Computer Floss

With the release of Indigo, Eclipse hackers have been exposed to a little change in the API of the org.osgi.framework.Version class. Specifically, a new method: compareTo(Version) has appeared. Prior to Eclipse 3.7, comparing two Version objects was done via the compareTo(Object) method.

This caused us in the Saros team to suffer a rather subtle bug. When Saros was compiled under Eclipse 3.6 it functioned as expected on the latest version of everyone’s favourite IDE. But when Saros was compiled under 3.7 it suffered an NoSuchMethodException under 3.6 when a call to compareTo() was made. That’s because it was compiled in an environment (Indigo) that inserted a call to compareTo(Version), but was run in an environment (Helios) that knew nothing about such a method.

The reverse situation (compiled in 3.6 and run under 3.7) still worked because a call was inserted that references compareTo(Object). Version inherits from the Comparable interface and thus recognises compareTo(Object) regardless of Eclipse version. I saw at least a couple of other projects complaining online about the same symptoms when I was seeking a solution. As I was unable to find a good solution, I thought I would share with you what I came up with.

To get around the problem, I used reflection. The solution searches first for compareTo(Version) and then for compareTo(Object). Whichever method the code finds, it then executes. Et voilà:  code that will run in any iteration of Eclipse regardless of the version it was compiled under… for now at least.

Here’s the gist of the code:

Class versionClass = Version.class;
Method compareToMethod = null;
 
try {
    // Works on Eclipse 3.7
    compareToMethod = versionClass.getMethod("compareTo", Version.class);
} catch (NoSuchMethodException e1) {
    log.debug("We're on Eclipse 3.6 or earlier. Fall back compareTo(Object).");
    try {
        // Works on Eclipse 3.6 and earlier
        compareToMethod = versionClass.getMethod("compareTo", Object.class);
    } catch (NoSuchMethodException e2) {
        log.error("Unexpected error: cannot find compareTo() in Version" + e2);
    }
}
 
// Throws InvocationTargetException, IllegalAccessException
compareToMethod.invoke(v1, v2);

Edit: Thanks to a couple of readers pointing out the OSGi-preferred solution of compiling strictly against OSGi v1.5, which limits us to either: a) compiling strictly against Eclipse 3.6 (the solution we have been using), or b) importing version OSGi v1.5 into versions of Eclipse 3.7 and later.

The whole problem was actually slightly different from what I explained above. Remember, I’m not trying to use a brand new method newly introduced. What essentially happened was the signature of compareTo() method was changed (compareTo(Object) was actually removed without being depreciated between OSGi 1.5 and 1.6), causing formerly working code to become problematic. Of course, the OSGi method is safer, but prevents us from accessing functionality from later versions of the framework and required.

My solution above allows compilation against any version of Eclipse.