一、什么是@ModelAttribute注解
@ModelAttribute注解主要用来将请求转换为使用此注解指定的对象。例如，如果在@ModelAttribute旁边指定了一个Article实例，则与Article的字段对应的所有请求参数将被用作Article的字段值。什么意思呢，例如，POST提交后参数title的值将被设置为Article的title 字段。
http://blog.csdn.net/hejingyuan6/article/details/49995987
因此，此注解允许开发人员通过请求来持久化一个对象。没有它，Spring认为必须创建一个新对象。另外，它直接显示一个对象模型来查看。你不需要在方法中再调用model.setAttribute()。在视图部分，可以通过注解中的指定值查找指定对象(例如，@ModelAttribute(“articleView”)可以在jsp中通过&{articleView}获取相应的值)或对象的类名称(例如@ModelAttribute()Article article将在视图层获取方式就是${article})。
二、@ModelAttribute注解相关代码详解
@ModelAttribute注解的方法是作用于整个Controller的，实际上在执行Controller的每个请求时都会执行@ModelAttribute注解的方法。
执行过程在org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter中查看，每次执行Controller时都会执行@ModelAttribute注解的方法:
/**
* Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
* if view resolution is required.
* @since 4.2 可以看到4.2开始启用了
* @see #createInvocableHandlerMethod(HandlerMethod)
*/
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ServletWebRequest webRequest = new ServletWebRequest(request, response);
try {
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
if (this.argumentResolvers != null) {
invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
invocableMethod.setDataBinderFactory(binderFactory);
invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
//执行@ModelAttribute注解的方法
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);
}
//执行Controller中的方法
invocableMethod.invokeAndHandle(webRequest, mavContainer);
if (asyncManager.isConcurrentHandlingStarted()) {
return null;
}
return getModelAndView(mavContainer, modelFactory, webRequest);
}
finally {
webRequest.requestCompleted();
}
}modelFactory.initModel(webRequest, mavContainer, invocableMethod)中会执行@ModelAttribute注解的方法(org.springframework.web.method.annotation.ModelFactory中可查看):
/**
* Populate the model in the following order:
* <ol>
* <li>Retrieve "known" session attributes listed as {@code @SessionAttributes}.
* <li>Invoke {@code @ModelAttribute} methods
* <li>Find {@code @ModelAttribute} method arguments also listed as
* {@code @SessionAttributes} and ensure they're present in the model raising
* an exception if necessary.
* </ol>
* @param request the current request
* @param container a container with the model to be initialized
* @param handlerMethod the method for which the model is initialized
* @throws Exception may arise from {@code @ModelAttribute} methods
*/
public void initModel(NativeWebRequest request, ModelAndViewContainer container,
HandlerMethod handlerMethod) throws Exception {
Map<String, ?> sessionAttributes = this.sessionAttributesHandler.retrieveAttributes(request);
container.mergeAttributes(sessionAttributes);
//执行@ModelAttribute注解的方法
invokeModelAttributeMethods(request, container);
////方法执行结果的值放到container
for (String name : findSessionAttributeArguments(handlerMethod)) {
if (!container.containsAttribute(name)) {
Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
if (value == null) {
throw new HttpSessionRequiredException("Expected session attribute '" + name + "'", name);
}
container.addAttribute(name, value);
}
}
}在private void invokeModelAttributeMethods(NativeWebRequest request, ModelAndViewContainer container)中会判断方法上是否被@ModelAttribute注解，如果是则会执行这个方法，并将返回值放到container中:
/**
* Invoke model attribute methods to populate the model.
* Attributes are added only if not already present in the model.
*/
private void invokeModelAttributeMethods(NativeWebRequest request, ModelAndViewContainer container)
throws Exception {
while (!this.modelMethods.isEmpty()) {
InvocableHandlerMethod modelMethod = getNextModelMethod(container).getHandlerMethod();
//判断方法是否被@ModelAttribute注解
ModelAttribute ann = modelMethod.getMethodAnnotation(ModelAttribute.class);
Assert.state(ann != null, "No ModelAttribute annotation");
if (container.containsAttribute(ann.name())) {
if (!ann.binding()) {
container.setBindingDisabled(ann.name());
}
continue;
}
//执行被@ModelAttribute注解的方法
Object returnValue = modelMethod.invokeForRequest(request, container);
if (!modelMethod.isVoid()){
String returnValueName = getNameForReturnValue(returnValue, modelMethod.getReturnType());
if (!ann.binding()) {
container.setBindingDisabled(returnValueName);
}
if (!container.containsAttribute(returnValueName)) {
container.addAttribute(returnValueName, returnValue);
}
}
}
}我们进入org.springframework.web.method.support.InvocableHandlerMethod 的invokeForRequest方法，在给定request请求的上下文中解析其参数值后调用该方法，参数值通常通过 HandlerMethodArgumentResolver来解析。
/**
* Invoke the method after resolving its argument values in the context of the given request.
* <p>Argument values are commonly resolved through {@link HandlerMethodArgumentResolver}s.
* The {@code providedArgs} parameter however may supply argument values to be used directly,
* i.e. without argument resolution. Examples of provided argument values include a
* {@link WebDataBinder}, a {@link SessionStatus}, or a thrown exception instance.
* Provided argument values are checked before argument resolvers.
* @param request the current request
* @param mavContainer the ModelAndViewContainer for this request
* @param providedArgs "given" arguments matched by type, not resolved
* @return the raw value returned by the invoked method
* @exception Exception raised if no suitable argument resolver can be found,
* or if the method raised an exception
*/
@Nullable
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
//看下面的方法
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"' with arguments " + Arrays.toString(args));
}
Object returnValue = doInvoke(args);
if (logger.isTraceEnabled()) {
logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"] returned [" + returnValue + "]");
}
return returnValue;
}
/**
* Get the method argument values for the current request.
*/
private Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
MethodParameter[] parameters = getMethodParameters();
Object[] args = new Object[parameters.length];
for (int i = 0; i < parameters.length; i++) {
MethodParameter parameter = parameters[i];
parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
args[i] = resolveProvidedArgument(parameter, providedArgs);
if (args[i] != null) {
continue;
}
if (this.argumentResolvers.supportsParameter(parameter)) {
try {
//又回归到解析参数的老路上了，就不多解析了
args[i] = this.argumentResolvers.resolveArgument(
parameter, mavContainer, request, this.dataBinderFactory);
continue;
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
}
throw ex;
}
}
if (args[i] == null) {
throw new IllegalStateException("Could not resolve method parameter at index " +
parameter.getParameterIndex() + " in " + parameter.getExecutable().toGenericString() +
": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));
}
}
return args;
}org.springframework.web.method.support.HandlerMethodArgumentResolverComposite
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/**
* Iterate over registered {@link HandlerMethodArgumentResolver}s and invoke the one that supports it.
* @throws IllegalStateException if no suitable {@link HandlerMethodArgumentResolver} is found.
*/
@Override
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
if (resolver == null) {
throw new IllegalArgumentException("Unknown parameter type [" + parameter.getParameterType().getName() + "]");
}
//又回到老版本的resolveArgument路上了
return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}
/**
* Find a registered {@link HandlerMethodArgumentResolver} that supports the given method parameter.
*/
@Nullable
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
if (result == null) {
for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
if (logger.isTraceEnabled()) {
logger.trace("Testing if argument resolver [" + methodArgumentResolver + "] supports [" +
parameter.getGenericParameterType() + "]");
}
if (methodArgumentResolver.supportsParameter(parameter)) {
result = methodArgumentResolver;
this.argumentResolverCache.put(parameter, result);
break;
}
}
}
return result;
}回到org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter,可以看到:
public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
implements BeanFactoryAware, InitializingBean {
@Nullable
private List<HandlerMethodArgumentResolver> customArgumentResolvers;
@Nullable
private HandlerMethodArgumentResolverComposite argumentResolvers;
@Nullable
private HandlerMethodArgumentResolverComposite initBinderArgumentResolvers;
@Nullable
private List<HandlerMethodReturnValueHandler> customReturnValueHandlers;http://blog.csdn.net/hejingyuan6/article/details/49995987
当@ModelAttribute注解方法时,这个方法在每次访问Controller时都会被执行,其执行到的原理就是在每次执行Controller时都会判断一次,并执行@ModelAttribute的方法,将执行后的结果值放到container中.
转载：
芋道源码