# Tuesday, 10 March 2009
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100% Pure Java

In the past weekend I tried running JRuby 1.2.0RC2's interactive command line jirb on IKVM.NET. It somehow reminded me of an experience I had a little over a decade ago getting an application certified for as 100% Pure Java.

At the time I didn't really see the point of the 100% Pure Java certification, but Sun convinced us (mostly by offering to pay the certification costs), so we went ahead and eventually got certified. As far as I remember we didn't have any major issues, mostly some hardcoded paths that included backslashes and some dependencies on a case insenstive file system (to make it easier to find these issues without having to buy Solaris, I actually patched the C runtime that came with the Windows JDK to make all file access case sensitive). The other problem was that in those days Swing was exceptionally unstable on Solaris, but they eventually conceded that this wasn't our fault ;-)


As some may remember, I have been running JRuby tests for a while. This is an older version of JRuby (I don't even know which version) and not used interactively. In any case, running the most recent JRuby revealed a number of issues:

  1. IKVM.NET bug in JNI that caused constructing an object of a dynamically loaded class via JNI to fail.
  2. IKVM.NET regression introduced a couple of days ago.
  3. An infinite recursion bug in the CLR x64 JIT.
  4. JRuby Issue.
  5. JNA Issue (JNA is a library used by JRuby that provides P/Invoke like functionality).

JRuby Issue

The first issue was that when I started jirb, it would not come up with the irb(main):001:0> prompt but simply read lines, echo them back and execute them. After debugging this I found that JRuby uses reflection to access the private handle field in java.io.FileDescriptor (when on Windows). IKVM's FileDescriptor didn't have a handle field, because it uses .NET's System.IO.Stream instead of the native Win32 API.

I solved this by adding a handle property to FileDescriptor (not the actual code):

@ikvm.lang.Property(get = "get_handle")
private long handle;

private long get_handle() {
  if (stream instanceof cli.System.IO.FileStream) {
    cli.System.IO.FileStream fs = (cli.System.IO.FileStream)stream;
    return fs.get_Handle().ToInt64();
  } else if (this == in) {
    return GetStdHandle(-10).ToInt64();
  } else if (this == out) {
    return GetStdHandle(-11).ToInt64();
  } else if (this == err) {
    return GetStdHandle(-12).ToInt64();
  return -1;

private static native cli.System.IntPtr GetStdHandle(int nStdHandle);

Now when JRuby uses reflection to get the handle field, it ends up calling the get_handle method and it will get the right handle back. The nice thing about implementing this as a property it that it means the cost is virtually zero when you're not accessing it and it also delays the full trust requiring native method invocation until it is really needed, which means that partial trust code won't be affected.

This solved the first problem, the right prompt now appeared.

JNA Issue

The next problem appeared after I ran jruby.jar through ikvmc and ran the resulting jruby.exe. After exiting I would get an exception:

irb(main):001:0> exit
java.io.IOException: Cannot run program "C:\.virtual-ikvm-home/bin/java": The system cannot find the file specified
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:474)
    at java.lang.Runtime.exec(Runtime.java:610)
    at java.lang.Runtime.exec(Runtime.java:483)
    at com.sun.jna.Native$DeleteNativeLibrary.run(Native.java:761)
    at java.lang.Thread.threadProc(Thread.java:2297)
    at java.lang.Thread$1.Invoke(Thread.java:797)
    at cli.System.Threading.ThreadHelper.ThreadStart_Context(Unknown Source)
    at cli.System.Threading.ExecutionContext.Run(Unknown Source)
    at cli.System.Threading.ThreadHelper.ThreadStart(Unknown Source)
Caused by: java.io.IOException: The system cannot find the file specified
    at java.lang.ProcessImpl.<init>(ProcessImpl.java:126)
    at java.lang.ProcessImpl.start(ProcessImpl.java:54)
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:467)
    ... 8 more

This didn't show up when running via ikvm.exe, because I recently added support for this, but when running jruby.exe the ikvm.exe file wasn't available. I debugged this because I was curious why a new JVM was being started at exit. It turns out that JNA extracts a JNI DLL into the temp directory and wants to delete that at exit. While the DLL is loaded by the JVM, it cannot be deleted so in a shutdown hook it fires up a new JVM to delete the temporary file. This is already pretty gross (and not 100% Pure Java :-)), but not the actual issue. Digging deeper I found that this is the alternative code path, because they first attempt to unload the DLL by explicitly calling java.lang.ClassLoader$NativeLibrary.finalize() via reflection on the NativeLibrary object that corresponds to their DLL. This is, of course, exceptionally evil. Nevertheless, I have now implemented support for this in IKVM.


Maybe 100% Pure Java isn't as useless as I thought a decade ago :-)

Tuesday, 10 March 2009 06:53:42 (W. Europe Standard Time, UTC+01:00)  #    Comments [5]