JSP comparison operator behaviour
el, java, jsp, spring
Solution
Behavior of `==` is not changed but behavior of `{expr}` is changed...
About versions :
In backward compatibility section of JSP Specification,
If the version specified is less than 2.1, then the {expr} syntax is simply processed as a String literal.
So, till EL 2.0 all will be treated as a string literal and compared with `.equals` as `==` will be converted to `equals` internally (Reference here), but in 2.1 It will not be converted to string and will throw exception saying that `javax.el.ELException: Cannot convert No of type class java.lang.String to class java.lang.Long`
About Comparision :
In JSP specification JSP.2.3.5.7 of EL version 2.1, following is specified...
If A is null or B is null return false for == or eq, true for != or ne
If A or B is Byte, Short, Character, Integer, or Long coerce both A and B to Long, apply operator
so, In first case,
${1 =="" } // ans is false as second one is null as per 1st rule.
and In second case,
${1 =="4" } // ans is false as both are different after coercing to Long as per 2nd rule.
Both will be coerced to long in above case with internal type conversion.
But not in the third case, `${1 =="Yes" }` where second one is string can not be converted (coerced) to Long and `java.el.ELException` will be thrown with message "Cannot convert No of type class java.lang.String to class java.lang.Long".
Problem
I want to compare two different types in `<c:if>` tag of JSP. Basically left one is `Number` always but right one is a String and If that string could be parse to a Number I receive no error but If the String cant be parsed to a `Number` I receive `javax.el.ELException: Cannot convert No of type class java.lang.String to class java.lang.Long`. Practically: ${1 =="" } //works fine ${1 =="4" } //works fine ${1 =="Yes" } //triggers the Exception. But even the 3rd comparison worked fine in previous versions of JSPs but now it causes exceptions. Has the behaviour of `==` changed over period of time? Any suggestions are highly appreciated