java_library support for building headers-only

Flag for java_library modules to build just the Turbine headers and
skip building an impl jar.

Test: go test java
Bug: 289776578
Change-Id: Iad0babf951710476bc32df93c25d17065a14ab84

java/base.go

@@ -192,6 +192,9 @@ type CommonProperties struct {
 
 	// Additional srcJars tacked in by GeneratedJavaLibraryModule
 	Generated_srcjars []android.Path `android:"mutated"`
+
+	// If true, then only the headers are built and not the implementation jar.
+	Headers_only bool
 }
 
 // Properties that are specific to device modules. Host module factories should not add these when
@@ -574,6 +577,17 @@ func (j *Module) checkPlatformAPI(ctx android.ModuleContext) {
 	}
 }
 
+func (j *Module) checkHeadersOnly(ctx android.ModuleContext) {
+	if _, ok := ctx.Module().(android.SdkContext); ok {
+		headersOnly := proptools.Bool(&j.properties.Headers_only)
+		installable := proptools.Bool(j.properties.Installable)
+
+		if headersOnly && installable {
+			ctx.PropertyErrorf("headers_only", "This module has conflicting settings. headers_only is true which, which means this module doesn't generate an implementation jar. However installable is set to true.")
+		}
+	}
+}
+
 func (j *Module) addHostProperties() {
 	j.AddProperties(
 		&j.properties,
@@ -1151,6 +1165,36 @@ func (j *Module) compile(ctx android.ModuleContext, extraSrcJars, extraClasspath
 	// final R classes from the app.
 	flags.classpath = append(android.CopyOf(extraClasspathJars), flags.classpath...)
 
+	// If compiling headers then compile them and skip the rest
+	if j.properties.Headers_only {
+		if srcFiles.HasExt(".kt") {
+			ctx.ModuleErrorf("Compiling headers_only with .kt not supported")
+		}
+		if ctx.Config().IsEnvFalse("TURBINE_ENABLED") || disableTurbine {
+			ctx.ModuleErrorf("headers_only is enabled but Turbine is disabled.")
+		}
+
+		_, j.headerJarFile =
+			j.compileJavaHeader(ctx, uniqueJavaFiles, srcJars, deps, flags, jarName,
+				extraCombinedJars)
+		if ctx.Failed() {
+			return
+		}
+
+		ctx.SetProvider(JavaInfoProvider, JavaInfo{
+			HeaderJars:                     android.PathsIfNonNil(j.headerJarFile),
+			TransitiveLibsHeaderJars:       j.transitiveLibsHeaderJars,
+			TransitiveStaticLibsHeaderJars: j.transitiveStaticLibsHeaderJars,
+			AidlIncludeDirs:                j.exportAidlIncludeDirs,
+			ExportedPlugins:                j.exportedPluginJars,
+			ExportedPluginClasses:          j.exportedPluginClasses,
+			ExportedPluginDisableTurbine:   j.exportedDisableTurbine,
+		})
+
+		j.outputFile = j.headerJarFile
+		return
+	}
+
 	if srcFiles.HasExt(".kt") {
 		// When using kotlin sources turbine is used to generate annotation processor sources,
 		// including for annotation processors that generate API, so we can use turbine for