Gmail internal application, Two-Legged OAuth2, Server to Server authentication, and Google API versions

I am working on a little tool at home in my free time to put some skills into practice.  The general idea (nothing novel) is this:

I have some financial alerts sent to a new email address I have spun up on my domain.  I am creating an AWS Lambda that will wake up on an hourly schedule, read those emails, and publish SNS messages with parsed financial transaction information.  I then will have an SQS queue listen to the SNS message topic that is consumed by a Step Function.  The Step Function will:

• store the financial transaction information into a database
• send an SMS to me if the transaction is above a certain threshold.

I could later extend this to do some aggregation reporting, etc if I wanted, too.  This will only work for my own financial transactions, and the data being gathered/stored will be sufficiently vague, so I am not really concerned about financial security for this project.

The biggest hurdle I have run into so far is connecting to Gmail securely.  I was able to quickly connect with JavaMail after I turned off enhanced security for the account.  I don't want to keep that basic authentication in place long term, though, so I started working on how to connect securely using OAuth2.  This is not a typical OAuth implementation for a couple of reasons:

1. A cron job-like task will check the email account.  This means there will be no pop-up like authentication and authorization screen that google can provide.
2. The email account is never meant to be checked manually; it is really a poor man's queue for purposes of this project.  That means I only want the app I am building to manage the email; no manual mailbox handling should be necessary.

To elaborate on the hurdle some more... OAuth2 is not new, so this is not a new problem.  So why was this such a hurdle?  The answer is that there is a lot of documentation and examples out there, and much of it is very out of date.  So finding the right library with the right examples took way more time than actually writing the final code that worked.

The final solution turned out to require the following:

• Create a "Project" in Google's dev console.  Make note of the project name - you will reference it in the code.
• Generate a "Service Account" allowed to manage the Project in Google's dev console.
• Generate a JSON key for the service account that is allowed to interact with the Project.
• Set the Service Account as an Owner role for the Project.
• Modify Domain Wide Delegation for the JSON key's client Id to have access to desired scope -- particularly "https://mail.google.com/" because the service account should be allowed to do anything in this case.  The name of the API Client for delegation can be anything you want - it is just a useful description of the thing we are creating, we don't have to reference the name from what I can tell.  The client Id is the important part, and this value can easily be found inside the JSON key file.

Once all of this is done, we can write the code.  

import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport
import com.google.api.services.gmail.Gmail
import com.google.auth.http.HttpCredentialsAdapter
import com.google.auth.oauth2.ServiceAccountCredentials
import net.inherency.finances.jsonFactory
import net.inherency.finances.vo.Email
import org.apache.commons.configuration.EnvironmentConfiguration
import java.time.Instant
import java.time.LocalDateTime
import java.time.ZoneOffset


class GmailOauth2MailServiceImpl(
        private val env: EnvironmentConfiguration
): MailService {

    override fun listMessages(): List {
        val gmail = login()
        val list = gmail.users().Messages().list(userId()).execute()
        return list.messages.map { msg->
            val message = gmail.users().Messages().get(userId(), msg.id).execute()
            val messageDate = LocalDateTime.ofInstant(
              Instant.ofEpochMilli(message.internalDate), ZoneOffset.UTC)
                .toLocalDate()
            val fromAddress = message.payload.headers
              .first { it.name == "From" }.value
            val subject = message.payload.headers
              .first { it.name == "Subject" }.value
            Email(messageDate, fromAddress, subject)
        }
    }

    private fun login(): Gmail {
        val serviceAccountCredentials = ServiceAccountCredentials
                .fromStream(credentialsKey().byteInputStream())
                .createScoped(gmailScopes())
                .createDelegated(userId())

        serviceAccountCredentials.refreshAccessToken()

        return Gmail.Builder(
                GoogleNetHttpTransport.newTrustedTransport(),
                jsonFactory(),
                HttpCredentialsAdapter(serviceAccountCredentials))
                .setApplicationName(googleApplicationName())
                .build()
    }


    private fun googleApplicationName(): String {
        return env.getString("app_name")
    }

    private fun userId(): String {
        return env.getString("email_login")
    }

    private fun gmailScopes(): List {
        return listOf("https://mail.google.com/")
    }

    private fun credentialsKey(): String {
        return env.map["auth_json"].toString()
    }
}