WEB框架 之 Struts控制流程

The Struts framework provides several components that make up the Control layer of a MVC-style application. These include a controller servlet, developer-defined request handlers, and several supporting objects.

The Struts custom tag libraries provide direct support for the View layer of a MVC application. Some of these access the control-layer objects. Others are generic tags found convenient when writing applications. Other taglibs, including JSTL, can also be used with Struts. Other presentation technologies, like Velocity Templates and XSLT can also be used with Struts.

The Model layer in a MVC application is often project-specific. Struts is designed to make it easy to access the business-end of your application, but leaves that part of the programming to other products, like JDBC, Enterprise Java Beans, Object Relational Bridge, or Simper, to name a few.

Let's step through how this all fits together.

When initialized, the controller parses a configuration file (struts-config.xml) and uses it to deploy other control layer objects. Together, these objects form the Struts Configuration. The Struts Configuration defines (among other things) the ActionMappings [org.apache.struts.action.ActionMappings] for an application.

The Struts controller servlet consults the ActionMappings as it routes HTTP requests to other components in the framework. Requests may be forwarded to JavaServer Pages or Action [org.apache.struts.action.Action] subclasses provided by the Struts developer. Often, a request is first forwarded to an Action and then to a JSP (or other presentation page). The mappings help the controller turn HTTP requests into application actions.

An individual ActionMapping [org.apache.struts.action.ActionMapping] will usually contain a number of properties including:

• a request path (or "URI"),
• the object type (Action subclass) to act upon the request, and
• other properties as needed.

The Action object can handle the request and respond to the client (usually a Web browser) or indicate that control should be forwarded elsewhere. For example, if a login succeeds, a login action may wish to forward the request onto the mainMenu page.

Action objects have access to the application's controller servlet, and so have access to that servlet's methods. When forwarding control, an Action object can indirectly forward one or more shared objects, including JavaBeans, by placing them in one of the standard contexts shared by Java Servlets.

For example, an Action object can create a shopping cart bean, add an item to the cart, place the bean in the session context, and then forward control to another mapping. That mapping may use a JavaServer Page to display the contents of the user's cart. Since each client has their own session, they will each also have their own shopping cart.

In a Struts application, most of the business logic can be represented using JavaBeans. An Action can call the properties of a JavaBean without knowing how it actually works. This encapsulates the business logic, so that the Action can focus on error handling and where to forward control.

JavaBeans can also be used to manage input forms. A key problem in designing Web applications is retaining and validating what a user has entered between requests. With Struts, you can define your own set of input bean classes, by subclassing ActionForm [org.apache.struts.action.ActionForm]. The ActionForm class makes it easy to store and validate the data for your application's input forms. The ActionForm bean is automatically saved in one of the standard, shared context collections, so that it can be used by other objects, like an Action object or another JSP.

The form bean can be used by a JSP to collect data from the user ... by an Action object to validate the user-entered data ... and then by the JSP again to re-populate the form fields. In the case of validation errors, Struts has a shared mechanism for raising and displaying error messages.

Another element of the Struts Configuration are the ActionFormBeans [org.apache.struts.action.ActionFormBeans]. This is a collection of descriptor objects that are used to create instances of the ActionForm objects at runtime. When a mapping needs an ActionForm, the servlet looks up the form-bean descriptor by name and uses it to create an ActionForm instance of the specified type.

Here is the sequence of events that occur when a request calls for an mapping that uses an ActionForm:

• The controller servlet either retrieves or creates the ActionForm bean instance.
• The controller servlet passes the bean to the Action object.
• If the request is being used to submit an input page, the Action object can examine the data. If necessary, the data can be sent back to the input form along with a list of messages to display on the page. Otherwise the data can be passed along to the business tier.
• If the request is being used to create an input page, the Action object can populate the bean with any data that the input page might need.

The Struts framework includes custom tags that can automatically populate fields from a JavaBean. All most JavaServer Pages really need to know about the rest of the framework is the field names to use and where to submit the form.

Other Struts tags can automatically output messages queued by an Action or ActionForm and simply need to be integrated into the page's markup. The messages are designed for localization and will render the best available message for a user's locale.

The Struts framework and its custom tag libraries were designed from the ground-up to support the internationalization features built into the Java platform. All the field labels and messages can be retrieved from a message resource. To provide messages for another language, simply add another file to the resource bundle.

Internationalism aside, other benefits to the message resources approach are consistent labeling between forms, and the ability to review all labels and messages from a central location.

For the simplest applications, an Action object may sometimes handle the business logic associated with a request. However, in most cases, an Action object should invoke another object, usually a JavaBean, to perform the actual business logic. This lets the Action focus on error handling and control flow, rather than business logic. To allow reuse on other platforms, business-logic JavaBeans should not refer to any Web application objects. The Action object should translate needed details from the HTTP request and pass those along to the business-logic beans as regular Java variables.

In a database application, for example:

• A business-logic bean will connect to and query the database,
• The business-logic bean returns the result to the Action,
• The Action stores the result in a form bean in the request,
• The JavaServer Page displays the result in a HTML form.

Neither the Action nor the JSP need to know (or care) from where the result comes. They just need to know how to package and display it.

Other chapters in this document cover the various Struts components in greater detail. The Struts release also includes several Developer Guides covering various aspects of the frameworks, along with sample applications, the standard Javadoc API, and, of course, the complete source code!

Struts is distributed under the Apache Software Foundation license. The code is copyrighted, but is free to use in any application.