My build of libgtk-java failed with this error:

java.lang.reflect.InvocationTargetException
   at java.lang.reflect.Method.invoke(libgcj.so.7rh)
   at
gnu.classpath.tools.gjdoc.Main.startDoclet(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at gnu.classpath.tools.gjdoc.Main.start(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at gnu.classpath.tools.gjdoc.Main.main(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
Caused by: java.lang.NoClassDefFoundError: gnu.classpath.tools.gjdoc.expr.Evaluator
   at java.lang.Class.initializeClass(libgcj.so.7rh)
   at
gnu.classpath.tools.gjdoc.FieldDocImpl.constantValue(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at
gnu.classpath.tools.gjdoc.FieldDocImpl.constantValue(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at
gnu.classpath.tools.doclets.htmldoclet.HtmlDoclet.printMemberDetails(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at
gnu.classpath.tools.doclets.htmldoclet.HtmlDoclet.printClassPage(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at
gnu.classpath.tools.doclets.htmldoclet.HtmlDoclet.run(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at
gnu.classpath.tools.doclets.AbstractDoclet.startInstance(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at
gnu.classpath.tools.doclets.AbstractDoclet.start(gnu-classpath-tools-gjdoc-0.7.7.jar.so)
   at java.lang.reflect.Method.invoke(libgcj.so.7rh)
   ...3 more
Caused by: java.lang.ClassNotFoundException: antlr.CharScanner not found in
gnu.gcj.runtime.SystemClassLoader{urls=[file:/usr/share/java/com-sun   at
java.net.URLClassLoader.findClass(libgcj.so.7rh)
   at gnu.gcj.runtime.SystemClassLoader.findClass(libgcj.so.7rh)
   at java.lang.ClassLoader.loadClass(libgcj.so.7rh)
   at java.lang.ClassLoader.loadClass(libgcj.so.7rh)
   at java.lang.VMClassLoader.defineClass(libgcj.so.7rh)
   at java.lang.ClassLoader.defineClass(libgcj.so.7rh)
   at java.security.SecureClassLoader.defineClass(libgcj.so.7rh)
   at java.net.URLClassLoader.findClass(libgcj.so.7rh)
   at gnu.gcj.runtime.SystemClassLoader.findClass(libgcj.so.7rh)
   at java.lang.ClassLoader.loadClass(libgcj.so.7rh)
   at java.lang.ClassLoader.loadClass(libgcj.so.7rh)
   at java.lang.Class.initializeClass(libgcj.so.7rh)
   ...11 more

adding BuildRequires: antlr works around the issue.

I guess gjdoc should require antlr.