Try building the JDK when it is in a Read-only file system. You will get
the following error:

/awt -I/usr/openwin/include/X11/extensions -o /tmp/XXX/jdk-outputdir/gensrc/sun/awt/X11/generator/sizer.32 /tmp/XXX/jdk-outputdir/gensrc/sun/awt/X11/generator/sizer.32.c
cg error (as) : cannot open output file "sizer.32.o" : Read-only file system
cc: cg failed for /tmp/XXX/jdk-outputdir/gensrc/sun/awt/X11/generator/sizer.32.c
gnumake[1]: *** [/tmp/XXX/jdk-outputdir/gensrc/sun/awt/X11/generator/sizer.32] Error 2
gnumake[1]: Leaving directory `/net/YYY/u/XXX/integration/sync/jdk/make/sun/xawt'
gnumake: *** [all] Error 1
gnumake: Leaving directory `/net/YYY/u/XXX/integration/sync/jdk/make/sun'

Despite the fact that /tmp/XXX is readable-writable, the Makefiles try to
write to the source directory, which is mounted Read-only.