String Concatenation in EL [duplicate]

问题:

This question already has an answer here:

• How to concatenate a String in EL? 5 answers

I would like to concatenate a string within a ternary operator in EL(Expression Language).

Suppose there is a variable named value. If it's empty, I want to use some default text. Otherwise, I need to append it with some static text.

${(empty value)? "none" : value + " enabled"}

This will not compile however. What would be a correct way to write this? Or is this even possible?

回答1:

This answer is obsolete. Technology has moved on. Unless you're working with legacy systems see Joel's answer.

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There is no string concatenation operator in EL. If you don't need the concatenated string to pass into some other operation, just put these expressions next to each other:

${value}${(empty value)? 'none' : ' enabled'}

回答2:

With EL 2 you can do the following:

#{'this'.concat(' is').concat(' a').concat(' test!')}

回答3:

If you're already on EL 3.0 (Java EE 7; WildFly, Tomcat 8, GlassFish 4, etc), then you could use the new += operator for this:

<c:out value="${empty value ? 'none' : value += ' enabled'}" />

If you're however not on EL 3.0 yet, and the value is a genuine java.lang.String instance (and thus not e.g. java.lang.Long), then use EL 2.2 (Java EE 7; JBoss AS 6/7, Tomcat 7, GlassFish 3, etc) capability of invoking direct methods with arguments, which you then apply on String#concat():

<c:out value="${empty value ? 'none' : value.concat(' enabled')}" />

Or if you're even not on EL 2.2 yet, then use JSTL <c:set> to create a new EL variable with the concatenated values just inlined in value:

<c:set var="enabled" value="${value} enabled" />
<c:out value="${empty value ? 'none' : enabled}" />

回答4:

Since Expression Language 3.0, it is valid to use += operator for string concatenation.

${(empty value)? "none" : value += " enabled"}  // valid as of EL 3.0

Quoting EL 3.0 Specification.

String Concatenation Operator

To evaluate

A += B 

• Coerce A and B to String.
• Return the concatenated string of A and B.

回答5:

Mc Dowell's answer is right. I just want to add an improvement if in case you may need to return the variable's value as:

${ empty variable ? '<variable is empty>' : variable }

回答6:

1.The +(operator) has not effect to that in using EL. 2.so this is the way,to use that

<c:set var="enabled" value="${value} enabled" />


<c:out value="${empty value ? 'none' : enabled}" />

is this helpful to You ?

回答7:

it also can be a great idea using concat for EL + MAP + JSON problem like in this example :

#{myMap[''.concat(myid)].content}