EL any function to get absolute value of a number?

2020-02-02 00:19发布

I know the ugly way

a>0 ? a : -a

but this is very annoyng when a is a relatively long expression.

OBJ1["x"]-someVar>0 ? OBJ1["x"]-someVar : -(OBJ1["x"]-someVar)

Is there any nicer way of doing it?

回答1:

For plain jsp, you can create a custom EL function which delegates to Math#abs(int).

If you first just create a /WEB-INF/functions.tld file which look like follows:

<?xml version="1.0" encoding="UTF-8" ?>
<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>1.0</tlib-version>
    <short-name>Custom_Functions</short-name>
    <uri>http://example.com/functions</uri>

    <function>
        <name>abs</name>
        <function-class>java.lang.Math</function-class>
        <function-signature>int abs(int)</function-signature>
    </function>
</taglib>

Then you'll be able to use it as follows:

<%@taglib uri="http://example.com/functions" prefix="f" %>
...
${f:abs(OBJ1["x"]-someVar)}

This solution also works for JSF, but if you are using JSF, OmniFaces has simplified this for you by providing its importFunctions (if not already using OmniFaces with JSF, you should start using it)

<o:importFunctions type="java.lang.Math" var="m" />
...
#{m:abs(-10)}