Springmvc中的数据绑定工作得从DiapatcherServlet的doDispatch方法说起：
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());在前面的章节介绍过，这里的ha就是HandlerAdapter，我们以AnnotationMethodHandlerAdapter为例看它的handle方法究竟进行了哪些操作。
public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
Class<?> clazz = ClassUtils.getUserClass(handler);
Boolean annotatedWithSessionAttributes = this.sessionAnnotatedClassesCache.get(clazz);
if (annotatedWithSessionAttributes == null) {
annotatedWithSessionAttributes = (AnnotationUtils.findAnnotation(clazz, SessionAttributes.class) != null);
this.sessionAnnotatedClassesCache.put(clazz, annotatedWithSessionAttributes);
}
if (annotatedWithSessionAttributes) {
checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);
}
else {
checkAndPrepare(request, response, true);
}
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
synchronized (mutex) {
return invokeHandlerMethod(request, response, handler);
}
}
}
return invokeHandlerMethod(request, response, handler);
}我们可以看到这个方法做了很多前期准备，最关键的一步是invokeHandlerMethod,下面我们来看看这个方法。
protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
//获取方法处理器
ServletHandlerMethodResolver methodResolver = getMethodResolver(handler);
//resolveHandlerMethod这个方法很复杂这里略过不做详细介绍，这个方法最后帮handler找到了url对应的方法
Method handlerMethod = methodResolver.resolveHandlerMethod(request);
//实例化方法调用者
ServletHandlerMethodInvoker methodInvoker = new ServletHandlerMethodInvoker(methodResolver);
//实例化ServletWebRequest
ServletWebRequest webRequest = new ServletWebRequest(request, response);
//实例化数据模型Model
ExtendedModelMap implicitModel = new BindingAwareModelMap();
//调用controller下的方法并返回结果
Object result = methodInvoker.invokeHandlerMethod(handlerMethod, handler, webRequest, implicitModel);
//从返回结果（这个结果有可能是ModelAndView）和传入的ModelMap中获取ModelAndView
ModelAndView mav =
methodInvoker.getModelAndView(handlerMethod, handler.getClass(), result, implicitModel, webRequest);
methodInvoker.updateModelAttributes(handler, (mav != null ? mav.getModel() : null), implicitModel, webRequest);
return mav;
}这个方法的methodInvoker.invokeHandlerMethod，我们再来看看这个方法：
public final Object invokeHandlerMethod(Method handlerMethod, Object handler,
NativeWebRequest webRequest, ExtendedModelMap implicitModel) throws Exception {
//获取桥接方法，实际上是对handlerMethod封装了一下
Method handlerMethodToInvoke = BridgeMethodResolver.findBridgedMethod(handlerMethod);
//省略代码若干...
boolean debug = logger.isDebugEnabled();
//将methodResolver中的要存入session属性（带@SessionAttributes）塞入ModelMap中
for (String attrName : this.methodResolver.getActualSessionAttributeNames()) {
Object attrValue = this.sessionAttributeStore.retrieveAttribute(webRequest, attrName);
if (attrValue != null) {
implicitModel.addAttribute(attrName, attrValue);
}
}
//处理带@ModelAttribute注解的方法，将他们的结果放到ModelMap中
for (Method attributeMethod : this.methodResolver.getModelAttributeMethods()) {
Method attributeMethodToInvoke = BridgeMethodResolver.findBridgedMethod(attributeMethod);
Object[] args = resolveHandlerArguments(attributeMethodToInvoke, handler, webRequest, implicitModel);
if (debug) {
logger.debug("Invoking model attribute method: " + attributeMethodToInvoke);
}
String attrName = AnnotationUtils.findAnnotation(attributeMethod, ModelAttribute.class).value();
if (!"".equals(attrName) && implicitModel.containsAttribute(attrName)) {
continue;
}
ReflectionUtils.makeAccessible(attributeMethodToInvoke);
Object attrValue = attributeMethodToInvoke.invoke(handler, args);
if ("".equals(attrName)) {
Class<?> resolvedType = GenericTypeResolver.resolveReturnType(attributeMethodToInvoke, handler.getClass());
attrName = Conventions.getVariableNameForReturnType(attributeMethodToInvoke, resolvedType, attrValue);
}
if (!implicitModel.containsAttribute(attrName)) {
implicitModel.addAttribute(attrName, attrValue);
}
}
Object[] args = resolveHandlerArguments(handlerMethodToInvoke, handler, webRequest, implicitModel);
if (debug) {
logger.debug("Invoking request handler method: " + handlerMethodToInvoke);
}
ReflectionUtils.makeAccessible(handlerMethodToInvoke);
return handlerMethodToInvoke.invoke(handler, args);
//省略代码若干...
}这个函数主要是将一些需要装入ModelMap的参数装入之。然后调用resolveHandlerArguments（）来处理参数，包括参数的绑定、验证、转换。接下来我们看看这个函数：
private Object[] resolveHandlerArguments(Method handlerMethod, Object handler,
NativeWebRequest webRequest, ExtendedModelMap implicitModel) throws Exception {
//获取参数数据类型
Class<?>[] paramTypes = handlerMethod.getParameterTypes();
//构造参数个数长度的数组
Object[] args = new Object[paramTypes.length];
//遍历args的每一个成员
for (int i = 0; i < args.length; i++) {
//实例化方法的参数，这是一个Springmvc的类，它描述的与该参数相联系的类、方法、参数名、注解等信息
MethodParameter methodParam = new MethodParameter(handlerMethod, i);
methodParam.initParameterNameDiscovery(this.parameterNameDiscoverer);
//设置参数的所属类和参数类型等信息
GenericTypeResolver.resolveParameterType(methodParam, handler.getClass());
//参数名
String paramName = null;
//http头的属性名
String headerName = null;
//http是否有body部分
boolean requestBodyFound = false;
//cookie中的属性名
String cookieName = null;
//路径变量
String pathVarName = null;
//属性名
String attrName = null;
boolean required = false;
String defaultValue = null;
boolean validate = false;
Object[] validationHints = null;
//是否有注解
int annotationsFound = 0;
//获取参数的注解
Annotation[] paramAnns = methodParam.getParameterAnnotations();
for (Annotation paramAnn : paramAnns) {
//处理有@RequestParam注解的参数
if (RequestParam.class.isInstance(paramAnn)) {
RequestParam requestParam = (RequestParam) paramAnn;
//@RequestParam的value属性
paramName = requestParam.value();
//@RequestParam的required属性
required = requestParam.required();
//@RequestParam的defaultValue属性
defaultValue = parseDefaultValueAttribute(requestParam.defaultValue());
annotationsFound++;
}
//处理带@RequestHeader注解的参数
else if (RequestHeader.class.isInstance(paramAnn)) {
RequestHeader requestHeader = (RequestHeader) paramAnn;
headerName = requestHeader.value();
required = requestHeader.required();
defaultValue = parseDefaultValueAttribute(requestHeader.defaultValue());
annotationsFound++;
}
//处理带@RequestBody注解的参数
else if (RequestBody.class.isInstance(paramAnn)) {
requestBodyFound = true;
annotationsFound++;
}
//处理带@CookieValue注解的参数
else if (CookieValue.class.isInstance(paramAnn)) {
CookieValue cookieValue = (CookieValue) paramAnn;
cookieName = cookieValue.value();
required = cookieValue.required();
defaultValue = parseDefaultValueAttribute(cookieValue.defaultValue());
annotationsFound++;
}
//处理带@PathVariable注解的参数
else if (PathVariable.class.isInstance(paramAnn)) {
PathVariable pathVar = (PathVariable) paramAnn;
pathVarName = pathVar.value();
annotationsFound++;
}
//处理带@ModelAttribute注解的参数
else if (ModelAttribute.class.isInstance(paramAnn)) {
ModelAttribute attr = (ModelAttribute) paramAnn;
attrName = attr.value();
annotationsFound++;
}
//处理带@Value注解的参数
else if (Value.class.isInstance(paramAnn)) {
defaultValue = ((Value) paramAnn).value();
}
//处理带需要验证的参数
else if (paramAnn.annotationType().getSimpleName().startsWith("Valid")) {
validate = true;
Object value = AnnotationUtils.getValue(paramAnn);
validationHints = (value instanceof Object[] ? (Object[]) value : new Object[] {value});
}
}
//如果一个参数带两个或两个以上注解抛出错误
if (annotationsFound > 1) {
throw new IllegalStateException("Handler parameter annotations are exclusive choices - " +
"do not specify more than one such annotation on the same parameter: " + handlerMethod);
}
//如果没有注解
if (annotationsFound == 0) {
Object argValue = resolveCommonArgument(methodParam, webRequest);
//如果是Reuqest则赋给WebRequest实例
if (argValue != WebArgumentResolver.UNRESOLVED) {
args[i] = argValue;
}
//为参数赋予默认值
else if (defaultValue != null) {
args[i] = resolveDefaultValue(defaultValue);
}
else {
Class<?> paramType = methodParam.getParameterType();
//如果有Model或者Map类型的参数则直接把ModelMap实例赋给他们
if (Model.class.isAssignableFrom(paramType) || Map.class.isAssignableFrom(paramType)) {
if (!paramType.isAssignableFrom(implicitModel.getClass())) {
throw new IllegalStateException("Argument [" + paramType.getSimpleName() + "] is of type " +
"Model or Map but is not assignable from the actual model. You may need to switch " +
"newer MVC infrastructure classes to use this argument.");
}
args[i] = implicitModel;
}
//如果是SessionStatus类型的参数赋给this.sessionStatus
else if (SessionStatus.class.isAssignableFrom(paramType)) {
args[i] = this.sessionStatus;
}
//如果是HttpEntity类型的参数单独处理
else if (HttpEntity.class.isAssignableFrom(paramType)) {
args[i] = resolveHttpEntityRequest(methodParam, webRequest);
}
//如果是Errors类型的参数抛出异常状态异常
else if (Errors.class.isAssignableFrom(paramType)) {
throw new IllegalStateException("Errors/BindingResult argument declared " +
"without preceding model attribute. Check your handler method signature!");
}
//如果是普通的参数
else if (BeanUtils.isSimpleProperty(paramType)) {
paramName = "";
}
else {
attrName = "";
}
}
}
if (paramName != null) {
//处理普通参数，包括MultipartFile参数，@initBinder注解处理，普通值处理
args[i] = resolveRequestParam(paramName, required, defaultValue, methodParam, webRequest, handler);
}
//http报文头信息处理
else if (headerName != null) {
args[i] = resolveRequestHeader(headerName, required, defaultValue, methodParam, webRequest, handler);
}
//http报文body信息处理
else if (requestBodyFound) {
args[i] = resolveRequestBody(methodParam, webRequest, handler);
}
//http请求cookie信息处理
else if (cookieName != null) {
args[i] = resolveCookieValue(cookieName, required, defaultValue, methodParam, webRequest, handler);
}
//路径变量信息处理
else if (pathVarName != null) {
args[i] = resolvePathVariable(pathVarName, methodParam, webRequest, handler);
}
//放入ModelMap的信息，一般是@ModelAttribute注解的
else if (attrName != null) {
WebDataBinder binder =
resolveModelAttribute(attrName, methodParam, implicitModel, webRequest, handler);
boolean assignBindingResult = (args.length > i + 1 && Errors.class.isAssignableFrom(paramTypes[i + 1]));
if (binder.getTarget() != null) {
doBind(binder, webRequest, validate, validationHints, !assignBindingResult);
}
args[i] = binder.getTarget();
if (assignBindingResult) {
args[i + 1] = binder.getBindingResult();
i++;
}
implicitModel.putAll(binder.getBindingResult().getModel());
}
}
return args;
}以上的代码完成了request参数到controller类下的函数的参数的绑定。resolveRequestParam会调用WebDataBinder的convertIfNecessary方法，这个方法会搜索所有注册的converter并找到支持当前参数转化的对应converter进行参数转换。doBind()方法会调用WebDataBinder的validator成员变量的binder.validate(validationHints)在绑定参数时对参数进行验证。
Springmvc对支持的类型的转换是使用PropertyEditor对象来表示的，这个是可以由用户扩展的，我们来看看Springmvc默认支持哪些类型的转换。
private void createDefaultEditors() {
this.defaultEditors = new HashMap<Class<?>, PropertyEditor>(64);
// Simple editors, without parameterization capabilities.
// The JDK does not contain a default editor for any of these target types.
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());
this.defaultEditors.put(Pattern.class, new PatternEditor());
this.defaultEditors.put(Properties.class, new PropertiesEditor());
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());
}
// Default instances of collection editors.
// Can be overridden by registering custom instances of those as custom editors.
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));
// Default editors for primitive arrays.
this.defaultEditors.put(byte[].class, new ByteArrayPropertyEditor());
this.defaultEditors.put(char[].class, new CharArrayPropertyEditor());
// The JDK does not contain a default editor for char!
this.defaultEditors.put(char.class, new CharacterEditor(false));
this.defaultEditors.put(Character.class, new CharacterEditor(true));
// Spring's CustomBooleanEditor accepts more flag values than the JDK's default editor.
this.defaultEditors.put(boolean.class, new CustomBooleanEditor(false));
this.defaultEditors.put(Boolean.class, new CustomBooleanEditor(true));
// The JDK does not contain default editors for number wrapper types!
// Override JDK primitive number editors with our own CustomNumberEditor.
this.defaultEditors.put(byte.class, new CustomNumberEditor(Byte.class, false));
this.defaultEditors.put(Byte.class, new CustomNumberEditor(Byte.class, true));
this.defaultEditors.put(short.class, new CustomNumberEditor(Short.class, false));
this.defaultEditors.put(Short.class, new CustomNumberEditor(Short.class, true));
this.defaultEditors.put(int.class, new CustomNumberEditor(Integer.class, false));
this.defaultEditors.put(Integer.class, new CustomNumberEditor(Integer.class, true));
this.defaultEditors.put(long.class, new CustomNumberEditor(Long.class, false));
this.defaultEditors.put(Long.class, new CustomNumberEditor(Long.class, true));
this.defaultEditors.put(float.class, new CustomNumberEditor(Float.class, false));
this.defaultEditors.put(Float.class, new CustomNumberEditor(Float.class, true));
this.defaultEditors.put(double.class, 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));
// Only register config value editors if explicitly requested.
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);
}
}Springmvc的验证功能并没有实现具体的验证规则，一般需要利用hibernate的validator这里不对hibernate做介绍。我们来看看Springmvc实现了哪些和验证有关的东西。
HandlerMethodInvoker类下有个WebBindingInitializer类型的的成员变量，这个类的默认实现类是这样的：
它有一个Validator成员和一个ConversionService成员这两个成员都依赖我们的配置。
到这里SpringMVC的数据绑定、验证、转换就算讲完了。
