Backing Beans - The Java EE 5 Tutorial (original) (raw)

Backing Beans

A typical JavaServer Faces application includes one or more backing beans, each of which is a JavaServer Faces managed bean that is associated with the UI components used in a particular page. Managed beans are JavaBeans components (see JavaBeans Components) that you can configure using the managed bean facility, which is described inConfiguring Beans. This section introduces the basic concepts on creating, configuring, and using backing beans in an application.

Creating a Backing Bean Class

In addition to defining a no-arg constructor, as all JavaBeans components must do, a backing bean class also defines a set of UI component properties and possibly a set of methods that perform functions for a component.

Each of the component properties can be bound to one of the following:

A component’s value
A component instance
A converter instance
A listener instance
A validator instance

The most common functions that backing bean methods perform include the following:

Validating a component’s data
Handling an event fired by a component
Performing processing to determine the next page to which the application must navigate

As with all JavaBeans components, a property consists of a private data field and a set of accessor methods, as shown by this code from the Guess Number example:

Integer userNumber = null; ... public void setUserNumber(Integer user_number) { userNumber = user_number; } public Integer getUserNumber() { return userNumber; } public String getResponse() { ... }

Because backing beans follow JavaBeans component conventions, you can reference beans you’ve already written from your JavaServer Faces pages.

When a bean property is bound to a component’s value, it can be any of the basic primitive and numeric types or any Java object type for which the application has access to an appropriate converter. For example, a property can be of type Date if the application has access to a converter that can convert the Date type to a String and back again. See Writing Bean Properties for information on which types are accepted by which component tags.

When a bean property is bound to a component instance, the property’s type must be the same as the component object. For example, if aUISelectBoolean is bound to the property, the property must accept and return a UISelectBoolean object.

Likewise, if the property is bound to a converter, validator, or listener instance then the property must be of the appropriate converter, validator, or listener type.

For more information on writing beans and their properties, see Writing Bean Properties.

Configuring a Bean

JavaServer Faces technology supports a sophisticated managed bean creation facility, which allows application architects to do the following:

Configure simple beans and more complex trees of beans
Initialize bean properties with values
Place beans in a particular scope
Expose the beans to the unified EL so that page authors can access them

An application architect configures the beans in the application configuration resource file. To learn how to configure a managed bean, see Configuring Beans. The managed bean configuration used by the Guess Number example is the following:

UserNumberBean guessNumber.UserNumberBean session minimum long 0 maximum long 10

The JavaServer Faces implementation processes this element on application startup time. When UserNumberBeanis first referenced from the page, the JavaServer Faces implementation initializes it and sets the values of the properties, maximum and minimum. The bean is then stored in session scope if no instance exists. As such, the bean is available for all pages in the application.

A page author can then access the bean properties from the component tags on the page using the unified EL, as shown here:

<h:outputText value="#{UserNumberBean.minimum}"/>

The part of the expression before the . matches the name defined by the managed-bean-name element. The part of the expression after the . matches the name defined by the property-name element corresponding to the same managed-bean declaration.

Notice that the application configuration resource file does not configure the userNumber property. Any property that does not have a corresponding managed-property element will be initialized to whatever the constructor of the bean class has the instance variable set to. The next section explains more about using the unified EL to reference backing beans.

For more information on configuring beans using the managed bean creation Facility, seeConfiguring Beans.

Using the Unified EL to Reference Backing Beans

To bind UI component values and objects to backing bean properties or to reference backing bean methods from UI component tags, page authors use the unified expression language (EL) syntax defined by JSP 2.1. As explained in Unified Expression Language, some of the features this language offers are:

Deferred evaluation of expressions
The ability to use a value expression to both read and write data
Method expressions

These features are all especially important for supporting the sophisticated UI component model offered by JavaServer Faces technology.

Deferred evaluation of expressions is important because the JavaServer Faces life cycle is split into separate phases so that component event handling, data conversion and validation, and data propagation to external objects are all performed in an orderly fashion. The implementation must be able to delay the evaluation of expressions until the proper phase of the life cycle has been reached. Therefore, its tag attributes always use deferred evaluation syntax, which is distinguished by the #{} delimiters. The Life Cycle of a JavaServer Faces Pagedescribes the life cycle in detail.

In order to store data in external objects, almost all JavaServer Faces tag attributes use lvalue value expressions, which are expressions that allow both getting and setting data on external objects.

Finally, some component tag attributes accept method expressions that reference methods that handle component events, or validate or convert component data.

To illustrate a JavaServer Faces tag using the unified EL, let’s suppose that the userNo tag of the guessNumber application referenced a method rather than usingLongRangeValidator to perform the validation of user input :

<h:inputText id="userNo" value="#{UserNumberBean.userNumber}" validator="#{UserNumberBean.validate}" />

This tag binds the userNo component’s value to the UserNumberBean.userNumber backing bean property using an lvalue expression. It uses a method expression to refer to theUserNumberBean.validate method, which performs validation of the component’s local value. The local value is whatever the user enters into the field corresponding to this tag. This method is invoked when the expression is evaluated, which is during the process validation phase of the life cycle.

Nearly all JavaServer Faces tag attributes accept value expressions. In addition to referencing bean properties, value expressions can also reference lists, maps, arrays, implicit objects, and resource bundles.

Another use of value expressions is binding a component instance to a backing bean property. A page author does this by referencing the property from thebinding attribute:

Those component tags that use method expressions are UIInput component tags and UICommandcomponent tags. See sections Using Text Components and Using Command Components for Performing Actions and Navigation for more information on how these component tags use method expressions.

In addition to using expressions with the standard component tags, you can also configure your custom component properties to accept expressions by creating ValueExpression or MethodExpressioninstances for them. See Creating Custom Component Classes and Enabling Component Properties to Accept Expressions for more information on enabling your component’s attributes to support expressions.

To learn more about using expressions to bind to backing bean properties, seeBinding Component Values and Instances to External Data Sources.

For information on referencing backing bean methods from component tags, see Referencing a Backing Bean Method.