使用SpringBoot的RestController来写接口后，不再使用httpServletResponse来操作请求的输出了，所以返回图片需要换一种方式

修改@RequestMapping

在RequestMapping / GetMapping新增produces属性，并把方法的返回值改为字节流byte[]类型：

@GetMapping(value = "/image",produces = MediaType.IMAGE_JPEG_VALUE)
public byte[] getPicture() throws Exception {...}

实际逻辑的代码实现

public byte[] getPicture() throws Exception {
    OutputStream out = null;
    File file = new File("path/1.jpeg");
    FileInputStream inputStream = new FileInputStream(file);
    byte[] bytes = new byte[inputStream.available()];
    inputStream.read(bytes, 0, inputStream.available());
    return bytes;
}

动态设置Content-type

如果逻辑中还需要实现鉴权 / 限流 / not found等报错;
分别让正常请求返回image/jpeg图片在异常时返回application/json错误信息，则可以如下改造：

@GetMapping("ct/{id}")
public ResponseEntity<Object> getPictureIfExist(@PathVariable Integer id) {
        HttpHeaders headers = new HttpHeaders();
        try {
            //设置Content-type为image/jpeg
            headers.add(HttpHeaders.CONTENT_TYPE, MediaType.IMAGE_JPEG_VALUE);
            File file = new File("path/1.jpeg");
            FileInputStream inputStream = new FileInputStream(file);
            byte[] bytes = new byte[inputStream.available()];
            inputStream.read(bytes, 0, inputStream.available());
            return new ResponseEntity<Object>(bytes, headers, HttpStatus.OK);
        } catch (Exception e) {
            //设置Content-type为application/json
            headers.add(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE);
            Map<String, Object> resultMap = new HashMap<>();
            resultMap.put("code", 500);
            resultMap.put("message", "not found");
            return new ResponseEntity<Object>(resultMap, headers, HttpStatus.OK);
        }
    }

也就是不使用RequestMapping的produces参数，改用ResponseEntity的特性来灵活定义返回header的Content-type，借此实现RESTful接口的定义。