diff --git a/java/aar.go b/java/aar.go
index 6426ac3..2c19487 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -535,6 +535,10 @@
 	return a.sdkVersion()
 }
 
+func (a *AARImport) javaVersion() string {
+	return ""
+}
+
 var _ AndroidLibraryDependency = (*AARImport)(nil)
 
 func (a *AARImport) ExportPackage() android.Path {
diff --git a/java/builder.go b/java/builder.go
index 0a5c79b..c67a26d 100644
--- a/java/builder.go
+++ b/java/builder.go
@@ -190,7 +190,7 @@
 	systemModules *systemModules
 	aidlFlags     string
 	aidlDeps      android.Paths
-	javaVersion   string
+	javaVersion   javaVersion
 
 	errorProneExtraJavacFlags string
 	errorProneProcessorPath   classpath
@@ -239,7 +239,7 @@
 	deps = append(deps, srcJars...)
 
 	var bootClasspath string
-	if flags.javaVersion == "1.9" {
+	if flags.javaVersion.usesJavaModules() {
 		var systemModuleDeps android.Paths
 		bootClasspath, systemModuleDeps = flags.systemModules.FormJavaSystemModulesPath(ctx.Device())
 		deps = append(deps, systemModuleDeps...)
@@ -279,7 +279,7 @@
 				"bootClasspath": bootClasspath,
 				"classpath":     flags.classpath.FormJavaClassPath("-classpath"),
 				"javacFlags":    flags.javacFlags,
-				"javaVersion":   flags.javaVersion,
+				"javaVersion":   flags.javaVersion.String(),
 				"outDir":        android.PathForModuleOut(ctx, "javac", "classes.xref").String(),
 				"processorpath": flags.processorPath.FormJavaClassPath("-processorpath"),
 				"processor":     processor,
@@ -318,7 +318,7 @@
 			"srcJars":       strings.Join(srcJars.Strings(), " "),
 			"classpath":     strings.Join(flags.classpath.FormTurbineClasspath("--classpath "), " "),
 			"outDir":        android.PathForModuleOut(ctx, "turbine", "classes").String(),
-			"javaVersion":   flags.javaVersion,
+			"javaVersion":   flags.javaVersion.String(),
 		},
 	})
 }
@@ -340,7 +340,7 @@
 	deps = append(deps, srcJars...)
 
 	var bootClasspath string
-	if flags.javaVersion == "1.9" {
+	if flags.javaVersion.usesJavaModules() {
 		var systemModuleDeps android.Paths
 		bootClasspath, systemModuleDeps = flags.systemModules.FormJavaSystemModulesPath(ctx.Device())
 		deps = append(deps, systemModuleDeps...)
@@ -388,7 +388,7 @@
 			"srcJarDir":     android.PathForModuleOut(ctx, intermediatesDir, srcJarDir).String(),
 			"outDir":        android.PathForModuleOut(ctx, intermediatesDir, outDir).String(),
 			"annoDir":       android.PathForModuleOut(ctx, intermediatesDir, annoDir).String(),
-			"javaVersion":   flags.javaVersion,
+			"javaVersion":   flags.javaVersion.String(),
 		},
 	})
 }
diff --git a/java/droiddoc.go b/java/droiddoc.go
index 1091331..32758f6 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -665,7 +665,7 @@
 	cmd := javadocSystemModulesCmd(ctx, rule, j.srcFiles, outDir, srcJarDir, srcJarList,
 		deps.systemModules, deps.classpath, j.sourcepaths)
 
-	cmd.FlagWithArg("-source ", javaVersion).
+	cmd.FlagWithArg("-source ", javaVersion.String()).
 		Flag("-J-Xmx1024m").
 		Flag("-XDignore.symbol.file").
 		Flag("-Xdoclint:none")
@@ -1432,12 +1432,12 @@
 	}
 }
 
-func metalavaCmd(ctx android.ModuleContext, rule *android.RuleBuilder, javaVersion string, srcs android.Paths,
+func metalavaCmd(ctx android.ModuleContext, rule *android.RuleBuilder, javaVersion javaVersion, srcs android.Paths,
 	srcJarList android.Path, bootclasspath, classpath classpath, sourcepaths android.Paths) *android.RuleBuilderCommand {
 	cmd := rule.Command().BuiltTool(ctx, "metalava").
 		Flag(config.JavacVmFlags).
 		FlagWithArg("-encoding ", "UTF-8").
-		FlagWithArg("-source ", javaVersion).
+		FlagWithArg("-source ", javaVersion.String()).
 		FlagWithRspFileInputList("@", srcs).
 		FlagWithInput("@", srcJarList)
 
diff --git a/java/java.go b/java/java.go
index 9ed7621..9aa75b2 100644
--- a/java/java.go
+++ b/java/java.go
@@ -865,8 +865,7 @@
 	return deps
 }
 
-func getJavaVersion(ctx android.ModuleContext, javaVersion string, sdkContext sdkContext) string {
-	var ret string
+func getJavaVersion(ctx android.ModuleContext, javaVersion string, sdkContext sdkContext) javaVersion {
 	v := sdkContext.sdkVersion()
 	// For PDK builds, use the latest SDK version instead of "current"
 	if ctx.Config().IsPdkBuild() &&
@@ -884,41 +883,69 @@
 		ctx.PropertyErrorf("sdk_version", "%s", err)
 	}
 	if javaVersion != "" {
-		ret = normalizeJavaVersion(ctx, javaVersion)
+		return normalizeJavaVersion(ctx, javaVersion)
 	} else if ctx.Device() && sdk <= 23 {
-		ret = "1.7"
+		return JAVA_VERSION_7
 	} else if ctx.Device() && sdk <= 29 {
-		ret = "1.8"
+		return JAVA_VERSION_8
 	} else if ctx.Device() &&
 		sdkContext.sdkVersion() != "" &&
 		sdkContext.sdkVersion() != "none" &&
 		sdkContext.sdkVersion() != "core_platform" &&
 		sdk == android.FutureApiLevel {
 		// TODO(ccross): once we generate stubs we should be able to use 1.9 for sdk_version: "current"
-		ret = "1.8"
+		return JAVA_VERSION_8
 	} else {
-		ret = "1.9"
+		return JAVA_VERSION_9
 	}
-
-	return ret
 }
 
-func normalizeJavaVersion(ctx android.ModuleContext, javaVersion string) string {
+type javaVersion int
+
+const (
+	JAVA_VERSION_UNSUPPORTED = 0
+	JAVA_VERSION_6           = 6
+	JAVA_VERSION_7           = 7
+	JAVA_VERSION_8           = 8
+	JAVA_VERSION_9           = 9
+)
+
+func (v javaVersion) String() string {
+	switch v {
+	case JAVA_VERSION_6:
+		return "1.6"
+	case JAVA_VERSION_7:
+		return "1.7"
+	case JAVA_VERSION_8:
+		return "1.8"
+	case JAVA_VERSION_9:
+		return "1.9"
+	default:
+		return "unsupported"
+	}
+}
+
+// Returns true if javac targeting this version uses system modules instead of a bootclasspath.
+func (v javaVersion) usesJavaModules() bool {
+	return v >= 9
+}
+
+func normalizeJavaVersion(ctx android.BaseModuleContext, javaVersion string) javaVersion {
 	switch javaVersion {
 	case "1.6", "6":
-		return "1.6"
+		return JAVA_VERSION_6
 	case "1.7", "7":
-		return "1.7"
+		return JAVA_VERSION_7
 	case "1.8", "8":
-		return "1.8"
+		return JAVA_VERSION_8
 	case "1.9", "9":
-		return "1.9"
+		return JAVA_VERSION_9
 	case "10", "11":
 		ctx.PropertyErrorf("java_version", "Java language levels above 9 are not supported")
-		return "unsupported"
+		return JAVA_VERSION_UNSUPPORTED
 	default:
 		ctx.PropertyErrorf("java_version", "Unrecognized Java language level")
-		return "unrecognized"
+		return JAVA_VERSION_UNSUPPORTED
 	}
 }
 
@@ -931,7 +958,7 @@
 
 	// javac flags.
 	javacFlags := j.properties.Javacflags
-	if flags.javaVersion == "1.9" {
+	if flags.javaVersion.usesJavaModules() {
 		javacFlags = append(javacFlags, j.properties.Openjdk9.Javacflags...)
 	}
 	if ctx.Config().MinimizeJavaDebugInfo() {
@@ -963,9 +990,8 @@
 
 	flags.processor = strings.Join(deps.processorClasses, ",")
 
-	if len(flags.bootClasspath) == 0 && ctx.Host() && flags.javaVersion != "1.9" &&
-		decodeSdkDep(ctx, sdkContext(j)).hasStandardLibs() &&
-		inList(flags.javaVersion, []string{"1.6", "1.7", "1.8"}) {
+	if len(flags.bootClasspath) == 0 && ctx.Host() && !flags.javaVersion.usesJavaModules() &&
+		decodeSdkDep(ctx, sdkContext(j)).hasStandardLibs() {
 		// Give host-side tools a version of OpenJDK's standard libraries
 		// close to what they're targeting. As of Dec 2017, AOSP is only
 		// bundling OpenJDK 8 and 9, so nothing < 8 is available.
@@ -989,7 +1015,7 @@
 		}
 	}
 
-	if j.properties.Patch_module != nil && flags.javaVersion == "1.9" {
+	if j.properties.Patch_module != nil && flags.javaVersion.usesJavaModules() {
 		// Manually specify build directory in case it is not under the repo root.
 		// (javac doesn't seem to expand into symbolc links when searching for patch-module targets, so
 		// just adding a symlink under the root doesn't help.)
@@ -1022,7 +1048,7 @@
 	deps := j.collectDeps(ctx)
 	flags := j.collectBuilderFlags(ctx, deps)
 
-	if flags.javaVersion == "1.9" {
+	if flags.javaVersion.usesJavaModules() {
 		j.properties.Srcs = append(j.properties.Srcs, j.properties.Openjdk9.Srcs...)
 	}
 	srcFiles := android.PathsForModuleSrcExcludes(ctx, j.properties.Srcs, j.properties.Exclude_srcs)
diff --git a/java/kotlin.go b/java/kotlin.go
index 8306907..f8ae229 100644
--- a/java/kotlin.go
+++ b/java/kotlin.go
@@ -141,8 +141,8 @@
 	}
 
 	encodedJavacFlags := kaptEncodeFlags([][2]string{
-		{"-source", flags.javaVersion},
-		{"-target", flags.javaVersion},
+		{"-source", flags.javaVersion.String()},
+		{"-target", flags.javaVersion.String()},
 	})
 
 	kotlinName := filepath.Join(ctx.ModuleDir(), ctx.ModuleSubDir(), ctx.ModuleName())