diff --git a/core/build-system.html b/core/build-system.html
index c7938cc..3d86e24 100644
--- a/core/build-system.html
+++ b/core/build-system.html
@@ -762,6 +762,19 @@
 Dialer, Contacts, etc.  This will probably change or go away when we switch
 to an ant-based build system for the apps.</p>
 
+<h4>LOCAL_PATCH_MODULE (experimental option)</h4>
+<p>As of January 2018, you almost certainly don't need this option, so please
+ask and only use it if you understand what you're doing. This feature is
+experimental and may go away in future.</p>
+<p>
+When compiling language level 9+ .java code in packages that are part of a
+a system module, <code>LOCAL_PATCH_MODULE</code> names the module that your
+sources and dependencies should be patched into. The Android runtime currently
+(Jan 2018) doesn't implement the JEP 261 module system so this option is only
+supported at compile time. It should only be needed to compile tests in packages
+that exist in libcore and which are inconvenient to move elsewhere.
+</p>
+
 <h4>LOCAL_PATH</h4>
 <p>The directory your Android.mk file is in. You can set it by putting the
 following as the first line in your Android.mk:</p>
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index b4a03ea..a151bef 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -181,6 +181,7 @@
 LOCAL_PACKAGE_NAME:=
 LOCAL_PACKAGE_SPLITS:=
 LOCAL_PACK_MODULE_RELOCATIONS:=
+LOCAL_PATCH_MODULE:=
 LOCAL_PICKUP_FILES:=
 LOCAL_POST_INSTALL_CMD:=
 LOCAL_POST_LINK_CMD:=
diff --git a/core/definitions.mk b/core/definitions.mk
index a20bf44..7aef290 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -2245,6 +2245,9 @@
       $(addprefix -bootclasspath ,$(strip \
           $(call normalize-path-list,$(PRIVATE_BOOTCLASSPATH)) \
           $(PRIVATE_EMPTY_BOOTCLASSPATH)))) \
+    $(if $(PRIVATE_USE_SYSTEM_MODULES), \
+      $(if $(PRIVATE_PATCH_MODULE), \
+        --patch-module=$(PRIVATE_PATCH_MODULE)=$(call normalize-path-list,. $(2)))) \
     $(addprefix -classpath ,$(strip \
         $(call normalize-path-list,$(2)))) \
     $(if $(findstring true,$(PRIVATE_WARNINGS_ENABLE)),$(xlint_unchecked),) \
diff --git a/core/java_common.mk b/core/java_common.mk
index cfc9d7f..436f3a3 100644
--- a/core/java_common.mk
+++ b/core/java_common.mk
@@ -359,6 +359,7 @@
 $(LOCAL_INTERMEDIATE_TARGETS): PRIVATE_BOOTCLASSPATH := $(full_java_bootclasspath_libs)
 $(LOCAL_INTERMEDIATE_TARGETS): PRIVATE_EMPTY_BOOTCLASSPATH := $(empty_bootclasspath)
 $(LOCAL_INTERMEDIATE_TARGETS): PRIVATE_SYSTEM_MODULES := $(my_system_modules_dir)
+$(LOCAL_INTERMEDIATE_TARGETS): PRIVATE_PATCH_MODULE := $(LOCAL_PATCH_MODULE)
 
 ifndef LOCAL_IS_HOST_MODULE
 # This is set by packages that are linking to other packages that export