Comment 1 for bug 364657

I get the same thing.
$ trang -I xml -O rng ~/public_html/linux/usb_ubuntu_9.04RC/index.xhtml junk.rng
Exception in thread "main" java.lang.NullPointerException
   at org.relaxng.datatype.helpers.DatatypeLibraryLoader$Service$Loader2.getResources(libgcj_bc.so.1)
   at org.relaxng.datatype.helpers.DatatypeLibraryLoader$Service.<init>(libgcj_bc.so.1)
   at org.relaxng.datatype.helpers.DatatypeLibraryLoader.<init>(libgcj_bc.so.1)
   at com.thaiopensource.relaxng.input.xml.Inferrer.infer(trang)
   at com.thaiopensource.relaxng.input.xml.XmlInputFormat.load(trang)
   at com.thaiopensource.relaxng.input.AbstractMultiInputFormat.load(trang)
   at com.thaiopensource.relaxng.translate.Driver.doMain(trang)
   at com.thaiopensource.relaxng.translate.Driver.main(trang)

$ ls -lgF /usr/lib/libgcj_bc.so.1
lrwxrwxrwx 1 root 12 2009-04-28 12:57 /usr/lib/libgcj_bc.so.1 -> libgcj.so.90

ii trang 20030619-6.1 Multi-format XML schema converter based on R
ii libgcj-bc 4.3.3-1ubuntu1 Link time only library for use with gcj
ii libgcj9-0 4.3.3-5ubuntu4 Java runtime library for use with gcj

libgcj-bc: /usr/lib/libgcj_bc.so.1
libgcj9-0: /usr/lib/gcj-4.3-90/libgcj_bc.so.1

Was it incorrectly compiled to use the link time only lib? Looks like it:
$ ldd /usr/bin/trang
 linux-vdso.so.1 => (0x00007fff6b7ff000)
 libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x00007fe7632ad000)
 libgcj_bc.so.1 => /usr/lib/libgcj_bc.so.1 (0x00007fe76037d000)
 libm.so.6 => /lib/libm.so.6 (0x00007fe7600f8000)
 libpthread.so.0 => /lib/libpthread.so.0 (0x00007fe75fedc000)
 librt.so.1 => /lib/librt.so.1 (0x00007fe75fcd4000)
 libz.so.1 => /lib/libz.so.1 (0x00007fe75fabc000)
 libdl.so.2 => /lib/libdl.so.2 (0x00007fe75f8b8000)
 libc.so.6 => /lib/libc.so.6 (0x00007fe75f546000)
 /lib64/ld-linux-x86-64.so.2 (0x00007fe7634c5000)

However, the following workaround doesn't work:
 LD_LIBRARY_PATH=/usr/lib/gcj-4.3-90/:/usr/lib:/lib trang -I xml -O rng ~/public_html/linux/usb_ubuntu_9.04RC/index.xhtml junk.rng
Distributor ID: Ubuntu
Description: Ubuntu 9.04
Release: 9.04
Codename: jaunty