Introduce classpath_fragment.go.

A skeleton of a classpath fragment's interface and base to be used by
any modules that provide entries for *CLASSPATH variables at runtime.

In follow up changes, this would be extended to generate
packages/modules/SdkExtensions/proto/classpaths.proto configs for such
modules to be bundled into system and individual apex binaries.

Bug: 180105615
Test: m
Change-Id: I2df550862e97222c5650c4d0480c90231fd78ef0

java/Android.bp

@@ -32,6 +32,7 @@ bootstrap_go_package {
         "boot_image.go",
         "boot_jars.go",
         "builder.go",
+        "classpath_fragment.go",
         "device_host_converter.go",
         "dex.go",
         "dexpreopt.go",

java/classpath_fragment.go

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package java
+
+import (
+	"android/soong/android"
+)
+
+// Build rules and utilities to generate individual packages/modules/SdkExtensions/proto/classpaths.proto
+// config files based on build configuration to embed into /system and /apex on a device.
+//
+// See `derive_classpath` service that reads the configs at runtime and defines *CLASSPATH variables
+// on the device.
+
+type classpathType int
+
+const (
+	// Matches definition in packages/modules/SdkExtensions/proto/classpaths.proto
+	BOOTCLASSPATH classpathType = iota
+	DEX2OATBOOTCLASSPATH
+	SYSTEMSERVERCLASSPATH
+)
+
+func (c classpathType) String() string {
+	return [...]string{"BOOTCLASSPATH", "DEX2OATBOOTCLASSPATH", "SYSTEMSERVERCLASSPATH"}[c]
+}
+
+type classpathFragmentProperties struct {
+}
+
+// classpathFragment interface is implemented by a module that contributes jars to a *CLASSPATH
+// variables at runtime.
+type classpathFragment interface {
+	android.Module
+
+	classpathFragmentBase() *classpathFragmentBase
+}
+
+// classpathFragmentBase is meant to be embedded in any module types that implement classpathFragment;
+// such modules are expected to call initClasspathFragment().
+type classpathFragmentBase struct {
+	properties classpathFragmentProperties
+
+	classpathType classpathType
+
+	outputFilepath android.OutputPath
+}
+
+// Initializes classpathFragmentBase struct. Must be called by all modules that include classpathFragmentBase.
+func initClasspathFragment(c classpathFragment) {
+	base := c.classpathFragmentBase()
+	c.AddProperties(&base.properties)
+}