(Go) OAuth2 for a GMail using a JSON Service Account Key

This example shows how to obtain an OAuth2 access token for Gmail using a Google Service Account and a JSON private key. Once acquired, the access token can be used to send emails. Remember, upon token expiration, this process needs to be repeated to obtain a new token. Note: This procedure is specific to OAuth2 with Google Service Account keys.

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    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // --------------------------------------------------------------------------------
    // For a step-by-step guide for setting up your Google Workspace service account,
    // see Setup Google Workspace Account for Sending SMTP GMail from a Service Account
    // --------------------------------------------------------------------------------

    // First load the JSON key into a string.
    fac := FileAccess_Ref.html">chilkat.NewFileAccess()
    jsonKey := fac.ReadEntireTextFile("qa_data/googleApi/chilkat25-b4214220e565.json","utf-8")
    if fac.LastMethodSuccess() != true {
        fmt.Println(fac.LastErrorText())
        fac.DisposeFileAccess()
        return
    }

    // A Google service account JSON private key looks like this:

    // {
    //   "type": "service_account",
    //   "project_id": "chilkat25",
    //   "private_key_id": "b4214220f565881e19eeb97c2699bf5a0d1e3e0b",
    //   "private_key": "-----BEGIN PRIVATE KEY-----\nMIIEvQ...NXcM=\n-----END PRIVATE KEY-----\n",
    //   "client_email": "chilkatsvc@chilkat25.iam.gserviceaccount.com",
    //   "client_id": "109122032928932715958",
    //   "auth_uri": "https://accounts.google.com/o/oauth2/auth",
    //   "token_uri": "https://oauth2.googleapis.com/token",
    //   "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
    //   "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/chilkatsvc%40chilkat25.iam.gserviceaccount.com",
    //   "universe_domain": "googleapis.com"
    // }

    gAuth := AuthGoogle_Ref.html">chilkat.NewAuthGoogle()
    gAuth.SetJsonKey(*jsonKey)

    // Specify a scope.
    gAuth.SetScope("https://mail.google.com/")

    // Request an access token that is valid for this many seconds.
    gAuth.SetExpireNumSeconds(3600)

    // When using a Google Workspace account with Gmail APIs, a service account can impersonate a user 
    // via a process called domain-wide delegation � and the "sub" claim in the JWT is what enables this.
    // Domain-wide delegation allows a Google Workspace administrator to authorize a service account to 
    // act on behalf of any user in the domain, without user interaction.

    // This is required for server-to-server access to user data � such as reading/sending Gmail from a background service.
    // This is your company email address.
    gAuth.SetSubEmailAddress("info@chilkat.xyz")

    // Connect to www.googleapis.com using TLS
    tlsSock := Socket_Ref.html">chilkat.NewSocket()
    success := tlsSock.Connect("www.googleapis.com",443,true,5000)
    if success != true {
        fmt.Println(tlsSock.LastErrorText())
        fac.DisposeFileAccess()
        gAuth.DisposeAuthGoogle()
        tlsSock.DisposeSocket()
        return
    }

    // Send the request to obtain the access token.
    success = gAuth.ObtainAccessToken(tlsSock)
    if success != true {
        fmt.Println(gAuth.LastErrorText())
        fac.DisposeFileAccess()
        gAuth.DisposeAuthGoogle()
        tlsSock.DisposeSocket()
        return
    }

    // Examine the access token:
    accessToken := gAuth.AccessToken()
    fmt.Println("Access Token: ", accessToken)

    // Sample output:
    // ya29.a0AW4XtxjGTD67Z8 .... IRw0218

    // The access token allows us to send unlimited emails while it's valid. Once it expires, we must obtain and use a new one.

    // -----------------------------------------------------------------------
    mailman := MailMan_Ref.html">chilkat.NewMailMan()

    // Set the properties for the GMail SMTP server:
    mailman.SetSmtpHost("smtp.gmail.com")
    mailman.SetSmtpPort(587)
    mailman.SetStartTLS(true)

    mailman.SetSmtpUsername("info@chilkat.xyz")
    mailman.SetOAuth2AccessToken(accessToken)

    // Create a new email object
    email := Email_Ref.html">chilkat.NewEmail()

    email.SetSubject("This is a test")
    email.SetBody("This is a test")
    email.SetFrom("Chilkat Test <info@chilkat.xyz>")
    success = email.AddTo("Chilkat Software","info@chilkatsoft.com")
    // To add more recipients, call AddTo, AddCC, or AddBcc once per recipient.

    success = mailman.SendEmail(email)
    if success != true {
        fmt.Println(mailman.LastErrorText())
        fac.DisposeFileAccess()
        gAuth.DisposeAuthGoogle()
        tlsSock.DisposeSocket()
        mailman.DisposeMailMan()
        email.DisposeEmail()
        return
    }

    success = mailman.CloseSmtpConnection()
    if success != true {
        fmt.Println("Connection to SMTP server not closed cleanly.")
    }

    fmt.Println("Successfully sent email using Gmail with a service account key.")

    fac.DisposeFileAccess()
    gAuth.DisposeAuthGoogle()
    tlsSock.DisposeSocket()
    mailman.DisposeMailMan()
    email.DisposeEmail()