springmvc源码解析(四),@ResponseBody返回json

r囧r小猫 2022-11-15 01:21 198阅读 0赞

### 文章目录 ###

*   *  处理返回结果
     *   *  HandlerMethodReturnValueHandler
         *  RequestResponseBodyMethodProcessor的supports方法
         *  RequestResponseBodyMethodProcessor的resolveArgument方法(解析请求参数)
         *   *  readWithMessageConverters
         *  RequestResponseBodyMethodProcessor的handleReturnValue方法(解析返回参数)
         *   *  writeWithMessageConverters跟上面差不多

## 处理返回结果 ##

前面说到,通过反射调用方法返回参数,参数都是使用Object接收的:  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 1]

### HandlerMethodReturnValueHandler ###

对于返回参数的处理,跟请求参数差不多,返回参数的处理器都实现了HandlerMethodReturnValueHandler 接口,通过supportsReturnType来进行适配,handleReturnValue方法进行处理.

public interface HandlerMethodReturnValueHandler { 
    
    	boolean supportsReturnType(MethodParameter returnType);
    
    
    	void handleReturnValue(Object returnValue, MethodParameter returnType,
    			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
    
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 2]  
现在使用的比较多的就是通过注解@ResponseBody返回json,对于json的返回,springmvc是RequestResponseBodyMethodProcessor类来对返回参数进行处理的.这个类叫RequestResponse,就是表示如果请求参数也是json也是根据这个类来进行处理的.  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 3]  
我们先看看RequestResponseBodyMethodProcessor的类图:  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 4]

### RequestResponseBodyMethodProcessor的supports方法 ###

supports方法就是判断是否有RequestBody,ResponseBody注解  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 5]

### RequestResponseBodyMethodProcessor的resolveArgument方法(解析请求参数) ###

@Override
    	public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
    			NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception { 
    		//判断入参是不是Optional类型，是则返回嵌套类型
    		parameter = parameter.nestedIfOptional();
    		//读取入参(主要)
    		Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
    		//获取参数类型的短名称
    		String name = Conventions.getVariableNameForParameter(parameter);
    
    		//数据绑定器
    		WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
    		if (arg != null) { 
    		  //判断是否使用了validate注解
    			validateIfApplicable(binder, parameter);
    			if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) { 
    			  // 是否有错误,有错误抛出异常
    				throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());
    			}
    		}
    		mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
    
    		//处理Optional类型入参的情况后返回
    		return adaptArgumentIfNecessary(arg, parameter);
    	}

#### readWithMessageConverters ####

@Override
    	protected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter,
    			Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException { 
    
    		HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
    		ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);
    // 主要
    		Object arg = readWithMessageConverters(inputMessage, parameter, paramType);
    		if (arg == null) { 
    			if (checkRequired(parameter)) { 
    				throw new HttpMessageNotReadableException("Required request body is missing: " +
    						parameter.getMethod().toGenericString());
    			}
    		}
    		return arg;
    	}
    
    
    	protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
    			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException { 
    
    		MediaType contentType;
    		boolean noContentType = false;
    		try { 
    			contentType = inputMessage.getHeaders().getContentType();
    		}
    		catch (InvalidMediaTypeException ex) { 
    			throw new HttpMediaTypeNotSupportedException(ex.getMessage());
    		}
    		if (contentType == null) { 
    			noContentType = true;
    			contentType = MediaType.APPLICATION_OCTET_STREAM;
    		}
    
    		// 获取入参类
    		Class<?> contextClass = (parameter != null ? parameter.getContainingClass() : null);
    		// 获取入参的类型
    		Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);
    		if (targetClass == null) { 
    			ResolvableType resolvableType = (parameter != null ?
    					ResolvableType.forMethodParameter(parameter) : ResolvableType.forType(targetType));
    			targetClass = (Class<T>) resolvableType.resolve();
    		}
    
    		HttpMethod httpMethod = ((HttpRequest) inputMessage).getMethod();
    		Object body = NO_VALUE;
    
    		try { 
    			inputMessage = new EmptyBodyCheckingHttpInputMessage(inputMessage);
    
    			for (HttpMessageConverter<?> converter : this.messageConverters) { 
    				Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
    				if (converter instanceof GenericHttpMessageConverter) { 
    					GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;
    					if (genericConverter.canRead(targetType, contextClass, contentType)) { 
    						if (logger.isDebugEnabled()) { 
    							logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");
    						}
    						if (inputMessage.getBody() != null) { 
    						  //循环调用ControllerAdvice切面
    							inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);
    							//调用转换器的read方法
    							body = genericConverter.read(targetType, contextClass, inputMessage);
    							//循环调用ControllerAdvice切面
    							body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType);
    						}
    						else { 
    							body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType);
    						}
    						break;
    					}
    				}
    				else if (targetClass != null) { 
    					if (converter.canRead(targetClass, contentType)) { 
    						if (logger.isDebugEnabled()) { 
    							logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");
    						}
    						if (inputMessage.getBody() != null) { 
    							inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);
    							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);
    		}
    
    		if (body == NO_VALUE) { 
    			if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||
    					(noContentType && inputMessage.getBody() == null)) { 
    				return null;
    			}
    			throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);
    		}
    
    		return body;
    	}

调用转换器的read方法,就读取到对应的对象了,转换器,一般使用默认的MappingJackson2HttpMessageConverter转换器就够了,具体的转换可以自己点到转换器里面去看,也比较简单了;  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 6]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 7]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4MzY2MDYz_size_16_color_FFFFFF_t_70 8]

### RequestResponseBodyMethodProcessor的handleReturnValue方法(解析返回参数) ###

@Override
    	public void handleReturnValue(Object returnValue, MethodParameter returnType,
    			ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
    			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException { 
    
    		//视图是否需要解析，如果是true就不需要解析
    		mavContainer.setRequestHandled(true);
    		//输入流
    		ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
    		//输出流
    		ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
    
    		// Try even with null return value. ResponseBodyAdvice could get involved.
    		//写出
    		writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
    	}

#### writeWithMessageConverters跟上面差不多 ####

protected <T> void writeWithMessageConverters(T value, MethodParameter returnType,
    			ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
    			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException { 
    
    		Object outputValue;
    		Class<?> valueType;
    		Type declaredType;
    
    		// 是不是CharSequence类型
    		if (value instanceof CharSequence) { 
    			outputValue = value.toString();
    			valueType = String.class;
    			declaredType = String.class;
    		}
    		else { 
    			//处理
    			outputValue = value;
    			//获取返回值类型
    			valueType = getReturnValueType(outputValue, returnType);
    			//获取目标类型,包括泛型
    			declaredType = getGenericType(returnType);
    		}
    
    		//获取对应的request
    		HttpServletRequest request = inputMessage.getServletRequest();
    		//获取允许的类型
    		List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
    		//获取可生产的类型
    		List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);
    
    		if (outputValue != null && producibleMediaTypes.isEmpty()) { 
    			throw new IllegalArgumentException("No converter found for return value of type: " + valueType);
    		}
    
    		//要使用的媒体类型
    		Set<MediaType> compatibleMediaTypes = new LinkedHashSet<MediaType>();
    		//遍历
    		for (MediaType requestedType : requestedMediaTypes) { 
    			for (MediaType producibleType : producibleMediaTypes) { 
    				if (requestedType.isCompatibleWith(producibleType)) { 
    					compatibleMediaTypes.add(getMostSpecificMediaType(requestedType, producibleType));
    				}
    			}
    		}
    		if (compatibleMediaTypes.isEmpty()) { 
    			if (outputValue != null) { 
    				throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
    			}
    			return;
    		}
    
    		List<MediaType> mediaTypes = new ArrayList<MediaType>(compatibleMediaTypes);
    		//排序
    		MediaType.sortBySpecificityAndQuality(mediaTypes);
    
    		MediaType selectedMediaType = null;
    		//找到对应的媒体类型
    		for (MediaType mediaType : mediaTypes) { 
    			if (mediaType.isConcrete()) { 
    				selectedMediaType = mediaType;
    				break;
    			}
    			else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) { 
    				selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
    				break;
    			}
    		}
    
    		if (selectedMediaType != null) { 
    			selectedMediaType = selectedMediaType.removeQualityValue();
    			//遍历消息转换器
    			for (HttpMessageConverter<?> messageConverter : this.messageConverters) { 
    				if (messageConverter instanceof GenericHttpMessageConverter) { 
    					//判断消息转换器能否写
    					if (((GenericHttpMessageConverter) messageConverter).canWrite(
    							declaredType, valueType, selectedMediaType)) { 
    						//循环调用ControllerAdvice切面
    						outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
    								(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
    								inputMessage, outputMessage);
    						if (outputValue != null) { 
    							addContentDispositionHeader(inputMessage, outputMessage);
    							//开始写
    							((GenericHttpMessageConverter) messageConverter).write(
    									outputValue, declaredType, selectedMediaType, outputMessage);
    							if (logger.isDebugEnabled()) { 
    								logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
    										"\" using [" + messageConverter + "]");
    							}
    						}
    						return;
    					}
    				}
    				else 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);
    						((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
    						if (logger.isDebugEnabled()) { 
    							logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
    									"\" using [" + messageConverter + "]");
    						}
    					}
    					return;
    				}
    			}
    		}
    
    		if (outputValue != null) { 
    			throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
    		}
    	}

也是在抽象类里面实现了writeInternal方法:

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