SWT in a DLL [long]

[Cristiano Paris <frodo at theshire dot org>]

Hi everyone,

I've posted this in the MingW mailing list but I haven't received much
feedback: maybe the topic in more appropriate for this mailing list than
the MingW one. I'm sorry if this mail is a duplicate for some of you.

I'm new to the mingw/gcj and I'm experimenting hard. I've successfully
built the GNU Toolchain for the mingw32 target both for Linux and Windows.

Details of the toolchain:

- gcc 4.0.2
- binutils 2.16.1
- w32api 3.5
- mingw-runtime 3.9

I could compile simple SWT/JFace applications using gcj and the
SWT/JFace libraries compiled in statically.

Now I'm experimenting with DLLs. I've compiled the following code:

mydll.java:
public class mydll
{
  public int doubling(int i)
  {
    return i*2;
  }
}

which implements a simple DLL, and:

main.java:
public class main
{
  public static void main(String[] args)
  {
    mydll dll = new mydll();
	
    System.out.println("Result = " + dll.doubling(5));
  }
}

which calls the class's method. I've compiled the DLLs and the EXE
following these steps:

gcj -c -findirect-dispatch -fjni mydll.java
gcj -shared -o mydll.dll -Wl,--out-implib,mydll.dll.a \
    -Wl,--export-all-symbols -Wl,-S -Wl,-s mydll.o
gcj -c --classpath=. main.java
gcj --main=main -o test.exe main.o -L. -lmydll

and it works as expected though I see these (I think harmless) messages:

Info: resolving __ZN5mydll6class$E by linking to
__imp___ZN5mydll6class$E (auto-import)

I've noticed that leaving out the indirect-dispatch switch when
compiling the mydll.java source makes test.exe complain at run-time.

Well, now for the great move.

I tried to compile the swt.jar that comes with SWT 3.1.1 the same way:

gcj -c -findirect-dispatch -fjni swt.jar
gcj -shared -o swtcom.dll -Wl,--out-implib,swtcom.dll.a \
    -Wl,--export-all-symbols -Wl,-S -Wl,-s swt.o
gcj -c --classpath=swt.jar SWTHello.java
gcj --main=SWTHello -o SWTHello.exe SWTHello.o -L. -lswtcom

As before gcj issues the following messages:

Info: resolving __ZN3org7eclipse3swt7widgets7Display6class$E by linking
to __imp___ZN3org7eclipse3swt7widgets7Display6class$E (auto-import)
Info: resolving __ZN3org7eclipse3swt7widgets5Shell6class$E by linking to
__imp___ZN3org7eclipse3swt7widgets5Shell6class$E (auto-import)
Info: resolving __ZN3org7eclipse3swt6layout9RowLayout6class$E by linking
to __imp___ZN3org7eclipse3swt6layout9RowLayout6class$E (auto-import)
Info: resolving __ZN3org7eclipse3swt7widgets5Label6class$E by linking to
__imp___ZN3org7eclipse3swt7widgets5Label6class$E (auto-import)

but now SWTHello.exe doesn't work:

Exception in thread "main" java.lang.NoClassDefFoundError: while
resolving class: org.eclipse.swt.widgets.Display
   at java.lang.VMClassLoader.transformException(java.lang.Class,
java.lang.Throwable)
(C:\home\paris\mingw\gcc-4.0.2\libjava\java\lang\VMClassLoader.java:138)
   at java.lang.VMClassLoader.resolveClass(java.lang.Class)
...

Notice that using static linkage everything works fine.

Am I trying to do something unachievable with current gcc/gcj and SWT?

Thank you for any reply,

Cristiano