要从Gmail获取数据，首先需要使用Gmail API进行身份验证和授权。下面是一个使用Java代码示例的解决方法：

import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp;
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.auth.oauth2.GoogleCredential;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.services.gmail.Gmail;
import com.google.api.services.gmail.GmailScopes;
import com.google.api.services.gmail.model.ListMessagesResponse;
import com.google.api.services.gmail.model.Message;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Collections;
import java.util.List;

public class GmailDataReader {

    private static final String APPLICATION_NAME = "Gmail Data Reader";
    private static final JsonFactory JSON_FACTORY = JacksonFactory.getDefaultInstance();
    private static final String TOKENS_DIRECTORY_PATH = "tokens";
    private static final List SCOPES = Collections.singletonList(GmailScopes.GMAIL_READONLY);
    private static final String CREDENTIALS_FILE_PATH = "/path/to/credentials.json";

    public static void main(String... args) throws Exception {
        // 初始化HTTP传输器和JSON工厂
        HttpTransport httpTransport = GoogleNetHttpTransport.newTrustedTransport();

        // 从JSON文件中加载客户端凭据
        InputStream credentialsStream = GmailDataReader.class.getResourceAsStream(CREDENTIALS_FILE_PATH);
        GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(credentialsStream));

        // 构建授权流
        GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
                httpTransport, JSON_FACTORY, clientSecrets, SCOPES)
                .setDataStoreFactory(new FileDataStoreFactory(new java.io.File(TOKENS_DIRECTORY_PATH)))
                .setAccessType("offline")
                .build();

        // 获取授权
        Credential credential = new AuthorizationCodeInstalledApp(flow, new LocalServerReceiver()).authorize("user");

        // 创建Gmail服务
        Gmail service = new Gmail.Builder(httpTransport, JSON_FACTORY, credential)
                .setApplicationName(APPLICATION_NAME)
                .build();

        // 获取邮件列表
        ListMessagesResponse response = service.users().messages().list("me").execute();
        List messages = response.getMessages();

        // 遍历邮件并获取附件
        for (Message message : messages) {
            Message fullMessage = service.users().messages().get("me", message.getId()).execute();
            List parts = fullMessage.getPayload().getParts();
            for (MessagePart part : parts) {
                if (part.getFilename() != null && part.getFilename().endsWith(".json")) {
                    String attachmentId = part.getBody().getAttachmentId();
                    MessagePartBody attachmentPart = service.users().messages().attachments().get("me", message.getId(), attachmentId).execute();
                    String data = new String(Base64.decodeBase64(attachmentPart.getData()));
                    System.out.println("附件内容：" + data);
                }
            }
        }
    }
}

上述代码使用了Google API客户端库，并假设您已经从Google开发者控制台获取了凭据文件（credentials.json）并将其放在您的项目路径中。您还需要确保您已经授权了Gmail API的访问权限。

请注意，上述代码仅获取了包含.json附件的邮件，并使用Base64解码将附件转换为原始数据。您可以根据实际需求进行修改和扩展。