1 快速上手

1.1 引入依赖

    io.github.dk900912
    easyexcel-spring-boot-starter
    0.0.7

1.2 导入与导出

@RestController
@RequestMapping(path = "/easyexcel")
public class ExcelController {

    @PostMapping(path = "/v1/upload")
    public ResponseEntity upload(
            @RequestExcel(sheets = {
                    @Sheet(index = 0, headClazz = User.class, headRowNumber = 1),
                    @Sheet(index = 1, headClazz = User.class, headRowNumber = 1),
                    @Sheet(index = 2, headClazz = User.class, headRowNumber = 1)
            })
            @Valid List> users) {
        return ResponseEntity.ok("OK");
    }

    @ResponseExcel(
            name="程序猿",
            sheets = {
                    @Sheet(name = "sheet-0", headClazz = User.class),
                    @Sheet(name = "sheet-1", headClazz = User.class),
                    @Sheet(name = "sheet-2", headClazz = User.class)
            },
            suffix = ExcelTypeEnum.XLSX)
    @GetMapping(path = "/v1/export")
    public List> export() {
        List data = Lists.newArrayList();
        for (int i = 0; i < 10000; i++) {
            User user = User.builder().name("暴风赤红" + (i+1))
                    .birth(LocalDate.now()).address("江苏省苏州市科技城昆仑山路58号")
                    .build();
            data.add(user);
        }
        return ImmutableList.of(data, data, data);
    }

    @ResponseExcel(name="templates/程序猿.xlsx", scene = TEMPLATE)
    @GetMapping(path = "/v1/template")
    public void template() {}
}

2 实现原理

一切 Java 程序都是基于 Thread 的，当一个 HTTP 请求到达后，Servlet Container 会从其线程池中捞出一个线程来处理该 HTTP 请求。具体地，该 HTTP 请求首先到达 Servlet Container 的FilterChain中；然后，FilterChain 将该 HTTP 请求委派给DispatcherServlet处理，而 DispatcherServlet 恰恰就是 Spring MVC 的门户。在 Spring MVC 中，所有 HTTP 请求都由 DispatcherServlet 进行路由分发。大致流程下图所示。

image.png

DispatcherServlet 在 HandlerMapping 的帮助下可以快速匹配到最终的 Controller，由于 Controller 大多由@RequestMapping注解标注，那么RequestMappingHandlerMapping最终脱颖而出。RequestMappingHandlerMapping 会将 HTTP 请求映射到一个HandlerExecutionChain实例中，每一个 HandlerExecutionChain 实例的内部维护了HandlerMethod和List。其中，HandlerMethod 实例持有一个Object类型的 bean 变量和java.lang.reflect.Method类型的 method 变量，bean 和 method 这俩成员变量组合起来最终可以确定究竟由哪一个 Controller 中的某一方法来处理当前 HTTP 请求。

重点来了！首先，我们需要一个实现 HandlerMethodArgumentResolver 接口的方法参数解析器，该解析器主要用于解析@RequestExcel注解，以读取 Excel 文档；此外，我们还需要一个实现 HandlerMethodReturnValueHandler 接口的方法返回值解析器，该解析器主要用于解析@ResponseExcel注解，以将目标方法所返回的数据写入到 Excel 文档中；最后，将这两个自定义的解析器分别添加到 RequestMappingHandlerAdapter 中的 argumentResolvers 与 returnValueHandlers 这俩成员变量中。

在 Spring MVC 中，由RequestResponseBodyMethodProcessor负责处理@RequestBody与@ResponseBody 注解。基于这一事实，笔者也没有单独设计两个解析器来分别应对 @RequestExcel 与 @ResponseExcel 注解，而是合二为一。

2.1 @RequestExcel 与 @ResponseExcel 解析器

public class RequestResponseExcelMethodProcessor implements HandlerMethodArgumentResolver,
        HandlerMethodReturnValueHandler {

    private final ResourceLoader resourceLoader;

    public RequestResponseExcelMethodProcessor(ResourceLoader resourceLoader) {
        this.resourceLoader = resourceLoader;
    }

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.hasParameterAnnotation(RequestExcel.class);
    }

    @Override
    public boolean supportsReturnType(MethodParameter returnType) {
        return returnType.hasMethodAnnotation(ResponseExcel.class);
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
                                  NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
        parameter = parameter.nestedIfOptional();
        Object data = readWithMessageConverters(webRequest, parameter);

        validateIfNecessary(data, parameter);

        return data;
    }

    @Override
    public void handleReturnValue(Object returnValue, MethodParameter returnType,
                                  ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
        // There is no need to render view
        mavContainer.setRequestHandled(true);
        writeWithMessageConverters(returnValue, returnType, webRequest);
    }

    // +----------------------------------------------------------------------------+
    // |                            private method for read                         |
    // +----------------------------------------------------------------------------+

    protected  Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter)
            throws IOException, UnsatisfiedMethodSignatureException {
        validateArgParamOrReturnValueType(parameter);

        HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
        Assert.state(servletRequest != null, "No HttpServletRequest");

        return readWithMessageConverters(servletRequest, parameter);
    }

    protected Object readWithMessageConverters(HttpServletRequest servletRequest, MethodParameter parameter)
            throws IOException {
        RequestExcelInfo requestExcelInfo =
                new RequestExcelInfo(parameter.getParameterAnnotation(RequestExcel.class));
        InputStream inputStream;
        if (servletRequest instanceof MultipartRequest) {
            inputStream = ((MultipartRequest) servletRequest)
                    .getMultiFileMap()
                    .values()
                    .stream()
                    .flatMap(Collection::stream)
                    .findFirst()
                    .map(multipartFile -> {
                        try {
                            return multipartFile.getInputStream();
                        } catch (IOException e) {
                            return null;
                        }
                    })
                    .get();
        } else {
            inputStream = servletRequest.getInputStream();
        }

        CollectorReadListener collectorReadListener = new CollectorReadListener();
        try (ExcelReader excelReader = EasyExcel.read(inputStream).build()) {
            List readSheetList = requestExcelInfo.getSheetInfoList()
                    .stream()
                    .map(sheetInfo -> EasyExcel.readSheet(sheetInfo.getIndex())
                            .head(sheetInfo.getHeadClazz())
                            .registerReadListener(collectorReadListener)
                            .build()
                    )
                    .collect(Collectors.toList());
            excelReader.read(readSheetList);
        }

        return collectorReadListener.groupByHeadClazz();
    }

    protected void validateIfNecessary(Object data, MethodParameter parameter) throws ExcelCellContentNotValidException {
        if (parameter.hasParameterAnnotation(Validated.class)
                || parameter.hasParameterAnnotation(Valid.class)) {
            List