diff --git a/core/definitions.mk b/core/definitions.mk
index 36f2026..fc18ee3 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1750,9 +1750,9 @@
 $(hide) if [ -d "$(PRIVATE_SOURCE_INTERMEDIATES_DIR)" ]; then \
           find $(PRIVATE_SOURCE_INTERMEDIATES_DIR) -name '*.java' >> $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list; \
 fi
-$(hide) if [ -s $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list ] ; then \
-    ( tr ' ' '\n' < $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list \
-     | sort -u > $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq ); \
+$(hide) tr ' ' '\n' < $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list \
+    | sort -u > $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq
+$(hide) if [ -s $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq ] ; then \
     $(1) -encoding UTF-8 \
     $(strip $(PRIVATE_JAVAC_DEBUG_FLAGS)) \
     $(if $(findstring true,$(PRIVATE_WARNINGS_ENABLE)),$(xlint_unchecked),) \
@@ -1815,10 +1815,8 @@
 $(hide) if [ -d "$(PRIVATE_SOURCE_INTERMEDIATES_DIR)" ]; then \
           find $(PRIVATE_SOURCE_INTERMEDIATES_DIR) -name '*.java' >> $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list; \
 fi
-$(hide) if [ -s $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list ] ; then \
-    tr ' ' '\n' < $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list \
-      | sort -u > $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list-uniq; \
-fi
+$(hide) tr ' ' '\n' < $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list \
+    | sort -u > $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list-uniq
 $(if $(PRIVATE_JACK_PROGUARD_FLAGS), \
     $(hide) echo -basedirectory $(CURDIR) > $@.flags; \
     echo $(PRIVATE_JACK_PROGUARD_FLAGS) >> $@.flags; \
@@ -1895,10 +1893,8 @@
 $(hide) if [ -d "$(PRIVATE_SOURCE_INTERMEDIATES_DIR)" ]; then \
           find $(PRIVATE_SOURCE_INTERMEDIATES_DIR) -name '*.java' >> $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list; \
 fi
-$(hide) if [ -s $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list ] ; then \
-    tr ' ' '\n' < $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list \
-      | sort -u > $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list-uniq; \
-fi
+$(hide) tr ' ' '\n' < $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list \
+    | sort -u > $(PRIVATE_JACK_INTERMEDIATES_DIR)/java-source-list-uniq
 $(if $(PRIVATE_JACK_PROGUARD_FLAGS), \
     $(hide) echo -basedirectory $(CURDIR) > $@.flags; \
     echo $(PRIVATE_JACK_PROGUARD_FLAGS) >> $@.flags; \
@@ -1940,6 +1936,69 @@
 $(if $(PRIVATE_JAR_MANIFEST), $(hide) echo unsupported options JAR_MANIFEST in $@; exit 53)
 endef
 
+# Override the above definitions if we want to do incremetal javac
+ifeq (true, $(ENABLE_INCREMENTALJAVAC))
+define compile-java
+$(hide) mkdir -p $(dir $@)
+$(hide) mkdir -p $(PRIVATE_CLASS_INTERMEDIATES_DIR)
+$(hide) touch $(PRIVATE_CLASS_INTERMEDIATES_DIR)/newstamp
+$(call unzip-jar-files,$(PRIVATE_STATIC_JAVA_LIBRARIES),$(PRIVATE_CLASS_INTERMEDIATES_DIR))
+$(hide) if [ -e $(PRIVATE_CLASS_INTERMEDIATES_DIR)/stamp ] ; then \
+        newerFlag=$$(echo -n "-newer $(PRIVATE_CLASS_INTERMEDIATES_DIR)/stamp") ; \
+    fi ; \
+    find $(PRIVATE_JAVA_SOURCES) $$newerFlag > $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list ; \
+    if [ -d "$(PRIVATE_SOURCE_INTERMEDIATES_DIR)" ]; then \
+        find $(PRIVATE_SOURCE_INTERMEDIATES_DIR) -name '*.java' $$newerFlag >> $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list; \
+    fi
+$(hide) tr ' ' '\n' < $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list \
+    | sort -u > $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq
+@echo "(Incremental) build source files:"
+@cat $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq
+$(hide) if [ -s $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq ] ; then \
+    $(1) -encoding UTF-8 \
+    $(strip $(PRIVATE_JAVAC_DEBUG_FLAGS)) \
+    $(if $(findstring true,$(PRIVATE_WARNINGS_ENABLE)),$(xlint_unchecked),) \
+    $(2) \
+    $(addprefix -classpath ,$(strip \
+        $(call normalize-path-list,$(PRIVATE_ALL_JAVA_LIBRARIES) $(PRIVATE_CLASS_INTERMEDIATES_DIR)))) \
+    $(if $(findstring true,$(PRIVATE_WARNINGS_ENABLE)),$(xlint_unchecked),) \
+    -extdirs "" -d $(PRIVATE_CLASS_INTERMEDIATES_DIR) \
+    $(PRIVATE_JAVACFLAGS) \
+    \@$(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq \
+    || ( exit 41 ) \
+fi
+$(hide) rm -f $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list
+$(hide) rm -f $(PRIVATE_CLASS_INTERMEDIATES_DIR)/java-source-list-uniq
+$(hide) rm -f $@
+$(if $(PRIVATE_JAR_EXCLUDE_FILES), $(hide) find $(PRIVATE_CLASS_INTERMEDIATES_DIR) \
+    -name $(word 1, $(PRIVATE_JAR_EXCLUDE_FILES)) \
+    $(addprefix -o -name , $(wordlist 2, 999, $(PRIVATE_JAR_EXCLUDE_FILES))) \
+    | xargs rm -rf)
+$(if $(PRIVATE_JAR_PACKAGES), \
+    $(hide) find $(PRIVATE_CLASS_INTERMEDIATES_DIR) -mindepth 1 -type f \
+        $(foreach pkg, $(PRIVATE_JAR_PACKAGES), \
+            -not -path $(PRIVATE_CLASS_INTERMEDIATES_DIR)/$(subst .,/,$(pkg))/\*) -delete ; \
+        find $(PRIVATE_CLASS_INTERMEDIATES_DIR) -empty -delete)
+$(if $(PRIVATE_JAR_EXCLUDE_PACKAGES), $(hide) rm -rf \
+    $(foreach pkg, $(PRIVATE_JAR_EXCLUDE_PACKAGES), \
+        $(PRIVATE_CLASS_INTERMEDIATES_DIR)/$(subst .,/,$(pkg))))
+$(if $(PRIVATE_RMTYPEDEFS), $(hide) $(RMTYPEDEFS) -v $(PRIVATE_CLASS_INTERMEDIATES_DIR))
+$(if $(PRIVATE_JAR_MANIFEST), \
+    $(hide) sed -e 's/%BUILD_NUMBER%/$(BUILD_NUMBER)/' \
+            $(PRIVATE_JAR_MANIFEST) > $(dir $@)/manifest.mf && \
+        jar -cfm $@ $(dir $@)/manifest.mf \
+            -C $(PRIVATE_CLASS_INTERMEDIATES_DIR) ., \
+    $(hide) jar -cf $@ -C $(PRIVATE_CLASS_INTERMEDIATES_DIR) .)
+$(if $(PRIVATE_EXTRA_JAR_ARGS),$(call add-java-resources-to,$@))
+$(hide) mv $(PRIVATE_CLASS_INTERMEDIATES_DIR)/newstamp $(PRIVATE_CLASS_INTERMEDIATES_DIR)/stamp
+endef
+
+define transform-java-to-classes.jar
+@echo "target Java: $(PRIVATE_MODULE) ($(PRIVATE_CLASS_INTERMEDIATES_DIR))"
+$(call compile-java,$(TARGET_JAVAC),$(PRIVATE_BOOTCLASSPATH))
+endef
+endif # ENABLE_INCREMENTALJAVAC
+
 define transform-classes.jar-to-emma
 $(hide) java -classpath $(EMMA_JAR) emma instr -outmode fullcopy -outfile \
     $(PRIVATE_EMMA_COVERAGE_FILE) -ip $< -d $(PRIVATE_EMMA_INTERMEDIATES_DIR) \
diff --git a/core/main.mk b/core/main.mk
index e04c4c7..3bf9cae 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -234,7 +234,23 @@
 # These are the modifier targets that don't do anything themselves, but
 # change the behavior of the build.
 # (must be defined before including definitions.make)
-INTERNAL_MODIFIER_TARGETS := showcommands all
+INTERNAL_MODIFIER_TARGETS := showcommands all incrementaljavac
+
+.PHONY: incrementaljavac
+incrementaljavac: ;
+
+# WARNING:
+# ENABLE_INCREMENTALJAVAC should NOT be enabled by default, because change of
+# a Java source file won't trigger rebuild of its dependent Java files.
+# You can only enable it by adding "incrementaljavac" to your make command line.
+# You are responsible for the correctness of the incremental build.
+# This may decrease incremental build time dramatically for large Java libraries,
+# such as core.jar, framework.jar, etc.
+ENABLE_INCREMENTALJAVAC :=
+ifneq (,$(filter incrementaljavac, $(MAKECMDGOALS)))
+ENABLE_INCREMENTALJAVAC := true
+MAKECMDGOALS := $(filter-out incrementaljavac, $(MAKECMDGOALS))
+endif
 
 # EMMA_INSTRUMENT_STATIC merges the static emma library to each emma-enabled module.
 ifeq (true,$(EMMA_INSTRUMENT_STATIC))