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344

Chapter 14 Creating Custom JSP Tag Libraries

Now, suppose that we want to define a set of tags that would be used like

this:

<%= someExpression %>

JSP to include if condition is true

JSP to include if condition is false

To accomplish this task, the first step is to define an 

IfTag

 class to handle

the 

csajsp:if

 tag. This handler should have methods to specify and check

whether the condition is true or false (

setCondition

 and 

getCondition

) as

well as methods to designate and check if the condition has ever been explic 

itly set (

setHasCondition

 and 

getHasCondition

), since we want to disal 

low 

csajsp:if

 tags that contain no 

csajsp:condition

 entry. Listing 14.23

shows the code for 

IfTag

.

The second step is to define a tag handler for 

csajsp:condition

. This

class, called 

IfConditionTag

, defines a 

doStartTag

  method  that  merely

checks if the tag appears within 

IfTag

. It returns 

EVAL_BODY_TAG

 if so and

throws an exception if not. The handler's 

doAfterBody

 method looks up

the body content (

getBodyContent

), converts it to a 

String

 (

getString

),

and compares that to 

"true"

. This approach means that an explicit value of

true

 can be substituted for a JSP expression like 

<%= expression %>

 if,

during initial page development, you want to temporarily designate that the

then

 portion should always be used. Using a comparison to 

"true"

 also

means that any other value will be considered  false.  Once this compari 

son is performed, the result is stored in the enclosing tag by means of the

setCondition

 method of 

IfTag

. The code for 

IfConditionTag

 is shown

in Listing 14.24.

The third step is to define a class to handle the 

csajsp:then

 tag. The

doStartTag

 method of this class verifies that it is inside 

IfTag

 and also

checks that an explicit condition has been set (i.e., that the 

IfConditionTag

has already appeared within the 

IfTag

). The 

doAfterBody

 method checks

for the condition in the 

IfTag

 class, and, if it is true, looks up the body con 

tent and prints it. Listing 14.25 shows the code.

The final step in defining tag handlers is to define a class for 

csa 

jsp:else

. This class is very similar to the one to handle the 

then

 part of

the tag, except that this handler only prints the tag body from 

doAfterBody

if the condition from the surrounding 

IfTag

 is false. The code is shown in

Listing 14.26. 

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