System dependencies

I’m so happy to be developing in Java. It takes away the drudgery of software development, like memory management. And it frees you of worrying about how certain features are implemented on different platforms: Write Once, Run Anywhere!

You do feel the but coming, don’t you? 😉

Well, sometimes I do run into system dependencies. And since I’m no longer used to it, I don’t expect it anymore. Ah well, this happens only once in a very, very little while. Let me tell you about one such event.

I described in my previous post how I used reflection to extract common code into a base class. I used Class.getMethods(), for which the JavaDoc reads:

public Method[] getMethods() throws SecurityException

Returns an array containing Method objects reflecting all the public member methods of the class or interface represented by this Class object, including those declared by the class or interface and those inherited from superclasses and superinterfaces. Array classes return all the (public) member methods inherited from the Object class. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if this Class object represents a class or interface that has no public member methods, or if this Class object represents a primitive type or void.

The interesting part is in italic. This is one of those sentences that you can easily overlook. I know I did.

What does it mean? Nothing more than that the order is undefined in the spec, and so depends on the system (in this case the particular JVM implementation) that you use. We use both Windows and GNU/Linux to test our stuff, but on both we have a Sun JVM. I guess most people will use this one too, since it’s from the makers of Java and it’s free (as in beer and now also as in speech).

But not IBM. For their AIX platform, they have built a custom JVM. And you guessed right: that JVM uses a different order for the methods in the array. Whereas the Sun implementation always gives you methods from the class, then those from it’s base class, etc. the IBM implementation uses the exact reverse order. My code subtly depended on that order, and so it failed on AIX.

BTW, if you want to find out more about which JVM implementation you are using, just issue java -version. This is from my machine at home:

java version "1.6.0_06"
Java(TM) SE Runtime Environment (build 1.6.0_06-b02)
Java HotSpot(TM) Server VM (build 10.0-b22, mixed mode)

The HotSpot part is what gives away that it’s from Sun. This is from our AIX box:

java version "1.5.0"
Java(TM) 2 Runtime Environment, Standard Edition (build
    pap64dev-20080315 (SR7))
IBM J9 VM (build 2.3, J2RE 1.5.0 IBM J9 2.3 AIX
    ppc64-64 j9vmap6423-20080315 (JIT enabled)
J9VM - 20080314_17962_BHdSMr
JIT  - 20080130_0718ifx2_r8
GC   - 200802_08
JCL  - 20080314