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| <title>Velocity - You make the decision - JavaBeans</title> |
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| <a name="JavaBeans"><strong>JavaBeans</strong></a> |
| </font> |
| </td></tr> |
| <tr><td> |
| <blockquote> |
| <p> |
| JavaBeans are the way to use Java objects from JSP pages in order to |
| follow the MVC design pattern. The point of doing this is to implement |
| something similar to the <a href="http://jakarta.apache.org/turbine/turbine-2/pullmodel.html">Pull |
| methodology</a>. For example: |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| <jsp:useBean id="name" scope="page|request|session|application" |
| class="className" type="typeName"> |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| Examining the syntax of the above code, the first thing that pops up |
| right away is the use of the scope attribute. How many HTML designers |
| understand the programming concepts of scope? It is safe to suggest that |
| a good portion of web designers barely understand the concept of how a |
| CGI works. By stating this, we are not trying to slight people. Instead, |
| we are simply pointing out that design and software engineering are |
| distinct skill sets. You wouldn't expect a Java programmer to select a |
| print and web safe color palette, would you? |
| </p> |
| <p> |
| The common response to an argument like this is that the designers |
| should simply ignore these tags and let others define and implement |
| them. The problem with that is that you have now given them the power to |
| accidentally wreck your entire application in such a way that it is |
| very difficult to debug because a complex scope issue might not show up |
| right away. |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| The Java code: |
| |
| public class HelloBean { |
| private String name = "World"; |
| |
| public void setName(String name) { |
| this.name = name; |
| } |
| |
| public String getName() { |
| return name; |
| } |
| } |
| |
| The JSP code: |
| |
| <jsp:useBean id="hello" class="HelloBean"> |
| <jsp:setProperty name="hello" property="*" /> |
| </jsp:useBean> |
| |
| <HTML> |
| <HEAD><TITLE>Hello</TITLE></HEAD> |
| <BODY> |
| <H1> |
| Hello, <jsp:getProperty name="hello" property="name" /> |
| </H1> |
| </BODY> |
| </HTML> |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| Above, we have a very simple example of using a bean in a page. Pass it |
| some properties and then retrieve the results. This is the right way to |
| do things when using JSP. However, if we look at an example of doing the |
| same exact thing in Velocity, the extra amount of needless typing that |
| one needs to perform to simply retrieve a property seems a just bit |
| absurd. Of course there are always GUI based drag and drop tools to make |
| typing a thing of the past. Really. |
| </p> |
| <p> |
| There are several commercial solutions available today which provide a |
| nice drag and drop view for doing development with JSP and Struts. |
| However many of these tools are still first generational tools. They |
| typicially only address parts of the problem and require digging down |
| into the nitty gritty stuff when things become difficult or even |
| impossible to do with the GUI (anyone remember a product called Tango?). |
| Often these tools also produce code that is not optimized for heavily |
| hit sites and getting an existing application to scale sometimes |
| requires a complete rewrite. Again, this is not our decision, it is |
| yours. Another item to note here is that these are costly (>$1000/seat) |
| development tools. In this .bomb economy, who really has the money to |
| spend on these tools? |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| The Java code: |
| |
| context.put ("hello", new HelloBean()); |
| |
| The Velocity code: |
| |
| $hello.setName("*") |
| <HTML> |
| <HEAD><TITLE>Hello</TITLE></HEAD> |
| <BODY> |
| <H1> |
| Hello, $hello.Name |
| </H1> |
| </BODY> |
| </HTML> |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| The example shows the creation of the HelloBean() object and then |
| placing it into the Context. Then, during runtime execution of the |
| template, that object is available as a $variable which uses the |
| JavaBean specification to do introspection on the object. For example, |
| Velocity uses Bean style introspection to permit the method call to be |
| shortened from <code>$hello.getName()</code> to simply typing what is |
| shown above. |
| </p> |
| <p> |
| When Velocity is combined with Turbine, the HelloBean object can be |
| added into the Context as a configuration option or it can be added at |
| any point of the processing. This is what provides the "scope" of the |
| object in the Context. |
| </p> |
| <p> |
| Another "gotcha" with using JavaBeans in JSP is again quoted from Jason's |
| book: |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| One thing to watch out for: On some servers (including Tomcat 3.2) if |
| you have a bean with a scope of "session" or "application" and you |
| change the bean class implementation, you may get a ClassCastException |
| on a later request. This exception occurs because the generated servlet |
| code has to do a cast on the bean instance as it's retrieved from the |
| session or application, and the old bean type stored in the session or |
| application doesn't match the new bean type expected. The simplest |
| solution is to restart the server. |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| You make the decision. |
| </p> |
| <p> |
| <strong>[ <a href="ymtd-error-handling.html">Error Handling</a> <- Previous | |
| Next -> <a href="./ymtd-sampleapp.html">Sample Application</a> ] |
| </strong></p> |
| </blockquote> |
| </p> |
| </td></tr> |
| <tr><td><br/></td></tr> |
| </table> |
| </td> |
| </tr> |
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| Copyright © 1999-2004, The Apache Software Foundation |
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