Programming Jakarta Struts, 2nd Edition [Electronic resources] نسخه متنی

8.5 Logic Tags

The Logic tag library contains tags that are
useful for managing conditional generation of output text, looping
over object collections for repetitive generation of output text, and
application flow management. Table 8-5 lists the
tags available within the Logic tag library.

empty

equal

forward

greaterEqual

greaterThan

iterate

lessEqual

lessThan

match

messagesNotPresent

messagesPresent

notEmpty

notEqual

notMatch

notPresent

present

redirect

The tags within the Logic tag library can be divided into four
separate categories based on how they are used:

• Value comparison

• Substring matching

• Redirecting and forwarding

• Collection utilities

This division into categories is done for the purpose of explanation.
The tags are not packaged or arranged into these categories; they all
belong to the Logic package.

8.5.1 Value Comparison

The value-comparison
tags print out the body of the tag if and only if the comparison
evaluates to true. There are several different types of comparison
tags that you can use, depending on your specific needs.

Each of the value-comparison tags takes a value and compares it to
the value of a comparison attribute. If the value given can be
successfully converted to a number, a number comparison is performed;
otherwise, a string comparison is performed.

The comparison tags share the common attributes listed in Table 8-6.

name

parameter

property

scope

value

A few examples will help solidify how these comparison tags can be
used. To check whether a particular request parameter is present, you
can use the Logic present tag:

<logic:present parameter="id">
<!-- Print out the request parameter id value -->
</logic:present>

To check whether a collection is empty before iterating over it, you
can use the notEmpty tag:

<logic:notEmpty name="userSummary" property="addresses">
<!-- Iterate and print out the user's addresses -->
</logic:notEmpty>

Finally, here's how to compare a number value
against a property within an ActionForm:

<logic:lessThan property="age" value="21">  
<!-- Display a message about the user's age -->
</logic:lessThan>

8.5.2 Substring Matching

The substring-matching tags take all the
same arguments as the value-comparison tags. You compare the string
specified by the value attribute to any of the comparison values you
give it, specified by cookie,
header, parameter,
property, or name. Matching
tags also have an additional location attribute that informs the tag
where to start matching from (either
"start" or
"end").

In this example, the matchTag is being used to
determine whether the request parameter action begins with the string
"processLogin":

<logic:matchTag parameter="action" value="processLogin" location="start">
Processing Login....
</logic:matchTag>

If the location attribute is not specified, a
match between the variable and the value may occur at any position
within the variable string.

8.5.3 Redirecting and Forwarding

The
forward and
redirect tags within the Logic
tag library might have been better suited to the HTML tag library.
However, the fact that they are in the Logic tag library
doesn't make them any less valuable. In fact,
combined with one of the other Logic tags, these two tags become
extremely useful.

The redirect tag is responsible for sending a
redirect to the client's browser, complete with
URL-rewriting if the container supports it. Its attributes are
consistent with the Struts HTML link tag. The base
URL is calculated based on which of the following attributes you
specify (you must specify exactly one of them):

forward

Use the value of this attribute as the name of a global
ActionForward to be looked up, and use the
context-relative URI found there.

href

Use the value of this attribute unchanged.

page

Use the value of this attribute as a context-relative URI, and
generate a server-relative URI by including the context path.

The forward tag is responsible for either
redirecting or forwarding to a specified global
ActionForward. The forward tag
has one attribute, name, which is the logical name
of the ActionForward from the configuration file.

8.5.4 Collection Utilities

One of the most useful and most widely
used tags within the Struts tag libraries is the
iterate
tag. The iterate tag is responsible for executing
its body content once for every element inside the specified
collection. It has one required attribute:

id

The name of a page-scope JSP bean that will contain the current
element during an iteration.

An example is the best way to understand how to use the
iterate tag:

<logic:iterate id="address" name="userSummary" property="addresses">
<!-- Print out the address obejct in a table -->
</logic:iterate>

Here, the iterate tag will get the collection of
addresses by calling the getAddresses() method on
the userSummary bean. During each iteration, an
individual address will be assigned to the address
variable. This variable can be used inside the body of the
iterate tag as if you had assigned it directly.
During the next iteration, the next address object will be assigned
to the address variable. This continues until the
entire collection of addresses has been traversed.

The iterate tag is very flexible in terms of where
it gets the collection to iterate over. The attributes that control
how the iterate tag performs this behavior are
listed in Table 8-7.

collection

id

indexed

length

name

offset

property

scope

type

8.5.5 Messages and Errors

The
messagesPresent and
messagesNotPresent tags evaluate
the body content, depending on whether an
ActionMessages or ActionErrors
object is present in the request scope. Table 8-8
lists the attributes of these two tags.

name

property

message