JSP EL String concatenation [duplicate]
How do I concatenate strings in EL?
I want to do something like this but it doesn't work:
${var1 == 0 ? 'hi' : 'hello ' + var2}
It throws an exception trying to cast 'hello'
to a Double
Solution 1:
Using java string concatenation works better.
#{var1 == 0 ? 'hi' : 'hello'.concat(var2)}
The benefit here is you can also pass this into a function, for instance
#{myCode:assertFalse(myVar == "foo", "bad myVar value: ".concat(myVar).concat(", should be foo"))}
Solution 2:
The +
operator always means numerical addition in JSP Expression Language. To do string concatenation you would have to use multiple adjacent EL expressions like ${str1}${str2}
.
If I read your example correctly this could be written as:
${var1 == 0 ? 'hi' : 'hello '}${var1 == 0 ? '' : var2}
Edit
Another possibility would be to use JSTL, which is longer but might be clearer if there is more text that depends on var1
:
<c:choose>
<c:when test="${var1 == 0}">hi</c:when>
<c:otherwise>hello <c:out value="${var2}"/></c:otherwise>
</c:choose>
The c:out
might not be needed, depending on the JSP version.
Solution 3:
I know this is an old topic, but the answer to this question has changed in the past six months, and it's important to note that change, IMO (since I found this by Googling "el concatenate strings").
As of EL Expression 3.0 (public ballot approved August 2012, released with Java EE 7), a twist on the syntax the questioner originally used is now valid:
${var1 == 0 ? 'hi' : 'hello ' += var2}
There was much disagreement with the use of +=
instead of +
, but it is what it is. This will correctly evaluate and concatenate the strings as expected. You can also use the cat
operator instead of the +=
operator:
${var1 == 0 ? 'hi' : 'hello ' cat var2}
While this is now legal, note that you won't be able to use it until your web container (Tomcat, Jetty, GlassFish, etc.) releases a new version that supports Java EE 7/EL 3.0. This is expected sometime before the end of 2013, perhaps as early as the fall.
Edited 2015-02-19 to note that the final operator was +=
and not +
as originally answered.