探索 SpringBoot （五）SpringMvc Rest 处理流程源码分析

﹏ヽ暗。殇╰゛Y 2022-04-02 14:20 246阅读 0赞

# 探索 SpringBoot （五）SpringMvc Rest 处理流程源码分析 #

### 文章目录 ###

*  探索 SpringBoot （五）SpringMvc Rest 处理流程源码分析
 *   *  1 缘起
     *  2 参数和返回值的解析转换
     *  3 HandlerMethodArgumentResolver 参数解析者
     *  4 InvocableHandlerMethod 请求方法执行者
     *  5 HandlerMethodReturnValueHandler 处理方法返回值
     *  6 通过 AbstractGenericHttpMessageConverter 自定义参数解析的处理
     *  自定义参数解析和返回值处理
     *   *  1 自定义 HandlerMethodArgumentResolver
         *  2 自定义 HandlerMethodReturnValueHandler
         *  3 向 requestMappingHandlerAdapter 适配者中添加解析者
         *  4 控制层写法

## 1 缘起 ##

> 接着上文 以 rest 请求为例子， 继续分析 springmvc 流程处理，本文建立上篇《探索 SpringBoot （四）SpringMvc 深度解析》的基础上继续分析

## 2 参数和返回值的解析转换 ##

> 重点涉及 InvocableHandlerMethod，HandlerMethodArgumentResolver，HandlerMethodReturnValueHandler

## 3 HandlerMethodArgumentResolver 参数解析者 ##

HandlerMethodArgumentResolver， 其实就是一个接口，主要定义了两个方法
    
    判断该解析者是否支持该方法
    boolean supportsParameter(MethodParameter parameter);
    
    解析参数，返回 object对象
    @Nullable
    Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
    		NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception;

## 4 InvocableHandlerMethod 请求方法执行者 ##

InvocableHandlerMethod， 主要作用是解析请求参数，获取 Object 的返回值
    
    @Nullable
    public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
    		Object... providedArgs) throws Exception {
    
    	Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
    	if (logger.isTraceEnabled()) {
    		logger.trace("Arguments: " + Arrays.toString(args));
    	}
    	return doInvoke(args);
    }
    
    我们先看看 getMethodArgumentValues 干了什么
    ...
    
    protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
    
    		MethodParameter[] parameters = getMethodParameters();
    		Object[] args = new Object[parameters.length];
    		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;
    			}
    			if (this.argumentResolvers.supportsParameter(parameter)) {
    				try {
    					args[i] = this.argumentResolvers.resolveArgument(
    							parameter, mavContainer, request, this.dataBinderFactory);
    					continue;
    				}
    				catch (Exception ex) {
    					// Leave stack trace for later, e.g. AbstractHandlerExceptionResolver
    					if (logger.isDebugEnabled()) {
    						String message = ex.getMessage();
    						if (message != null && !message.contains(parameter.getExecutable().toGenericString())) {
    							logger.debug(formatArgumentError(parameter, message));
    						}
    					}
    					throw ex;
    				}
    			}
    			if (args[i] == null) {
    				throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
    			}
    		}
    		return args;
    	}
    	
    
    其实主要就是干了这两件事
    
    1 判断是否支持该参数
    2 解析该参数
    
    this.argumentResolvers.supportsParameter(parameter)
    
    args[i] = this.argumentResolvers.resolveArgument(
    						parameter, mavContainer, request, this.dataBinderFactory);
    						
    我们再看看 supportsParameter，resolveArgument具体实现逻辑是啥。
    
    public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
    			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
    
    	HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
    	if (resolver == null) {
    		throw new IllegalArgumentException(
    				"Unsupported parameter type [" + parameter.getParameterType().getName() + "]." +
    						" supportsParameter should be called first.");
    	}
    	return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
    }
    
    这里的 resolver 就是 RequestResponseBodyMethodProcessor

## 5 HandlerMethodReturnValueHandler 处理方法返回值 ##

ServletInvocableHandlerMethod
    
    public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
    
            // 上面讲解过的 执行方法，并且获取返回值
    		Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    		setResponseStatus(webRequest);
    
    		if (returnValue == null) {
    			if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
    				mavContainer.setRequestHandled(true);
    				return;
    			}
    		}
    		else if (StringUtils.hasText(getResponseStatusReason())) {
    			mavContainer.setRequestHandled(true);
    			return;
    		}
    
    		mavContainer.setRequestHandled(false);
    		Assert.state(this.returnValueHandlers != null, "No return value handlers");
    		try {
    			this.returnValueHandlers.handleReturnValue(
    					returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    		}
    		catch (Exception ex) {
    			if (logger.isTraceEnabled()) {
    				logger.trace(formatErrorForReturnValue(returnValue), ex);
    			}
    			throw ex;
    		}
    	}
    
    关键在这里 处理了返回值
    this.returnValueHandlers.handleReturnValue(
    					returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    
    
    我们接着往下看					
    public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
    			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
    
            // 选择了最佳处理者，这里是RequestResponseBodyMethodProcessor，rest 判断了方法上是否有ResponseBody注解
    		HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
    		if (handler == null) {
    			throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
    		}
    		handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
    	}
    	
    handleReturnValue 值得我们看看
    
    
    这里面选择了指定的http消息转换者去转换输出信息 HttpMessageConverter，我们看看这个接口的部分代码
    
    这里获取者支持的媒体类型
    List<MediaType> getSupportedMediaTypes();
    
    // 服务端写出到客户端的时候进行返回值的处理
    void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
    			throws IOException, HttpMessageNotWritableException;
    
    // 客户端传入到服务端进行的参数处理
    protected abstract T readInternal(Class<? extends T> clazz, HttpInputMessage inputMessage)
    			throws IOException, HttpMessageNotReadableException;

## 6 通过 AbstractGenericHttpMessageConverter 自定义参数解析的处理 ##

*  6.1 需求场景，有时候我们需要对用户传入的参数进行自定义的解析和返回处理
 *  6.2

-- 请求
    
    post:localhost:8080/add/props
    
    Content-Type：text/properties
    Accept：text/properties
    
    body:
    
    id:1
    username:jun
    
    -- 接口
    
    @PostMapping(value = "/add/props")
    public Properties addProperties( @RequestBody Properties properties) {
        return properties;
    }

6.3 实现 PropertiesHttpMessageConverter 继承 AbstractGenericHttpMessageConverter

public class PropertiesHttpMessageConverter extends AbstractGenericHttpMessageConverter<Properties> {
    
        public PropertiesHttpMessageConverter() {
            // 设置支持的 MediaType
            super(new MediaType("text", "properties"));
        }
    
        @Override
        protected void writeInternal(Properties properties, Type type, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
            // Properties -> String
            // OutputStream -> Writer
            HttpHeaders httpHeaders = outputMessage.getHeaders();
            MediaType mediaType = httpHeaders.getContentType();
            // 获取字符编码
            Charset charset = mediaType.getCharset();
            // 当 charset 不存在时，使用 UTF-8
            charset = charset == null ? Charset.forName("UTF-8") : charset;
            // 字节输出流
            OutputStream outputStream = outputMessage.getBody();
            // 字符输出流
            Writer writer = new OutputStreamWriter(outputStream, charset);
            // Properties 写入到字符输出流
            properties.store(writer,"From PropertiesHttpMessageConverter");
        }
    
        @Override
        protected Properties readInternal(Class<? extends Properties> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
    
            // 字符流 -> 字符编码
            // 从 请求头 Content-Type 解析编码
            HttpHeaders httpHeaders = inputMessage.getHeaders();
            MediaType mediaType = httpHeaders.getContentType();
            // 获取字符编码
            Charset charset = mediaType.getCharset();
            // 当 charset 不存在时，使用 UTF-8
            charset = charset == null ? Charset.forName("UTF-8") : charset;
    
            // 字节流
            InputStream inputStream = inputMessage.getBody();
            InputStreamReader reader = new InputStreamReader(inputStream, charset);
            Properties properties = new Properties();
            // 加载字符流成为 Properties 对象
            properties.load(reader);
            return properties;
        }
    
        @Override
        public Properties read(Type type, Class<?> contextClass, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
            return readInternal(null, inputMessage);
        }
    }

这样 mvc 就能自动解析和处理我们的返回值了

## 自定义参数解析和返回值处理 ##

> 我们上面的自定义解析还是依赖与 @RequestBody，我们如何不依赖呢？

### 1 自定义 HandlerMethodArgumentResolver ###

public class PropertiesHandlerMethodArgumentResolver implements HandlerMethodArgumentResolver {
        @Override
        public boolean supportsParameter(MethodParameter parameter) {
            return Properties.class.equals(parameter.getParameterType());
        }
    
        @Override
        public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
                                      NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
    
            // 复用 PropertiesHttpMessageConverter
            PropertiesHttpMessageConverter converter = new PropertiesHttpMessageConverter();
    
            ServletWebRequest servletWebRequest = (ServletWebRequest) webRequest;
            // Servlet Request API
            HttpServletRequest request = servletWebRequest.getRequest();
    
            HttpInputMessage httpInputMessage = new ServletServerHttpRequest(request);
    
            return converter.read(null, null, httpInputMessage);
        }
    }

### 2 自定义 HandlerMethodReturnValueHandler ###

public class PropertiesHandlerMethodReturnValueHandler implements HandlerMethodReturnValueHandler {
    
        @Override
        public boolean supportsReturnType(MethodParameter returnType) {
            // 判断方法的返回类型，是否与 Properties 类型匹配
            return Properties.class.equals(returnType.getMethod().getReturnType());
        }
    
        @Override
        public void handleReturnValue(Object returnValue, MethodParameter returnType,
                                      ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
    
            // 强制装换
            Properties properties = (Properties) returnValue;
            // 复用 PropertiesHttpMessageConverter
            PropertiesHttpMessageConverter converter = new PropertiesHttpMessageConverter();
    
            ServletWebRequest servletWebRequest = (ServletWebRequest) webRequest;
            // Servlet Request API
            HttpServletRequest request = servletWebRequest.getRequest();
            String contentType = request.getHeader("Content-Type");
            // 获取请求头 Content-Type 中的媒体类型
            MediaType mediaType = MediaType.parseMediaType(contentType);
    
            // 获取 Servlet Response 对象
            HttpServletResponse response = servletWebRequest.getResponse();
            HttpOutputMessage message = new ServletServerHttpResponse(response);
            // 通过 PropertiesHttpMessageConverter 输出
            converter.write(properties, mediaType, message);
            // 告知 Spring Web MVC 当前请求已经处理完毕
            mavContainer.setRequestHandled(true);
        }
    }

### 3 向 requestMappingHandlerAdapter 适配者中添加解析者 ###

@Configuration
    public class RestWebMvcConfigurer implements WebMvcConfigurer {
    
        @Autowired
        private RequestMappingHandlerAdapter requestMappingHandlerAdapter;
    
        @PostConstruct
        public void init() {
            // 获取当前 RequestMappingHandlerAdapter 所有的 Resolver 对象
            List<HandlerMethodArgumentResolver> resolvers = requestMappingHandlerAdapter.getArgumentResolvers();
            List<HandlerMethodArgumentResolver> newResolvers = new ArrayList<>(resolvers.size() + 1);
            // 添加 PropertiesHandlerMethodArgumentResolver 到集合首位
            newResolvers.add(new PropertiesHandlerMethodArgumentResolver());
            // 添加 已注册的 Resolver 对象集合
            newResolvers.addAll(resolvers);
            // 重新设置 Resolver 对象集合
            requestMappingHandlerAdapter.setArgumentResolvers(newResolvers);
    
            // 获取当前 HandlerMethodReturnValueHandler 所有的 Handler 对象
            List<HandlerMethodReturnValueHandler> handlers = requestMappingHandlerAdapter.getReturnValueHandlers();
            List<HandlerMethodReturnValueHandler> newHandlers = new ArrayList<>(handlers.size() + 1);
            // 添加 PropertiesHandlerMethodReturnValueHandler 到集合首位
            newHandlers.add(new PropertiesHandlerMethodReturnValueHandler());
            // 添加 已注册的 Handler 对象集合
            newHandlers.addAll(handlers);
            // 重新设置 Handler 对象集合
            requestMappingHandlerAdapter.setReturnValueHandlers(newHandlers);
        }
    }

### 4 控制层写法 ###

@Controller
    public class PropertiesRestController {
    
        @PostMapping(value = "/add/props")
        public Properties addProperties(Properties properties) {
            return properties;
        }
    }