Friday, October 17, 2008

Redirect from a JSP page (to another server)

Keywords:
best practice tips JSP redirect XML

Problem:
<jsp:forward> is not for redirecting to an external server (and the address you forward to will have access the request attributes and parameters, which may be desirable, see discussion here: when to use response redirect and jsp forward.

Is using response.sendRedirect(...) ok/best-practice?
Solution:
The short answer, is it's fine. In summary, there are four approaches I can think of:

Approach #1: Scriptlet to sendRedirect

<%
    response.sendRedirect("http://www.example.com");
%>

This has worked since the introduction of JSPs (see: Implementing a Redirect in a JSP Page) ... sometimes you need the servlet API in your JSP page either because there's other content (eg HTML) that makes it reasonable not to be a pure servlet. Other times it's just convenient to have a text file compiled on the fly.

Though there are many that would disagree (Sciptlet snobs? see How to redirect a page in JSP - I don't see why you'd get wound up about scriptlets. Why would you write a servlet when one line of code in a text file gives you the same result?).

Approach #2: Use Apache JSTL and the c:redirect tag

<c:redirect url="http://www.example.com"/>

or scriptlets again:
<c:redirect url="<%=scriptlet logic%>"/>

Sure, it's 'pure' XML but you'll need the taglib definition in the JSP file and include the JSTL jars in your web application - as discussed in the notes for a previous post.

Approach #3: Refresh meta tag


You could get the JSP to produce a HTML page that contains the Refresh meta tag.
<html>
<head>
  <meta http-equiv="Refresh" content="0; url=http://www.example.com/">
</head>
<body></body>
</html>

Where 0 is the delay in seconds. If more than zero, you'd probably want to put some text on the page to explain what's happening.

Approach #4: Javascript


As above, you could get the JSP to produce a HTML page that contains javascript to perform the redirect.
<html>
<head>
<script type="text/javascript">
window.location = "http://www.example.com/";
</script>
</head>
<body></body>
</html>

You could use setTimeout to get the redirect to happen after a period of milliseconds.


Notes:
Out of curiosity I checked TCP monitor for what the browser is actually receiving when you use Approach #1 or #2 (sendRedirect or c:redirect)
HTTP/1.1 302 Moved Temporarily
Location: http://www.example.com

So it saves you from 2 lines of servlet (or scriptlet) code:
response.setStatus(302);
response.setHeader("Location", "http://www.example.com");

So the difference to Approaches #3 & #4 isn't great or much more inefficient really, in all cases it's up to the browser to go to the specified address.

Friday, October 10, 2008

Comments in JSON

Keywords:
comments JSON javascript

Problem:
Can you have comments in JSON?

Solution:
A discussion leading to the answer is on the Bytes IT forum.

Basically, though it's not defined in the JSON Grammar - on json.org - you can use slash-star /* ... */ comments to get most(?) javascript engines to ignore the comment text.

For example:
<script type="text/javascript">
    var nested_arrays = {
        cities: [
            ["Sydney", "Melbourne", "Canberra"]  /* Australia */
          , ["London", "Birmingham"]             /* UK */
          , ["Los Angeles", "New York"]          /* US */
        ]
    };
</script>