JavaBean Notes

I got my information researching various internet sites and the book Core Servlet and JavaServer Pages.

1. A bean class must have an empty constructor.
2. A bean class should have no public fields.
3. Use consistent naming convention for access data (setXxx/getXxx or isXxx/setXxx for accessing xxx).

Loading a JavaBean to be Used in a JSP page

Warning!!
Since jsp:useBean uses XML syntax, its format (along with any jsp:xxx elements) differs from HTML syntax in 3 ways:

• Attribute names are case sensitive
• Attribute values must be enclosed in single or double quotes
• Noncontainer elements should end the tag with / > instead of >

Simple syntax:
< jsp:useBean id="variableName" class="package.Class" / > 
means instantiate an object of type Class and bind it to the variable variableName 
ex:
< jsp:useBean id="emp" class="EmployeeDatabase.Worker" / > 
is equivalent to the scriplet
< % EmployeeDatabase.Worker emp =  new EmployeeDatabase.Worker(); % >

Accessing a JavaBean's Properties

To print out the property, there are 2 ways. The 2nd is useful when using
the property in code other than displaying. name should match the id used in the jsp:useBean tag.
    < jsp:getProperty name = "beanName" property = "propertyName" / >
    or
    < % = beanName.getPropertyName() % >  
     FYI: A bean property is a private data field of the class that has 
methods to get and set the value of the field.

Setting a JavaBean's Properties

The simplest ways to set a bean's property are as follows:
    < jsp:setProperty name="beanName" property = "propertyName"  value = "theValue" / >
    or
    < % beanName.setPropertyName(theValue); % > 
    Put theValue in double quotes if string type is expected

To associate a property with an input parameter, use param instead of value:
< jsp:setProperty name="beanName" property = "propertyName" param = "paramName" >
This action is nice because it automatically converts the param type to property's type.

If all properties of a bean have input parameters with the EXACT same name, 
you can assign all the properties to equal their corresponding parameters this way:
< jsp:setProperty name="beanName" property="*" /> 

Sharing Beans

 Beans, all bound to local variables, are stored in 1 out of 4 different locations 
depending on the value of the optional scope attribute of jsp:useBean:

• page: (default) Means bean should be placed in the PageContext object for the duration of the request. This means that servlet code can access it by calling getAttribute on the predefined pageContext variable. Usually, beans created with this scope are accessed by scriplets, expressions later in the same page, jsp:getProperty, or jsp:setProperty.
• application: Means bean will be saved in the shared ServletContext available through the predefined application variable or by a call to getServletContext(). ServletContext is shared by all servlets in the same Web application, or it's shared by all servlets in the same server or servlet engine if no explicit Web applications are defined. The getAttribute method retrieves the values in the ServletContext.

  Ramifications of sharing:
  1) Provides a simple mechanism for multiple servlets & JSP pages to access the same object. 2)Allows a servlet to create a bean that'll be used in JSP pages, not just access one that was previously created. Lets a servlet handle complex user requests by setting up beans, storing them inteh ServletConext, then forwarding the request to on e of several possible JSP pages ot present results appropriate to the rquest data.

• session: Means bean should be stored in the HttpSession object associated with the current request. It's accessible by getValue. An error will be produced at page translation if the page directive specifies that the current page is not participating in sessions.
• request: Means bean should be placed in the ServletRequest object for the extent of the present request. It's available by the getAttribute method.