Archive for Java

ng-admin + JAX-RS: 400 Bad Request on DELETE

I’m tried of building admin UIs and I’m trying out ng-admin. It’s pretty straightforward to setup given the guides and demos.

List, create, updates were fine until I got to the DELETE method. The server was throwing 400 Bad Requests and upon Chrome network inspection I discover that ng-admin was sending a JSON body in the request. I don’t really care who is “following the standard” as long they work together (think browsers and jquery), so I’m fine to fix either side to either the client not send the body, or the server accepting the non-empty body.

ng-admin uses Restangular under the hood to make REST requests. Restangular did have this FAQ about DELETEs with body(s).

A little refactoring and presto! DELETE now works.


app.config(['RestangularProvider', function(RestangularProvider) {
  RestangularProvider.setRequestInterceptor(function(elem, operation) {
    return (operation === "remove") ? undefined : elem;
  });
}]);

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Wrong encoding when JSP includes HTML

We had a standard JSP header with logo and menu, and we used it to include a few static HTML pages (e.g. privacy policy and about us). We had multiple versions of the static content in different languages, and the non-English ones shows up in gibberish, although the menu shows up fine.

We already specified the encoding in our enclosing JSP page.

<%@ page pageEncoding="UTF-8"%>

as well as specified the charset in the meta header

<meta http-equiv="content-type" content="text/html; charset=utf-8">

A common “workaround” is to convert the html to JSP to specify the encoding. However being lazy developers, we want a better solution. Eventually we found the hint to set the encoding in web.xml. We tweaked the extension from .jsp to .html and it works!


<jsp-config>
  <jsp-property-group>
    <url-pattern>*.html</url-pattern>
    <page-encoding>UTF-8</page-encoding>
  </jsp-property-group>
</jsp-config>

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double HTTP requests

We first discovered that our webapp was receiving duplicate HTTP AJAX requests from clients that results in a database insert. Fortunately jQuery had a nocache timestamp as part of the AJAX request so we could recognize it as a duplicate and reject the 2nd request.

As we tried to narrow down the cause we found that it happens on both GET/POST, as well as on a variety of browsers and network providers. After days of trying to reproducing the behavior we resorted to using iMacro to do browser automation and finally manage to reproduce it occasionally. What was surprising was that we were receiving the response of the 2nd response, which was the rejection message! (while the database insert was successful) It was absolutely confusing and we set up the automation again with Wireshark.

The packet analysis confirmed that the client browser was sending the duplicate request. However we also scanned Firebug and confirmed that only a single AJAX call was made! It didn’t make any sense to any of us until I noticed a pattern in the Wireshark logs that the 1st request made was always terminated without a response (either a TCP Connection Reset or a proper TCP Connection Close initiated by the server), and the 2nd request would be made on another kept-alive TCP connection. Following that symptom with Google I chanced upon a blog that highlighted HTTP 1.1 RFC 8.2.4.

If an HTTP/1.1 client sends a request which includes a request body, but which does not include an Expect request-header field with the “100-continue” expectation, and if the client is not directly connected to an HTTP/1.1 origin server, and if the client sees the connection close before receiving any status from the server, the client SHOULD retry the request.

Could this be it? We mocked up a HTTP server using raw Java ServerSockets by intentionally closing the first connection and allowing the second. We logged the requests on the server-side and voila! The mock server showed that it received two HTTP requests, Firebug showed that it sent one only, and Wireshark showed that the browser sent two.

Essentially, we just proved that the browsers adhered to HTTP 1.1 RFC 8.2.4…….

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Trimpath unterminated string literal in IE

We’re using an ancient library called TrimPath to render templated data with JavaScript, and a peer hit the “unterminated string literal” in eval() on IE only. I assisted to solve it, by extracting the offending string and isolated the cause to a ‘\n’ within the string. It was easy to cut away the additional newlines and IE is happy. (This is on the assumption that the newline are decorative and not necessary as part of the output)

However, I wasn’t satisfied what caused the newline to be parsed wrongly, and why IE only. The good thing with Open Source is we get to dig in ourselves to troubleshoot issues.

After stepping through the cause is identified: multiple continuous \n not parsed correctly. Apparently someone hit this at least 5 years ago and there was a fix for it, by detecting the additional \n and eating it up. But it still fails if there are more \n.

Why IE only? We were using script tags as the template, and in IE script.innerHTML is prefixed with an additional \n.

The following snippet will fail in IE, but pass in FF.


<html>
    <head>
        <script src="trimpath-template-1.1.2.js"></script>
    </head>
    <body>
        <script id="src" type="text/html">

            Test
        </script>
        <div id="out"></div>
        <script>
            document.getElementById("out").innerHTML = 
                TrimPath.processDOMTemplate("src", null);
        </script>
    </body>
</html>

If another newline is added (with no additional whitepsace), FF will fail with the same error.
If <textarea> instead of <script> is used, it will pass, but adding more newlines will also cause it to fail.

As an anecdote, I’m not sure how our project got to use 1.1.2, when the last on the download site is 1.0.38 (legacy stuff…). But a search on the web shows it’s not just us

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TimeUnit

How many times have you got a millisecond long timestamp and had to compare it to a duration e.g. 1000 * 60 * 60 * 24 * … ? I stumbled upon this more readable and less error prone way of representing, say, 5 hours, by using the java.util.concurrent.TimeUnit.


private static final long FIVE_HOURS = TimeUnit.MILLISECONDS.convert(5, TimeUnit.HOURS);
...
if (timestamp < System.currentTimeMillis() - FIVE_HOURS) {
    // do something.
}

The above will check if a timestamp is overdue for more than five hours. The variable twoDays will have the value 172800000. TimeUnit supports from nanoseconds, micro-, milli-, all the way up to days.

However, if the date/locale is important to you (e.g. daylight savings) then you should use the Calendar API/JodaTime rather than this “duration”-based TimeUnit.

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