Recipe 16.4 Accessing or Removing ServletContext Attributes in JSPs

Problem

You want to access or remove a ServletContext attribute in a JSP.

Solution

Use the c:out JSTL core tag to display the value of an attribute and the c:remove tag to remove the attribute from the ServletContext .

Discussion

By now you are probably familiar with the object attribute that the previous recipes stored in the ServletContext under the name com.jspservletcookbook.ContextObject . If you are not, Recipe 16.1 and Recipe 16.2 show the source code for this class and how it is bound as an attribute to a servlet and a JSP. This recipe shows the JSTL tags that you can use in JSP code to access this attribute and optionally remove or unbind it.

Example 16-6 includes the taglib directive that is required for using JSTL 1.0 tags in a JSP. The c:out tag then accesses the ServletContext attribute in the tag's value attribute. The tag gets the value of the ServletContext attribute by using the applicationScope JSTL implicit object, which is a java.util.Map type.

Example 16-6. Accessing an application attribute in a JSP

 <%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %> //HTML or other presentation code here... <c:out value=     "${applicationScope[\"com.jspservletcookbook.ContextObject\"].values}"         escapeXml="false" />

An implicit object is an object that the JSTL automatically makes available to the developer. You use the term applicationScope within ${...} characters , and this term evaluates to a java.util.Map of any object attributes that are bound to the ServletContext .

The code:

 ${applicationScope[\"com.jspservletcookbook.ContextObject\"].values}

uses EL syntax to access the ServletContext attribute named com.jspservletcookbook.ContextObject and get its values property, which effectively calls the getValues( ) method on the ContextObject object. This method displays all the keys of the Map contained by ContextObject , separated by an HTML line break ( <br> ). The attribute escapeXml="false " prevents the < and > characters in <br> from being escaped (and being replaced by < and > , respectively), which would prevent its proper display in a web browser.

If I wanted to make the ContextObject more universal, I could include a JavaBean property allowing the user of the class to set the line separator, so that the output of the getValues( ) method could be used in different contexts, not just HTML.

Figure 16-1 shows the result of accessing a JSP that uses this code in a browser.

Figure 16-1. Accessing a ServletContext bound attribute in a JSP

To remove the attribute from the ServletContext , use the c:remove JSTL tag. This tag removes the named variable from the specified scope:

 <c:remove var=     "com.jspservletcookbook.ContextObject" scope="application" />

application is an alias for the ServletContext . After a JSP that contains this tag is executed, any further attempts to access a ServletContext attribute of the same name will return null .