Taglib in Java: tag with array parameter

Question

How can I define a tag that receives a array as a parameter?

Thanks

Answer 1

JSTL does it, and you can too.

I happen to have an el function example handy, and then I'll paste a portion of the c:forEach definition to give you an idea:

You could pass it as a delimited string, but if you want a collection or array, you can use something like this:

<function>
  <name>join</name>
  <function-class>mypackage.Functions</function-class>
  <function-signature>String join(java.lang.Object, java.lang.String)</function-signature>
</function>

and

/**
 * jstl's fn:join only works with String[].  This one is more general.
 * 
 * usage: ${nc:join(values, ", ")}
 */
public static String join(Object values, String seperator)
{
    if (values == null)
        return null;
    if (values instanceof Collection<?>)
        return StringUtils.join((Collection<?>) values, seperator);
    else if (values instanceof Object[])
        return StringUtils.join((Object[]) values, seperator);
    else
        return values.toString();
}

Obviously, instead of an Object input you can use an array if you don't want to handle collections as well.

Here is the c:forEach definition:

<tag>
    <description>
        The basic iteration tag, accepting many different
        collection types and supporting subsetting and other
        functionality
    </description>
    <name>forEach</name>
    <tag-class>org.apache.taglibs.standard.tag.rt.core.ForEachTag</tag-class>
    <tei-class>org.apache.taglibs.standard.tei.ForEachTEI</tei-class>
    <body-content>JSP</body-content>
    <attribute>
        <description>
            Collection of items to iterate over.
        </description>
        <name>items</name>
        <required>false</required>
        <rtexprvalue>true</rtexprvalue>
        <type>java.lang.Object</type>
    </attribute>
    ...

Answer 2

If the array data is Strings, you could pass the values as a delimited list, maybe in an attribute.

<mytag myattribute="value1,value2,value3"/>

You could do the same with the tag body, or a jsp:param or some such, but I suspect that the attribute approach is probably easiest to code and understand.