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August 4, 2024

Send Gmail in Kotlin

kotlin

springboot

Contents

Dependencies

dependencies {
    implementation("com.google.api-client:google-api-client:2.0.0")
    implementation("com.google.oauth-client:google-oauth-client-jetty:1.34.1")
    implementation("com.google.apis:google-api-services-gmail:v1-rev20220404-2.0.0")
    implementation("javax.mail:mail:1.4.7")
}

OAuth Credential and Code Implementation

  • In the past we have studied Gmail API (not just sending email) from this post.

    • This time we assume that we have all got the credential.json in the same way, make sure that in the OAuth Consent Screen session we choose the scope correctly:

      • Filter the scope:

      • Choose the privilege we want:

    • And make sure that when we create a credential we choose Desktop app (then we don't need to provide the return_url which is not needed in backend application)

      • The credential.json must be obtained from the account that you want to send email. A company boss account has no permission to send email on behalf of his/her employee.

        If we plan to send email using 3 accounts (for different purpose), make sure to prepare 3 set of credential.json and StoredCredential (to be generated by the sdk once we try to send any email).

  • Let's translate the code in Gmail API Documentation into kotlin, then we get the following:

    1import com.google.api.client.auth.oauth2.Credential
2import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp
3import com.google.api.client.extensions.jetty.auth.oauth2.LocalServerReceiver
4import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow
5import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets
6import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport
7import com.google.api.client.googleapis.json.GoogleJsonResponseException
8import com.google.api.client.http.javanet.NetHttpTransport
9import com.google.api.client.json.gson.GsonFactory
10import com.google.api.client.util.store.FileDataStoreFactory
11import com.google.api.services.gmail.Gmail
12import com.google.api.services.gmail.GmailScopes
13import com.google.api.services.gmail.model.Message
14import org.apache.commons.codec.binary.Base64
15import org.springframework.beans.factory.annotation.Value
16
17import org.springframework.stereotype.Service
18import java.io.*
19import java.nio.file.Paths
20import java.util.*
21import javax.mail.Session
22import javax.mail.internet.InternetAddress
23import javax.mail.internet.MimeMessage
24
25
26@Service
27class GmailService(
28    @Value("\${gmail.sender}") private val sender: String,
29    @Value("\${gmail.credential-path}") private val credentialPath: String
30) {
31
32
33    private val httpTransport = GoogleNetHttpTransport.newTrustedTransport()
34    private val jsonFactory = GsonFactory.getDefaultInstance()
35
36    private val tokenPath = "tokens"
37
38    init {
39        println("Sender: $sender")
40        println("credentialPath: $credentialPath")
41    }
42
43    @Throws(IOException::class)
44    private fun getCredentials(httpTransport: NetHttpTransport, jsonFactory: GsonFactory): Credential {
45        // Load client secrets.
46        val inputStream: InputStream = GmailService::class.java.getResourceAsStream(credentialPath)
47            ?: throw FileNotFoundException("Resource not found: $credentialPath")
48        val clientSecrets =
49            GoogleClientSecrets.load(jsonFactory, InputStreamReader(inputStream))
50
51        // Build flow and trigger user authorization request.
52        val flow = GoogleAuthorizationCodeFlow.Builder(
53            httpTransport, jsonFactory, clientSecrets, setOf(GmailScopes.GMAIL_SEND)
54        )
55            .setDataStoreFactory(FileDataStoreFactory(Paths.get(tokenPath).toFile()))
56            // .setDataStoreFactory(FileDataStoreFactory(File(System.getProperty("user.dir"))))

    • Note that a request to login and get StoredCredential can only be applied when we actually use it (like sending an email).

    • To store this StoredCredential we must specify a writable directory (which cannot be resources folder).

    • Step 1. Therefore we choose user.dir which points to the current project root directory (where we execute the program), and execute the program to send email.

    • Step 2. A link will be shown in the terminal to ask for logging-in in a browser.

      On successful login, and on credential.json matched, a StoredCredential will be generated.

    • Step 3. We can move that StoredCredential to the resources directory, then we can write tokenPath (which is the directory that contains the StoredCredential token) in place of "user.dir".

    • Example. When we have 3 emails in an application and if we want to use noreply:

      then we set tokenPath to classpath:email/noreply_billieonsite.

    57          .setAccessType("offline")
58          .build()
59      val receiver = LocalServerReceiver.Builder().setPort(8888).build()
60      val credential: Credential = AuthorizationCodeInstalledApp(flow, receiver).authorize("user")
61      // returns an authorized Credential object.
62      return credential
63  }
64
65  fun sendEmail(subject: String, bodyText: String, toEmail: String): Message? {
66      val service = Gmail.Builder(httpTransport, jsonFactory, getCredentials(httpTransport, jsonFactory))
67          .setApplicationName("payment")
68          .build()
69
70      val props = Properties()
71      val session: Session = Session.getDefaultInstance(props, null)
72      val email: MimeMessage = MimeMessage(session)
73      email.setFrom(InternetAddress(sender))
74      email.addRecipient(
75          javax.mail.Message.RecipientType.TO,
76          InternetAddress(toEmail)
77      )
78      email.subject = subject
79      email.setText(bodyText)
80
81      val buffer = ByteArrayOutputStream()
82      email.writeTo(buffer)
83      val rawMessageBytes = buffer.toByteArray()
84      val encodedEmail = Base64.encodeBase64URLSafeString(rawMessageBytes)
85      var message = Message()
86      message.setRaw(encodedEmail)
87
88      try {
89          // Create send message
90          message = service.users().messages().send("me", message).execute()
91          println("Message id: " + message.id)
92          println(message.toPrettyString())
93          return message
94      } catch (e: GoogleJsonResponseException) {
95          val error = e.details
96          if (error.code == 403) {
97              println("Unable to create draft: " + e.details)
98              throw Exception("403 not found")
99          } else {
100              throw e
101          }
102      }
103  }
104}