Implemented a Bootstrapper class which reads a local bootstrap file.

xds/build.gradle

@@ -26,11 +26,18 @@ dependencies {
     compile project(':grpc-protobuf'),
             project(':grpc-stub'),
             project(':grpc-core'),
-            project(':grpc-services')
+            project(':grpc-services'),
+            project(':grpc-auth')
     compile (libraries.protobuf_util) {
         // prefer 26.0-android from libraries instead of 20.0
         exclude group: 'com.google.guava', module: 'guava'
     }
+    compile (libraries.google_auth_oauth2_http) {
+        // prefer 26.0-android from libraries instead of 25.1-android
+        exclude group: 'com.google.guava', module: 'guava'
+        // prefer 0.19.2 from libraries instead of 0.18.0
+        exclude group: 'io.opencensus', module: 'opencensus-api'
+    }
 
     testCompile project(':grpc-core').sourceSets.test.output
 

xds/src/main/java/io/grpc/xds/Bootstrapper.java

@@ -0,0 +1,135 @@
+/*
+ * Copyright 2019 The gRPC Authors
+ *
+ * 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 io.grpc.xds;
+
+import com.google.auth.oauth2.ComputeEngineCredentials;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.protobuf.InvalidProtocolBufferException;
+import com.google.protobuf.util.JsonFormat;
+import io.envoyproxy.envoy.api.v2.core.ApiConfigSource;
+import io.envoyproxy.envoy.api.v2.core.ApiConfigSource.ApiType;
+import io.envoyproxy.envoy.api.v2.core.Node;
+import io.grpc.CallCredentials;
+import io.grpc.auth.MoreCallCredentials;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * Loads configuration information to bootstrap xDS load balancer.
+ */
+@Immutable
+abstract class Bootstrapper {
+
+  private static final String BOOTSTRAP_PATH_SYS_ENV_VAR = "GRPC_XDS_BOOTSTRAP";
+  private static volatile Exception failToBootstrapException;
+  private static volatile Bootstrapper DEFAULT_INSTANCE;
+
+  static Bootstrapper getInstance() throws Exception {
+    if (DEFAULT_INSTANCE == null && failToBootstrapException == null) {
+      synchronized (Bootstrapper.class) {
+        if (DEFAULT_INSTANCE == null && failToBootstrapException == null) {
+          try {
+            DEFAULT_INSTANCE = new FileBasedBootstrapper();
+          } catch (Exception e) {
+            failToBootstrapException = e;
+          }
+        }
+      }
+    }
+    if (DEFAULT_INSTANCE == null) {
+      throw failToBootstrapException;
+    }
+    return DEFAULT_INSTANCE;
+  }
+
+  /**
+   * Returns the canonical name of the traffic director to be connected to.
+   */
+  abstract String getBalancerName();
+
+  /**
+   * Returns a {@link Node} message with project/network metadata in it to be included in
+   * xDS requests.
+   */
+  abstract Node getNode();
+
+  /**
+   * Returns the credentials to use when communicating with the xDS server.
+   */
+  abstract CallCredentials getCallCredentials();
+
+  @VisibleForTesting
+  static final class FileBasedBootstrapper extends Bootstrapper {
+
+    private final String balancerName;
+    private final Node node;
+    // TODO(chengyuanzhang): Add configuration for call credentials loaded from bootstrap file.
+    //  hard-coded for alpha release.
+
+    private FileBasedBootstrapper() throws IOException {
+      this(Bootstrapper.readConfig());
+    }
+
+    @VisibleForTesting
+    FileBasedBootstrapper(Bootstrap bootstrapConfig) {
+      ApiConfigSource serverConfig = bootstrapConfig.getXdsServer();
+      if (!serverConfig.getApiType().equals(ApiType.GRPC)) {
+        throw new RuntimeException("Unexpected api type: " + serverConfig.getApiType().toString());
+      }
+      if (serverConfig.getGrpcServicesCount() != 1) {
+        throw new RuntimeException(
+            "Unexpected number of gRPC services: expected: 1, actual: "
+                + serverConfig.getGrpcServicesCount());
+      }
+      balancerName = serverConfig.getGrpcServices(0).getGoogleGrpc().getTargetUri();
+      node = bootstrapConfig.getNode();
+    }
+
+    @Override
+    String getBalancerName() {
+      return balancerName;
+    }
+    
+    @Override
+    Node getNode() {
+      return node;
+    }
+
+    @Override
+    CallCredentials getCallCredentials() {
+      return MoreCallCredentials.from(ComputeEngineCredentials.create());
+    }
+  }
+
+  private static Bootstrap readConfig() throws IOException {
+    String filePath = System.getenv(BOOTSTRAP_PATH_SYS_ENV_VAR);
+    if (filePath == null) {
+      throw new RuntimeException("Environment variable "
+          + BOOTSTRAP_PATH_SYS_ENV_VAR + " not found.");
+    }
+    return parseConfig(new String(Files.readAllBytes(Paths.get(filePath)), "UTF-8"));
+  }
+
+  @VisibleForTesting
+  static Bootstrap parseConfig(String rawData) throws InvalidProtocolBufferException {
+    Bootstrap.Builder bootstrapBuilder = Bootstrap.newBuilder();
+    JsonFormat.parser().merge(rawData, bootstrapBuilder);
+    return bootstrapBuilder.build();
+  }
+}