Now that we understand how to constrUCt the Model and View components of your application, it is time to focus on the Controller components. Struts includes a servlet that implements the PRimary function of mapping a request URI to an Action class. Therefore, your primary responsibilities related to the Controller are:

• Write an ActionForm class to mediate between the Model and the View. (See also Building an ActionForm).
• Write an Action class for each logical request that may be received (extend org.apache.struts.action.Action).
• Configure a ActionMapping (in xml) for each logical request that may be submitted. The XML configuration file is usually named struts-config.xml.

To deploy your application, you will also need to:

• Update the web application deployment descriptor file (in XML) for your application to include the necessary Struts components.
• Add the appropriate Struts components to your application.

The latter two items are covered in the "Configuring Applications" chapter.

For those of you familiar with MVC architecture, the ActionServlet represents the C - the controller. The job of the controller is to:

• process user requests,
• determine what the user is trying to achieve according to the request,
• pull data from the model (if necessary) to be given to the appropriate view, and
• select the proper view to respond to the user.

The Struts controller delegates most of this grunt work to the Request Processor and Action classes.

In addition to being the front controller for your application, the ActionServlet instance also is responsible for initialization and clean-up of resources. When the controller initializes, it first loads the application config corresponding to the "config" init-param. It then goes through an enumeration of all init-param elements, looking for those elements who's name starts with config/. For each of these elements, Struts loads the configuration file specified by the value of that init-param, and assigns a "prefix" value to that module's ModuleConfig instance consisting of the piece of the init-param name following "config/". For example, the module prefix specified by the init-param config/foo would be "foo". This is important to know, since this is how the controller determines which module will be given control of processing the request. To access the module foo, you would use a URL like:

http://localhost:8080/myApp/foo/someAction.do

For each request made of the controller, the method process(HttpServletRequest, HttpServletResponse) will be called. This method simply determines which module should service the request and then invokes that module's RequestProcessor's process method, passing the same request and response.

The RequestProcessor is where the majority of the core processing occurs for each request. Let's take a look at the helper functions the process method invokes in-turn:

An ActionForm represents an Html form that the user interacts with over one or more pages. You will provide properties to hold the state of the form with getters and setters to access them. ActionForms can be stored in either the session (default) or request scopes. If they're in the session it's important to implement the form's reset method to initialize the form before each use. Struts sets the ActionForm's properties from the request parameters and sends the validated form to the appropriate Action's execute method.

When you code your ActionForm beans, keep the following principles in mind:

• The ActionForm class itself requires no specific methods to be implemented. It is used to identify the role these particular beans play in the overall architecture. Typically, an ActionForm bean will have only property getter and property setter methods, with no business logic.
• The ActionForm object also offers a standard validation mechanism. If you override a "stub" method, and provide error messages in the standard application resource, Struts will automatically validate the input from the form (using your method). See "Automatic Form Validation" for details. Of course, you can also ignore the ActionForm validation and provide your own in the Action object.
• Define a property (with associated getXxx and setXxx methods) for each field that is present in the form. The field name and property name must match according to the usual javaBeans conventions (see the Javadoc for the java.beans.Introspector class for a start on information about this). For example, an input field named username will cause the setUsername method to be called.
• Buttons and other controls on your form can also be defined as properties. This can help determine which button or control was selected when the form was submitted. Remember, the ActionForm is meant to represent your data-entry form, not just the data beans.
• Think of your ActionForm beans as a firewall between HTTP and the Action. Use the validate method to ensure all required properties are present, and that they contain reasonable values. An ActionForm that fails validation will not even be presented to the Action for handling.
• You may also place a bean instance on your form, and use nested property references. For example, you might have a "customer" bean on your ActionForm, and then refer to the property "customer.name" in your presentation page. This would correspond to the methods customer.getName() and customer.setName(string Name) on your customer bean. See the Tag Library Developer Guides for more about using nested syntax with the Struts jsp tags.
• Caution: If you nest an existing bean instance on your form, think about the properties it exposes. Any public property on an ActionForm that accepts a single String value can be set with a query string. It may be useful to place beans that can affect the business state inside a thin "wrapper" that exposes only the properties required. This wrapper can also provide a filter to be sure runtime properties are not set to inappropriate values.

Maintaining a separate concrete ActionForm class for each form in your Struts application is time-consuming. It is particularly frustrating when all the ActionForm does is gather and validate simple properties that are passed along to a business JavaBean.

This bottleneck can be alleviated through the use of DynaActionForm classes. Instead of creating a new ActionForm subclass and new get/set methods for each of your bean's properties, you can list its properties, type, and defaults in the Struts configuration file.

For example, add the following to struts-config.xml for a UserForm bean that stores a user's given and family names:

<form-bean     name="UserForm"     type="org.apache.struts.action.DynaActionForm">    <form-property         name="givenName"         type="java.lang.String"         initial="John"/>    <form-property         name="familyName"         type="java.lang.String"         initial="Smith"/></form-bean>

The types supported by DynaActionForm include:

• java.lang.BigDecimal
• java.lang.BigInteger
• boolean and java.lang.Boolean
• byte and java.lang.Byte
• char and java.lang.Character
• java.lang.Class
• double and java.lang.Double
• float and java.lang.Float
• int and java.lang.Integer
• long and java.lang.Long
• short and java.lang.Short
• java.lang.String
• java.sql.Date
• java.sql.Time
• java.sql.Timestamp

You may also specify Arrays of these types (e.g. String[]). You may also specify a concrete implementation of the Map Interface, such as java.util.HashMap, or a List implementation, such as java.util.ArrayList.

If you do not supply an initial attribute, numbers will be initialized to 0 and objects to null.

In JSP pages using the original Struts custom tags, attributes of DynaActionForm objects can be referenced just like ordinary ActionForm objects. Wherever a Struts tag refers to a "property", the tags will automatically use the DynaActionForm properties just like those of a conventional JavaBean. You can even expose DynaActionForm properties using bean:define. (Although, tou can't use bean:define to instantiate a DynaActionForm, since it needs to be setup with the appropriate dyna-properties).

If you are using the Struts JSTL EL taglib, the references are different, however. Only properties of ordinary ActionForm objects can be directly accessed through the JSTL expression language syntax. The DynaActionForm properties must be accessed through a slightly different syntax. The JSTL EL syntax for referencing a property of an ActionForm goes like this:

${formbean.prop}

The syntax for referencing a property of a DynaActionForm would be:

${dynabean.map.prop}

The map property is a property of DynaActionForm which represents the HashMap containing the DynaActionForm properties.

DynaActionForms are meant as an easy solution to a common problem: Your ActionForms use simple properties and standard validations, and you just pass these properties over to another JavaBean (say using BeanUtils.copyProperties(myBusinessBean,form)).

DynaActionForms are not a drop-in replacement for ActionForms. If you need to access ActionForm properties in your Action, you will need to use the map-style accessor, like myForm.get("name"). If you actively use the ActionForm object in your Action, then you may want to use conventional ActionForms instead.

DynaActionForms cannot be instantiated using a no-argument constructor. In order to simulate the extra properties, there is a lot of machinery involved in their construction. You must rely on Struts to instantiate a DynaActionForm for you, via the ActionMapping.

If need be, you can extend the DynaActionForm to add custom validate and reset methods you might need. Simply specify your subclass in the struts-config instead. However, you cannot mix conventional properties and DynaProperties. A conventional getter or setter on a DynaActionForm won't be found by the reflection utilities.

To use DynaActionForms with the Struts Validator, specify org.apache.struts.validator.ValidatorActionForm (or your subclass) as the form-bean class.

And, of course, while the DynaActionForm may support various binary types, properties used with the html:text tag should still be String properties.

DynaActionForms relieve developers of maintaining simple ActionForms. For even less maintenance, try Niall Pemberton's LazyActionForm.

The DynaActionForm classes offer the ability to create ActionForm beans at initialization time, based on a list of properties enumerated in the Struts configuration file. However, many HTML forms are generated dynamically at request time. Since the properties of these forms' ActionForm beans are not all known ahead of time, we need a new approach.

Struts allows you to make one or more of your ActionForm's properties' values a Map instead of a traditional atomic object. You can then store the data from your form's dynamic fields in that Map. Here is an example of a map-backed ActionForm class:

public FooForm extends ActionForm {    private final Map values = new HashMap();    public void setValue(String key, Object value) {        values.put(key, value);    }    public Object getValue(String key) {        return values.get(key);    }}

In its corresponding JSP page, you can access objects stored in the values map using a special notation: mapname(keyname). The parentheses in the bean property name indicate that:

• The bean property named mapname is indexed using Strings (probably backed by a Map), and that
• Struts should look for get/set methods that take a String key parameter to find the correct sub-property value. Struts will, of course, use the keyname value from the parentheses when it calls the get/set methods.

Here is a simple example:

<html:text property="value(foo)"/>

This will call the getValue method on FooForm with a key value of "foo" to find the property value. To create a form with dynamic field names, you could do the following:

<% 	for (int i = 0; i < 10; i++) {		String name = "value(foo-" + i + ")";%>		<html:text property="<%= name %>"/>		<br/><%	}%>

Note that there is nothing special about the name value. Your map-backed property could instead be named property, thingy, or any other bean property name you prefer. You can even have multiple map-backed properties on the same bean.

In addition to map-backed properties, you can also create list-backed properties. You do so by creating indexed get/set methods on your bean:

public FooForm extends ActionForm {    private final List values = new ArrayList();    public void setValue(int key, Object value) {        values.set(key, value);    }    public Object getValue(int key) {        return values.get(key);    }}

In your presentation pages, you access individual entries in a list-backed property by using a different special notation: listname[index]. The braces in the bean property name indicate that the bean property named listname is indexed (probably backed by a List), and that Struts should look for get/set methods that take an index parameter in order to find the correct sub-property value.

While map-backed ActionForms provide you with more flexibility, they do not support the same range of syntax available to conventional or DynaActionForms. You might have difficulty referencing indexed or mapped properties using a map-backed ActionForm. The validwhen validator (since Struts 1.2.1) also does not support map-backed ActionForms.

The Action class defines two methods that could be executed depending on your servlet environment:

public ActionForward execute(ActionMapping mapping,                     ActionForm form,                     ServletRequest request,                     ServletResponse response)throws Exception;public ActionForward execute(ActionMapping mapping,                     ActionForm form,                     HttpServletRequest request,                     HttpServletResponse response)throws Exception;

Since the majority of Struts projects are focused on building web applications, most projects will only use the "HttpServletRequest" version. A non-HTTP execute() method has been provided for applications that are not specifically geared towards the HTTP protocol.

The goal of an Action class is to process a request, via its execute method, and return an ActionForward object that identifies where control should be forwarded (e.g. a JSP, Tile definition, Velocity template, or another Action) to provide the appropriate response. In the MVC/Model 2 design pattern, a typical Action class will often implement logic like the following in its execute method:

• Validate the current state of the user's session (for example, checking that the user has successfully logged on). If the Action class finds that no logon exists, the request can be forwarded to the presentation page that displays the username and passWord prompts for logging on. This could occur because a user tried to enter an application "in the middle" (say, from a bookmark), or because the session has timed out, and the servlet container created a new one.
• If validation is not complete, validate the form bean properties as needed. If a problem is found, store the appropriate error message keys as a request attribute, and forward control back to the input form so that the errors can be corrected.
• Perform the processing required to deal with this request (such as saving a row into a database). This can be done by logic code embedded within the Action class itself, but should generally be performed by calling an appropriate method of a business logic bean.
• Update the server-side objects that will be used to create the next page of the user interface (typically request scope or session scope beans, depending on how long you need to keep these items available).
• Return an appropriate ActionForward object that identifies the presentation page to be used to generate this response, based on the newly updated beans. Typically, you will acquire a reference to such an object by calling findForward on either the ActionMapping object you received (if you are using a logical name local to this mapping), or on the controller servlet itself (if you are using a logical name global to the application).

In Struts 1.0, Actions called a perform method instead of the now-preferred execute method. These methods use the same parameters and differ only in which exceptions they throw. The elder perform method throws SerlvetException and IOException. The new execute method simply throws Exception. The change was to facilitate the Declarative Exception handling feature introduced in Struts 1.1.

The perform method may still be used in Struts 1.1 but is deprecated. The Struts 1.1 method simply calls the new execute method and wraps any Exception thrown as a ServletException.

Remember the following design guidelines when coding Action classes:

• Write code for a multi-threaded environment - The controller servlet creates only one instance of your Action class, and uses this one instance to service all requests. Thus, you need to write thread-safe Action classes. Follow the same guidelines you would use to write thread-safe Servlets. Here are two general guidelines that will help you write scalable, thread-safe Action classes:
  • Only Use Local Variables - The most important principle that aids in thread-safe coding is to use only local variables, not instance variables, in your Action class. Local variables are created on a stack that is assigned (by your JVM) to each request thread, so there is no need to worry about sharing them. An Action can be factored into several local methods, so long as all variables needed are passed as method parameters. This assures thread safety, as the JVM handles such variables internally using the call stack which is associated with a single Thread.
  • Conserve Resources - As a general rule, allocating scarce resources and keeping them across requests from the same user (in the user's session) can cause scalability problems. For example, if your application uses JDBC and you allocate a separate JDBC connection for every user, you are probably going to run in some scalability issues when your site suddenly shows up on Slashdot. You should strive to use pools and release resources (such as database connections) prior to forwarding control to the appropriate View component -- even if a bean method you have called throws an exception.
• Don't throw it, catch it! - Ever used a commercial website only to have a stack trace or exception thrown in your face after you've already typed in your credit card number and clicked the purchase button? Let's just say it doesn't inspire confidence. Now is your chance to deal with these application errors - in the Action class. If your application specific code throws expections you should catch these exceptions in your Action class, log them in your application's log (servlet.log("Error message", exception)) and return the appropriate ActionForward.

It is wise to avoid creating lengthy and complex Action classes. If you start to embed too much logic in the Action class itself, you will begin to find the Action class hard to understand, maintain, and impossible to reuse. Rather than creating overly complex Action classes, it is generally a good practice to move most of the persistence, and "business logic" to a separate application layer. When an Action class becomes lengthy and procedural, it may be a good time to refactor your application architecture and move some of this logic to another conceptual layer; otherwise, you may be left with an inflexible application which can only be accessed in a web-application environment. Struts should be viewed as simply the foundation for implementing MVC in your applications. Struts provides you with a useful control layer, but it is not a fully featured platform for building MVC applications, soup to nuts.

The MailReader example application included with Struts stretches this design principle somewhat, because the business logic itself is embedded in the Action classes. This should be considered something of a bug in the design of the example, rather than an intrinsic feature of the Struts architecture, or an approach to be emulated. In order to demonstrate, in simple terms, the different ways Struts can be used, the MailReader application does not always follow best practices.

You can define an ExceptionHandler to execute when an Action's execute method throws an Exception. First, you need to subclass org.apache.struts.action.ExceptionHandler and override the execute method. Your execute method should process the Exception and return an ActionForward object to tell Struts where to forward to next. Then you configure your handler in struts-config.xml like this:

<global-exceptions>    <exception       key="some.key"       type="java.io.IOException"       handler="com.yourcorp.ExceptionHandler"/></global-exceptions>

This configuration element says that com.yourcorp.ExceptionHandler.execute will be called when any IOException is thrown by an Action. The key is a key into your message resources properties file that can be used to retrieve an error message.

You can override global exception handlers by defining a handler inside an action definition.

A common use of ExceptionHandlers is to configure one for java.lang.Exception so it's called for any exception and log the exception to some data store.

The PlugIn interface extends Action and so that applications can easily hook into the ActionServlet lifecycle. This interface defines two methods, init() and destroy(), which are called at application startup and shutdown, respectively. A common use of a Plugin Action is to configure or load application-specific data as the web application is starting up.

At runtime, any resource setup by init would be accessed by Actions or business tier classes. The PlugIn interface allows you to setup resources, but does not provide any special way to access them. Most often, the resource would be stored in application context, under a known key, where other components can find it.

PlugIns are configured using <plug-in> elements within the Struts configuration file. See PlugIn Configuration for details.

In order to Operate successfully, the Struts controller servlet needs to know several things about how each request URI should be mapped to an appropriate Action class. The required knowledge has been encapsulated in a Java class named ActionMapping, the most important properties are as follows:

• type - Fully qualified Java class name of the Action implementation class used by this mapping.
• name - The name of the form bean defined in the config file that this action will use.
• path - The request URI path that is matched to select this mapping. See below for examples of how matching works and how to use wildcards to match multiple request URIs.
• unknown - Set to true if this action should be configured as the default for this application, to handle all requests not handled by another action. Only one action can be defined as a default within a single application.
• validate - Set to true if the validate method of the action associated with this mapping should be called.
• forward - The request URI path to which control is passed when this mapping is invoked. This is an alternative to declaring a type property.

How does the controller servlet learn about the mappings you want? It would be possible (but tedious) to write a small Java class that simply instantiated new ActionMapping instances, and called all of the appropriate setter methods. To make this process easier, Struts uses the Jakarta Commons Digester component to parse an XML-based description of the desired mappings and create the appropriate objects initialized to the appropriate default values. See the Jakarta Commons website for more information about the Digester.

The developer's responsibility is to create an XML file named struts-config.xml and place it in the WEB-INF Directory of your application. This format of this document is described by the Document Type Definition (DTD) maintained at http://struts.apache.org/dtds/struts-config_1_2.dtd. This chapter covers the configuration elements that you will typically write as part of developing your application. There are several other elements that can be placed in the struts-config file to customize your application. See "Configuring Applications" for more about the other elements in the Struts configuration file.

The controller uses an internal copy of this document to parse the configuration; an Internet connection is not required for operation.

The outermost XML element must be <struts-config>. Inside of the <struts-config> element, there are three important elements that are used to describe your actions:

• <form-beans>
• <global-forwards>
• <action-mappings>

<form-beans>