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Spring参数的自解析--还在自己转换？你out了！

背景

前段时间开发一个接口，因为调用我接口的同事脾气特别好，我也就不客气，我就直接把源代码发给他当接口定义了。

没想到同事看到我的代码问：要么 get a,b,c 要么 post [a,b,c]。这么写可以自动解析？他们一直都是自己转换成list。

我很肯定的说可以，但是已经习惯这么用了，没有了解底层的机制，这里其实RequestParam这个注解是不能省略的，普通的字符串参数可以自动绑定，需要这种内部转换的不可以。

参数绑定原理

Spring的参数解析使用HandlerMethodArgmentResolver类型的组件完成。不同类型的使用不同的ArgumentResolver来解析。具体参考 RequestMappingHandlerAdapter类的源码。里面有个方法是很好的诠释：

// 获取默认的 HandlerMethodArgumentResolver
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() { 
    List<HandlerMethodArgumentResolver> resolvers = new ArrayList<HandlerMethodArgumentResolver>();
    // 1.基于注解的参数解析 <-- 解析的数据来源主要是 HttpServletRequest | ModelAndViewContainer
    // Annotation-based argument resolution
    // 解析被注解 @RequestParam, @RequestPart 修饰的参数, 数据的获取通过 HttpServletRequest.getParameterValues
    resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
    // 解析被注解 @RequestParam 修饰, 且类型是 Map 的参数, 数据的获取通过 HttpServletRequest.getParameterMap
    resolvers.add(new RequestParamMapMethodArgumentResolver());
    // 解析被注解 @PathVariable 修饰, 数据的获取通过 uriTemplateVars, 而 uriTemplateVars 却是通过 RequestMappingInfoHandlerMapping.handleMatch 生成, 其实就是 uri 中映射出的 key <-> value
    resolvers.add(new PathVariableMethodArgumentResolver());
    // 解析被注解 @PathVariable 修饰 且数据类型是 Map, 数据的获取通过 uriTemplateVars, 而 uriTemplateVars 却是通过 RequestMappingInfoHandlerMapping.handleMatch 生成, 其实就是 uri 中映射出的 key <-> value
    resolvers.add(new PathVariableMapMethodArgumentResolver());
    // 解析被注解 @MatrixVariable 修饰, 数据的获取通过 URI提取了;后存储的 uri template 变量值
    resolvers.add(new MatrixVariableMethodArgumentResolver());
    // 解析被注解 @MatrixVariable 修饰 且数据类型是 Map, 数据的获取通过 URI提取了;后存储的 uri template 变量值
    resolvers.add(new MatrixVariableMapMethodArgumentResolver());
    // 解析被注解 @ModelAttribute 修饰, 且类型是 Map 的参数, 数据的获取通过 ModelAndViewContainer 获取, 通过 DataBinder 进行绑定
    resolvers.add(new ServletModelAttributeMethodProcessor(false));
    // 解析被注解 @RequestBody 修饰的参数, 以及被@ResponseBody修饰的返回值, 数据的获取通过 HttpServletRequest 获取, 根据 MediaType通过HttpMessageConverter转换成对应的格式, 在处理返回值时 也是通过 MediaType 选择合适HttpMessageConverter, 进行转换格式, 并输出
    resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
    // 解析被注解 @RequestPart 修饰, 数据的获取通过 HttpServletRequest.getParts()
    resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
    // 解析被注解 @RequestHeader 修饰, 数据的获取通过 HttpServletRequest.getHeaderValues()
    resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
    // 解析被注解 @RequestHeader 修饰且参数类型是 Map, 数据的获取通过 HttpServletRequest.getHeaderValues()
    resolvers.add(new RequestHeaderMapMethodArgumentResolver());
    // 解析被注解 @CookieValue 修饰, 数据的获取通过 HttpServletRequest.getCookies()
    resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
    // 解析被注解 @Value 修饰, 数据在这里没有解析
    resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
    // 解析被注解 @SessionAttribute 修饰, 数据的获取通过 HttpServletRequest.getAttribute(name, RequestAttributes.SCOPE_SESSION)
    resolvers.add(new SessionAttributeMethodArgumentResolver());
    // 解析被注解 @RequestAttribute 修饰, 数据的获取通过 HttpServletRequest.getAttribute(name, RequestAttributes.SCOPE_REQUEST)
    resolvers.add(new RequestAttributeMethodArgumentResolver());

    // 2.基于类型的参数解析器
    // Type-based argument resolution
    // 解析固定类型参数(比如: ServletRequest, HttpSession, InputStream 等), 参数的数据获取还是通过 HttpServletRequest
    resolvers.add(new ServletRequestMethodArgumentResolver());
    // 解析固定类型参数(比如: ServletResponse, OutputStream等), 参数的数据获取还是通过 HttpServletResponse
    resolvers.add(new ServletResponseMethodArgumentResolver());
    // 解析固定类型参数(比如: HttpEntity, RequestEntity 等), 参数的数据获取还是通过 HttpServletRequest
    resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
    // 解析固定类型参数(比如: RedirectAttributes), 参数的数据获取还是通过 HttpServletResponse
    resolvers.add(new RedirectAttributesMethodArgumentResolver());
    // 解析固定类型参数(比如: Model等), 参数的数据获取通过 ModelAndViewContainer
    resolvers.add(new ModelMethodProcessor());
    // 解析固定类型参数(比如: Model等), 参数的数据获取通过 ModelAndViewContainer
    resolvers.add(new MapMethodProcessor());
    // 解析固定类型参数(比如: Errors), 参数的数据获取通过 ModelAndViewContainer
    resolvers.add(new ErrorsMethodArgumentResolver());
    // 解析固定类型参数(比如: SessionStatus), 参数的数据获取通过 ModelAndViewContainer
    resolvers.add(new SessionStatusMethodArgumentResolver());
    // 解析固定类型参数(比如: UriComponentsBuilder), 参数的数据获取通过 HttpServletRequest
    resolvers.add(new UriComponentsBuilderMethodArgumentResolver());
    // 3.自定义参数解析器
    // Custom arguments
    if (getCustomArgumentResolvers() != null) {
        resolvers.addAll(getCustomArgumentResolvers());
    }
    // Catch-all
    //这两个解析器可以解析所有类型的参数
    resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
    resolvers.add(new ServletModelAttributeMethodProcessor(true));
    return resolvers;
}

在一步步跟踪源码之后，最终在 PropertyEditorRegistrySupport这个类中，有一个 createDefaultEditors的私有方法。里面定义了各种类型转换：

private void createDefaultEditors() {
        this.defaultEditors = new HashMap(64);
        this.defaultEditors.put(Charset.class, new CharsetEditor());
        this.defaultEditors.put(Class.class, new ClassEditor());
        this.defaultEditors.put(Class[].class, new ClassArrayEditor());
        this.defaultEditors.put(Currency.class, new CurrencyEditor());
        this.defaultEditors.put(File.class, new FileEditor());
        this.defaultEditors.put(InputStream.class, new InputStreamEditor());
        this.defaultEditors.put(InputSource.class, new InputSourceEditor());
        this.defaultEditors.put(Locale.class, new LocaleEditor());
        if(pathClass != null) {
            this.defaultEditors.put(pathClass, new PathEditor());
        }

        this.defaultEditors.put(Pattern.class, new PatternEditor());
        this.defaultEditors.put(Properties.class, new PropertiesEditor());
        this.defaultEditors.put(Reader.class, new ReaderEditor());
        this.defaultEditors.put(Resource[].class, new ResourceArrayPropertyEditor());
        this.defaultEditors.put(TimeZone.class, new TimeZoneEditor());
        this.defaultEditors.put(URI.class, new URIEditor());
        this.defaultEditors.put(URL.class, new URLEditor());
        this.defaultEditors.put(UUID.class, new UUIDEditor());
        if(zoneIdClass != null) {
            this.defaultEditors.put(zoneIdClass, new ZoneIdEditor());
        }

        this.defaultEditors.put(Collection.class, new CustomCollectionEditor(Collection.class));
        this.defaultEditors.put(Set.class, new CustomCollectionEditor(Set.class));
        this.defaultEditors.put(SortedSet.class, new CustomCollectionEditor(SortedSet.class));
        this.defaultEditors.put(List.class, new CustomCollectionEditor(List.class));
        this.defaultEditors.put(SortedMap.class, new CustomMapEditor(SortedMap.class));
        this.defaultEditors.put(byte[].class, new ByteArrayPropertyEditor());
        this.defaultEditors.put(char[].class, new CharArrayPropertyEditor());
        this.defaultEditors.put(Character.TYPE, new CharacterEditor(false));
        this.defaultEditors.put(Character.class, new CharacterEditor(true));
        this.defaultEditors.put(Boolean.TYPE, new CustomBooleanEditor(false));
        this.defaultEditors.put(Boolean.class, new CustomBooleanEditor(true));
        this.defaultEditors.put(Byte.TYPE, new CustomNumberEditor(Byte.class, false));
        this.defaultEditors.put(Byte.class, new CustomNumberEditor(Byte.class, true));
        this.defaultEditors.put(Short.TYPE, new CustomNumberEditor(Short.class, false));
        this.defaultEditors.put(Short.class, new CustomNumberEditor(Short.class, true));
        this.defaultEditors.put(Integer.TYPE, new CustomNumberEditor(Integer.class, false));
        this.defaultEditors.put(Integer.class, new CustomNumberEditor(Integer.class, true));
        this.defaultEditors.put(Long.TYPE, new CustomNumberEditor(Long.class, false));
        this.defaultEditors.put(Long.class, new CustomNumberEditor(Long.class, true));
        this.defaultEditors.put(Float.TYPE, new CustomNumberEditor(Float.class, false));
        this.defaultEditors.put(Float.class, new CustomNumberEditor(Float.class, true));
        this.defaultEditors.put(Double.TYPE, new CustomNumberEditor(Double.class, false));
        this.defaultEditors.put(Double.class, new CustomNumberEditor(Double.class, true));
        this.defaultEditors.put(BigDecimal.class, new CustomNumberEditor(BigDecimal.class, true));
        this.defaultEditors.put(BigInteger.class, new CustomNumberEditor(BigInteger.class, true));
        if(this.configValueEditorsActive) {
            StringArrayPropertyEditor sae = new StringArrayPropertyEditor();
            this.defaultEditors.put(String[].class, sae);
            this.defaultEditors.put(short[].class, sae);
            this.defaultEditors.put(int[].class, sae);
            this.defaultEditors.put(long[].class, sae);
        }
    }