Struts Logic Tags: An Introduction

Struts logic tags are conditional tags that replaces scriptlets in the jsp files.

This tag library contains tags that do the following things :
    conditional generation of the output text,
    generate iterations to output the text,
    application flow management,
    matching strings and substrings...

Logic tags available in the Struts Framework

Struts logic tags uses equal, greaterEqual, greaterThan, lessEqual, lessThan, notEqual tags to compare the values. There are rules that must be  followed :

• Initially the specified variable needed to be compared is examined (whether it can be converted to a double or to a long). Then a  numeric (either floating point or integer) type comparison is done. If numeric conversion fails than a String comparison is performed.
• The variable to be compared is retrieved from the request attribute(s)  that may be a cookie, header, name, parameter or property available to the specified tag. This variable is then converted to the appropriate type needed for the comparison..
• If the variable or property to be compared returns null, it is converted into a zero-length string before the comparison occurs.
• If the comparison logic returns a true value then only the specific comparison is performed and subsequently the nested body content is evaluated .

Struts logic tags  can also do the substring matching using the tags (match, notMatch). There are rules that must be  followed :

• The variable to be compared is retrieved from the request attribute(s)  that may be a cookie, header, name, parameter or property available on this tag. Then this variable is converted to a String, if necessary.
• If the retrieval of the variable fails or it contains a null value then a request time exception is thrown.  
• The existence of the variable as a substring is examined at the position specified by the location attribute Location can be specified at the beginning (if location is set to start), at the end (if location is set to end), or anywhere (if location is not specified).

A JspException is thrown at runtime if any tag is utilized incorrectly or any invalid combination of tag attributes are used. To process the actual exception that caused the problem,  a request attribute under key org.apache.struts.action.EXCEPTION  is passed to the error page.

In the subsequent section we will show you the examples of these tags.