SpringMVC(三)：参数绑定、输入输出转换

一、参数解析绑定

1. 自定义绑定：不绑定某些项

   @InitBinder
    private void initBinder(WebDataBinder dataBinder) {
        dataBinder.setDisallowedFields("company", "dateCreated", "lastUpdated");
    }

2. 什么时候使用DataBinder

RequestMappingHandlerAdapter 的 invokeHandlerMethod 方法

    protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

        ServletWebRequest webRequest = new ServletWebRequest(request, response);
　　　　　//获取WebDataBinderFactory　
        WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
        ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

        ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
        invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
        invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
        invocableMethod.setDataBinderFactory(binderFactory);
        invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

        ModelAndViewContainer mavContainer = new ModelAndViewContainer();
        mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
        modelFactory.initModel(webRequest, mavContainer, invocableMethod);
        mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

        AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
        asyncWebRequest.setTimeout(this.asyncRequestTimeout);

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.setTaskExecutor(this.taskExecutor);
        asyncManager.setAsyncWebRequest(asyncWebRequest);
        asyncManager.registerCallableInterceptors(this.callableInterceptors);
        asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

        if (asyncManager.hasConcurrentResult()) {
            Object result = asyncManager.getConcurrentResult();
            mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
            asyncManager.clearConcurrentResult();
            if (logger.isDebugEnabled()) {
                logger.debug("Found concurrent result value [" + result + "]");
            }
            invocableMethod = invocableMethod.wrapConcurrentResult(result);
        }

　　　　　//注入方法参数
        invocableMethod.invokeAndHandle(webRequest, mavContainer);
        if (asyncManager.isConcurrentHandlingStarted()) {
            return null;
        }

        return getModelAndView(mavContainer, modelFactory, webRequest);
    }

3.  RequestMappingHandlerAdapter

1. RequestMappingHandlerAdapter中两个和参数解析器相关的成员变量

    private List<HandlerMethodArgumentResolver> customArgumentResolvers;

    private HandlerMethodArgumentResolverComposite argumentResolvers;

2. RequestMappingHandlerAdapter实例化后，会调用afterPropertiesSet()，在这里把系统默认的解析器和我们写的自定义解析器汇总

    @Override
    public void afterPropertiesSet() {
        // Do this first, it may add ResponseBody advice beans
        initControllerAdviceCache();

        if (this.argumentResolvers == null) {
            List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
            this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
        }
        if (this.initBinderArgumentResolvers == null) {
            List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
            this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
        }
        if (this.returnValueHandlers == null) {
            List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
            this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
        }
    }

3. 调用匹配的methodArgumentResolver解析数据

参数解析绑定总结

1. SpringMVC初始化时，RequestMappingHandlerAdapter类会把一些默认的参数解析器添加到argumentResolvers中。当SpringMVC接收到请求后首先根据url查找对应的HandlerMethod。
2. 遍历HandlerMethod的MethodParameter数组
3. 根据MethodParameter的类型来查找确认使用哪个HandlerMethodArgumentResolver，遍历所有的argumentResolvers的supportsParameter(MethodParameter parameter)方法。如果返回true，则表示查找成功，当前MethodParameter，使用该HandlerMethodArgumentResolver。这里确认大多都是根据参数的注解已经参数的Type来确认。
4. 解析参数，从request中解析出MethodParameter对应的参数，这里解析出来的结果都是String类型。
5. 转换参数，把对应String转换成具体方法所需要的类型，这里就包括了基本类型、对象、List、Set、Map。

二、输入输出转换

1.

public interface ConverterFactory<S, R> {

    /**
     * Get the converter to convert from S to target type T, where T is also an instance of R.
     * @param <T> the target type
     * @param targetType the target type to convert to
     * @return A converter from S to T
     */
    <T extends R> Converter<S, T> getConverter(Class<T> targetType);

}

Spring实现ConvertFactory示例

final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {

    @Override
    public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
        Class<?> enumType = targetType;
        while (enumType != null && !enumType.isEnum()) {
            enumType = enumType.getSuperclass();
        }
        if (enumType == null) {
            throw new IllegalArgumentException "The target type " + targetType.getName() + " does not refer to an enum");
        }
        return new StringToEnum(enumType);
    }

    private class StringToEnum<T extends Enum> implements Converter<String, T> {

        private final Class<T> enumType;

        public StringToEnum(Class<T> enumType) {
            this.enumType = enumType;
        }

        @Override
        public T convert(String source) {
            if (source.length() == 0) {
                // It's an empty enum identifier: reset the enum value to null.
                return null;
            }
            return (T) Enum.valueOf(this.enumType, source.trim());
        }
    }

}

上面的示例不支持基数转换为枚举，改进如下：

public class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {

    public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
        return new StringToEnumConverter(targetType);
    }

    private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> {

        private Class<T> enumType;

        public StringToEnumConverter(Class<T> enumType) {
            this.enumType = enumType;
        }

        public T convert(String source) {

            try {
                Integer ordinal = Integer.parseInt(source);
　　　　　　　　　　//Enum的values是编译器加的
                T[] values = (T[]) enumType.getDeclaredMethod("values", new Class[0]).invoke(null, new Object[0]);
                return values[ordinal];
            } catch (Exception e) {

            }

            try {
                Method valueOf = enumType.getDeclaredMethod("valueOf", String.class);
                return (T) valueOf.invoke(null, source);
            } catch (Exception e) {

            }

            throw new IllegalStateException("非法参数值:" + source);

        }
    }
}

二、json输出转换:  ObjectMapper

spring提供三种不同级别的自定义方式

参考：

spring reference: how to customer ObjectMapper

spring 参数解析绑定