我可以在沒有任何配置的情況下使用 Java 透過本機 SMTP 伺服器發送電子郵件嗎？

使用Java 發送電子郵件

問題：

由於連接原因，嘗試使用Java 發送電子郵件時發生錯誤本地主機SMTP問題

問題：

提供的程式碼可以用來傳送電子郵件嗎？

答案：

提供的使用 Java 發送電子郵件的程式碼（使用郵件伺服器的預設）可能無法在所有版本中工作案例。具體來說，本地主機 SMTP 伺服器預設情況下不太可能正常運作。

解決方案：

要使用 Java 可靠地發送電子郵件，請考慮使用第三方 SMTP 伺服器，例如如Google郵件。下面的代碼片段演示瞭如何使用Google 的API 和oAuth2 身份驗證通過Google 的SMTP 服務器發送電子郵件：

import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.jetty.auth.oauth2.LocalServerReceiver;
import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow;
import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.json.gson.GsonFactory;
import com.google.api.client.util.store.FileDataStoreFactory;
import com.google.api.services.gmail.Gmail;
import com.google.api.services.gmail.model.Message;
import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.GeneralSecurityException;
import java.util.HashSet;
import java.util.Properties;
import java.util.Set;
import javax.mail.MessagingException;
import javax.mail.Session;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;

public class GoogleMail {
    private static final GsonFactory JSON_FACTORY = GsonFactory.getDefaultInstance();
    private static final HttpTransport HTTP_TRANSPORT;
    private static final File DATA_STORE_DIRECTORY = getGmailDataDirectory();

    static {
        try {
            HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
        } catch (GeneralSecurityException | IOException e) {
            throw new RuntimeException(e);
        }
    }

    private static Gmail getGmailService(Credential credential) throws IOException {
        return new Gmail.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential).setApplicationName("My Gmail App").build();
    }

    private static File getGmailDataDirectory() {
        return new File(org.yccheok.jstock.gui.Utils.getUserDataDirectory() + "authentication" + File.separator + "gmail");
    }

    public static void sendEmail(String to, String subject, String message) throws MessagingException, IOException {
        // Get the email account from saved credentials
        String email = loadEmail(DATA_STORE_DIRECTORY);
        if (email == null) {
            // If no credentials saved, request user authorization
            Pair<Pair<Credential, String>, Boolean> credentials = authorizeGmail();
            if (!credentials.second) {
                throw new RuntimeException("Failed to get credentials from user");
            }
            email = credentials.first.second;
            // Save the email address for future use
            saveEmail(DATA_STORE_DIRECTORY, email);
        }

        // Create a MIME message
        Properties props = new Properties();
        Session session = Session.getDefaultInstance(props, null);
        MimeMessage emailMessage = new MimeMessage(session);

        // Set the sender's email address
        InternetAddress sender = new InternetAddress(email);
        emailMessage.setFrom(sender);

        // Set the recipient's email address
        InternetAddress recipient = new InternetAddress(to);
        emailMessage.addRecipient(Message.RecipientType.TO, recipient);

        // Set the subject and the message body
        emailMessage.setSubject(subject);
        emailMessage.setText(message);

        // Get the authorized credentials
        Credential credential = credentialsPair.first.first;

        // Create a Gmail service object
        Gmail gmailService = getGmailService(credential);

        // Create a message object and send the email
        Message messageObject = createMessageWithEmail(emailMessage);
        gmailService.users().messages().send("me", messageObject).execute();
    }

    private static Message createMessageWithEmail(MimeMessage emailMessage) throws MessagingException, IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        emailMessage.writeTo(baos);
        String encodedEmail = Base64.encodeBase64URLSafeString(baos.toByteArray());
        return new Message().setRaw(encodedEmail);
    }

    private static Pair<Pair<Credential, String>, Boolean> authorizeGmail() throws IOException {
        // Load client secrets from a resource file
        GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY,
                new InputStreamReader(GoogleMail.class.getResourceAsStream("/client_secrets.json")));

        // Define the OAuth 2.0 scopes to request
        Set<String> scopes = new HashSet<>();
        scopes.add(GmailScopes.GMAIL_SEND);

        // Build the Google Authorization Code Flow object
        GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
                HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, scopes)
                .setDataStoreFactory(new FileDataStoreFactory(DATA_STORE_DIRECTORY))
                .build();

        // Request the user's authorization
        return new MyAuthorizationCodeInstalledApp(flow, new LocalServerReceiver()).authorize("user");
    }
}

// Call the sendEmail method to send the email
GoogleMail.sendEmail("recipient@example.com", "Subject", "Message");

附加說明：

• authorizeGmail( ) 方法執行oAuth2 驗證並傳回一對憑證和電子郵件地址。
• 為了請求使用者授權，使用 MyAuthorizationCodeInstalledApp 類，顯示一個簡單的使用者介面。
• sendEmail() 方法使用 Gmail 服務和 MIME 訊息發送電子郵件。
• 為了避免對敏感資訊進行硬編碼，請考慮對客戶端機密和其他機密使用環境變數或機密管理器憑證。

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