Special characters--why do they keep bothering me?
Finally... have we also solved the problems with double quotes and
newlines? Not at all! Take for example the call
initText(pname,"<%=pname%>");
If the Java string "pname" contains a double quote, we
have the same problem as previously described, and this time we will
receive a JavaScript error message in the browser. This is actually
worse than before. We will simply have to consider what characters we
safely can accept inside a pair of double quotes. Using single quotes
in stead of double quotes--which JavaScript allows--to enclose
<%=pname%>, will only result in single quotes in the data
becoming a problem.
The characters that we will have to handle in some way are: double
quote, single quote (for completeness), backslash (the JavaScript
"escape"-character), and "newline". JavaScript
has rules for how these characters are created:
|
character |
must be written as
("escape"-sequence)
|
|
double quote |
\" |
|
single quote |
\' |
|
backslash |
\\ |
|
newline (CR + LF) |
\r\n |
Java utilities come to our rescue
So why not create a Java utility that will do this conversion for us?
You could put it into a Java bean or directly in your JSP-page (a
nice way to do this is to include it from a file using the
"<%@ include file"-construct). Here is a definition of
a Java method, convJS, for insertion in your JSP-page:
<%!
String convJS(Object s) {
// Convert problem characters to JavaScript Escaped values
if (s == null) {
return "";
}
String t = (String)s;
t = replace(t,"\\","\\\\"); // replace backslash with \\
t = replace(t,"'","\\\'"); // replace an single quote with \'
t = replace(t,"\"","\\\""); // replace a double quote with \"
t = replace(t,"\r","\\r"); // replace CR with \r;
t = replace(t,"\n","\\n"); // replace LF with \n;
return t;
}
%>
The "replace" method which we have used is also a utility
that you can not live without. Here's the code for it:
<%!
String replace(String s, String one, String another) {
// In a string replace one substring with another
if (s.equals("")) return "";
String res = "";
int i = s.indexOf(one,0);
int lastpos = 0;
while (i != -1) {
res += s.substring(lastpos,i) + another;
lastpos = i + one.length();
i = s.indexOf(one,lastpos);
}
res += s.substring(lastpos); // the rest
return res;
}
%>
We will place these two methods in a file called "util.inc".
We're now ready to modify the example from above to handle special
characters as well. We assume once more that we have
obtained name, address, etc. from a Java Bean or another source:
<%@ include file = "util.inc" %>
<%
// Get data for the form from somewhere...
String pname = ...
pname = convJS(pname); // Convert for use in JavaScript
String paddress = ...
paddress = convJS(paddress); // . . .
String age = ...
String mail = ...
String comments ...
comments = convJS(comments); // . . .
%>
<html>
<head>
<script language="javascript" src="init.js"></script>
<script>
function initAll(form) {
// Initialize all form controls
with (form) {
initText(pname,"<%=pname%>");
initText(paddress,"<%=paddress%>");
initSelect(age,"<%=age%>");
initCheckBox(mail,"<%=mail%>");
initTextArea(comments,"<%=comments%>");
}
}
</script>
etc.
As you may have noticed we only need to convert text fields and text
areas. Checkboxes, radio buttons and selection lists should not
contain any of the problematic characters.
Conclusion
By using a few, general JavaScript functions, we get a simple, yet
robust method of implementing the initialization of the controls in
a form. It requires minimal JSP coding, and thus leaves the
HTML-section almost free of JSP-constructs. This results in a more
readable, maintainable and therefore less error-prone application.
Finally, to have a real happy ending, I can add that all JavaScript
functions have been tested with success on Internet Explorer version
3 and up (yes, the old 3), as well as Netscape version 3 and up,
including version 6. "initRadio" does not work in Opera version 5
because of a bug in the browser's handling of the "checked" property.
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