JSP比较运算符的行为 [英] JSP comparison operator behaviour

问题描述

我想比较JSP的<c:if>标记中的两种不同类型.基本上，左一个总是Number，右一个是字符串，如果该字符串可以解析为数字，我不会收到任何错误，但是如果字符串不能解析为Number，我会收到javax.el.ELException: Cannot convert No of type class java.lang.String to class java.lang.Long.

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.

实际上:

$ {1 =="}//正常运行
$ {1 =="4"}//正常运行
$ {1 ==是"} //触发异常.

${1 =="" } //works fine
${1 =="4" } //works fine
${1 =="Yes" } //triggers the Exception.

但是，即使第3次比较在以前的JSP版本中也能正常工作，但现在它会导致异常.

But even the 3rd comparison worked fine in previous versions of JSPs but now it causes exceptions.

==的行为是否在一段时间内发生了变化?

Has the behaviour of == changed over period of time?

任何建议都将受到赞赏

推荐答案

==的行为未更改，但{expr}的行为已更改...

Behavior of == is not changed but behavior of {expr} is changed...

关于版本:

在 JSP规范

如果指定的版本小于2.1，则{expr}语法为 只需作为String文字处理即可.

If the version specified is less than 2.1, then the {expr} syntax is simply processed as a String literal.

因此，直到EL 2.0都将被视为字符串文字并与.equals进行比较，因为==将在内部转换为equals(

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

关于比较:

在 EL版本2.1的JSP规范JSP.2.3.5.7 ，指定了以下内容...

In JSP specification JSP.2.3.5.7 of EL version 2.1, following is specified...

1. 如果A为null或B为null，则对于==或eq返回false，对于！=或ne返回true

1. If A is null or B is null return false for == or eq, true for != or ne

如果A或B为字节，短，字符，整数或长强制A 和B到Long，应用运算符

If A or B is Byte, Short, Character, Integer, or Long coerce both A and B to Long, apply operator

所以，在第一种情况下

${1 =="" } // ans is false as second one is null as per 1st rule.

在第二种情况下，

${1 =="4" } // ans is false as both are different after coercing to Long as per 2nd rule.

在上述情况下，通过内部类型转换，两者都将被强制为long.

Both will be coerced to long in above case with internal type conversion.

但是在第三种情况下， ${1 =="Yes" }其中第二个字符串是字符串，不能被转换(强制)为Long，并且将抛出java.el.ELException并显示消息无法将类型java.lang.String类型的No转换为java.lang.Long类".

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".

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