itextsharp barcode example vb.net FIGURE 4-2 The evolution of the EL over time in Java

Make DataMatrix in Java FIGURE 4-2 The evolution of the EL over time

FIGURE 4-2 The evolution of the EL over time
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Managed Beans and the JSF Expression Language
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E XPERT GROUP I NSIGHT In keeping with the design goal of being extremely extensible, it is
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possible to customize the semantics of the EL since the beginning of JSF For more on how to do so, please see 12
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PART I PART I PART I
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Unified EL Concepts
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The goal of having a Unified EL is to provide an easy and compact way to access application objects from any point in the application A major force behind the need for a compact language to refer to objects was the increased usage of tag attributes in JSP or, in general, XML Consider a tag with an attribute:
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<MyTag attribute="value" />
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If value had to be derived dynamically at runtime, it could necessitate a long Java statement to obtain the value Expression language provides a solution to this by: 1 Eliminating the need to always refer to container-managed parent objects (request, session, and application) 2 Shortening the expression by omitting get and set from object properties/methods 3 Allowing navigation of an arbitrarily complex hierarchy of JavaBeans objects
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Value Expressions
Value expressions (referred to as Value Binding Expressions in JSF 11) are by far the most common usage of expressions in JSF applications They allow for presenting (getting) dynamically evaluated results or setting bean properties at runtime using a compact expression Value expressions can be used to evaluate a single value such as the value of a property of a managed bean or an expression that evaluates a final value from multiple sources including sources outside of Faces managed beans Recall an expression used in previous examples:
<h:outputText value="#{userBeanfirstName}"/>
The expression #{userBeanfirstName} is a shorthand way to call the getfirstName( ) method of the userBean managed bean In general, value expressions are evaluated behind the scenes by a getValue( ) method from a ValueExpression instance that is provided by the JSF runtime
What Really Happens When an EL Expression Is Evaluated
As the expression #{userBeanfirstName} is evaluated at runtime, the EL system breaks the expression down into two parts The part before the first dot (or square bracket) is called the base In this case, the base is userBean The part after the first dot (or square bracket) is called a property and is recursively broken down into smaller parts; in this case it is just firstName Generally, the base says where to look for the bean instance and the property parts are interpreted as beans properties to get from the base The first thing the EL does is check if the value of the base is one of the implicit objects (described shortly) In this case, it is not, so the EL sets about looking for userBean in progressively wider scopes It does this by first calling getAttribute("userBean") on the ServletRequest Because the userBean managed
Part I:
The JavaServer Faces Framework
bean was declared in the session scope, it is not found in the ServletRequest The EL then calls getAttribute("userBean") on the HttpSession In this case, the bean is found, so it is returned (If the bean had not been found in the session, getAttribute( userBean ) would have been called on the ServletContext for this application) Once the EL has obtained the base it then proceeds to resolve properties against the base With firstName, the getFirstName( ) method is called on the UserBean instance In addition to simply displaying dynamically derived values from value expressions, the output of a value expression can also be used to set an attribute of a UI component A common usage is to set the rendered attribute to a value expression that evaluates to a boolean value Consider the example:
<h:outputText rendered="#{userBeanmanager}" value="#{Employeesalary}"/>
Notice the rendered attribute is defined with a value expression that derives a boolean value (assuming the manager field is of type boolean) by calling the method isManager( ) This allows the UIOutput component to render conditionally based on whether or not the user is a manager
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