diff --git a/config/config.mk b/config/config.mk
index affb1fd971a..6b5a048bfa4 100644
--- a/config/config.mk
+++ b/config/config.mk
@@ -898,3 +898,14 @@ DEFINES += -DBUILD_ID=$(BUILD_ID)
 ifeq (,$(filter WINCE WINNT OS2,$(OS_ARCH)))
 RUN_TEST_PROGRAM = $(DIST)/bin/run-mozilla.sh
 endif
+
+#
+# Java macros
+#
+
+# Make sure any compiled classes work with at least JVM 1.4
+JAVAC_FLAGS += -source 1.4
+
+ifdef MOZ_DEBUG
+JAVAC_FLAGS += -g
+endif
diff --git a/extensions/java/xpcom/interfaces/Makefile.in b/extensions/java/xpcom/interfaces/Makefile.in
index 8c6acad8c85..bd8a7d3eef1 100644
--- a/extensions/java/xpcom/interfaces/Makefile.in
+++ b/extensions/java/xpcom/interfaces/Makefile.in
@@ -68,10 +68,6 @@ GARBAGE_DIRS += _javagen org
 
 include $(topsrcdir)/config/rules.mk
 
-ifdef MOZ_DEBUG
-JAVAC_FLAGS = -g
-endif
-
 ifeq ($(OS_ARCH),WINNT)
 RUN =
 OUTPUT_DIR = $(shell cygpath -w $(CURDIR)/_javagen/org/mozilla/xpcom)
diff --git a/extensions/java/xpcom/src/Makefile.in b/extensions/java/xpcom/src/Makefile.in
index 81e67a3a8a2..f7240a4e763 100755
--- a/extensions/java/xpcom/src/Makefile.in
+++ b/extensions/java/xpcom/src/Makefile.in
@@ -97,10 +97,6 @@ GARBAGE_DIRS += _javagen org
 
 include $(topsrcdir)/config/rules.mk
 
-ifdef MOZ_DEBUG
-JAVAC_FLAGS = -g
-endif
-
 ifeq ($(OS_ARCH),WINNT)
 RUN =
 SEP = ;
diff --git a/extensions/java/xpcom/tests/Makefile.in b/extensions/java/xpcom/tests/Makefile.in
index 2c80f53b047..bb490f6c1ee 100644
--- a/extensions/java/xpcom/tests/Makefile.in
+++ b/extensions/java/xpcom/tests/Makefile.in
@@ -78,8 +78,8 @@ include $(topsrcdir)/config/rules.mk
 
 %.class: %.java Makefile Makefile.in
 	$(REPORT_BUILD)
-	$(ELOG) $(CYGWIN_WRAPPER) $(JAVAC) -classpath $(_JAVA_CLASSPATH) -d . \
-			$(_VPATH_SRCS)
+	$(ELOG) $(CYGWIN_WRAPPER) $(JAVAC) $(JAVAC_FLAGS) \
+			-classpath $(_JAVA_CLASSPATH) -d . $(_VPATH_SRCS)
 
 # make sure to rebuild Java interfaces if our XPIDLSRCS change
 $(REBUILD_JAR): $(XPIDLSRCS)