Expression language
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The document defines and describes Expression Language (EL) which allows accessing data stored in JavaBeans from JSP pages. It discusses how EL expressions are used to retrieve and manipulate web application data. The key types of EL expressions are defined as value expressions to access object properties, method expressions to call methods, and immediate and deferred expressions based on when they are evaluated. Benefits of using EL to reduce scriptlets in JSP pages are also highlighted.
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• Syntax of value expression to access properties and elements of a
collection in java uses two operators. ( . ) and ( [] ).
• For example.
The name property of the employee JavaBean can be referred as
${employee.name} or ${ employee[“name"] } expression.
To access a specific element of a collections such as list or an
array inside a JavaBean we can use ${ employee.name[0] }, ${
employee.name[1] }, etc...](https://image.slidesharecdn.com/expressionlanguage-160504191022/85/Expression-language-10-320.jpg)
Expression language
- 1. GUJARAT TECHNOLOGICAL UNIVERSITY Chandkheda, Ahmedabad Affiliated G. H. Patel College of Engineering and Technology, VallabhVidyanagar A Presentation On Expression Language Under subject of ADVANCED JAVA Semester – VI Computer Engineering Submitted by: Hemant Suthar(130110107058) Submitted to: Prof. Hetal Gaudani
- 2. Content • Definition • Methods used to call Java Code • Benefits of EL • Types of EL • References
- 3. Definition • EL is a language that allows JSP programmers to fetch application data stored in JavaBeans component.
- 4. Methods used to call Java Code • The following methods are used to call the java code from a JSP page: • 1. Placing the entire Java code in a JSP page • 2. Defining separate helper classes that encapsulate the entire Java code and calling these helper classes from a JSP page • 3. Using JavaBeans and JSP page action tags • 4. Using JSP EL • 5. Create tag handler classes to embed the Java code
- 5. Benefits of EL • The incorporation of EL to the JSP has helped reduce the use of scriptles in JSP page. • EL expressions provide short hand notations to retrieve, present, and manipulate Web application data.
- 6. Basics of EL • EL expressions are enclosed between the ${ and} characters. • The most general example of an EL expression is ${object.data} • Where, object can be any Java object representing different scope, such as request, session.
- 7. Types of EL • EL expression tags can be categorized into the following types: • 1. Immediate and deferred expressions • 2. Value expressions • 3. Method expressions
- 8. Immediate and deferred expressions • Two constructs are used to represent EL expressions, JSP{ expr } used for expressions that need to be evaluated immediately, and #{ expr } used for expressions that need to be evaluated at later time. • An expression that uses ${ expr } is called immediate expression and • The expression that uses #{ expr } is called deferred expression
- 9. Value Expressions • Value expressions are used to refer to objects such as JavaBeans, Collections, enumerations and implicit objects and their properties. • An object is referred to by using the value expression containing the name of the object • Suppose the ${ employee } expression is used in a JSP page, where employee refers to the name of a JavaBean • Which searches for the employee JavaBean in the request, session and application scopes • If the employee JavaBean does not exist. a null value is returned.
- 10. Continue… • Syntax of value expression to access properties and elements of a collection in java uses two operators. ( . ) and ( [] ). • For example. The name property of the employee JavaBean can be referred as ${employee.name} or ${ employee[“name"] } expression. To access a specific element of a collections such as list or an array inside a JavaBean we can use ${ employee.name[0] }, ${ employee.name[1] }, etc...
- 11. Continue… • The value expressions are of two types: • rvalue rvalue expression can only read data, and not write data. • lvalue lvalue expression can read as well as write data.
- 12. Method Expressions • Method expressions are used to call public methods. which returns a value or object. • Such expressions are usually deferred expressions. • Tags usually use method expressions to call functions that perform operations such as validating a UI component or handling the events generated on a UI component. • The following code snippet shows the use of method expressions in JSF page:
- 13. Continue… <h:form> <h:inputText id="email" va1ue="#{employee.email} validator="# {employee.validateEmail} " /> <h:commandButton id="submit" action="#{customer.submit}" /> </h:form>
- 14. Continue… • The various elements shown in the preceding code snippet can be briefly described as follows: • The inputText tag : shows the Ullnput component in the form of a text field on a webpage. • The validator attribute : calls the validateEmail method of the employee JavaBean. • The action attribute of the commandButton tag : calls the submit method, which carries out processing after submitting the webpage. • The submit method : Refers to the method that is called during the invoke application phase of the JSP life cycle.
- 15. References • http://www.darshan.ac.in/Upload/DIET/Documents/CE/Advanced%2 0Java%20technology_Unit-7_05012015_081036AM.pdf • Advance Java Technology by M.T. Savaliya Published by Dreamtech Press/Wiley India, 2011