NullPointerException only after I have setServiceAccountUser() for GoogleCredential object (Grails/Java)

Question

I receive a NullPointerException with very little detail when I attempt to setServiceAccountUser(ACCOUNT_TO_IMPERSONATE) on my GoogleCredential, build a Google Tasks Service Object and subsequently try to send a request for a list of Tasks from ACCOUNT_TO_IMPERSONATE.

def credential = new GoogleCredential.Builder()
    .setTransport(HTTP_TRANSPORT)
    .setJsonFactory(JSON_FACTORY)
    .setServiceAccountId(SERVICE_ACCOUNT_ID)
    .setServiceAccountPrivateKeyFromP12File(P12_FILE)
    .setServiceAccountScopes(GOOGLE_TASK_SCOPES)
    .setServiceAccountUser(ACCOUNT_TO_IMPERSONATE)
    .build()

    credential.refreshToken()
    log.debug("Google Credential Impersonating: ${credential.getServiceAccountUser()}")

    def service = new com.google.api.services.tasks.Tasks.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential).build()

    return service

Please note that the credential.getServiceAccountUser() logs out the expected ACCOUNT_TO_IMPERSONATE.

There was a similar issue reported in https://github.com/googleads/googleads-java-lib/issues/19

StackTrace :
Caused by: java.lang.NullPointerException at com.google.api.client.repackaged.com.google.common.base.Preconditions.checkNotNull(Preconditions.java:191)
at com.google.api.client.util.Preconditions.checkNotNull(Preconditions.java:127)
at com.google.api.client.json.jackson2.JacksonFactory.createJsonParser(JacksonFactory.java:96)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:85)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:81)
at com.google.api.client.auth.oauth2.TokenResponseException.from(TokenResponseException.java:88)
at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:287)
at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:307)
at com.google.api.client.auth.oauth2.Credential.executeRefreshToken(Credential.java:570)
at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:247)
at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:489)
at com.google.api.ads.common.lib.auth.OAuth2Helper.callRefreshToken(OAuth2Helper.java:70)
at com.google.api.ads.common.lib.auth.OfflineCredentials.generateCredential(OfflineCredentials.java:144)
...

The lack of detail in the error was said to be an issue in older versions of Google Tasks but I have upgraded to the latest version and see no difference.

compile "com.google.api-client:google-api-client:1.19.0"
compile 'com.google.apis:google-api-services-tasks:v1-rev41-1.19.0'

EDIT: After tinkering with the code in the grails console in our web application I can see that executing credential.refreshToken() causes the NPE when ServiceAccountUser is set.

When I unset the ServiceAccountUser, refreshToken() is successful. Additionally, trying to retrieve a list of Google Tasks is succcessful for the Service account when the ServiceAccountUser is not set.

Any help would be greatly appreciated.

Answer 1

I discovered the reason for the behaviour we were experiencing, but thanks for your suggestions, none-the-less, user1807337 & Srinivasan.

We need to get our application's users to delegate domain-wide authority to our application. This can be done by the organisataion's google apps admin, following the instructions in the sub-section entitled Delegating domain-wide authority to the service account

https://developers.google.com/accounts/docs/OAuth2ServiceAccount

Here is the explanation given in the same resource: "If you have delegated domain-wide access to the service account and you want to impersonate a user account, specify the email address of the user account with the setServiceAccountUser method of the GoogleCredential factory..."