SpringMVC(三)：参数绑定、输入输出转换

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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

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。
遍历HandlerMethod的MethodParameter数组
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。
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。
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

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); }
}
}