAMPLE OpenAccount.html NewAccount.jsp

  23. ‘WILDCARDS’ EXAMPLE

  24. 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

  25. 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

  26. 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

  27. 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.

  28. EXAMPLE: Lets assume we have a ‘back’ link on the PayRent.jsp ??? Balance should be 350.00 

  29. Problem is that when we return to the Bank.jsp page the setProperty sets the balance to 500 again

  30. 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

  31. EXAMPLE: Now we have Balance is correct at 350.00 

  32. 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

  33. Tomcat: LocalHost

  34. Tomcat Directory Structure

  35. 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

  36. 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

  37. 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/

  38. 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

More Related