Beginning Apache Struts - From Novice To Professional (2006)

execute() function. Actions must be programmed in a thread-safe manner, because Struts will share the same instance for multiple simultaneous requests. This means you should design with the following points in mind:

•Instance and static variables must not be used to store information related to the state of a particular request.

•They may be used to share global resources across requests for the same Action.

•Access to other resources (JavaBeans, session variables, etc.) must be synchronized if those resources require protection. (Generally, however, resource classes should be designed to provide their own protection where necessary.)

org.apache.struts.action.ActionForm

An ActionForm is a JavaBean optionally associated with one or more ActionMappings. Such a bean will have had its properties initialized from the corresponding request parameters before the corresponding Action.execute() function is called.

When the properties of this bean have been populated, but before the execute() function of the Action is called, this bean’s validate() function will be called, which gives the bean a chance to verify that the properties submitted by the user are correct and valid. If this function finds problems, it returns an error messages object that encapsulates those problems, and Struts will return control to the corresponding input form. Otherwise, the validate() function returns null, indicating that everything is acceptable and the corresponding Action.execute() function should be called.

The ActionForm class must be subclassed in order to be instantiated.

Subclasses should provide property getter and setter methods for all of the bean properties they wish to expose. In addition, they should override any of the public or protected methods for which they wish to provide modified functionality.

Because ActionForms are JavaBeans, subclasses should also implement Serializable, as required by the JavaBean specification. Some containers require that an object meet all JavaBean requirements in order to use the introspection API upon which ActionForms rely.

org.apache.struts.upload.FormFile

This interface is used to represent a file uploaded by a client.

org.apache.struts.tiles.ComponentContext

This class is the equivalent of HttpServletRequest for a Tile.

org.apache.struts.action.ExceptionHandler

An ExceptionHandler is a special class to handle exceptions generated in your ActionForm or Action subclasses. Refer to Chapter 9 on how to declare and use ExceptionHandlers.

A P P E N D I X C

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Struts Tag Reference

This appendix is a reference on all Struts 1.2 tags, from the five tag libraries: HTML, Bean, Logic, Nested, and Tiles. We’ve included usage examples and information on Struts-EL and JSTL (see Chapter 10 for both), as well as JSF and Struts-Faces Integration Library (see Chapter 20 for both) equivalent tags where appropriate. This reference does not include the obsolete Template tag library, which has been superseded by the Tiles tag library.

To make this reference more compact, we’ve collected attributes that occur in more than one tag under one or more common attribute sets. These are introduced at the start of each section describing a tag library.

The HTML Tag Library

The custom tags in the HTML library are essentially in one-to-one relationship to the ordinary HTML <form> tag and its associated input tags, like the various <input> tags.

The purpose of this tag library is to enable you to connect the View tier to the Controller tier of your Struts webapp. The linkage between the two is done statically in struts-config.xml.

The tags in the HTML library can be divided into four groups based on function:

•Form tags: These tags transfer data from View to Controller. The tags are mostly in one-to-one correspondence with their HTML equivalents (<form> and the various HTML data input tags), but there are extra Struts-specific tags (e.g., multibox) that do much more to facilitate the easy transfer of data between View and Controller.

•Message tags: These tags display (in the View tier) messages that originate from the Controller.

•URL tags: These tags are counterparts to ordinary HTML tags that require a URL to function. The Struts versions exist primarily to allow you to use the names of global forwards instead of actual URLs.

•Miscellaneous tags: These tags have no clear functional grouping and have to be treated individually.

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Table C-5. The evt-attrs Attribute Set

The Accessibility Attribute Set

The next set is also HTML related, and these attributes aid in accessibility.

Table C-6 lists these tags, which we’ll refer to as the Accessibility attribute set, or acc-attrs for short.

Table C-6. The acc-attrs Attribute Set

Most attributes in Table C-6 are simply mirror images of their HTML counterparts. The exceptions are altKey and titleKey, which use Struts’ message resource file to produce a localized version of the actual attribute (alt and title, respectively).

The Error Style Attribute Set (Struts 1.2.5+)

The Error Style attribute set (err-attrs) was newly introduced with Struts 1.2.5. These attributes enable you to specify an error style if a given input element hits a validation error. Table C-9 describes these attributes.

Table C-9. The err-attrs Attribute Set

Struts-EL Tags for the HTML Tag Library

All tags in the HTML library have EL-enabled versions.

■Note EL-enabled tags are those that allow you to use EL expressions. Refer to Chapter 10 for examples.

base

This tag is used to allow you to conveniently create an HTML <base> tag.

The HTML <base> tag allows you to specify the base URL from which to calculate relative paths (e.g., ../../myscripts.js or mypage.html) in your JSPs. You might think that such URLs are resolved using the absolute path of the enclosing JSP page, but this isn’t necessarily the case. The only way to guarantee this is to use the <base> tag, or much more conveniently, <html:base>.

Usage Restrictions

Both <base> and <html:base> must be a child tag of HTML’s <head> tag.