# Monday, December 8, 2008
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Modularizing OpenJDK

Recently Mark Reinhold started blogging about modularizing the JDK. I've been getting requests to split up IKVM.OpenJDK.ClassLibrary.dll for a long time. It's good to see that the Java platform is also moving in that direction, but we needn't wait for that.

I've been working on making ikvmc support "multi target" mode for a while now. Last week I used this mode to compile the OpenJDK classes into 743 different assemblies (one per package) . I then tried to use NDepend to dig through the dependencies, but it turns out that 743 assemblies is a bit too much to handle (NDepend handled it resonably well, but the dependency graph was way too large to be useful). So I started moving some obvious stuff together. The result was this NDepend report. Still a lot of data and a lot of dependencies, but some patterns are starting to emerge.

Here's my preliminary view of how things could be split:

IKVM.OpenJDK.Core.dll 5 MB
IKVM.OpenJDK.Security.dll 3 MB
IKVM.OpenJDK.Util.dll 1 MB
IKVM.OpenJDK.Xml.dll 8 MB
IKVM.OpenJDK.SwingAWT.dll 3 MB
IKVM.OpenJDK.Charsets.dll 5 MB
IKVM.OpenJDK.Corba.dll 2 MB
IKVM.OpenJDK.Management.dll      2 MB
IKVM.OpenJDK.Misc.dll 3 MB

(The sizes are only approximate.)

I had originally hoped to make IKVM.OpenJDK.Core.dll smaller by keeping java.util, java.net and java.io out of it, but it looks like you won't be able to run anything non-trivial without requiring classes from these packages, so it makes more sense to put them into the core assembly. The IKVM.OpenJDK.Security.dll and  IKVM.OpenJDK.Util.dll assemblies contain other util and security related packages that shouldn't be needed as often.

It is possible to split packages across assemblies (e.g. java.awt.AWTPermission will be in IKVM.OpenJDK.Core.dll because java.lang.SecurityManager depends on it), but given the potential for confusion my current thinking is that it is probably best to only move individual classes into Core should it be necessary (because, realistically, you can't develop without having a reference to Core you're less likely to be confused when trying to locate the class).

To avoid confusion or expectations that are too high: I haven't yet built the runtime infrastructure to support this. So while I can compile the class library into all these parts, the runtime won't actually be able to work correctly, because it still expects all the boot class loader classes in a single assembly.

As always, feedback on the proposed split is very welcome.


I really like NDepend's ability to show the dependencies in different ways and the interactive dependency matrix that allows you to drill down into a dependency to see exactly where it comes from. Beyond dependency analysis it also has a power SQL like query language to allows use to query dependencies and compute all the code metrics you can come up with. It also includes a number of code metrics out of the box, but those aren't really my cup of tea, so I can't comment how useful they are.

One other small but really nice thing about it is that you can run it without installing. IMO this is very nice compared with installers that do who knows what to your system (and require administrator access).

An evaluation copy can be downloaded from their website.

Full disclosure: I was given a free copy of NDepend Professional Edition.

Monday, December 8, 2008 6:51:28 AM (W. Europe Standard Time, UTC+01:00)  #    Comments [0]