Accessing java variable from javascript on same jsp

Question

Is it possible to access a String type variable defined in jsp from a javascript on the same page?

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1255">
<title>Insert title here</title>
<script type="text/javascript">  
foo();
function foo()  
{  
var value = "<%=myVar%>";  
alert(value);   
}  
</script>  
</head>
<body>

<%

String myVar="blabla";
%>

</body>

</html>

In eclipse I am getting an error

myVar cannot be resolved to a variable

`<%` and `<%=` blocks are evaluated in the order in which they're found on the page. You need to put the one that declares `myVar` before the one that uses it. — millimoose, Sep 24 '13 at 19:51
possible duplicate of [JSP Variable Accessing in JavaScript](http://stackoverflow.com/questions/4803906/jsp-variable-accessing-in-javascript) — Rafi Kamal, Sep 24 '13 at 20:00
@RafiKamal it's not a duplicate. Current OP's question is why its current approach doesn't work. — Luiggi Mendoza, Sep 24 '13 at 20:05

score 27 · Accepted Answer · edited May 23 '17 at 11:46

27

This won't work since you're trying to use an undefined variable. The code is generated like this:

... = myVar;
//...
String myVar = "blabla";

Doesn't make sense, right? So, in order to make this work you should declare the variable before using it (as always):

<%
    String myVar="blabla";
%>
<script type="text/javascript">
    foo();
    function foo() {
        var value = "<%=myVar%>";
        alert(value);
    }
</script>

Still, usage of scriptlets is extremely discouraged. Assuming you're using JSTL and Expression Language (EL), this can be rewritten to:

<c:set name="myVar" value="blabla" />
<script type="text/javascript">  
    foo();
    function foo() {
        var value = "${myVar}";
        alert(value);
    }
</script>

If your variable has characters like " inside, then this approach will faile. You can escape the result by using <c:out> from JSTL:

var value = "<c:out value='${myVar}' />";

More info:

How to avoid Java code in JSP files?

edited May 23 '17 at 11:46

Community

1
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answered Sep 24 '13 at 19:59

Luiggi Mendoza

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2

@VgeShi I know, that was a recommendation. – Luiggi Mendoza Sep 24 '13 at 20:52
2

what if myVar contains " characters? Isn't there a safe way to use it without escaping such characters manually?? – DonkeyKong Mar 03 '15 at 19:29
@LuiggiMendoza i want the opposite of that . i want to get javascript value and pass it to java variable – Shaaban Ebrahim Jun 05 '18 at 12:13
@ShaabanEbrahim for that purpose, send a request to the server from your client side (javascript). – Luiggi Mendoza Jun 05 '18 at 14:54
@LuiggiMendoza i want to encrypt password by method in java without need to send request to server – Shaaban Ebrahim Jun 06 '18 at 08:50
@ShaabanEbrahim Javascript is in client side, Java is in server side. There's no way to communicate to a Java process from Javascript, unless you're running Javascript on the server side as well, but I doubt that's what you're doing. – Luiggi Mendoza Jun 06 '18 at 13:45

Accessing java variable from javascript on same jsp

1 Answers1