Using JavaBeans in JavaServer Pages - Chương 3

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Session Objectives  Describe the various features of a JavaBean  Differentiate between a JavaBean and a Java class  Describe the basic structure of a JavaBean  Describe the various ways of creating beans  Describe the advantages of using beans  Explain how to access beans through JSP scriptlets  Describe the use of various JSP bean tags  Define the scope of beans

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Nội dung Text: Using JavaBeans in JavaServer Pages - Chương 3

Using JavaBeans in JavaServer Pages Chương 3
Session Objectives  Describe the various features of a JavaBean  Differentiate between a JavaBean and a Java class  Describe the basic structure of a JavaBean  Describe the various ways of creating beans  Describe the advantages of using beans  Explain how to access beans through JSP scriptlets  Describe the use of various JSP bean tags  Define the scope of beans / 2 of 36
What is a JavaBean? JavaBeans brings component technology to the Java platform With the help of the JavaBeans API, you can create reusable and platformindependent components These components can be combined into applets, applications, or composite components JavaBean or a Bean, is a simple Java class that follows a set of naming and design conventions, outlined by the JavaBeans specifications / 3 of 36
Difference between a JavaBean and a Java class A bean is a Java class with the following additional characteristics:  The class must be instantiable  It must have a default constructor  It must be serializable  It must follow the JavaBeans design patterns / 4 of 36
JavaBeans Design Patterns  Naming conventions for bean methods and data members  Design patterns are important for introspection  Introspection: mechanism using components makes its internal structure known to the outside world?  Methods are exposed through reflection / 5 of 36
An Example of a JavaBean Property public class Tower { private float height; public float getHeight() { getPropertyName() return height; } public void setHeight(float h) { height = h; } setPropertyName() public boolean isGreaterHeight(int initialHeight, int finalHeight) { if((finalHeight - initialHeight) > 0) { return true; } else { return false;} } public Tower() { height = (float)10.5; } } / 6 of 36
Using JSP Bean Tags  JSP provides three basic bean tags:  To find and use the bean – jsp:useBean  To set one or more properties – jsp:setProperty  To get a property – jsp:getProperty  These are also known as actions and are specific tags that affect the following:  the runtime behavior of the JSP  the responses that are sent back to the client / 7 of 36
Using JSP Bean Tags – (1) getProperty_Name() setProperty_Name(value) The JSP need not know the inner structure of the bean Bean The Black box approach / 8 of 36
jsp:useBean  This is used to associate a JavaBean in JSP.  It ensures that the object can be referenced from JSP, using an ID and scope The bean class must be available  The syntax is as follows: to the JSP engine
jsp:useBean – (1) beanDetails is one of:  class = "className"  class = "className" type = "typeName"  beanName = "beanName" type = "typeName"  type = "typeName" / 10 of 36
Scope of Beans  page  The bean will disappear as soon as the current page finishes generating  References to this object will only be released after the response is sent back to the client  request  The bean instance will be available as long as the request object is available  References to this object will be released only after the request is processed completely / 11 of 36
Scope of Beans – (1)  session  The instance will be available across a particular session between the browser of a particular machine and the Web server  References to this object will be released only after the associated sessions end  application  The instance will be available to all users and pages of the application  It is released only when the runtime environment reclaims the ServletContext / 12 of 36
An Example The scope is set . to the session . This is used to instantiate the bean Hello World! / 13 of 36
Session Scope  Beans with session scope are accessible only within pages processing requests that are in the same session as the one in which the bean was created  Beans cannot be defined in a page whose page directive has an attribute session=false  References to the session scope are stored in the session object  The bean is distinct for every client and is valuable as long as the client’s session is valid / 14 of 36
Session Scope – (1) An Example The first JSP . . . Instantiate the Counter bean A Session bean: The First Example The current count for the counter bean is: . . . / 15 of 36
Session Scope – (2) An Example The second JSP . . . Instantiate the Counter bean A Session bean: The Second Example The current count for the counter bean is: . . . / 16 of 36
Session Scope – (3)  When you browse the first JSP for the first time, the “current count of the counter bean” will be equal to 1.  When you browse the second JSP with the same instance of the browser, the counter increases from the last value shown in the first JSP.  If a new instance of the browser is opened, the current count attribute will be reset. Each instance of a client creates its own instance of the HttpSession, which is where the Counter bean is stored. / 17 of 36
Application Scope  Beans with the application scope are accessible within pages that are in the same application  References to the object will be released only when the runtime environment reclaims the ServletContext object.  Beans with the application scope are best used when there is a need for sharing the information among JSPs and servlets for the life of an application.  All clients access the same object / 18 of 36
Application Scope – (1) An Example The first JSP . . . Instantiate the Counter bean An Application bean: The First Example The current count for the counter bean is: . . . / 19 of 36
Application Scope – (2) An Example The second JSP . . . Instantiate the Counter bean An Application bean: The Second Example The current count for the counter bean is: . . . / 20 of 36