认识Spring参数解析器

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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。

前端传值 后端接收 结果
{“id”: 3,”name”:”xxx”} User(id,name) 成功注入
{“id”: 3,”name”:”xxx”} （String name,int id） 并不支持这个方式

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。

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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;

}

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

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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;
}

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。

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@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);
}

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

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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;
}

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

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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);
	}
}

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

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@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 "";
}

实现参数解析器

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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;
        }
    }
}

添加自定义参数解析器

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@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));
    }
}

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。