详解SpringMVC中Controller的方法中参数的工作原理

Spring MVC中Controller的处理方法的参数可以是Integer，String，自定义对象，ServletRequest，ServletResponse，ModelAndView等等，非常灵活。本文将分析SpringMVC是如何对这些参数进行处理的，使读者能够处理自定义的一些参数。

先来看几个示例：

@Controller
public class UserController {

    @RequestMapping(value = "/user/testRequestBody")
    @ResponseBody
    public User testRequestBody(@RequestBody User user) {
        return user;
    }

    @RequestMapping(value = "/user/testCustomObj")
    @ResponseBody
    public User testCustomObj(User user) {
        return user;
    }

    @RequestMapping(value = "/user/testRequestParam")
    @ResponseBody
    public User testRequestParam(@RequestParam User user) {
        return user;
    }

    @RequestMapping("/user/testDate")
    @ResponseBody
    public Date testDate(Date date) {
        return date;
    }
}

首先这是一个Controller，有4个方法。他们对应的参数分别是带有@RequestBody的自定义对象、自定义对象、带有@RequestParam的自定义对象、日期对象。

接下来我们一个一个方法进行访问看对应的现象是如何的。

首先第一个testRequestBody：

第二个testCustomObj：

第三个testRequestParam：

第四个testDate：

为何User参数会被解析，带有@RequestParam的User参数不会被解析，甚至报错？

为何日期类型不能被解析？

SpringMVC到底是如何处理这些方法的参数的？

@RequestBody、@RequestParam这两个注解有什么区别？

带着这几个问题。我们开始进行分析。

在分析源码之前，首先让我们来看下SpringMVC中两个重要的接口。

两个接口分别对应请求方法参数的处理、响应返回值的处理，分别是HandlerMethodArgumentResolver和HandlerMethodReturnValueHandler

/**
 * Strategy interface for resolving method parameters into argument values in
 * the context of a given request.
 */
public interface HandlerMethodArgumentResolver {

    /**
     * Whether the given {@linkplain MethodParameter method parameter} is
     * supported by this resolver.
     */
    boolean supportsParameter(MethodParameter parameter);

    /**
     * Resolves a method parameter into an argument value from a given request.
     */
    Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;

}

/**
 * Strategy interface to handle the value returned from the invocation of a
 * handler method .
 */
public interface HandlerMethodReturnValueHandler {

    /**
     * Whether the given {@linkplain MethodParameter method return type} is
     * supported by this handler.
     */
    boolean supportsReturnType(MethodParameter returnType);

    /**
     * Handle the given return value by adding attributes to the model and
     * setting a view or setting the
     * {@link ModelAndViewContainer#setRequestHandled} flag to {@code true}
     * to indicate the response has been handled directly.
     */
    void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;

}

SpringMVC处理请求大致是这样的：

首先被DispatcherServlet截获，DispatcherServlet通过handlerMapping获得HandlerExecutionChain，然后请求HandlerAdapter。

HandlerAdapter在内部对于每个请求，都会实例化一个ServletInvocableHandlerMethod进行处理，ServletInvocableHandlerMethod在进行处理的时候，会分两部分别对请求跟响应进行处理。

之后HandlerAdapter得到ModelAndView，然后做相应的处理。

本文将重点介绍ServletInvocableHandlerMethod对请求以及响应的处理。

/**
 * Invokes the method and handles the return value through one of the
 * configured {@link HandlerMethodReturnValueHandler}s.
 */
public void invokeAndHandle(ServletWebRequest webRequest,
        ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
    /**
    * Invoke the method after resolving its argument values in the context of the given request.
    * Argument values are commonly resolved through HandlerMethodArgumentResolvers.
    */
    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    setResponseStatus(webRequest);

    if (returnValue == null) {
        if (isRequestNotModified(webRequest) || hasResponseStatus() || mavContainer.isRequestHandled()) {
            mavContainer.setRequestHandled(true);
            return;
        }
    }
    else if (StringUtils.hasText(this.responseReason)) {
        mavContainer.setRequestHandled(true);
        return;
    }

    mavContainer.setRequestHandled(false);
    try {
        /**
        * Handle the given return value by adding attributes to the model and
        * setting a view or setting the ModelAndViewContainer#setRequestHandled flag to true
        * to indicate the response has been handled directly.
         */
        this.returnValueHandlers.handleReturnValue(
                returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    }
    catch (Exception ex) {
        if (logger.isTraceEnabled()) {
            logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
        }
        throw ex;
    }
}

1. 处理请求的时候，会根据ServletInvocableHandlerMethod的属性argumentResolvers（这个属性是它的父类InvocableHandlerMethod中定义的）进行处理，其中argumentResolvers属性是一个HandlerMethodArgumentResolverComposite类(这里使用了组合模式的一种变形)，这个类是实现了HandlerMethodArgumentResolver接口的类，里面有各种实现了HandlerMethodArgumentResolver的List集合。

public class HandlerMethodArgumentResolverComposite implements HandlerMethodArgumentResolver {

    protected final Log logger = LogFactory.getLog(getClass());

    private final List<HandlerMethodArgumentResolver> argumentResolvers =
            new LinkedList<HandlerMethodArgumentResolver>();

    private final Map<MethodParameter, HandlerMethodArgumentResolver> argumentResolverCache =
            new ConcurrentHashMap<MethodParameter, HandlerMethodArgumentResolver>(256);

}

2. 处理响应的时候，会根据ServletInvocableHandlerMethod的属性returnValueHandlers(自身属性)进行处理，returnValueHandlers属性是一个HandlerMethodReturnValueHandlerComposite类(这里使用了组合模式的一种变形)，这个类是实现了HandlerMethodReturnValueHandler接口的类，里面有各种实现了HandlerMethodReturnValueHandler的List集合。

public class HandlerMethodReturnValueHandlerComposite implements HandlerMethodReturnValueHandler {

    protected final Log logger = LogFactory.getLog(getClass());

    private final List<HandlerMethodReturnValueHandler> returnValueHandlers =
        new ArrayList<HandlerMethodReturnValueHandler>();
}

ServletInvocableHandlerMethod的returnValueHandlers和argumentResolvers这两个属性都是在ServletInvocableHandlerMethod进行实例化的时候被赋值的(使用RequestMappingHandlerAdapter的属性进行赋值)。

private ServletInvocableHandlerMethod createRequestMappingMethod(
        HandlerMethod handlerMethod, WebDataBinderFactory binderFactory) {
    ServletInvocableHandlerMethod requestMethod;
    requestMethod = new ServletInvocableHandlerMethod(handlerMethod);
    requestMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
    requestMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
    requestMethod.setDataBinderFactory(binderFactory);
    requestMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
    return requestMethod;
}

RequestMappingHandlerAdapter的argumentResolvers和returnValueHandlers这两个属性是在RequestMappingHandlerAdapter进行实例化的时候被Spring容器注入的。

@Override
public void afterPropertiesSet() {
    // Do this first, it may add ResponseBody advice beans
    initControllerAdviceCache();
    if (this.argumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
        this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.initBinderArgumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
        this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.returnValueHandlers == null) {
        List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
        this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
    }
}

其中默认的ArgumentResolvers:

/**
 * Return the list of argument resolvers to use including built-in resolvers
 * and custom resolvers provided via {@link #setCustomArgumentResolvers}.
 */
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
    List<HandlerMethodArgumentResolver> resolvers = new ArrayList<HandlerMethodArgumentResolver>();
    // Annotation-based argument resolution
    resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
    resolvers.add(new RequestParamMapMethodArgumentResolver());
    resolvers.add(new PathVariableMethodArgumentResolver());
    resolvers.add(new PathVariableMapMethodArgumentResolver());
    resolvers.add(new MatrixVariableMethodArgumentResolver());
    resolvers.add(new MatrixVariableMapMethodArgumentResolver());
    resolvers.add(new ServletModelAttributeMethodProcessor(false));
    resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters()));
    resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters()));
    resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
    resolvers.add(new RequestHeaderMapMethodArgumentResolver());
    resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
    resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
    // Type-based argument resolution
    resolvers.add(new ServletRequestMethodArgumentResolver());
    resolvers.add(new ServletResponseMethodArgumentResolver());
    resolvers.add(new HttpEntityMethodProcessor(getMessageConverters()));
    resolvers.add(new RedirectAttributesMethodArgumentResolver());
    resolvers.add(new ModelMethodProcessor());
    resolvers.add(new MapMethodProcessor());
    resolvers.add(new ErrorsMethodArgumentResolver());
    resolvers.add(new SessionStatusMethodArgumentResolver());
    resolvers.add(new UriComponentsBuilderMethodArgumentResolver());
    // Custom arguments
    if (getCustomArgumentResolvers() != null) {
        resolvers.addAll(getCustomArgumentResolvers());
    }
    // Catch-all
    resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
    resolvers.add(new ServletModelAttributeMethodProcessor(true));
    return resolvers;
}

默认的returnValueHandlers：

/**
 * Return the list of return value handlers to use including built-in and
 * custom handlers provided via {@link #setReturnValueHandlers}.
 */
private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
    List<HandlerMethodReturnValueHandler> handlers = new ArrayList<HandlerMethodReturnValueHandler>();
    // Single-purpose return value types
    handlers.add(new ModelAndViewMethodReturnValueHandler());
    handlers.add(new ModelMethodProcessor());
    handlers.add(new ViewMethodReturnValueHandler());
    handlers.add(new HttpEntityMethodProcessor(
            getMessageConverters(), this.contentNegotiationManager, this.responseBodyAdvice));
    handlers.add(new HttpHeadersReturnValueHandler());
    handlers.add(new CallableMethodReturnValueHandler());
    handlers.add(new DeferredResultMethodReturnValueHandler());
    handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));
    handlers.add(new ListenableFutureReturnValueHandler());
    // Annotation-based return value types
    handlers.add(new ModelAttributeMethodProcessor(false));
    handlers.add(new RequestResponseBodyMethodProcessor(
            getMessageConverters(), this.contentNegotiationManager, this.responseBodyAdvice));
    // Multi-purpose return value types
    handlers.add(new ViewNameMethodReturnValueHandler());
    handlers.add(new MapMethodProcessor());
    // Custom return value types
    if (getCustomReturnValueHandlers() != null) {
        handlers.addAll(getCustomReturnValueHandlers());
    }
    // Catch-all
    if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
        handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
    }
    else {
        handlers.add(new ModelAttributeMethodProcessor(true));
    }
    return handlers;
}

使用@ResponseBody注解的话最终返回值会被RequestResponseBodyMethodProcessor这个HandlerMethodReturnValueHandler实现类处理。

我们通过源码发现，RequestResponseBodyMethodProcessor这个类其实同时实现了HandlerMethodReturnValueHandler和HandlerMethodArgumentResolver这两个接口。

public class RequestResponseBodyMethodProcessor extends AbstractMessageConverterMethodProcessor {

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

    @Override
    public boolean supportsReturnType(MethodParameter returnType) {
        return (AnnotationUtils.findAnnotation(returnType.getContainingClass(), ResponseBody.class) != null ||
                returnType.getMethodAnnotation(ResponseBody.class) != null);
    }
}

RequestResponseBodyMethodProcessor支持的请求类型是Controller方法参数中带有@RequestBody注解，支持的响应类型是Controller方法带有@ResponseBody注解。

@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
        throws IOException, HttpMediaTypeNotAcceptableException {
    mavContainer.setRequestHandled(true);
    // Try even with null return value. ResponseBodyAdvice could get involved.
    writeWithMessageConverters(returnValue, returnType, webRequest);
}

RequestResponseBodyMethodProcessor响应的具体处理是使用消息转换器。

@Override
public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
        NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
    Object argument = readWithMessageConverters(webRequest, parameter, parameter.getGenericParameterType());
    String name = Conventions.getVariableNameForParameter(parameter);
    WebDataBinder binder = binderFactory.createBinder(webRequest, argument, name);
    if (argument != null) {
        validate(binder, parameter);
    }
    mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
    return argument;
}

处理请求的时候使用内部的readWithMessageConverters方法。

@Override
protected <T> Object readWithMessageConverters(NativeWebRequest webRequest,
        MethodParameter methodParam,  Type paramType) throws IOException, HttpMediaTypeNotSupportedException {
    final HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
    HttpInputMessage inputMessage = new ServletServerHttpRequest(servletRequest);
    InputStream inputStream = inputMessage.getBody();
    if (inputStream == null) {
        return handleEmptyBody(methodParam);
    }
    else if (inputStream.markSupported()) {
        inputStream.mark(1);
        if (inputStream.read() == -1) {
            return handleEmptyBody(methodParam);
        }
        inputStream.reset();
    }
    else {
        final PushbackInputStream pushbackInputStream = new PushbackInputStream(inputStream);
        int b = pushbackInputStream.read();
        if (b == -1) {
            return handleEmptyBody(methodParam);
        }
        else {
            pushbackInputStream.unread(b);
        }
        inputMessage = new ServletServerHttpRequest(servletRequest) {
            @Override
            public InputStream getBody() throws IOException {
                // Form POST should not get here
                return pushbackInputStream;
            }
        };
    }
    return super.readWithMessageConverters(inputMessage, methodParam, paramType);
}
private Object handleEmptyBody(MethodParameter param) {
    if (param.getParameterAnnotation(RequestBody.class).required()) {
        throw new HttpMessageNotReadableException("Required request body content is missing: " + param);
    }
    return null;
}

然后会执行父类(AbstractMessageConverterMethodArgumentResolver)的readWithMessageConverters方法。

/**
 * Create the method argument value of the expected parameter type by reading
 * from the given HttpInputMessage.*/
@SuppressWarnings("unchecked")
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage,
        MethodParameter methodParam, Type targetType) throws IOException, HttpMediaTypeNotSupportedException {
    MediaType contentType;
    try {
        contentType = inputMessage.getHeaders().getContentType();
    }
    catch (InvalidMediaTypeException ex) {
        throw new HttpMediaTypeNotSupportedException(ex.getMessage());
    }
    if (contentType == null) {
        contentType = MediaType.APPLICATION_OCTET_STREAM;
    }
    Class<?> contextClass = methodParam.getContainingClass();
    for (HttpMessageConverter<?> converter : this.messageConverters) {
        if (converter instanceof GenericHttpMessageConverter) {
            GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;
            if (genericConverter.canRead(targetType, contextClass, contentType)) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Reading [" + targetType + "] as \"" +
                            contentType + "\" using [" + converter + "]");
                }
                return genericConverter.read(targetType, contextClass, inputMessage);
            }
        }
        Class<T> targetClass = (Class<T>)
                ResolvableType.forMethodParameter(methodParam, targetType).resolve(Object.class);
        if (converter.canRead(targetClass, contentType)) {
            if (logger.isDebugEnabled()) {
                logger.debug("Reading [" + targetClass.getName() + "] as \"" +
                        contentType + "\" using [" + converter + "]");
            }
            return ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);
        }
    }
    throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);
}

下面来我们来看看常用的HandlerMethodArgumentResolver实现类(本文粗略讲下，有兴趣的读者可自行研究)。

1. RequestParamMethodArgumentResolver

支持带有@RequestParam注解的参数或带有MultipartFile类型的参数

2. RequestParamMapMethodArgumentResolver

支持带有@RequestParam注解的参数 && @RequestParam注解的属性value存在 && 参数类型是实现Map接口的属性

3. PathVariableMethodArgumentResolver

支持带有@PathVariable注解的参数 且如果参数实现了Map接口，@PathVariable注解需带有value属性

4. MatrixVariableMethodArgumentResolver

支持带有@MatrixVariable注解的参数 且如果参数实现了Map接口，@MatrixVariable注解需带有value属性

5. RequestResponseBodyMethodProcessor

本文已分析过

6. ServletRequestMethodArgumentResolver

参数类型是实现或继承或是WebRequest、ServletRequest、MultipartRequest、HttpSession、Principal、Locale、TimeZone、InputStream、Reader、HttpMethod这些类。

（这就是为何我们在Controller中的方法里添加一个HttpServletRequest参数，Spring会为我们自动获得HttpServletRequest对象的原因）

7. ServletResponseMethodArgumentResolver

参数类型是实现或继承或是ServletResponse、OutputStream、Writer这些类

8. RedirectAttributesMethodArgumentResolver

参数是实现了RedirectAttributes接口的类

9. HttpEntityMethodProcessor

参数类型是HttpEntity

从名字我们也看的出来， 以Resolver结尾的是实现了HandlerMethodArgumentResolver接口的类，以Processor结尾的是实现了HandlerMethodArgumentResolver和HandlerMethodReturnValueHandler的类。

下面来我们来看看常用的HandlerMethodReturnValueHandler实现类。

1. ModelAndViewMethodReturnValueHandler

返回值类型是ModelAndView或其子类

2. ModelMethodProcessor

返回值类型是Model或其子类

3. ViewMethodReturnValueHandler

返回值类型是View或其子类

4. HttpHeadersReturnValueHandler

返回值类型是HttpHeaders或其子类

5. ModelAttributeMethodProcessor

返回值有@ModelAttribute注解

6. ViewNameMethodReturnValueHandler

返回值是void或String

其余没讲过的读者可自行查看源码。

下面开始解释为何本文开头出现那些现象的原因：

1. 第一个方法testRequestBody以及地址 http://localhost:8080/user/testRequestBody?name=zhangsan&age=10

这个方法的参数使用了@RequestBody，被RequestResponseBodyMethodProcessor进行处理。之后根据http请求头部的contentType然后选择合适的消息转换器进行读取。

很明显，我们的消息转换器只有默认的那些跟部分json以及xml转换器，且传递的参数name=1&age=3，传递的头部中没有content-type，默认使用了application/octet-stream，因此触发了HttpMediaTypeNotSupportedException异常

解放方案： 我们将传递数据改成json，同时http请求的Content-Type改成application/json即可(post请求？)。

2. testCustomObj方法以及地址 http://localhost:8080/user/testCustomObj?name=zhangsan&age=10

这个请求会找到ServletModelAttributeMethodProcessor这个resolver。默认的resolver中有两个ServletModelAttributeMethodProcessor，只不过实例化的时候属性annotationNotRequired一个为true，1个为false。这个ServletModelAttributeMethodProcessor处理参数支持@ModelAttribute注解，annotationNotRequired属性为true的话，参数不是简单类型就通过，因此选择了ServletModelAttributeMethodProcessor，最终通过DataBinder实例化User对象，并写入对应的属性。

3 testRequestParam方法以及地址 http://localhost:8080/user/testRequestParam?name=zhangsan&age=10

这个请求会找到RequestParamMethodArgumentResolver(使用了@RequestParam注解)。RequestParamMethodArgumentResolver在处理参数的时候使用request.getParameter(参数名)即request.getParameter("user")得到，很明显我们的参数传的是name=zhangsan&age=10。因此得到null，RequestParamMethodArgumentResolver处理missing value会触发MissingServletRequestParameterException异常。

解决方案：去掉@RequestParam注解，让ServletModelAttributeMethodProcessor来处理。

4. testDate方法以及地址 http://localhost:8080/user/testDate?date=2014-05-15

这个请求会找到RequestParamMethodArgumentResolver。因为这个方法与第二个方法一样，有两个RequestParamMethodArgumentResolver，属性useDefaultResolution不同。RequestParamMethodArgumentResolver支持简单类型，ServletModelAttributeMethodProcessor是支持非简单类型。最终步骤跟第三个方法一样，我们的参数名是date，于是通过request.getParameter("date")找到date字符串(这里参数名如果不是date，那么最终页面是空白的，因为没有@RequestParam注解，参数不是必须的，RequestParamMethodArgumentResolver处理null值返回null)。最后通过DataBinder找到合适的属性编辑器进行类型转换。最终找到java.util.Date对象的构造函数 public Date(String s)，由于我们传递的格式不是标准的UTC时间格式，因此最终触发了IllegalArgumentException异常。

Ref：

https://www.cnblogs.com/fangjian0423/p/springMVC-request-param-analysis.html