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 &lt; and &gt;, 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.

See Also

Chapter 23 on using the JSTL; Recipe 16.1 and Recipe 16.2 on setting ServletContext attributes in servlets and JSPs; Recipe 16.3 on accessing or removing ServletContext attributes in servlets; Recipe 16.5-Recipe 16.8 on handling session attributes in servlets and JSPs; Recipe 16.9-Recipe 16.12 on handling request attributes in servlets and JSPs; Recipe 14.5 on using a ServletContext event listener; the Javadoc for javax.servlet.ServletContextAttributeListener: http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletContextAttributeListener.html.