Question about some calculation using JSP EL?
I am new to JSP EL. I am reading some EL text and got the following code:
<body>
<h1>Hello World!</h1>
<%
String s1 = 开发者_如何学C"aaa ";
String s2 = "22";
Double dObj = new Double(3.5);
int a;
a = 12;
%>
s1 + s2 = ${ s1 + s2 } <br>
a + 2 = ${a + 2} <br>
dObj + 2 = ${dObj + 2} <br>
2 + 2 = ${"2" + 2}
</body>
And the results:
s1 + s2 = 0 (1)
a + 2 = 2 (2)
dObj + 2 = 2 (3)
2 + 2 = 4 (4)
I can understand the result of (4) but have no ideas with the results of (1), (2), and (3). Can anyone elaborate on that?
The EL variables reference a page-, or request-, or session-, or application-scope attribute (each scope is inspected, in that order). They don't reference a local scriptlet variable.
So, since you don't have any attribute in any scope named s1, s2, a or dObj, default values are used (0).
The results would be the ones that you're expecting if the scriptlet code would be the following:
<%
String s1 = "aaa ";
String s2 = "22";
Double dObj = new Double(3.5);
int a;
a = 12;
request.setAttribute("s1", s1);
request.setAttribute("s2", s2);
request.setAttribute("dObj", dObj);
request.setAttribute("a", a);
%>
In a well-architected application, these attributes would be set by a servlet which would dispatch to your JSP, or by a JSP tag (<c:set> for example), but not by a scriptlet. Scriptlets should not be used anymore.
Just google "JSP EL arithmetic" and you should find a lot of examples, like here: http://www.roseindia.net/jsp/simple-jsp-example/ExpressionLanguageBasicArithmetic.shtml
The results are dependent on how the objects are evaluated inside EL {} tags.
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