Java(TM) Boutique - Introducing: the Struts bean and logic Tag Libraries

Tutorials : Introducing: the Struts bean and logic Tag Libraries :

The Struts logic tags

These tags are used to control the generation of page output by allowing conditions to be tested, loops to be set up and application flow to be directed to other pages. A comparison will often involve a bean created with one of the bean tags. A very simple example with a bean containing only a single property:

<bean:define id="string" value="Struts "/>

<logic:equal name="string" value="Struts">
The string contained the word Struts
</logic:equal>

- Figure 10: logic:equal example

This will result in this output being shown in the browser:

The string contained the word Struts

Let's make a more realistic--but still simple--example. We'll use the DVD class shown above. The storing of the DVD instance in session scope is done in a scriptlet to simplify the example. Normally you'd do this in your Action class:

<%
List actors = new LinkedList();
actors.add("Tom Hulce");
actors.add("Elizabeth Berridge");

DVD dvd = new DVD("Amadeus", 158, actors);
session.setAttribute("dvd", dvd);
%>

<logic:notEqual name="dvd" property="title" value="The Matrix">
This DVD is not The Matrix<br>
</logic:notEqual> 

<logic:equal name="dvd" property="title" value="Amadeus">
This DVD is Amadeus<br>
</logic:equal>

- Figure 11: logic:equal and logic:notEqual example

When run we'll see this output:

This DVD is not The Matrix
This DVD is Amadeus

The comparison tags are these:

equal	notEqual
greaterEqual	lessEqual
greaterThan	lessThan
match	notMatch
present	notPresent

- Figure 12: The comparison tags

The first 2*3 comparison tags use this rule (taken from the Struts tag documentation):

"If the value given can be successfully converted to a float or double, then a number comparison is performed on the value given and the value of the comparison attribute. Otherwise a String comparison is performed."

Besides comparing values with bean properties you may also compare with cookie, header, and request parameter data. For example:

<logic:match header="User-Agent" value="Mozilla">
Hi Mozilla browser
</logic:match>
<logic:notMatch header="User-Agent" value="Mozilla">
You browser is not Mozilla - it is
<bean:header id="agent" name="User-Agent"/>
<bean:write name="agent"/>
</logic:notMatch>

- Figure 13: logic:match and logic:notMatch example

The present/notPresent tags are useful if you don't know if a property is available:

<logic:present header="referer">
You came to this page from
<bean:header id="ref" name="referer"/>
<bean:write name="ref"/>
</logic:present>

- Figure 14: logic:present example

The iterate tag

Probably the most interesting tag in the logic library is the iterate tag. It is used to iterate over a collection, and you saw in Figure 5 how it is used:

<logic:iterate id="dvd" name="dvds" property="DVDList">

. . . (process each item in the collection returned by getDVDList)

</logic:iterate>

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