GoogleJsonResponseException：401 未经授权的具有 OAuth2 保护的调用端点

我正在尝试制作一个以 App Engine 作为移动后端的 Android 应用程序。 当我尝试调用受身份验证保护的端点时，出现此错误：

12-21 18:58:05.120    4452-4477/com.test.myapplication W/System.err﹕ com.google.api.client.googleapis.json.GoogleJsonResponseException: 401 Unauthorized
12-21 18:58:05.120    4452-4477/com.test.myapplication W/System.err﹕ {
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ {
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ "domain": "global",
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ "location": "Authorization",
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ "locationType": "header",
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ "message": "Authorization required",
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ "reason": "required"
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ }
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ ],
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ "message": "Authorization required"
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ }
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:113)
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:40)
12-21 18:58:05.130    4452-4477/com.test.myapplication W/System.err﹕ at com.google.api.client.googleapis.services.AbstractGoogleClientRequest$1.interceptResponse(AbstractGoogleClientRequest.java:312)

这是 App Engine 上的端点代码

@ApiMethod(
        name = "signInWithGoogle",
        path = "signInWithGoogle",
        httpMethod = ApiMethod.HttpMethod.POST
)
public Profile signInWithGoogle(final User user) throws UnauthorizedException {
    if (user == null) {
        throw new UnauthorizedException("Authorization required");
    }
    Key<Profile> profileKey = Key.create(Profile.class, user.getEmail());
    return OfyService.ofy().load().key(profileKey).now();
}

这是调用端点的活动代码

public class MainActivity extends Activity {

public static final String WEB_CLIENT_ID = "XXXX.apps.googleusercontent.com";
public static final String AUDIENCE = "server:client_id:" + WEB_CLIENT_ID;
protected static final HttpTransport HTTP_TRANSPORT = AndroidHttp.newCompatibleTransport();
protected static final JsonFactory JSON_FACTORY = new AndroidJsonFactory();

GoogleAccountCredential credential;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    credential = GoogleAccountCredential.usingAudience(this, AUDIENCE);

    startActivityForResult(credential.newChooseAccountIntent(), 0);
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    String accountName = data.getExtras().getString(AccountManager.KEY_ACCOUNT_NAME);
    credential.setSelectedAccountName(accountName);
    final SignUser signUser = new SignUser.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential).build();
    Log.d("TEST", "start");
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Profile profile = signUser.signInWithGoogle().execute();
                Log.d("TEST", "success");
            } catch (IOException e) {
                e.printStackTrace();
                Log.d("TEST", "failed");
            }
        }
    }).start();
}
}

似乎signUser.signInWithGoogle().execute()不创建端点所需的 User 对象，因此，端点抛出 UnauthorizedException。 实际上，如果我从端点删除 OAuth2 保护，一切都会正常。

谁能解释一下错误是什么？

UPDATE

这里是java类的@Api注解。

@Api(
    name = "signUser",
    version = "v1",
    scopes = {Constants.EMAIL_SCOPE},
    clientIds = {Constants.ANDROID_CLIENT_ID, Constants.API_EXPLORER_CLIENT_ID},
    audiences = {Constants.ANDROID_AUDIENCE},
    description = "APIs for sign user"
)

where EMAIL_SCOPE is https://www.googleapis.com/auth/userinfo.email and ANDROID_AUDIENCE等于 WEB_CLIENT_ID

我通过向凭证对象设置自定义请求初始值设定项解决了这个问题，如下所示

GoogleCredential credential = new GoogleCredential.Builder()
.setTransport(this.TRANSPORT).setJsonFactory(this.JSON_FACTORY)
.setClientSecrets(Constants.CLIENT_ID,     Constants.CLIENT_SECRET).setRequestInitializer((new HttpRequestInitializer(){
            @Override
            public void initialize(HttpRequest request)
                    throws IOException {
                request.getHeaders().put("Authorization", "Bearer " + accessToken);
            }
        })).build()