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The following are the list of changes that occurred using the Struts tag library

By Charles Warren,2014-09-16 12:52
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The following are the list of changes that occurred using the Struts tag library

    ;3

作者?friendcn

日期?2001-2-22 12:50:46

Page changes

The following are the list of changes that occurred

    using the Struts tag library:

Imports

<%@ taglib uri="/WEB-INF/struts.tld"

    prefix="struts" %>

    The <%@page import? for Java has been replaced by <%@ taglib uri? for the Struts tag library.

Text

The resource property file contains the text for

    join.title. In this example, ApplicationResources property file contains the name-value pair. This

    makes string review and changes for internationalization easier.

Errors

    ActionServlet or ActionForm builds the error message to display. These error messages can also be contained in the property file. ApplicationResources also provides a way of formatting the error by setting error.header and error.footer.

HTML Form

    JSP rm> tags and attributes replace HTML

tags and attributes. name="join"> has changed to

    action="join.do" focus="email" >.

HTML tag has been replaced by .

HTML tag has been replaced by

    .

Model -- Session state

    JoinForm subclasses ActionForm and contains the form

data. The form data in this example is simply the e-

    mail address. I have added a setter and getter for the e-mail address for the framework to access. For demonstration purposes, I overwrote the validate() method and used the error tracking feature of Struts. Struts will create JoinForm and set the state information.

Model -- Business logic

    As we discussed earlier, Action is the interface between the Controller and the actual business

    object. JoinAction wraps the calls to the business.jar that was originally in join.jsp. The perform() method for JoinAction is displayed in Listing 5.

Listing 5. - JoinAction.perform()

public ActionForward perform(ActionMapping mapping,

     ActionForm form,

     HttpServletRequest

    request,

     HttpServletResponse

    response)

     throws IOException,

    ServletException {

     // Extract attributes and parameters we will need

     JoinForm joinForm = (JoinForm) form;

     String email = joinForm.getEmail();

     ActionErrors errors = new ActionErrors();

     // store input....

     try {

     business.db.MailingList.AddEmail(email);

     } catch (Exception e) {

     // log, print stack

     // display error back to user

     errors.add("email",new ActionError("error.mailing.db.add"));

     }

     // If any messages is required, save the

    specified error messages keys

     // into the HTTP request for use by the

     tag.

     if (!errors.empty()) {

     saveErrors(request, errors);

     // return to the original form

     return (new

    ActionForward(mapping.getInput()));

     }

     // Forward control to the specified 'success' URI that is in the Action.xml

     return (mapping.findForward("success"));

}

    Note: The perform() returns a class called ActionForward that tells the Controller where to go next. In this example, I am using the mapping passed in from the Controller to determine where to go.

Controller

    I have modified the JSP file and created two new classes: one to contain form data and one to call the business package. Finally, I glue it all together with changes to the configuration file struts-config.xml. Listing 6 displays the action element I added to control the flow of joinMVC.jsp.

    Listing 6. Action Configuration path="/join"

     name="joinForm"

     type="web.mailinglist.JoinAction"

     scope="request"

     input="/joinMVC.jsp"

     validate="true">

    

    The action element describes a mapping from a request path to the corresponding Action classes that should be used to process the requests. Each request type should have a corresponding action element describing how to process the request. On a join request:

joinForm is used to hold the form data.

    Since validate is marked true, joinForm will try to validate itself.

    web.mailinglist.JoinAction is the action class used to process requests for this mapping.

If everything works correctly, the request will

    forward to welcome.jsp.

    If there is a business logic failure, the flow will

    return to joinMVC.jsp, which is the original page that made the request. Why is this? In the action element in Listing 6 is an attribute called input with a value of "/joinMVC.jsp". In my

    JoinAction.perform(), displayed in Listing 5, if the business logic fails, perform() returns an ActionForward using mapping.getInput() as the parameter. The getInput() in this instance is "/joinMVC.jsp". If the business logic fails, it will return to joinMVC.jsp, which is the original page that made the request.

Before and after Struts

    As we can see from Figure 9, a lot of complexity and layers have been added. No more direct calls from the JSP file to the Service layer.

Figure 9. Before and after Struts

Struts pros

Use of JSP tag mechanism

    The tag feature promotes reusable code and abstracts Java code from the JSP file. This feature allows nice integration into JSP-based development tools

    that allow authoring with tags.

Tag library

    Why re-invent the wheel, or a tag library? If you cannot find something you need in the library, contribute. In addition, Struts provides a starting point if you are learning JSP tag technology.

Open source

    You have all the advantages of open source, such as being able to see the code and having everyone else using the library reviewing the code. Many eyes make for great code review.

Sample MVC implementation

Struts offers some insight if you want to create

    your own MVC implementation.

Manage the problem space

    Divide and conquer is a nice way of solving the problem and making the problem manageable. Of course, the sword cuts both ways. The problem is more complex and needs more management.

Struts cons

Youth

    Struts development is still in preliminary form.

    They are working toward releasing a version 1.0, but as with any 1.0 version, it does not provide all the bells and whistles.

Change

    The framework is undergoing a rapid amount of change. A great deal of change has occurred between Struts 0.5 and 1.0. You may want to download the most current Struts nightly distributions, to avoid deprecated methods. In the last 6 months, I have seen the Struts library grow from 90K to over 270K. I had to modify my examples several times because of changes in Struts, and I am not going to guarantee my examples will work with the version of Struts you download.

Correct level of abstraction

Does Struts provide the correct level of

    abstraction? What is the proper level of abstraction for the page designer? That is the $64K question. Should we allow a page designer access to Java code in page development? Some frameworks like Velocity say no, and provide yet another language to learn for Web development. There is some validity to

    limiting Java code access in UI development. Most importantly, give a page designer a little bit of Java, and he will use a lot of Java. I saw this happen all the time in Microsoft ASP development. In ASP development, you were supposed to create COM

    objects and then write a little ASP script to glue it all together. Instead, the ASP developers would go crazy with ASP script. I would hear "Why wait for a COM developer to create it when I can program it

directly with VBScript?" Struts helps limit the

    amount of Java code required in a JSP file via tag libraries. One such library is the Logic Tag, which manages conditional generation of output, but this does not prevent the UI developer from going nuts with Java code. Whatever type of framework you

    decide to use, you should understand the environment in which you are deploying and maintaining the framework. Of course, this task is easier said than done.

Limited scope

    Struts is a Web-based MVC solution that is meant be implemented with HTML, JSP files, and servlets.

J2EE application support

    Struts requires a servlet container that supports JSP 1.1 and Servlet 2.2 specifications. This alone will not solve all your install issues, unless you

    are using Tomcat 3.2. I have had a great deal of problems installing the library with Netscape iPlanet 6.0, which is supposedly the first J2EE-

    compliant application server. I recommend visiting the Struts User Mailing List archive (see Resources)

    when you run into problems.

Complexity

    Separating the problem into parts introduces

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