【Spring】@RequestBody的实现原理
@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：
- 调用
getHandler方法获取请求对应的Handler处理器； - 根据
handler获取对应的适配器，这里用到了适配器模式； - 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：
- 创建
ServletInvocableHandlerMethod； - 调用invokeAndHandle方法继续请求处理；
- 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：
-
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:
-
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：
-
调用
getArgumentResolver方法获取对应的参数处理器resolver； -
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 <T> 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：
-
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；
-
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 <T> 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<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) 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<T>) 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：
-
获取参数的Class类型，从断点中可以看出是[class com.example.demo.model.UserInfo];
-
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<Object> {
@Override
public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException {
// 获取参数的Class类型
JavaType javaType = getJavaType(type, contextClass);
// 解析参数
return readJavaType(javaType, inputMessage);
}
private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) throws IOException {
// 获取ContentType
MediaType contentType = inputMessage.getHeaders().getContentType();
// 获取字符集
Charset charset = getCharset(contentType);
ObjectMapper objectMapper = selectObjectMapper(javaType.getRawClass(), contentType);
Assert.state(objectMapper != null, "No ObjectMapper for " + javaType);
boolean isUnicode = ENCODINGS.containsKey(charset.name());
try {
// ...
if (isUnicode) {
// 获取HTTP请求体中的JSON数据，转为JAVA对象
return objectMapper.readValue(inputMessage.getBody(), javaType);
}
else {
Reader reader = new InputStreamReader(inputMessage.getBody(), charset);
return objectMapper.readValue(reader, javaType);
}
}
// ....
}
}
到这里已经成功从HTTP请求体中的JSON数据，并转为JAVA对象，完成了参数的设置。
Spring版本:5.3.4