Wednesday, August 25, 2010


I ran across an interesting issue today involving JVM internals, and how they behave differently based on the host OS.

The issue was with the following code snippet:

MyEntity a = new MyEntity();
a.setUid("foo" + System.currentTimeMillis());
a.setName("my entity");;

This was part of some object creation code I was using in a unit test, which ran successfully every time on my macbook but was failing on other's windows machines. When I found out the reason behind this, I was definitely surprised.

Believe it or not, it is more or less a known issue that the resolution of currentTimeMillis() on windows machines hovers around 10-15ms (linux and mac have a resolution of around 1ms). This was causing duplicate foreign key violations to occur.

So, as a general rule of thumb, it is a bad idea to rely on currentTimeMillis() to have a millisecond-resolution on windows, especially if you are using it to generate unique things like file names or db IDs. Looks like System.nanoTime() might tick faster.

Here are some links I found on StackOverflow about this:

On Windows, you'll more or less be limited to 10 ms resolution at best. Here's a bit from the Inside Windows NT High Resolution Timers article from TechNet:

Windows NT bases all of its timer support off of one system clock interrupt, which by default runs at a 10 millisecond granularity. This is therefore the resolution of standard Windows timers.

In my experience, using System.currentTimeMillis method gives about 15-16 ms resolution on Windows. Getting better time than the operating system timer will probably require more exotic methods.

Here's the StackOverflow question which helped me solve this issue:

At the end of the day, I decided it was simpler for my test to use a static synchronized counter variable rather than depending upon currentTimeMillis() for generating my UIDs. Another possible solution could be to use UUID.randomUUID().toString().

Wednesday, August 11, 2010

Fixing ClassNotFoundExceptions when working with Eclipse WTP, Maven, Tomcat

Lately, I've been struggling off and on with ClassNotFoundExceptions when starting tomcat from within Eclipse (Springsource Tool Suite, to be exact). My projects are all dependency-managed by the Maven m2eclipse plugin (side note: its really important to install the m2eclipse WTP add-on, found here: so that the web dependencies are all configured correctly).

Anyways, I would tend to encounter this error after restarting Eclipse. When I would investigate, by looking into the [tomcat_home]/wtpwebapps/[app]/WEB-INF/lib folder, it would of course be empty (hence, class not found). I would always flail about aimlessly for hours until it would magically start working again.

I have in the past week discovered a new trick, which has worked *both* times I've tried it: when I got the class not found error, I stopped tomcat, right clicked on my project in the Package Explorer, and clicked Maven -> Update Dependencies. For whatever reason, this caused WTP to publish the dependent jars into WEB-INF/lib, and everything worked again. :-)

Hope that helps save some debugging hours for some!

Thursday, June 3, 2010

DOM Events for HTML Radio Buttons

An interesting bug came up today, and it had to do with the way IE 8 processes DOM events differently than the other browsers.

Here's the scenario:

We were attaching to a radio button's onchange event to do some processing. When you click on an option, we show you a particular div relevant to your selected option. This worked just fine on Firefox and Chrome, but was wacky on IE8. It turns out that IE8 doesn't seem to fire the onchange DOM event until after the element *loses focus*, meaning that our divs didn't show up until we clicked somewhere else on the screen.

We fixed the issue by switching to the onclick event, which fires nicely, on the click, as you would expect, on all three browsers.

In summary: when you want to trigger something when a user selects one of your radio buttons, attach to the onclick event, and not the onchange event, which has variable behavior across browsers.

Friday, February 19, 2010

Unsigned Java Primitives

This one's just a quick note about unsigned primitives and how to work with them in Java, which technically has no notion of 'unsigned'.

If you need to find the unsigned value of a primitive (e.g. you are porting code in another language, or dealing with byte-level networking), you basically need to promote it to the next higher data type, and mask out the bits you don't need (anything above the size of your unsigned data type).


To find the unsigned value of a byte (8 bits):

byte b = ...;
short unsignedB = (0x00FF & ((short)b));

And for a short (16 bits):

short s = ...;
int unsignedShort = (0x0000FFFF & ((int)s));

And so on.

You can read more on this and other exciting topics such as *-endian byte ordering, and why Java doesn't have unsigned in the first place here:

(hint: Gosling didn't think Java developers could understand them!)