auth: GoogleAuthLibraryCallCredentials uses Credentials Builder

auth/src/main/java/io/grpc/auth/GoogleAuthLibraryCallCredentials.java

@@ -28,12 +28,11 @@ import io.grpc.SecurityLevel;
 import io.grpc.Status;
 import io.grpc.StatusException;
 import java.io.IOException;
-import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.net.URI;
 import java.net.URISyntaxException;
-import java.security.PrivateKey;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
@@ -245,31 +244,75 @@ final class GoogleAuthLibraryCallCredentials extends io.grpc.CallCredentials2 {
     return rawGoogleCredentialsClass.asSubclass(Credentials.class);
   }
 
+  private static class MethodPair {
+    private final Method getter;
+    private final Method builderSetter;
+
+    private MethodPair(Method getter, Method builderSetter) {
+      this.getter = getter;
+      this.builderSetter = builderSetter;
+    }
+
+    private void apply(Credentials credentials, Object builder)
+        throws InvocationTargetException, IllegalAccessException {
+      builderSetter.invoke(builder, getter.invoke(credentials));
+    }
+  }
+
   @VisibleForTesting
   static class JwtHelper {
     private final Class<? extends Credentials> serviceAccountClass;
-    private final Constructor<? extends Credentials> jwtConstructor;
+    private final Method newJwtBuilder;
+    private final Method build;
     private final Method getScopes;
-    private final Method getClientId;
-    private final Method getClientEmail;
-    private final Method getPrivateKey;
-    private final Method getPrivateKeyId;
+    private final List<MethodPair> methodPairs;
 
     public JwtHelper(Class<?> rawServiceAccountClass, ClassLoader loader)
         throws ClassNotFoundException, NoSuchMethodException {
       serviceAccountClass = rawServiceAccountClass.asSubclass(Credentials.class);
       getScopes = serviceAccountClass.getMethod("getScopes");
-      getClientId = serviceAccountClass.getMethod("getClientId");
-      getClientEmail = serviceAccountClass.getMethod("getClientEmail");
-      getPrivateKey = serviceAccountClass.getMethod("getPrivateKey");
-      getPrivateKeyId = serviceAccountClass.getMethod("getPrivateKeyId");
       Class<? extends Credentials> jwtClass = Class.forName(
           "com.google.auth.oauth2.ServiceAccountJwtAccessCredentials", false, loader)
           .asSubclass(Credentials.class);
-      jwtConstructor
-          = jwtClass.getConstructor(String.class, String.class, PrivateKey.class, String.class);
+      newJwtBuilder = jwtClass.getDeclaredMethod("newBuilder");
+      Class<?> builderClass = newJwtBuilder.getReturnType();
+      build = builderClass.getMethod("build");
+
+      methodPairs = new ArrayList<>();
+
+      {
+        Method getter = serviceAccountClass.getMethod("getClientId");
+        Method setter = builderClass.getMethod("setClientId", getter.getReturnType());
+        methodPairs.add(new MethodPair(getter, setter));
+      }
+      {
+        Method getter = serviceAccountClass.getMethod("getClientEmail");
+        Method setter = builderClass.getMethod("setClientEmail", getter.getReturnType());
+        methodPairs.add(new MethodPair(getter, setter));
+      }
+      {
+        Method getter = serviceAccountClass.getMethod("getPrivateKey");
+        Method setter = builderClass.getMethod("setPrivateKey", getter.getReturnType());
+        methodPairs.add(new MethodPair(getter, setter));
+      }
+      {
+        Method getter = serviceAccountClass.getMethod("getPrivateKey");
+        Method setter = builderClass.getMethod("setPrivateKey", getter.getReturnType());
+        methodPairs.add(new MethodPair(getter, setter));
+      }
     }
 
+    /**
+     * This method tries to convert a {@link Credentials} object to a
+     * ServiceAccountJwtAccessCredentials.  The original credentials will be returned if:
+     * <ul>
+     *   <li> The Credentials is not a ServiceAccountCredentials</li>
+     *   <li> The ServiceAccountCredentials has scopes</li>
+     *   <li> Something unexpected happens </li>
+     * </ul>
+     * @param creds the Credentials to convert
+     * @return either the original Credentials or a fully formed ServiceAccountJwtAccessCredentials.
+     */
     public Credentials tryServiceAccountToJwt(Credentials creds) {
       if (!serviceAccountClass.isInstance(creds)) {
         return creds;
@@ -282,17 +325,20 @@ final class GoogleAuthLibraryCallCredentials extends io.grpc.CallCredentials2 {
           // Leave as-is, since the scopes may limit access within the service.
           return creds;
         }
-        return jwtConstructor.newInstance(
-            getClientId.invoke(creds),
-            getClientEmail.invoke(creds),
-            getPrivateKey.invoke(creds),
-            getPrivateKeyId.invoke(creds));
+        // Create the JWT Credentials Builder
+        Object builder = newJwtBuilder.invoke(null);
+
+        // Get things from the credentials, and set them on the builder.
+        for (MethodPair pair : this.methodPairs) {
+          pair.apply(creds, builder);
+        }
+
+        // Call builder.build()
+        return (Credentials) build.invoke(builder);
       } catch (IllegalAccessException ex) {
         caughtException = ex;
       } catch (InvocationTargetException ex) {
         caughtException = ex;
-      } catch (InstantiationException ex) {
-        caughtException = ex;
       }
       if (caughtException != null) {
         // Failure is a bug in this class, but we still choose to gracefully recover