| The BodyTag interface extends IterationTag by defining additional
methods that let a tag handler manipulate the content of evaluating its body.
It is the responsibility of the tag handler to manipulate the body
content. For example the tag handler may take the body content,
convert it into a String using the bodyContent.getString
method and then use it. Or the tag handler may take the body
content and write it out into its enclosing JspWriter using
the bodyContent.writeOut method.
A tag handler that implements BodyTag is treated as one that
implements IterationTag, except that the doStartTag method can
return SKIP_BODY, EVAL_BODY_INCLUDE or EVAL_BODY_BUFFERED.
If EVAL_BODY_INCLUDE is returned, then evaluation happens
as in IterationTag.
If EVAL_BODY_BUFFERED is returned, then a BodyContent object will be
created (by code generated by the JSP compiler) to capture the body
evaluation.
The code generated by the JSP compiler obtains the BodyContent object by
calling the pushBody method of the current pageContext, which
additionally has the effect of saving the previous out value.
The page compiler returns this object by calling the popBody
method of the PageContext class;
the call also restores the value of out.
The interface provides one new property with a setter method and one
new action method.
Properties
There is a new property: bodyContent, to contain the BodyContent
object, where the JSP Page implementation object will place the
evaluation (and reevaluation, if appropriate) of the body. The setter
method (setBodyContent) will only be invoked if doStartTag() returns
EVAL_BODY_BUFFERED and the corresponding action element does not have
an empty body.
Methods
In addition to the setter method for the bodyContent property, there
is a new action method: doInitBody(), which is invoked right after
setBodyContent() and before the body evaluation. This method is only
invoked if doStartTag() returns EVAL_BODY_BUFFERED.
Lifecycle
Lifecycle details are described by the transition diagram below.
Exceptions that are thrown during the computation of doStartTag(),
setBodyContent(), doInitBody(), BODY, doAfterBody() interrupt the
execution sequence and are propagated up the stack, unless the
tag handler implements the TryCatchFinally interface; see that
interface for details.
Empty and Non-Empty Action
If the TagLibraryDescriptor file indicates that the action must
always have an empty element body, by an <body-content> entry
of "empty", then the doStartTag() method must return SKIP_BODY.
Otherwise, the doStartTag() method may return SKIP_BODY,
EVAL_BODY_INCLUDE, or EVAL_BODY_BUFFERED.
Note that which methods are invoked after the doStartTag() depends on
both the return value and on if the custom action element is empty
or not in the JSP page, not how it's declared in the TLD.
If SKIP_BODY is returned the body is not evaluated, and doEndTag() is
invoked.
If EVAL_BODY_INCLUDE is returned, and the custom action element is not
empty, setBodyContent() is not invoked,
doInitBody() is not invoked, the body is evaluated and
"passed through" to the current out, doAfterBody() is invoked
and then, after zero or more iterations, doEndTag() is invoked.
If the custom action element is empty, only doStart() and
doEndTag() are invoked.
If EVAL_BODY_BUFFERED is returned, and the custom action element is not
empty, setBodyContent() is invoked,
doInitBody() is invoked, the body is evaluated, doAfterBody() is
invoked, and then, after zero or more iterations, doEndTag() is invoked.
If the custom action element is empty, only doStart() and doEndTag()
are invoked.
|