Two Random things I like about Java...

1) the fact that each and every class file needs to have 0xCAFEBABE in the first 4 byte positions

This actually identifies the file as a valid Java class file in addition to the file extension, no non-class file is likely to have this number in that location.

2 the fact that there is a class called java.text.DontCareFieldPosition!!

The name actually makes sense, it's used by the DecimalFormatter and is package-local, so clients of the API should never be aware of it, and is a null implementation of the FieldPosition interface. Cudos to the developer that argued that name past the API police!!

The importance of trim()... or how much I hate whitespace!!

So... One of my colleagues went on leave last friday, and Murphys law dictates that that is the perfect time for the application he maintains to suddenly and precipitously cease working in production. Of course this problem fell into my lap to resolve so I set to it around about lunchtime...

The situation is that a third party regularly sends us a variety of data in plain text or spreadsheet format, in this particular case it's spreadsheet. Once we get this, we read it off the filesystem, look up data from the spreadsheet and compare it to data in our systems and perform various actions based on it. This has been in production for about 8months and has worked perfectly the whole time. Until the maintainer went on leave... :-)

Digging in, the first problem was that it wasn't reading dates properly. Hmmm, ok, so we grab the last spreadsheet that worked and compare with that, ah, they'd changed the date format from YYYY-MM-DD to DD/MM/YYYY for no reason at all. Ok, not a biggie, one excel macro later it looks fine, reload but still no workie... what tha?

After more digging we were being bitten by Excel date handling. A cell may look like a date, but if the cell is formatted as a date then internally it's actually just an integer, which needs to be converted. In my checked-out version of code this was handled transparently, but in prod and QUAL this broke as numeric dates were being missed, resulting in nulls. Ok, another excel macro and we have columns of strings that look like dates instead of numbers that look like dates. Now loading this file into QUAL the dates are picked up fine, at which point I noticed that a lookup between the file and our system wasn't working. WTF mate? Each row has several cells that contain codes that need to be converted into rich data structures from a lookup, this data structure then being used for calculations later.

The lookup was broken, why? Well, after several hours of checking differences in environment, checking datasources, checking versions of code and finally in desperation stepping through the lookup code, it dawned on me... The spreadsheet had whitespace in it.

Normally when I read tabular data from disk, or really any untrusted data at all, trim()'ing and validation is the order of the day. If the field is an enumeration, then validate that. If it's a number, check for alphas, dollar signs, commas etc and so on. It's the sort of paranoia that maintenance programming gives you. I had ASSumed that my colleague would have done the same by default, oops no. Which is fine, we all make mistakes.

So, a quick google, this handy KB article and some VBA later, and the spreadsheet was all clean again. Re-running the load it came up roses, which is to say it started working like billy-o where it had previously just cruised through, and all was well with the world.

Moral of the story is, Excel is a pain to work with. Not because it's the spawn of pure evil, but rather because although it *looks* like a datasource with strong data rules, it isn't, and you can never, ever assume that anything inside it is what you think it is until you've checked and made sure. Columns will move around, dates will randomly be strings or numbers, any and every field will randomly be space/tab/zero padded. You simply have to treat it as a hostile datasource.

And never, ever fail silently when something is wrong with the data!! Die loudly and with great gusto!! Please for my sanity!!!

else
{
// No record exists may be something wrong with **** in the input file
record.setVaild****Prev(false);
}

java.lang.OutOfMemoryError: PermGen space

You know those things that don't crop up very often, but when they do they're a complete pain in the a**? well this is one of them!

Developing a plugin for Atlassian Jira, I don't care about heap size, or space allocated to PermGen, all I care about is getting functionality to the business.

I've read this:

http://wiki.caucho.com/Java.lang.OutOfMemoryError:_PermGen_space

and this:

http://www.jroller.com/agileanswers/entry/preventing_java_s_java_lang

among many, enough to know that PermGen isn't as simple as "make the heap bigger", enough to know that it's probably the fact that I'm running the app server in debug mode that contributes to this problem, but still, I'd rather not have to deal with it.

I understand that there's a lot of complexity involved in the Java Memory Model, I understand that people a lot smarter than me came up with it and decided on restrictions like the fact that the JVM can't give memory back to the OS once it's freed it, to make the problem of Performant Garbage Collection solvable, I'm cool with all that!

But... I just want it to work! I wish there was a --XX:PromiscuosMemory flag that I could set to say, hey, I'm developing here, this won't be used in production, I know what I'm doing, I don't mind if I take a bit of a hit on performance, but I would like you to eat memory as needed until you reach the system limit, running the garbage collector whenever you want, and you know, if you get a whole lot to spare? Just go on and give it back to the OS, thanks.

Of course that doesn't exist, so I find myself adding "-XX:MaxPermSize=512m -Xms128m -Xmx512m" to my startup script, and not caring about the memory model anyway. fun fun.

Using Reflection to get around hard-coded dependencies

Spring famously has no cycles in it's dependency graph:

http://www.springsource.org/node/310

Obviously writing applications is different to writing frameworks, and we can't always take the time to get things so polished. Also by the nature of the beast the domain model will (should!) pervade the entire application (domain model should be the root of your graph, with services/repositories/ etc depending on them).

Sometimes though you may want to enable some functionality if an object is an instance of some special class, without introducing explicit dependencies on those classes. reflection is your friend!!

Of course the price is that you loose type-safety, so be warned here be dragons!!

simply at runtime see if the string representing the class name is the class you want to twiddle, if it is, reflectively invoke your methods enabling additional functionality, otherwise continue on in the boring type-safe way that only deals with the interface or bace class you're dealing with. The key here is that by default you deal with a standard interface or base class (such as javax.activation.DataSource), and only if the instance of the object has the extra bits you want (eg my.cool.SuperSmartDataSource), then you selectively elect to do extra. Spring does this quite a lot with it's own DataSource wrappers internally.

handy sometimes...

Classloading in Application Servers: JARmaggedon!

original issue with trying to access the tomcat security principal in IMGWS, but since class was loaded from a sibling classloader, not a direct parent was unable to cast the object to the correct class to access it, even though debugging revealed it to be the correct object. Problem: class loaded from an unavailable classloader, so unable to use any methods or fields related to the class, have to break in through reflection.

current issue: upgrading JBoss 4.0.4 to JBoss 4.2.3, except that we use spring hibernate, and JBoss tries to be helpful by providing those jars as well, thanks JBoss, now I get

java.lang.ClassCastException: org.hibernate.search.event.FullTextIndexEventListener cannot be cast to org.hibernate.event.PostInsertEventListener

Solution? remove hibernate jars from jboss server/default/lib. this works ok in dev, but will probably break advanced jboss functionality which relies on those jars, lucky I don't use it... :-)

Our deployment scenario is amenable to not having spring/hibernate etc jars provided by app server, instead provided by each applications WAR. for us this works as we have one app per app server, but as more come online this may change. On the other hand, each application providing all it's own dependencies may cause some duplication of libraries and cost disk space, but this is a cheap price to pay for applications to evolve at their own rates and not be constrained by server dependencies, eg - I need a feature in hibernate 3.3.0, but in prod we only have 3.2.2. No thanks!

Personal opinion is that applications should be as self-contained as possible to simplify deployment and dependency management (leave that to maven). the exception to that is infrastructure explicitly provided by the app server, eg servlets, ejb3 persistence maybe, JDBC drivers definitely. Big fat WAR is Not A Problem, compared to Jarmeggadon...

System Antics

I've had the 3rd edition of this book in my amazon wishlist for ages, now I finally found an online version of the first edition which is short & pithy. read the preface then if you like it check out the rest. fantastic discussion of systems in general (in the sense of systems that humans set up, including but not limited to IT systems, including organisations, governments & companies etc) especially relating to failure modes.

http://www.ece.osu.edu/~fasiha/systemantics/

Thought vs Action

Circus saying "Thinking kills circus skills"

thinking about what you're doing in the middle of execution detracts from performance, but you need to constantly self-evaluate and adjust course. This creates a tension between thought & action, discuss.