# Monday, 26 May 2014
New Development Snapshot

More progress on the OpenJDK 8 integration.

Changes:

  • Integrated Nashorn.
  • Added workaround for bug in .NET 2.0 implementation of ModuleBuilder.GetMethodToken() exposed by Nashorn.
  • Bug fix. A .NET type should always have access to its nested types.
  • Fixed javap build.
  • Bug fix. The MethodHandle code requires the volatile field accessors in Unsafe for volatile field access.
  • Bug fix. Handle more than 8 parameters in invokedynamic.
  • Bug fix. DynamicMethod's ILGenerator doesn't like unbaked array types.
  • Bug fix. MethodHandle.invoke[Exact] methods should have VarArgs flag set.
  • Added workaround for javac MethodHandle.invoke[Exact] detection bug.
  • Moved cldrdata and localedata into separate assemblies.
  • Implemented native methods for HostLocaleProviderAdapterImpl. The system property "java.locale.providers" can now be set to "HOST" to use the .NET locale implementation.

Binaries available here: ikvmbin-8.0.5259.zip
Sources: ikvmsrc-8.0.5259.zip and openjdk-8-b132-stripped.zip

Monday, 26 May 2014 10:22:29 (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Tuesday, 20 May 2014
First OpenJDK 8 Based Development Snapshot

I finished the first major step for integrating OpenJDK 8. The class libraries are now built from the OpenJDK 8 sources. Not all new functionality is implemented yet (both in the class library and in the VM).

Lambda's work, but they are not yet optimized, so they are very slow. This will be fixed in a future development snapshot.

Another area that still needs a lot of work is the assembly partitioning of the class libraries. Currently IKVM.OpenJDK.Core.dll has grown to 13MB. That's clearly not a good thing. Also, a bunch of work has gone into OpenJDK 8 to better factor it and to support making parts of the class libaries optional, so that should be exposed as well.

Finally, there is no Nashorn support yet.

Changes:

  • Enabled Java 8 class file support by default.
  • Several fixes for Java 8 static, private and default interface methods.
  • Allow classes defined by Unsafe.defineAnonymousClass() access to private methods of their host class.
  • Add ExtensionAttribute to IKVM.OpenJDK.Core assembly, because Roslyn requires that for the extension methods to be visible.
  • Bug fix. Unsafe.park(false, 0) means infinite wait timeout, contrary to LockSupport.parkNanos(0).
  • Bug fix. Miranda method override stubs should be handled by the normal code, because a Miranda method can implement multiple interface methods.
  • Bug fix. Bridge method names should not include prefix in stack trace.
  • Fixed handling of ghost array types in remapped type methods.
  • Fixed class file validation of method and field names to disallow '[' character.
  • Fixed class file validation to disallow zero length bytecode stream.
  • Bug fix. Exceptions in bootstrap method arguments should not be wrapped in BootstrapMethodError.
  • Fixed ikvmc to emit fatal error instead of crashing when a field referenced in remap file isn't found.
  • Added ikvmc option -assemblyattributes: to explicitly specify the assembly attribute container class and deprecated the previous behavior.
  • Bug fix. If ikvmc -warnaserror is used, "warnings" should not be suppressed after an error was emitted.
  • IKVM.Reflection: Fixed several bugs in Type.GetInterfaceMap().
  • IKVM.Reflection: It is legal for a MethodBuilder to not have a signature, in that case it defaults to returning void with no parameters.

Binaries available here: ikvmbin-8.0.5252.zip
Sources: ikvmsrc-8.0.5252.zip and openjdk-8-b132-stripped.zip

Tuesday, 20 May 2014 09:06:50 (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Thursday, 15 May 2014
Default and Static Interface Methods

Java 8 introduces default and static interface methods.

Here's an example Java 8 interface:

public interface Foo {
  default void m1() {
    System.out.println("Hello from default interface method");
  }
  static void m2() {
    System.out.println("Hello from static interface method");
  }
}

When this interface is compiled by ikvmc,  it produces the approximate equivalent of the following pseudo C# code:

public interface Foo {
  public static class __DefaultMethods {
    public static void m1(Foo obj) {
      if (obj == null) throw new NullReferenceException();
      Foo.<default>m1(obj);
    }
  }
  public static class __Methods {
    public static void m2() {
      Foo.m2();
    }
  }
  public static void <default>m1(Foo obj) {
    System.out.println("Hello from default interface method");
  }
  public abstract void m1();
  public static void m2() {
    System.out.println("Hello from static interface method");
  }
}

There are a number of things going on here, so let's go over them one by one.

__DefaultMethods

This nested type exists to allow C# code to call the default method implemenation. So when your C# class implements the Foo interface, it may implement the m1 method by calling the default method. The reason it is a nested type is to avoid method name clashes and because the current C# compiler doesn't allow access to static interface members.

__Methods

This only exists because the current C# compiler doesn't allow access to static interface members (the new Roslyn C# compiler does). It is similar to the nested __Fields type that already exists to expose interface fields.

<default>m1

This is a static method that contains the body of the default interface method. Its name is mangled to avoid conflict with the "real" abstract method. Internally ikvmc calls this method when a class inherits the default method.

m1

Regular abstract interface method.

m2

Regular static method.

The result of this implementation choice is that it can be quite painful to implement a Java 8 interface in C#, because you manually have to forward each default method.

Given the large number of default methods introduced in Java 8 this is going to make interop quite a bit more inconvenient. A partial workaround for this is to write more interop code in Java or create a Java base class that implements the interface so that it can inherit the default methods and then write the implementation as a C# subclass.

I'm considering changing ikvmc to automatically generate a __BaseClass nested type inside all public interfaces to at least make it easy to write a class that implements a single Java interface.

JVM interfaces can also have private methods. The current C# compiler doesn't like interfaces with non-public members, so the private methods are put into another nested type by ikvmc (and instance methods are compiled as static methods).

Thursday, 15 May 2014 12:06:11 (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Thursday, 01 May 2014
Java Method Overriding Is FUBAR Part 9 of ∞

Java 8 introduces default methods. In the current HotSpot implementation this makes adding a private method to a non-final class a binary breaking change (contrary to what the JLS says about this).

Here's an example. Suppose you have two separate code bases Lib and App, shipped by different parties. App depends on Lib.

Lib defines a class B:

public class B {
}

App defines an interface I and a class D:

public interface I {
  default void m() {
    System.out.println("I.m");
  }
}

public class D extends B implements I {
  public static void main(String[] args) {
    D d = new D();
    d.m();
  }
}

All is well in the world. Now Lib ships a new version that includes a new version of class B:

public class B {
  private void m() { }
}

Now when you run D the output is:

Exception in thread "main" java.lang.IllegalAccessError: tried to access method B.m()V from class D
        at D.main(D.java:4)

You could argue that this is "just" an implementation bug, but I posted it as part of this series because it is symptomatic of the mess that is Java's method dispatch story.

Thursday, 01 May 2014 09:38:36 (W. Europe Daylight Time, UTC+02:00)  #    Comments [1]