Default value on JSP custom-tag attribute

Question

When defining an attribute for a custom JSP tag, is it possible to specify a default value? The attribute directive doesn't have a default value attribute. Currently I'm making do with:

<%@ attribute name="myAttr" required="false" type="java.lang.String" %>

<c:if test="${empty myAttr}" >
 <c:set var="myAttr" value="defaultValue" />
</c:if>

Is there a better way?

Solution 1

There is a better way:

<c:set var="title" value="${(empty title) ? 'Default title' : title}" />

No need for custom tag in Java nor tld. Just plain JSP EL and conditional operator.

---

In my opinion it is shorter and cleaner than old:

<c:if test="${empty title}" >
 <c:set var="title" value="Default title" />
</c:if>

Solution 2

So I wasn't able to figure out a way to add this to the attribute directive itself; it appears that the directive does not support this functionality. I was, however, able to create a tag that encapsulates the <c:if>...</c:if> logic. I had to write the tag in Java since there is no way (that I know of) to use an attribute value as a variable name.

First I wrote the tag file as a Java class:

DefaultTag.java

public class DefaultTag extends BodyTagSupport {

    private String var;
    private Object value;

    //for tag attribute
    public void setVar(String var) {
        this.var = var;
    }

    //for tag attribute
    public void setValue(Object value) {
        this.value = value;
    }

    public int doEndTag() throws JspException {
        Object oldValue = pageContext.getAttribute(var);
        Object newValue;

        if(value != null) {
            newValue = value;
        }

        else {
            if(bodyContent == null || bodyContent.getString() == null) {
                newValue = "";
            }

            else {
                newValue = bodyContent.getString().trim();
            }
        }

        if(oldValue == null) {
            pageContext.setAttribute(var, newValue);
        }

        else if(oldValue.toString().trim().length() == 0) {
            pageContext.setAttribute(var, newValue);
        }

        return EVAL_PAGE;
    }
}

Then I made a tld file:

utils.tld:

<?xml version="1.0" encoding="ISO-8859-1"?>
<taglib xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
        version="2.1">
    <tlib-version>2.0</tlib-version>
    <short-name>utils</short-name>
    <uri>http://utils</uri>
    <tag>
        <name>default</name>
        <tag-class>com.mystuff.mvc.tag.DefaultTag</tag-class>
        <body-content>JSP</body-content>
        <attribute>
            <name>var</name>
            <required>true</required>
            <rtexprvalue>true</rtexprvalue>
        </attribute>
        <attribute>
            <name>value</name>
            <required>true</required>
            <rtexprvalue>true</rtexprvalue>
        </attribute>
    </tag>
</taglib>

Then I made a custom tag that uses this tag:

defaultTest.tag

<%@ taglib prefix="utils" uri="/WEB-INF/tlds/utils.tld" %>
<%@ attribute name="value" required="true"%>
<%@ attribute name="optValue" required="false"%>

<utils:default var="optValue" value="optional monkeys"/>

${value} ${optValue}

After that I made a page to test the tag I just created:

tagTest.jsp

<mystuff:defaultTest value="helloThar" /><br/><br/>

<mystuff:defaultTest value="helloThere" optValue="monkeys" /><br/><br/>

<mystuff:defaultTest value="helloYou" optValue="${1 + 2 + 4 + 10}" /><br/><br/>

And that gave me:

helloThar optional monkeys

helloThere monkeys

helloYou 17

Comments

[TheBilTheory]

<c:if test="${empty title ? 'Default Title' : 'New Title'}"> ... </c:if>

[izogfif]

OK, what about the case when attribute is specified and an empty string is passed (and it's supposed to use this empty string instead of default value)? How to distinguish between cases "no attribute is specified", "attribute is specified but an empty value is passed", "attribute is specified but a null value is passed", "attribute is not specified, but there is a non-empty servlet variable with the same name"?