JavaBeans Components - The J2EE 1.4 Tutorial

JavaBeans Components

JavaBeans components are Java classes that can be easily reused and composed together into applications. Any Java class that follows certain design conventions is a JavaBeans component.

JavaServer Pages technology directly supports using JavaBeans components with standard JSP language elements. You can easily create and initialize beans and get and set the values of their properties.

JavaBeans Component Design Conventions

JavaBeans component design conventions govern the properties of the class and govern the public methods that give access to the properties.

A JavaBeans component property can be

Read/write, read-only, or write-only
Simple, which means it contains a single value, or indexed, which means it represents an array of values

A property does not have to be implemented by an instance variable. It must simply be accessible using public methods that conform to the following conventions:

For each readable property, the bean must have a method of the form

  PropertyClass getProperty() { ... } 

For each writable property, the bean must have a method of the form

  setProperty(PropertyClass pc) { ... } 

In addition to the property methods, a JavaBeans component must define a constructor that takes no parameters.

The Duke's Bookstore application JSP pages bookstore.jsp, bookdetails.jsp, catalog.jsp, and showcart.jsp use the database.BookDB and database.BookDetails JavaBeans components. BookDB provides a JavaBeans component front end to the access object database.BookDBAO. The JSP pages showcart.jsp and cashier.jsp access the bean cart.ShoppingCart, which represents a user's shopping cart.

The BookDB bean has two writable properties, bookId and database, and three readable properties: bookDetails, numberOfBooks, and books. These latter properties do not correspond to any instance variables but rather are a function of the bookId and database properties.

package database;
public class BookDB {
  private String bookId = "0";
  private BookDBAO database = null;
  public BookDB () {
  }
  public void setBookId(String bookId) {
  this.bookId = bookId;
  }
  public void setDatabase(BookDBAO database) {
  this.database = database;
  }
  public BookDetails getBookDetails() throws 
    BookNotFoundException {
    return (BookDetails)database.getBookDetails(bookId); 
  }
  public List getBooks() throws BooksNotFoundException {
    return database.getBooks();
  }
  public void buyBooks(ShoppingCart cart) 
    throws OrderException {
    database.buyBooks(cart);
  }
  public int getNumberOfBooks() throws BooksNotFoundException {
    return database.getNumberOfBooks();
  }
} 

Creating and Using a JavaBeans Component

To declare that your JSP page will use a JavaBeans component, you use a jsp:useBean element. There are two forms:

<jsp:useBean id="beanName"
  class="fully_qualified_classname" scope="scope"/> 

and

<jsp:useBean id="beanName"
  class="fully_qualified_classname" scope="scope">
  <jsp:setProperty .../>
</jsp:useBean> 

The second form is used when you want to include jsp:setProperty statements, described in the next section, for initializing bean properties.

The jsp:useBean element declares that the page will use a bean that is stored within and is accessible from the specified scope, which can be application, session, request, or page. If no such bean exists, the statement creates the bean and stores it as an attribute of the scope object (see Using Scope Objects). The value of the id attribute determines the name of the bean in the scope and the identifier used to reference the bean in EL expressions, other JSP elements, and scripting expressions (see Chapter 16). The value supplied for the class attribute must be a fully qualified class name. Note that beans cannot be in the unnamed package. Thus the format of the value must be package_name.class_name.

The following element creates an instance of mypkg.myLocales if none exists, stores it as an attribute of the application scope, and makes the bean available throughout the application by the identifier locales:

<jsp:useBean id="locales" scope="application"
  class="mypkg.MyLocales"/> 

Setting JavaBeans Component Properties

The standard way to set JavaBeans component properties in a JSP page is by using the jsp:setProperty element. The syntax of the jsp:setProperty element depends on the source of the property value. Table 12-3 summarizes the various ways to set a property of a JavaBeans component using the jsp:setProperty element.

Table 12-3 Valid Bean Property Assignments from String Values 

Value Source	Element Syntax
String constant	<jsp:setProperty name="beanName"   property="propName" value="string constant"/>
Request parameter	<jsp:setProperty name="beanName"   property="propName" param="paramName"/>
Request parameter name that matches bean property	<jsp:setProperty name="beanName"   property="propName"/>   <jsp:setProperty name="beanName"   property="*"/>
Expression	<jsp:setProperty name="beanName"   property="propName" value="expression"/>   <jsp:setProperty name="beanName"   property="propName" >   <jsp:attribute name="value">     expression   </jsp:attribute> </jsp:setProperty>
	1. beanName must be the same as that specified for the id attribute in a useBean element. 2. There must be a setPropName method in the JavaBeans component. 3. paramName must be a request parameter name.

A property set from a constant string or request parameter must have one of the types listed in Table 12-4. Because constants and request parameters are strings, the web container automatically converts the value to the property's type; the conversion applied is shown in the table.

String values can be used to assign values to a property that has a PropertyEditor class. When that is the case, the setAsText(String) method is used. A conversion failure arises if the method throws an IllegalArgumentException.

The value assigned to an indexed property must be an array, and the rules just described apply to the elements.

Table 12-4 Valid Property Value Assignments from String Values 

Property Type	Conversion on String Value
Bean Property	Uses setAsText(string-literal)
boolean or Boolean	As indicated in java.lang.Boolean.valueOf(String)
byte or Byte	As indicated in java.lang.Byte.valueOf(String)
char or Character	As indicated in java.lang.String.charAt(0)
double or Double	As indicated in java.lang.Double.valueOf(String)
int or Integer	As indicated in java.lang.Integer.valueOf(String)
float or Float	As indicated in java.lang.Float.valueOf(String)
long or Long	As indicated in java.lang.Long.valueOf(String)
short or Short	As indicated in java.lang.Short.valueOf(String)
Object	new String(string-literal)

You use an expression to set the value of a property whose type is a compound Java programming language type. The type returned from an expression must match or be castable to the type of the property.

The Duke's Bookstore application demonstrates how to use the setProperty element to set the current book from a request parameter in the database bean in bookstore2/web/bookdetails.jsp:

<c:set var="bid" value="${param.bookId}"/>
<jsp:setProperty name="bookDB" property="bookId"
  value="${bid}" /> 

The following fragment from the page bookstore2/web/bookshowcart.jsp illustrates how to initialize a BookDB bean with a database object. Because the initialization is nested in a useBean element, it is executed only when the bean is created.

<jsp:useBean id="bookDB" class="database.BookDB" scope="page">
  <jsp:setProperty name="bookDB" property="database" 
    value="${bookDBAO}" />
</jsp:useBean> 

Retrieving JavaBeans Component Properties

The main way to retrieve JavaBeans component properties is by using the JSP EL expressions. Thus, to retrieve a book title, the Duke's Bookstore application uses the following expression:

${bookDB.bookDetails.title} 

Another way to retrieve component properties is to use the jsp:getProperty element. This element converts the value of the property into a String and inserts the value into the response stream:

<jsp:getProperty name="beanName" property="propName"/> 

Note that beanName must be the same as that specified for the id attribute in a useBean element, and there must be a getPropName method in the JavaBeans component. Although the preferred approach to getting properties is to use an EL expression, the getProperty element is available if you need to disable expression evaluation.

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