我可以在没有任何配置的情况下使用 Java 通过本地 SMTP 服务器发送电子邮件吗？

使用 Java 发送电子邮件

问题：

由于连接原因，尝试使用 Java 发送电子邮件时发生错误本地 SMTP 服务器出现问题。

问题：

提供的代码可以用来发送电子邮件吗？

答案：

所提供的使用 Java 发送电子邮件的代码（使用邮件服务器的默认设置）可能无法在所有情况下工作。具体来说，本地主机 SMTP 服务器默认情况下不太可能正常工作。

解决方案：

要使用 Java 可靠地发送电子邮件，请考虑使用第三方 SMTP 服务器，例如如谷歌邮件。下面的代码片段演示了如何使用 Google 的 API 和 oAuth2 身份验证通过 Google 的 SMTP 服务器发送电子邮件：

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");

附加说明：

• authorizeGmail( ) 方法执行 oAuth2 身份验证并返回一对凭证和电子邮件地址。
• 为了请求用户授权，使用 MyAuthorizationCodeInstalledApp 类，显示一个简单的用户界面。
• sendEmail()方法使用 Gmail 服务和 MIME 消息发送电子邮件。
• 为了避免对敏感信息进行硬编码，请考虑对客户端密钥和其他凭据使用环境变量或密钥管理器。

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