springmvc源码解析(三)-----@RequestBody和@ResponseBody原理解析
本文转载自:Spring MVC源码(三) ----- @RequestBody和@ResponseBody原理解析
概述
在SpringMVC的使用时,往往会用到@RequestBody和@ResponseBody两个注解,尤其是处理ajax请求必然要使用@ResponseBody注解。这两个注解对应着Controller方法的参数解析和返回值处理,开始时都是只知其用,不知原理。我们来看个例子。
@RequestMapping("/requestBody") public void requestBody(@RequestBody String body, Writer writer) throws IOException{ writer.write(body); } @RequestMapping(value="/responseBody", produces="application/json") @ResponseBody public Map<String, Object> responseBody(){ Map<String, Object> retMap = new HashMap<>(); retMap.put("param1", "abc"); return retMap; }
第一个requestBody请求,使用@RequestBody将HTTP请求体转换成String类型,第二个responseBody请求,将Map对象转换成json格式输出到HTTP响应中。这两个请求方法没有什么特殊,就是一个在参数前加了@RequestBody注解,一个在方法上加了@ResponseBody注解。而这两个注解是怎么完成HTTP报文信息同Controller方法中对象的转换的呢?
SpringMVC处理请求和响应时,支持多种类型的请求参数和返回类型,而此种功能的实现就需要对HTTP消息体和参数及返回值进行转换,为此SpringMVC提供了大量的转换类,所有转换类都实现了HttpMessageConverter接口。
public interface HttpMessageConverter<T> { // 当前转换器是否能将HTTP报文转换为对象类型 boolean canRead(Class<?> clazz, MediaType mediaType); // 当前转换器是否能将对象类型转换为HTTP报文 boolean canWrite(Class<?> clazz, MediaType mediaType); // 转换器能支持的HTTP媒体类型 List<MediaType> getSupportedMediaTypes(); // 转换HTTP报文为特定类型 T read(Class<? extends T> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException; // 将特定类型对象转换为HTTP报文 void write(T t, MediaType contentType, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException; }
HttpMessageConverter接口定义了5个方法,用于将HTTP请求报文转换为java对象,以及将java对象转换为HTTP响应报文。
对应到SpringMVC的Controller方法,read方法即是读取HTTP请求转换为参数对象,write方法即是将返回值对象转换为HTTP响应报文。SpringMVC定义了两个接口来操作这两个过程:参数解析器HandlerMethodArgumentResolver和返回值处理器HandlerMethodReturnValueHandler。
// 参数解析器接口 public interface HandlerMethodArgumentResolver { // 解析器是否支持方法参数 boolean supportsParameter(MethodParameter parameter); // 解析HTTP报文中对应的方法参数 Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception; } // 返回值处理器接口 public interface HandlerMethodReturnValueHandler { // 处理器是否支持返回值类型 boolean supportsReturnType(MethodParameter returnType); // 将返回值解析为HTTP响应报文 void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception; }
参数解析器和返回值处理器在底层处理时,都是通过HttpMessageConverter进行转换。流程如下:
SpringMVC为@RequestBody和@ResponseBody两个注解实现了统一处理类RequestResponseBodyMethodProcessor,实现了HandlerMethodArgumentResolver和HandlerMethodReturnValueHandler两个接口。
由上一篇文章我们可以知道,Controller方法被封装成ServletInvocableHandlerMethod类,并且由invokeAndHandle方法完成请求处理。
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception { // 执行请求 Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs); // 返回值处理 try { this.returnValueHandlers.handleReturnValue( returnValue, getReturnValueType(returnValue), mavContainer, webRequest); } catch (Exception ex) {throw ex; } } public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception { // 参数解析 Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs); // invoke Controller方法 Object returnValue = doInvoke(args); return returnValue; }
在invoke Controller方法的前后分别执行了方法参数的解析和返回值的处理,我们分别来看。
参数解析
private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception { MethodParameter[] parameters = getMethodParameters(); Object[] args = new Object[parameters.length]; // 遍历所有参数,逐个解析 for (int i = 0; i < parameters.length; i++) { MethodParameter parameter = parameters[i]; parameter.initParameterNameDiscovery(this.parameterNameDiscoverer); args[i] = resolveProvidedArgument(parameter, providedArgs); if (args[i] != null) { continue; } // 参数解析器解析HTTP报文到参数 if (this.argumentResolvers.supportsParameter(parameter)) { args[i] = this.argumentResolvers.resolveArgument( parameter, mavContainer, request, this.dataBinderFactory); continue; } } return args; }
getMethodArgumentValues方法中的argumentResolvers就是多个HandlerMethodArgumentResolver的集合体,supportsParameter方法寻找参数合适的解析器,resolveArgument调用具体解析器的resolveArgument方法执行。
我们从RequestResponseBodyMethodProcessor看看@RequestBody的解析过程。RequestResponseBodyMethodProcessor的supportsParameter定义了它支持的参数类型,即必须有@RequestBody注解。
public boolean supportsParameter(MethodParameter parameter) { return parameter.hasParameterAnnotation(RequestBody.class); }
再来看resolveArgument方法
public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception { parameter = parameter.nestedIfOptional(); // 通过HttpMessageConverter读取HTTP报文 Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType()); String name = Conventions.getVariableNameForParameter(parameter); WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name); if (arg != null) { validateIfApplicable(binder, parameter); if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) { throw new MethodArgumentNotValidException(parameter, binder.getBindingResult()); } } mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult()); return adaptArgumentIfNecessary(arg, parameter); }
具体实现由HttpMessageConverter来完成
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter, Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException { .... try { inputMessage = new EmptyBodyCheckingHttpInputMessage(inputMessage); for (HttpMessageConverter<?> converter : this.messageConverters) { Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass(); .... // 判断转换器是否支持参数类型 if (converter.canRead(targetClass, contentType)) { if (inputMessage.getBody() != null) { inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType); // read方法执行HTTP报文到参数的转换 body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage); body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType); } else { body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType); } break; } ... } } catch (IOException ex) { throw new HttpMessageNotReadableException("I/O error while reading input message", ex); } .... return body; }
代码部分省略了,关键部分即是遍历所有的HttpMessageConverter,通过canRead方法判断转换器是否支持对参数的转换,然后执行read方法完成转换。
返回值处理
完成Controller方法的调用后,在ServletInvocableHandlerMethod的invokeAndHandle中,使用返回值处理器对返回值进行转换。
this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
这里的returnValueHandlers也是HandlerMethodReturnValueHandler的集合体HandlerMethodReturnValueHandlerComposite
public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception { // 选择合适的HandlerMethodReturnValueHandler,如果没有用@ResposeBody注解和用了注解其返回值处理器肯定不同 HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType); if (handler == null) { throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName()); } // 执行返回值处理 handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest); }
selectHandler方法遍历所有HandlerMethodReturnValueHandler,调用其supportsReturnType方法选择合适的HandlerMethodReturnValueHandler,然后调用其handleReturnValue方法完成处理。
这里还是以RequestResponseBodyMethodProcessor来分析下@ResponseBody的处理,它的具体实现在AbstractMessageConverterMethodProcessor抽象基类中。
public boolean supportsReturnType(MethodParameter returnType) { return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) || returnType.hasMethodAnnotation(ResponseBody.class)); }
RequestResponseBodyMethodProcessor要求方法上有@ResponseBody注解或者方法所在的Controller类上有@ResponseBody的注解。这就是常常用@RestController注解代替@Controller注解的原因,因为@RestController注解自带@ResponseBody。
handleReturnValue方法实际也是调用HttpMessageConverter来完成转换处理
public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException { mavContainer.setRequestHandled(true); ServletServerHttpRequest inputMessage = createInputMessage(webRequest); ServletServerHttpResponse outputMessage = createOutputMessage(webRequest); // 调用HttpMessageConverter执行 writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage); } protected <T> void writeWithMessageConverters(T value, MethodParameter returnType, ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage) throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException { .... if (selectedMediaType != null) { selectedMediaType = selectedMediaType.removeQualityValue(); for (HttpMessageConverter<?> messageConverter : this.messageConverters) { // 判断是否支持返回值类型,返回值类型很有可能不同,如String,Map,List等 if (messageConverter.canWrite(valueType, selectedMediaType)) { outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType, (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(), inputMessage, outputMessage); if (outputValue != null) { addContentDispositionHeader(inputMessage, outputMessage); // 执行返回值转换 ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage); ... } return; } } } .... }
使用canWrite方法选择合适的HttpMessageConverter,然后调用write方法完成转换。
我们看看传入的参数 ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
protected ServletServerHttpResponse createOutputMessage(NativeWebRequest webRequest) { //获取HttpServletResponse HttpServletResponse response = (HttpServletResponse)webRequest.getNativeResponse(HttpServletResponse.class); Assert.state(response != null, "No HttpServletResponse"); return new ServletServerHttpResponse(response); } public class ServletServerHttpResponse implements ServerHttpResponse { private final HttpServletResponse servletResponse; private final HttpHeaders headers; private boolean headersWritten = false; private boolean bodyUsed = false; public ServletServerHttpResponse(HttpServletResponse servletResponse) { Assert.notNull(servletResponse, "HttpServletResponse must not be null"); //将获取的HttpServletResponse作为ServletServerHttpResponse的属性值 this.servletResponse = servletResponse; this.headers = new ServletServerHttpResponse.ServletResponseHttpHeaders(); } } public interface ServletResponse { String getCharacterEncoding(); String getContentType(); //ServletResponse有一个输出流对象,保存需要相应客户端的字节流 ServletOutputStream getOutputStream() throws IOException; PrintWriter getWriter() throws IOException; void setCharacterEncoding(String var1); void setContentLength(int var1); void setContentLengthLong(long var1); void setContentType(String var1); void setBufferSize(int var1); int getBufferSize(); void flushBuffer() throws IOException; void resetBuffer(); boolean isCommitted(); void reset(); void setLocale(Locale var1); Locale getLocale(); }
我们具体看看 ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
protected void writeInternal(Object obj, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException { HttpHeaders headers = outputMessage.getHeaders(); //创建一个数组字节流缓冲对象 ByteArrayOutputStream outnew = new ByteArrayOutputStream(); //将obj对象转换成JSON并写入ByteArrayOutputStream中 int len = JSON.writeJSONString(outnew, this.fastJsonConfig.getCharset(), obj, this.fastJsonConfig.getSerializeConfig(), this.fastJsonConfig.getSerializeFilters(), this.fastJsonConfig.getDateFormat(), JSON.DEFAULT_GENERATE_FEATURE, this.fastJsonConfig.getSerializerFeatures()); headers.setContentLength((long)len); //获取ServletResponse的输出流对象 OutputStream out = outputMessage.getBody(); //将转换后的outnew写入ServletResponse的输出流对象,这样就可以给客户端响应数据了 outnew.writeTo(out); outnew.close(); } public OutputStream getBody() throws IOException { this.bodyUsed = true; this.writeHeaders(); //获取ServletResponse的输出流对象 //ServletOutputStream getOutputStream() throws IOException; return this.servletResponse.getOutputStream(); }
最后我们看看JSON是怎么将obj对象转换成JSON对象的流
就是做一些循环拼接。
至此我们基本走完了一个HTTP请求报文经过处理后到HTTP响应报文的转换过程。现在你可能有个疑惑,SpringMVC我们都是开箱即用,这些参数解析器和返回值处理器在哪里定义的呢?在核心的HandlerAdapter实现类RequestMappingHandlerAdapter的初始化方法中定义的。
而在RequestMappingHandlerAdapter构造时,也同时初始化了众多的HttpMessageConverter,以支持多样的转换需求。
WebMvcConfigurationSupport.java protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) { StringHttpMessageConverter stringConverter = new StringHttpMessageConverter(); stringConverter.setWriteAcceptCharset(false); messageConverters.add(new ByteArrayHttpMessageConverter()); messageConverters.add(stringConverter); messageConverters.add(new ResourceHttpMessageConverter()); messageConverters.add(new SourceHttpMessageConverter<Source>()); messageConverters.add(new AllEncompassingFormHttpMessageConverter()); if (romePresent) { messageConverters.add(new AtomFeedHttpMessageConverter()); messageConverters.add(new RssChannelHttpMessageConverter()); } if (jackson2XmlPresent) { ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.xml().applicationContext(this.applicationContext).build(); messageConverters.add(new MappingJackson2XmlHttpMessageConverter(objectMapper)); } else if (jaxb2Present) { messageConverters.add(new Jaxb2RootElementHttpMessageConverter()); } if (jackson2Present) { ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.json().applicationContext(this.applicationContext).build(); messageConverters.add(new MappingJackson2HttpMessageConverter(objectMapper)); } else if (gsonPresent) { messageConverters.add(new GsonHttpMessageConverter()); } }
对于json或xml的转换方式,只要引入了jackson的依赖,即可自动发现,并注册相关的转换器。
<dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-databind</artifactId> <version>2.9.0</version> </dependency> <dependency> <groupId>com.fasterxml.jackson.dataformat</groupId> <artifactId>jackson-dataformat-xml</artifactId> <version>2.9.0</version> </dependency>
posted on 2020-05-21 18:21 阿里-马云的学习笔记 阅读(933) 评论(0) 编辑 收藏 举报