# Tuesday, 09 March 2004
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Object Model Mapping Part 4

Yesterday I checked in a major change set that implements the new object model mapping infrastructure. Today I put the new snapshots online as well. The new implementation is about a thousand lines less code than the previous.

What's new

  • Many code changes to implement the new model.
  • When compiling classpath.dll, ikvmc now requires the -remap:map.xml option. This is the only time the mapping information is read from the XML. When code actually runs, or when other classes are compiled, the remapping information is read from custom attributes in classpath.dll.
  • Tracing infrastructure. Interesting points in the runtime now contain trace calls that can be enabled with a command line switch (or app.config setting). In addition, when Java code is compiled it can optionally be instrumented so that each method called writes its name and signature to the tracer. This has a big performance impact (it will be optimized a little bit in the future, but don't expect too much), so it is not enabled for classpath.dll, by default.
  • classpath.dll now contains the remapped types (java.lang.Object, java.lang.Throwable, java.lang.String and java.lang.Comparable). This means that if you want to create a Java like class in C# you can now extend java.lang.Object. Note however that you should never define your references as java.lang.Object, use System.Object instead. If you want to call a java.lang.Object instance method on a System.Object reference, use the corresponding static instancehelper method on java.lang.Object.


From the Java side of the fence, finalization continues to work as it always has, but when C# code is subclassing Java code, you should use the C# destructor if you need finalization. If you override the finalize method, you run the risk that it isn't called (it only gets called if one of your Java base classes actually overrides it). The C# destructor does the right thing. If you use another .NET language, you have to override Finalize and make sure that you call the base class Finalize. More complicated mixed scenarios (e.g. Java code subclassing C# code that subclasses Java code) are not supported at the moment (wrt finalization, other aspects should work fine).

What's next?

It's not quite done yet, but I'll be going through a stabilization phase before making any more changes. I have some ideas for changes to the way the remapped .NET types appear on the Java side (e.g. should it be possible to extend cli.System.Object in Java?). There are also some optimizations that can be done and there still remains some restructuring to be done.


I've tested this snapshot pretty well, but considering the scale of the changes, I expect some regressions. Bug reports are appreciated (as always).

New snapshots: just the binaries and source plus binaries.


Next month I'm speaking again at the rOOts conference in Bergen, Norway, where I had a very good time last year. Come and say hi if you're there. Also, I'm happy to be speaking again at the excellent (and fun) Colorado Software Summit in Keystone, Colorado in October.

Tuesday, 09 March 2004 10:42:12 (W. Europe Standard Time, UTC+01:00)  #    Comments [1]
Tuesday, 09 March 2004 14:58:48 (W. Europe Standard Time, UTC+01:00)
I'm not convinced that cli.System.Object should be visible to Java at all. AIUI, Java code will never see instances of cli.System.Object, because all such objects appear to inherit from java.lang.Object instead.

If cli.System.Object *is* visible to Java code, it introduces a paradox: java.lang.Object inherits from cli.System.Object (per the way it's actually implemented) but cli.System.Object should appear to inherit from java.lang.Object (per Java's rule that *everything* inherits from java.lang.Object). Now, it may be possible to create magic glue code that inverts the apparent inheritance relationship like that, but do you really want to go there? :)
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I apologize for the lameness of this, but the comment spam was driving me nuts. In order to be able to post a comment, you need to answer a simple question. Hopefully this question is easy enough not to annoy serious commenters, but hard enough to keep the spammers away.

Anti-Spam Question: What method on java.lang.System returns an object's original hashcode (i.e. the one that would be returned by java.lang.Object.hashCode() if it wasn't overridden)? (case is significant)

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