@RequestBody注解可以用于POST请求接收请求体中的参数，使用方式如下：

@Controller
public class IndexController {
    
    @PostMapping(value = "/submit", produces = MediaType.APPLICATION_JSON_VALUE)
    public void submit(@RequestBody UserInfo userInfo) {
        System.out.println(userInfo.toString());
    }
}

那么是如何从请求中解析数据设置到对应的参数中呢，接下来就从源码的角度一探究竟。

DispatcherServlet是Spring MVC的核心，它对请求进行调度，收到请求后会进入DispatcherServlet的doDispatch方法中：

1. 调用getHandler方法获取请求对应的Handler处理器；
2. 根据handler获取对应的适配器，这里用到了适配器模式；
3. 调用适配器的handle方法处理请求，它会返回一个ModelAndView对象；

public class DispatcherServlet extends FrameworkServlet {
    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		try {
			ModelAndView mv = null;
			Exception dispatchException = null;
			try {
				// 检查是否有Multipart
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);
				// 根据请求获取对应的处理器
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}
				// 根据handler获取对应的适配器
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
				// ...
				// 处理请求
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
				// ...
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			catch (Throwable err) {
				dispatchException = new NestedServletException("Handler dispatch failed", err);
			}
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		// ...
	}
}

通过POSTMAN模拟请求，在代码中打断点可以看到HandlerAdapter的类型为对RequestMappingHandlerAdapter:

handle方法在其父类AbstractHandlerMethodAdapter中实现，在它的handle方法中，又调用了handleInternal方法处理请求，handleInternal是一个抽象方法，由具体的子类实现：

public abstract class AbstractHandlerMethodAdapter extends WebContentGenerator implements HandlerAdapter, Ordered {
	@Override
	@Nullable
	public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
			throws Exception {
	    // 处理请求
		return handleInternal(request, response, (HandlerMethod) handler);
	}
	@Nullable
	protected abstract ModelAndView handleInternal(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception;
}

所以回到RequestMappingHandlerAdapter的handleInternal方法，里面调用了invokeHandlerMethod方法进行处理：

1. 创建ServletInvocableHandlerMethod；
2. 调用invokeAndHandle方法继续请求处理；
3. 调用getModelAndView方法返回ModelAndView;

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
		implements BeanFactoryAware, InitializingBean {
    @Override
	protected ModelAndView handleInternal(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
		ModelAndView mav;
		checkRequest(request);
		if (this.synchronizeOnSession) {
			HttpSession session = request.getSession(false);
			if (session != null) {
				Object mutex = WebUtils.getSessionMutex(session);
				synchronized (mutex) {
					// 执行请求
					mav = invokeHandlerMethod(request, response, handlerMethod);
				}
			}
			else {
				// 执行请求
				mav = invokeHandlerMethod(request, response, handlerMethod);
			}
		}
		else {
			// 执行请求
			mav = invokeHandlerMethod(request, response, handlerMethod);
		}
		// ...
		return mav;
	}
	@Nullable
	protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
		ServletWebRequest webRequest = new ServletWebRequest(request, response);
		try {
			// ...
            // 创建ServletInvocableHandlerMethod
            ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
			// 调用invokeAndHandle方法处理请求
			invocableMethod.invokeAndHandle(webRequest, mavContainer);
			if (asyncManager.isConcurrentHandlingStarted()) {
				return null;
			}
			// 返回ModelAndView
			return getModelAndView(mavContainer, modelFactory, webRequest);
		}
		finally {
			webRequest.requestCompleted();
		}
	}
}

ServletInvocableHandlerMethod的invokeAndHandle中调用了invokeForRequest方法执行请求，它的实现在其父类InvocableHandlerMethod中：

public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
    public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {
    	// 执行请求
		Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
		setResponseStatus(webRequest);
		// ...
	}
}

invokeForRequest中又调用了getMethodArgumentValues方法获取请求中的参数，处理逻辑如下：

1. 调用getMethodParameters获取方法中的参数，也就是我们的请求处理器方法中的所有参数，上面看到submit只接收了一个UserInfo类型的参数，这里可以从断点中看到parameters中只有一个元素，类型为UserInfo:

   

2. 对获取到方法中的所有参数进行遍历，通过处理器调用resolveArgument方法解析请求中的数据，解析每一个参数对应的值；

public class InvocableHandlerMethod extends HandlerMethod {
	@Nullable
	public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {
		// 获取请求中的参数
		Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
		if (logger.isTraceEnabled()) {
			logger.trace("Arguments: " + Arrays.toString(args));
		}
		return doInvoke(args);
	}
    
    protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {
		// 获取方法的所有参数
		MethodParameter[] parameters = getMethodParameters();
		if (ObjectUtils.isEmpty(parameters)) {
			return EMPTY_ARGS;
		}
		Object[] args = new Object[parameters.length];
		// 对方法中的所有参数进行遍历
		for (int i = 0; i < parameters.length; i++) {
			MethodParameter parameter = parameters[i];
			parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
			// ...
			try {
				// 调用resolveArgument从请求中解析对应的数据
				args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
			}
			// ...
		}
		return args;
	}
}

resolveArgument方法在HandlerMethodArgumentResolverComposite中实现：

1. 调用getArgumentResolver方法获取对应的参数处理器resolver；

2. 调用resolver的resolveArgument方法进行参数解析；

public class HandlerMethodArgumentResolverComposite implements HandlerMethodArgumentResolver {
	@Override
	@Nullable
	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
        // 获取对应的参数处理器
		HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
		if (resolver == null) {
			throw new IllegalArgumentException("Unsupported parameter type [" +
					parameter.getParameterType().getName() + "]. supportsParameter should be called first.");
		}
		// 解析参数
		return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
	}
}

从断点中可以看到此时的resolver是RequestResponseBodyMethodProcessor类型的:

进入到RequestResponseBodyMethodProcessor的resolveArgument方法中，它又调用了readWithMessageConverters方法解析参数，最终会进入到
AbstractMessageConverterMethodArgumentResolve中的readWithMessageConverters方法:

public class RequestResponseBodyMethodProcessor extends AbstractMessageConverterMethodProcessor {
    @Override
	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
		parameter = parameter.nestedIfOptional();
		// 通过转换器进行参数解析
		Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
		String name = Conventions.getVariableNameForParameter(parameter);
		// ...
		return adaptArgumentIfNecessary(arg, parameter);
	}
	@Override
	protected  Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter,
			Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
		HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
		Assert.state(servletRequest != null, "No HttpServletRequest");
		ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);
		// 调用AbstractMessageConverterMethodArgumentResolver中readWithMessageConverters方法读取参数
		Object arg = readWithMessageConverters(inputMessage, parameter, paramType);
		if (arg == null && checkRequired(parameter)) {
			throw new HttpMessageNotReadableException("Required request body is missing: " +
					parameter.getExecutable().toGenericString(), inputMessage);
		}
		return arg;
	}
}

readWithMessageConverters方法处理逻辑如下：

1. 遍历所有HTTP消息转换器，判断是否支持解析当前的请求参数类型；

2. 如果转换器支持解析当前的参数类型并且有消息体内容，调用转换器的read方法进行解析；

public abstract class AbstractMessageConverterMethodArgumentResolver implements HandlerMethodArgumentResolver {
    @Nullable
	protected  Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
		// ...
		try {
			message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
			// 遍历所有的消息转换器
			for (HttpMessageConverter converter : this.messageConverters) {
				Class> converterType = (Class>) converter.getClass();
				GenericHttpMessageConverter genericConverter =
						(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter) converter : null);
				// 判断是否支持当前参数类型的读取
				if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
						(targetClass != null && converter.canRead(targetClass, contentType))) {
					// 如果有消息体
					if (message.hasBody()) {
						HttpInputMessage msgToUse =
								getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
						// 调用read方法进行读取
						body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
								((HttpMessageConverter) converter).read(targetClass, msgToUse));
						body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
					}
					else {
						body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
					}
					break;
				}
			}
		}
		catch (IOException ex) {
			throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);
		}
		// ...
		return body;
	}
}

这里列举一些消息转换器的类型：

对于application/json;charset=UTF-8类型会进入到MappingJackson2HttpMessageConverter，read方法在其父类AbstractJackson2HttpMessageConverter，处理逻辑如下：

1. 获取参数的Class类型，从断点中可以看出是[class com.example.demo.model.UserInfo];

   

2. 调用readJavaType方法解析参数

   （1）获取ContentType，前面可以看到请求接收的类型为application/json;

   （2）获取字符集，这里的字符集为UTF-8；

   （3）创建ObjectMapper对象，并从请求体中读取JSON数据，转为JAVA对象；

public abstract class AbstractJackson2HttpMessageConverter extends AbstractGenericHttpMessageConverter