The empty operator in Java

Creator QR-Code in Java The empty operator

The empty operator solves the problem of an identifier being non-null yet essentially empty, such as a zero-length string Using the unary empty opera tor on an identifier returns true if the identifier is either null or has an empty value This applies to Strings, arrays, Maps, and Lists Here is an example of its use:

The EL engine has numerous objects available that you can use in expres sions to retrieve values You reference the implicit objects by name Here are their names and descriptions:

pageContext: The PageContext object pageScope: A Map object that maps page-scoped attributes to their

param: A Map object that maps parameter names to a single String value paramValues: A Map object that maps parameter names to a String

header: A Map object that maps header names to a single String value headerValues: A Map object that maps header names to a String array

cookie: A Map object that maps cookie names to a single Cookie object initParam: A Map object that maps context-initialization parameter names to their String parameter value

For example, to retrieve a value from a session attribute called userName, you would use the following:

The Struts-EL library is designed to work with the Struts framework The implementations of the tags know about the configuration of your Struts application and make reference to various Struts components when generat ing the page content You can find the complete syntax for all the Struts-EL tags in Appendix A

To use the Struts-EL library, you must include the standard Struts tag library, because the Struts-EL library classes are inherited from the Struts library

classes You also need to include the JSTL tag libraries because Struts-El uses the expression engine from the JSTL libraries The libraries strutsjar, struts-eljar, jstljar, and standardjar come with the Struts 11 distribution in the contrib\struts-el\lib folder If you intend to use the tags in any of the JSTL libraries and you should you need to include also the libraries that JSTL needs You need to download these additional libraries separately by retrieving the JSTL implementation from the Jakarta Taglibs Web site at

An implementation of JSTL version 11 is available, but we have chosen to use version 10 because version 11 requires a Web container that supports the JSP 20 specification Tomcat 50 supports JSP 20, but it is still in beta testing at the time of this writing To summarize, to use Struts-EL and JSTL, be sure that you include the follow ing twelve JAR files in your WEB-INF/lib folder

strutsjar: The standard Struts tag library, as well as the Struts frame work class files struts-eljar: The EL version of the standard tag library jstljar: The JSTL API classes standardjar: The JSTL standard taglib implementation domjar: Library needed by JSTL jaxen-fulljar: Library needed by JSTL jaxp-apijar: Library needed by JSTL jdbc2_0-stextjar: Library needed by JSTL saxjar: Library needed by JSTL saxpathjar: Library needed by JSTL xalanjar: Library needed by JSTL xercesImpljar: Library needed by JSTL

The Struts-EL library consists of three separate tag libraries Each library encompasses a particular set of functionality The Beans-EL tag library is for handling JavaBeans The largest library is HTML-EL, which contains Strutsrelated HTML tags Finally, Logic-EL provides functionality for performing cer tain logical operations

The Beans-EL library contains five tags The common convention is to refer ence them with the prefix bean, as follows:

<bean:>: Renders an internationalized message string to the response The JSTL fmt:message tag offers the same functionality, so we recom mend you use it instead of bean:message <bean:page>: Exposes a specified item from the page context as a bean The JSTL c:set tag offers the same functionality plus a whole lot more