认识Spring参数解析器

怼烎@ 2023-09-23 18:17 22阅读 0赞

使用Spring MVC开发的，应该都使用过@RequstBody接收json参数，转换成pojo对象，非常方便阿，但是功能并不是很全面，有点点瑕疵，并不能支持json key 方式注入到String、Integer 这类类型对象。

<table> 
 <thead> 
  <tr> 
   <th>前端传值</th> 
   <th>后端接收</th> 
   <th>结果</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>{"id": 3,"name":"xxx"}</td> 
   <td>User(id,name)</td> 
   <td>成功注入</td> 
  </tr> 
  <tr> 
   <td>{"id": 3,"name":"xxx"}</td> 
   <td>（String name,int id）</td> 
   <td>并不支持这个方式</td> 
  </tr> 
 </tbody> 
</table>

有时候一个接口只有两个参数上传，我们并不想为此创建一个单独的对象，这样可能会导致每一个hangdler 方法可能都存在一个pojo对象，大部分对象可能并不能被其他handler 共享。 而是希望使用一两个参数就可以接收到。先看下`@RequestBody`这个注解如何实现将json 转换成pojo对象的，通过spring HandlerMethodArgumentResolver 参数解析器来实现的，了解下接口。

public interface HandlerMethodArgumentResolver {
    
    	/**
    	 *   是否支持方法上参数的处理，只有返回ture，才会执行下面方法
    	 * @param parameter the method parameter to check
    	 * @return {@code true} if this resolver supports the supplied parameter;
    	 * {@code false} otherwise
    	 */
    	boolean supportsParameter(MethodParameter parameter);
    
    	/**
    	 * 将方法参数解析为给定请求的参数值
    	 * A {@link ModelAndViewContainer} provides access to the model for the
    	 * request. A {@link WebDataBinderFactory} provides a way to create
    	 * a {@link WebDataBinder} instance when needed for data binding and
    	 * type conversion purposes.
    	 * @param parameter the method parameter to resolve. This parameter must
    	 * have previously been passed to {@link #supportsParameter} which must
    	 * have returned {@code true}.
    	 * @param mavContainer the ModelAndViewContainer for the current request
    	 * @param webRequest the current request
    	 * @param binderFactory a factory for creating {@link WebDataBinder} instances
    	 * @return the resolved argument value, or {@code null} if not resolvable
    	 * @throws Exception in case of errors with the preparation of argument values
    	 */
    	@Nullable
    	Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
    			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception;
    
    }
    复制代码

在`RequestMappingHandlerAdapter`下可以看见默认参数解析器

private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
    		List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(30);
    
    		// Annotation-based argument resolution
    		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));  //@RequestPart  文件注入
    		resolvers.add(new RequestParamMapMethodArgumentResolver()); //@RequestParam 
    		resolvers.add(new PathVariableMethodArgumentResolver()); //@PathVariable
    		resolvers.add(new PathVariableMapMethodArgumentResolver()); //@PathVariable 会返回一个Map对象
    		resolvers.add(new MatrixVariableMethodArgumentResolver()); //@MatrixVariable
    		resolvers.add(new MatrixVariableMapMethodArgumentResolver()); //MatrixVariable 会返回Map 对象
    		resolvers.add(new ServletModelAttributeMethodProcessor(false)); //属性板顶
    		resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice)); //@RequestBody 后面重点讲解
    		resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice)); //@RequestPart
    		resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory())); // @RequestHeader
    		resolvers.add(new RequestHeaderMapMethodArgumentResolver()); //@RequestHeader Map对象
    		resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory())); //Cookie 值 注入
    		resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory())); //@Value
    		resolvers.add(new SessionAttributeMethodArgumentResolver()); //@SessionAttribute
    		resolvers.add(new RequestAttributeMethodArgumentResolver()); //@RequestAttribute
    
    		// Type-based argument resolution
    		resolvers.add(new ServletRequestMethodArgumentResolver());  //servlet api对象 HttpServletRequest  这类
    		resolvers.add(new ServletResponseMethodArgumentResolver()); //ServletResponse 对象注入
    		resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice)); //RequestEntity、HttpEntity
    		resolvers.add(new RedirectAttributesMethodArgumentResolver()); //重定向
    		resolvers.add(new ModelMethodProcessor()); //返回Model 对象
    		resolvers.add(new MapMethodProcessor()); // 处理方法参数返回一个Map
    		resolvers.add(new ErrorsMethodArgumentResolver()); //处理错误方法参数，返回最后一个对象
    		resolvers.add(new SessionStatusMethodArgumentResolver()); //SessionStatus
    		resolvers.add(new UriComponentsBuilderMethodArgumentResolver());  //UriComponentsBuilder
    		if (KotlinDetector.isKotlinPresent()) {
    			resolvers.add(new ContinuationHandlerMethodArgumentResolver());
    		}
    
    		// Custom arguments 自定义
    		if (getCustomArgumentResolvers() != null) {
    			resolvers.addAll(getCustomArgumentResolvers());
    		}
    
    		// Catch-all
    		resolvers.add(new PrincipalMethodArgumentResolver()); 
    		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
    		resolvers.add(new ServletModelAttributeMethodProcessor(true));
    
    		return resolvers;
    	}
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可以看见大部分方法参数都是通过上面处理器去实现的，重点放在RequestResponseBodyMethodProcessor 如何注入json的，主要看两个方法如何实现的。

@Override
    	public boolean supportsParameter(MethodParameter parameter) {
                    //只有使用了@RequestBody 注解默认就开启处理
    		return parameter.hasParameterAnnotation(RequestBody.class); 
    	}
    
    	@Override
    	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
    			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
    
    		parameter = parameter.nestedIfOptional();
                  //转换成对象了
    		Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
                   //获取参数变量名称
    		String name = Conventions.getVariableNameForParameter(parameter);
    
    		if (binderFactory != null) { //使用WebDatabinder 对已经序列化对象属性绑定处理
    			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());
    				}
    			}
    			if (mavContainer != null) {
    				mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
    			}
    		}
    
    		return adaptArgumentIfNecessary(arg, parameter);
    	}
    
    复制代码

将json 序列号成对象实现都是在readWithMessageConverters 方法中

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;
    		}
    
    		Class<?> contextClass = parameter.getContainingClass();
    		Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);
    		if (targetClass == null) {
    			ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);
    			targetClass = (Class<T>) resolvableType.resolve();
    		}
    
    		HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);
    		Object body = NO_VALUE;
    
    		EmptyBodyCheckingHttpInputMessage message;
    		try {
                            //创建重复读写流
    			message = new EmptyBodyCheckingHttpInputMessage(inputMessage); 
                           //通过HttpMessageConverter 来对String json 转换成对象
    			for (HttpMessageConverter<?> converter : this.messageConverters) {
    				Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
    				GenericHttpMessageConverter<?> genericConverter =
    						(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
    				if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
    						(targetClass != null && converter.canRead(targetClass, contentType))) {
    					if (message.hasBody()) {
    						HttpInputMessage msgToUse =
    								getAdvice().beforeBodyRead(message, parameter, targetType, converterType); //前置处理
    						body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
    								((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
    						body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType); //后置处理 类型通知
    					}
    					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);
    		}
                    //缩减部分代码
    		return body;
    	}
    复制代码

http 请求内容调用GenericHttpMessageConverter read 方法转换成对象，这时调用的是`AbstractJackson2HttpMessageConverter`

private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) throws IOException {
    		MediaType contentType = inputMessage.getHeaders().getContentType();
    		Charset charset = getCharset(contentType);
    
    		ObjectMapper objectMapper = selectObjectMapper(javaType.getRawClass(), contentType);
    		Assert.state(objectMapper != null, "No ObjectMapper for " + javaType);
    
    		boolean isUnicode = ENCODINGS.containsKey(charset.name()) ||
    				"UTF-16".equals(charset.name()) ||
    				"UTF-32".equals(charset.name());
    		try {
    			if (inputMessage instanceof MappingJacksonInputMessage) { //这个是使用了JsonView 
    				Class<?> deserializationView = ((MappingJacksonInputMessage) inputMessage).getDeserializationView();
    				if (deserializationView != null) {
    					ObjectReader objectReader = objectMapper.readerWithView(deserializationView).forType(javaType);
    					if (isUnicode) {
    						return objectReader.readValue(inputMessage.getBody());
    					}
    					else {
    						Reader reader = new InputStreamReader(inputMessage.getBody(), charset);
    						return objectReader.readValue(reader);  //反序列化成Java 对象
    					}
    				}
    			}
    			if (isUnicode) {
    				return objectMapper.readValue(inputMessage.getBody(), javaType);
    			}
    			else {
    				Reader reader = new InputStreamReader(inputMessage.getBody(), charset);
    				return objectMapper.readValue(reader, javaType);
    			}
    		}
    		catch (InvalidDefinitionException ex) {
    			throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
    		}
    		catch (JsonProcessingException ex) {
    			throw new HttpMessageNotReadableException("JSON parse error: " + ex.getOriginalMessage(), ex, inputMessage);
    		}
    	}
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可以看见@RequestBody 是通过GenericHttpMessageConverter class 调用ObjectMapper readValue 进行转换的，只能将json转换成pojo 对象， 并不支持简单类型 String、Integer、Long这类转换的，有着天然缺陷。如果想将json key 注入到参数，需要手动实现一个参数解析器，下面展示简单实现代码。

#### 动手实现 ####

目标： 我们希望创建一个类似@RequestBody的注解，标注当前参数支持json key直接注入，这个注解也可以直接在方法修饰，表示整个方法参数都是用注解，当然也可以支持整个类，表示所有方法的参数都支持。在实现一个对应注解的参数解析器。

创建注解类

@Target({ElementType.PARAMETER,ElementType.METHOD,ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    public @interface JsonKeyValue {
    
        /**
         *  json key 如何没有则使用 类型变量名
         * @return
         */
        @AliasFor("name")
        String value() default "";
    
        @AliasFor("value")
        String name() default "";
    }
    
    复制代码

实现参数解析器

public class RequestJsonKeyValueMethodProcessor  implements HandlerMethodArgumentResolver {
    
        private ObjectMapper objectMapper ;
    
        public RequestJsonKeyValueMethodProcessor(ObjectMapper objectMapper){
            this.objectMapper = objectMapper;
        }
    
        @Override
        public boolean supportsParameter(MethodParameter parameter) {
            boolean b = parameter.hasParameterAnnotation(JsonKeyValue.class);
            if (!b) {
                JsonKeyValue value = parameter.getMethodAnnotation(JsonKeyValue.class);  //从方法上找注解
                b = value != null;
                if (!b){
                    value = parameter.getContainingClass().getAnnotation(JsonKeyValue.class); //从类上找注解
                    b = value != null;
                }
            }
            return b;
        }
    
        @Override
        public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
                                      NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
            parameter = parameter.nestedIfOptional();
            HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
            Assert.state(servletRequest != null, "No HttpServletRequest");
            ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);
            MediaType contentType = inputMessage.getHeaders().getContentType();
            Charset charset = getCharset(contentType);
            Reader reader = new InputStreamReader(inputMessage.getBody(), charset);
            JsonNode jsonNode = objectMapper.readTree(reader);
            String parameterName = parameterName(parameter);
            JsonNode path = jsonNode.path(parameterName);
            Object o = objectMapper.convertValue(path, parameter.getNestedParameterType());
            return o;
        }
    
    
        private String parameterName(MethodParameter parameter){
            JsonKeyValue annotation = parameter.getParameterAnnotation(JsonKeyValue.class);
            if (annotation != null){
                String name = annotation.name();
                if (StringUtils.hasText(name))
                    return annotation.name();
            }
            return parameter.getParameterName();
        }
    
        //抄袭  AbstractMessageConverterMethodArgumentResolver  主要是防止将流读入后，controller 方法不能在读了
        private static class EmptyBodyCheckingHttpInputMessage implements HttpInputMessage {
    
            private final HttpHeaders headers;
    
            @Nullable
            private final InputStream body;
    
            public EmptyBodyCheckingHttpInputMessage(HttpInputMessage inputMessage) throws IOException {
                this.headers = inputMessage.getHeaders();
                InputStream inputStream = inputMessage.getBody();
                if (inputStream.markSupported()) {
                    inputStream.mark(1);
                    this.body = (inputStream.read() != -1 ? inputStream : null);
                    inputStream.reset();
                }
                else {
                    PushbackInputStream pushbackInputStream = new PushbackInputStream(inputStream);
                    int b = pushbackInputStream.read();
                    if (b == -1) {
                        this.body = null;
                    }
                    else {
                        this.body = pushbackInputStream;
                        pushbackInputStream.unread(b);
                    }
                }
            }
    
            @Override
            public HttpHeaders getHeaders() {
                return this.headers;
            }
    
            @Override
            public InputStream getBody() {
                return (this.body != null ? this.body : StreamUtils.emptyInput());
            }
    
            public boolean hasBody() {
                return (this.body != null);
            }
        }
    
        private Charset getCharset(@Nullable MediaType contentType) {
            if (contentType != null && contentType.getCharset() != null) {
                return contentType.getCharset();
            }
            else {
                return StandardCharsets.UTF_8;
            }
        }
    }
    复制代码

添加自定义参数解析器

@EnableWebMvc
    @Configuration
    public class WebMvcConfig implements WebMvcConfigurer{
    
        private Logger logger = LoggerFactory.getLogger(WebMvcConfig.class);
    
        @Autowired
        private ObjectMapper objectMapper;
    
        @Override
        public void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {
            resolvers.add(new RequestJsonKeyValueMethodProcessor(objectMapper));
        }
    }
    复制代码

这里不用担心自定义参数解析器会覆盖spring 原生的解析器，这里会将addArgumentResolvers 添加解析器自动放入到CustomArgumentResolvers中。 一个简单自定义参数解析器就完成了，支持普通类的String、Integer 这些参数的解析。注意的是这个参数解析器并没有考虑参数为空的处理，Optional 这类情况只满足一些简单、快速开发的场景。不知道各位同行是否在日常开发中有没有使用到这些知识，我公司的基础框架就是使用这些技术来做快速开发的，对开发效率提升特别大的。