SpringMVC返回图片的几种方式

@RequestMapping(value = {"/img/render"}, method = {RequestMethod.GET, RequestMethod.POST, RequestMethod.OPTIONS})
@CrossOrigin(origins = "*")
@ResponseBody
public String execute(HttpServletRequest httpServletRequest,
             HttpServletResponse httpServletResponse) {
    // img为图片的二进制流
    byte[] img = xxx;
    httpServletResponse.setContentType("image/png");
    OutputStream os = httpServletResponse.getOutputStream();
    os.write(img);
    os.flush();
    os.close();
    return "success";
}

@Data
public class BaseRequest {
    private static final long serialVersionUID = 1146303518394712013L;

    /**
     * 输出图片方式:
     *
     *  url : http地址 （默认方式）
     *  base64 : base64编码
     *  stream : 直接返回图片
     *
     */
    private String outType;

    /**
     * 返回图片的类型
     * jpg | png | webp | gif
     */ 
    private String mediaType;
    

    public ReturnTypeEnum returnType() {
        return ReturnTypeEnum.getEnum(outType);
    }


    public MediaTypeEnum mediaType() {
        return MediaTypeEnum.getEnum(mediaType);
    }
}

public enum ReturnTypeEnum {

    URL("url"),
    STREAM("stream"),
    BASE64("base");

    private String type;

    ReturnTypeEnum(String type) {
        this.type = type;
    }


    private static Map<String, ReturnTypeEnum> map;

    static {
        map = new HashMap<>(3);
        for(ReturnTypeEnum e: ReturnTypeEnum.values()) {
            map.put(e.type, e);
        }
    }

    public static ReturnTypeEnum getEnum(String type) {
        if (type == null) {
            return URL;
        }

        ReturnTypeEnum e = map.get(type.toLowerCase());
        return e == null ? URL : e;
    }
}

@Data
public enum MediaTypeEnum {
    ImageJpg("jpg", "image/jpeg", "FFD8FF"),
    ImageGif("gif", "image/gif", "47494638"),
    ImagePng("png", "image/png", "89504E47"),
    ImageWebp("webp", "image/webp", "52494646"),

    private final String ext;

    private final String mime;

    private final String magic;

    MediaTypeEnum(String ext, String mime, String magic) {
        this.ext = ext;
        this.mime = mime;
        this.magic = magic;
    }

    private static Map<String, MediaTypeEnum> map;

    static {
        map = new HashMap<>(4);
        for (MediaTypeEnum e: values()) {
            map.put(e.getExt(), e);
        }
    }

    public static MediaTypeEnum getEnum(String type) {
        if (type == null) {
            return ImageJpg;
        }

        MediaTypeEnum e = map.get(type.toLowerCase());
        return e == null ? ImageJpg : e;
    }
}

@Data
public class BaseResponse {

    /**
     * 返回图片的相对路径
     */
    private String path;


    /**
     * 返回图片的https格式
     */
    private String url;


    /**
     * base64格式的图片
     */
    private String base;
}

protected void buildResponse(BaseRequest request,
                             BaseResponse response,
                             byte[] bytes) throws SelfError {
    switch (request.returnType()) {
        case URL:
            upload(bytes, response);
            break;
        case BASE64:
            base64(bytes, response);
            break;
        case STREAM:
            stream(bytes, request);
    }
}


private void upload(byte[] bytes, BaseResponse response) throws SelfError {
    try {
        // 上传到图片服务器，根据各自的实际情况进行替换
        String path = UploadUtil.upload(bytes);

        if (StringUtils.isBlank(path)) { // 上传失败
            throw new InternalError(null);
        }

        response.setPath(path);
        response.setUrl(CdnUtil.img(path));
    } catch (IOException e) { // cdn异常
        log.error("upload to cdn error! e:{}", e);
        throw new CDNUploadError(e.getMessage());
    }
}

// 返回base64
private void base64(byte[] bytes, BaseResponse response) {
    String base = Base64.getEncoder().encodeToString(bytes);
    response.setBase(base);
}

// 返回二进制图片
private void stream(byte[] bytes, BaseRequest request) throws SelfError {
    try {
        MediaTypeEnum mediaType = request.mediaType();
        HttpServletResponse servletResponse = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getResponse();
        servletResponse.setContentType(mediaType.getMime());
        OutputStream os = servletResponse.getOutputStream();
        os.write(bytes);
        os.flush();
        os.close();
    } catch (Exception e) {
        log.error("general return stream img error! req: {}, e:{}", request, e);
        if (StringUtils.isNotBlank(e.getMessage())) {
            throw new InternalError(e.getMessage());
        } else {
            throw new InternalError(null);
        }
    }
}

如，在统一的地方进行异常计数，然后超过某个阀值之后，报警给负责人，这样就不需要在每个出现异常case的地方来主动埋点了

- 这个主要针对web服务，一般是在返回的json串中，会包含对应的错误状态码，错误信息
- 而异常case是可能出现在任何地方的，为了保持这个异常信息，要么将这些数据层层传递到controller；要么就是存在ThreadLocal中；显然这两种方式都没有抛异常的使用方便

@Override
public synchronized Throwable fillInStackTrace() {
    return this;
}