上传文件至S3时遇java.net.UnknownHostException问题求助

问题描述 #

上传文件至Amazon S3时，间歇性抛出java.net.UnknownHostException: bucketName.s3.ap-south-1.amazonaws.com异常。异常有时在重新部署WAR包后恢复，有时自行消失。当前使用AWS Java SDK 1.12.305版本，需排查该异常属于代码问题、DNS问题还是AWS相关问题。

代码片段 #

连接S3客户端方法 #

public static AmazonS3 connectToAmazonS3(String accessKeyID, String secretAccessKey) {

        LOGGER.info("Connect to the amazon s3 service using the credentials");
        try {
            final BasicAWSCredentials awsCreds = new BasicAWSCredentials(accessKeyID, secretAccessKey);

            final AmazonS3 s3Client = new AmazonS3Client(awsCreds);
            LOGGER.info("Successfully connected to Amazon S3 service");

            return s3Client;
        } catch (Exception e) {
            LOGGER.error("Error occured while connecting to Amazon S3 service" + e, e);
        }

        LOGGER.info("Connection not established with Amazon S3 service");
        return null;
    }

上传文件至S3方法 #

public static void uploadFileToAmazonS3(final AmazonS3 s3Client, final String bucketName, final String keyName, String cmdStr) {
        LOGGER.info("Started upload file to Amazon S3 for key: " + keyName);

        try {
            byte[] contentAsBytes = cmdStr.getBytes(EPSCommonConstants.CHARSET_UTF8);
            ByteArrayInputStream contentAsStream = new ByteArrayInputStream(contentAsBytes);

            ObjectMetadata metadata = new ObjectMetadata();
            metadata.setContentLength(contentAsBytes.length);

            PutObjectRequest req = new PutObjectRequest(bucketName, keyName, contentAsStream, metadata);
            s3Client.putObject(req);

        } catch (AmazonServiceException ase) {

            LOGGER.error("Caught an AmazonServiceException, which " + "means your request made it "
                    + "to Amazon S3, but was rejected with an error response" + " for some reason.", ase);

            LOGGER.error("Error Message:    " + ase.getMessage() + " " + "HTTP Status Code: " + ase.getStatusCode()
                    + " " + "AWS Error Code:   " + ase.getErrorCode() + " " + "Error Type:       " + ase.getErrorType()
                    + " " + "Request ID:       " + ase.getRequestId());
        } catch (AmazonClientException ace) {
            LOGGER.error(
                    "Caught an AmazonClientException, which " + "means the client encountered "
                            + "an internal error while trying to " + "communicate with S3, "
                            + "such as not being able to access the network." + "Error Message: " + ace.getMessage(),
                    ace);
        } catch (IOException ioe) {
            LOGGER.error("Caught an IOException Error Message: " + ioe.getMessage(), ioe);
        }
    }

排查方向 #

1. DNS相关问题（最可能） #

UnknownHostException本质是无法解析目标域名，间歇性出现且自行恢复的特征完全匹配DNS解析故障：

• 应用服务器本地DNS缓存过期或失效，重启JVM（重新部署WAR包）会清除旧缓存，可能恢复解析
• 网络运营商或本地DNS服务器的解析服务不稳定，出现间歇性解析失败
• 异常发生时，可在应用服务器上执行nslookup bucketName.s3.ap-south-1.amazonaws.com或dig bucketName.s3.ap-south-1.amazonaws.com，手动验证域名解析是否正常

2. 代码/客户端配置问题 #

当前代码存在潜在优化点，虽不直接导致间歇性异常，但可能加重解析负担：

• 未指定S3区域：AmazonS3Client默认使用US East 1区域，而bucket在ap-south-1，SDK自动重定向可能增加解析风险，建议初始化时指定区域：

  final AmazonS3 s3Client = AmazonS3Client.builder()
          .withCredentials(new AWSStaticCredentialsProvider(awsCreds))
          .withRegion(Regions.AP_SOUTH_1)
          .build();
  

• 客户端实例管理：频繁创建AmazonS3实例可能引发连接池或DNS缓存问题，建议复用客户端实例（单例模式）
• JVM DNS缓存设置：可通过调整networkaddress.cache.ttl（正缓存时长）和networkaddress.cache.negative.ttl（负缓存时长）参数，避免缓存过期的无效解析结果

3. AWS相关问题（可能性极低） #

AWS S3区域性解析故障概率极低，可通过AWS状态页面查看ap-south-1区域S3服务状态。另外，确认bucket名称符合S3命名规则（特殊字符可能导致解析异常，但不会是间歇性的）

总结 #

优先排查DNS解析问题，其次优化代码中的客户端配置与实例管理，最后确认AWS服务状态。

内容的提问来源于stack exchange，提问作者Sanket Vidwans