Spring Web文件上传下载注意点

我们使用如下的Controller来上传下载文件：

java">@RestController
@RequestMapping("/hello")
@Slf4j
public class FileController {@GetMapping("/upload")public void uploadFile(@RequestParam("file") MultipartFile file) {log.info("{} uploaded, {} bytes", file.getOriginalFilename(), file.getSize());}@GetMapping("/download")public void downloadFile(HttpServletResponse response) {List<TestVO> voList = new ArrayList<>();for (int i = 0; i < 100; i++) {TestVO vo = new TestVO();vo.setId(String.valueOf(i));vo.setName(i + "name");voList.add(vo);}try {response.setCharacterEncoding(StandardCharsets.UTF_8.name());response.setContentType(MediaType.APPLICATION_OCTET_STREAM_VALUE);response.setHeader(HttpHeaders.CONTENT_DISPOSITION, String.format("attachment; filename=%s", "test.xlsx"));EasyExcel.write(response.getOutputStream()).sheet(0).doWrite(voList);// OutputStream outputStream = response.getOutputStream();// easyexcel会flush，这里不用// outputStream.flush();log.info("response: {}", JsonUtil.toJson(response));} catch (IOException e) {log.error("下载失败：e:", e);throw new RuntimeException("文件下载失败");}}
}

Upload

当我们调用upload接口分别上传一个15MB和8MB的文件时会出现以下两个报错：

// 上传15MB文件时
org.apache.tomcat.util.http.fileupload.impl.SizeLimitExceededException: the request was rejected because its size (15579467) exceeds the configured maximum (10485760)// 上传8MB的文件时
org.apache.tomcat.util.http.fileupload.impl.FileSizeLimitExceededException: The field file exceeds its maximum permitted size of 1048576 bytes.

原来SpringBoot会对tomcat上传form/multipart的大小做限制，multipart可一次上传多个文件，
默认每个文件大小不超过1MB，文件总大小不超过10MB。参见如下默认配置：

java">@ConfigurationProperties(prefix = "spring.servlet.multipart", ignoreUnknownFields = false)
public class MultipartProperties {// 默认开启private boolean enabled = true;// 默认1MBprivate DataSize maxFileSize = DataSize.ofMegabytes(1);// 默认10MBprivate DataSize maxRequestSize = DataSize.ofMegabytes(10);...
}

可通过配置文件设置修改默认配置:

spring.servlet.multipart.max-request-size=200MB
spring.servlet.multipart.max-file-size=200MB

也可注入自定义配置类Bean:

java">    @Beanpublic MultipartConfigElement multipartConfigElement() {MultipartConfigFactory factory = new MultipartConfigFactory();factory.setMaxFileSize(DataSize.ofMegabytes(200));factory.setMaxRequestSize(DataSize.ofMegabytes(200));return factory.createMultipartConfig();}

但是上面的配置方式无法动态变化，只能修改代码重新上线。此时可继承MultipartConfigElement重写get方法配合配置中心达到动态配置的目的，代码如下：

java">@Component
public class DynamicMultipartConfigElement extends MultipartConfigElement {public DynamicMultipartConfigElement() {super("");}@Overridepublic long getMaxFileSize() {// 从配置中心获取最新配置return super.getMaxFileSize();}@Overridepublic long getMaxRequestSize() {// // 从配置中心获取最新配置return super.getMaxRequestSize();}
}

Download

运行以上下载文件的代码，发现会报错：

com.fasterxml.jackson.databind.JsonMappingException: getOutputStream() has already been called for this response (through reference chain: org.apache.catalina.connector.ResponseFacade["writer"])

看提示发现是在outputSteam#flush完再次通过打印日志的JsonUtil.toJson操作调用了outputStream，而stream只能被消费一次。
通常打印日志是通过AOP注解实现，使用时不会关注注解切面的内部逻辑，如果在切面执行完再调用打印入参的response则会报此错。
当然，打印日志即使报错也不该影响正常的业务逻辑，所以在日志AOP切面处理时要对打日志的逻辑加上try-catch