18、SpringMVC源码分析 - @ModelAttribute的使用及原理
一、使用
1、给ModelMap添加属性
可以通过给方法参数Model添加属性，也可以通过方法上注解@ModelAttribute来指定model的属性名称，返回值会作为属性值，下面方法执行后，在springmvc中ModelMap就会存在两个model。
在Controller中定义方法：
//指定返回值属性为user1
@ModelAttribute("user1")
public User getUser1(Model model) {
//给model添加属性userId
model.addAttribute("userId", 10);
User user = new User();
user.setId(1);
return user;
}
2、使用ModelMap中的model
在方法参数上加上@ModelAttribute注解，并值指定名称
@GetMapping(value = "/modelAttribute/test", produces = {
MediaType.APPLICATION_JSON_VALUE})
public ResponseEntity modelAttributeTest(Model model, @ModelAttribute(name = "user1") User user1, @ModelAttribute(name = "user2") User user2) throws BaseException {
ResponseEntity responseEntity = new ResponseEntity();
System.out.println("userId: " + model.getAttribute("userId"));
System.out.println("user1: " + user1);
System.out.println("user2: " + user2);
return responseEntity;
}
结果
userId: 10
user1: User(id=1, username=null, password=null, role=null, email=null, phone=188, realName=null, unit=null)
user2: User(id=2, username=null, password=null, role=null, email=null, phone=188, realName=null, unit=null)
3、全局的ModelAttribute
@RestControllerAdvice
public class ControllerAdviceTest {
@ModelAttribute("user2")
public User getUser() {
User user = new User();
user.setId(2);
return user;
}
}
二、@ModelAttribute方法解析
1、查找@ModelAttribute方法
在RequestMappingHandlerAdapter调用invokeHandlerMethod( )方法中，会组装ModelFactory
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
private ModelFactory getModelFactory(HandlerMethod handlerMethod, WebDataBinderFactory binderFactory) {
SessionAttributesHandler sessionAttrHandler = getSessionAttributesHandler(handlerMethod);
Class<?> handlerType = handlerMethod.getBeanType();
Set<Method> methods = this.modelAttributeCache.get(handlerType);
if (methods == null) {
//查找controller中标识有@ModelAttribute注解，并且没有@RequestMapping注解的方法
methods = MethodIntrospector.selectMethods(handlerType, MODEL_ATTRIBUTE_METHODS);
this.modelAttributeCache.put(handlerType, methods);
}
List<InvocableHandlerMethod> attrMethods = new ArrayList<>();
// Global methods first
//从@ControllerAdvice类中查找全局的@ModelAttribute注解方法
this.modelAttributeAdviceCache.forEach((controllerAdviceBean, methodSet) -> {
if (controllerAdviceBean.isApplicableToBeanType(handlerType)) {
Object bean = controllerAdviceBean.resolveBean();
for (Method method : methodSet) {
attrMethods.add(createModelAttributeMethod(binderFactory, bean, method));
}
}
});
for (Method method : methods) {
Object bean = handlerMethod.getBean();
attrMethods.add(createModelAttributeMethod(binderFactory, bean, method));
}
return new ModelFactory(attrMethods, binderFactory, sessionAttrHandler);
}
2、调用@ModelAttribute方法
同样在RequestMappingHandlerAdapter调用invokeHandlerMethod( )方法中
modelFactory.initModel(webRequest, mavContainer, invocableMethod);
public void initModel(NativeWebRequest request, ModelAndViewContainer container, HandlerMethod handlerMethod)
throws Exception {
//@SessionAttributes 标识的一些属性名
Map<String, ?> sessionAttributes = this.sessionAttributesHandler.retrieveAttributes(request);
//将sessionAttributes合并到modelMap中
container.mergeAttributes(sessionAttributes);
//调用@ModelAttribute方法
invokeModelAttributeMethods(request, container);
//从sessionAttributesHandler查找属性
for (String name : findSessionAttributeArguments(handlerMethod)) {
//ModelMap中不包含该属性时，从sessionAttributesHandler获取
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);
}
}
}
private void invokeModelAttributeMethods(NativeWebRequest request, ModelAndViewContainer container)
throws Exception {
while (!this.modelMethods.isEmpty()) {
InvocableHandlerMethod modelMethod = getNextModelMethod(container).getHandlerMethod();
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;
}
//调用方法
Object returnValue = modelMethod.invokeForRequest(request, container);
if (modelMethod.isVoid()) {
if (StringUtils.hasText(ann.value())) {
if (logger.isDebugEnabled()) {
logger.debug("Name in @ModelAttribute is ignored because method returns void: " +
modelMethod.getShortLogMessage());
}
}
continue;
}
//参数名
String returnValueName = getNameForReturnValue(returnValue, modelMethod.getReturnType());
if (!ann.binding()) {
container.setBindingDisabled(returnValueName);
}
if (!container.containsAttribute(returnValueName)) {
//添加到ModelMap中
container.addAttribute(returnValueName, returnValue);
}
}
}
二、@ModelAttribute参数解析
在ModelAttributeMethodProcessor类中
public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
Assert.state(mavContainer != null, "ModelAttributeMethodProcessor requires ModelAndViewContainer");
Assert.state(binderFactory != null, "ModelAttributeMethodProcessor requires WebDataBinderFactory");
//参数名
String name = ModelFactory.getNameForParameter(parameter);
ModelAttribute ann = parameter.getParameterAnnotation(ModelAttribute.class);
if (ann != null) {
mavContainer.setBinding(name, ann.binding());
}
Object attribute = null;
BindingResult bindingResult = null;
if (mavContainer.containsAttribute(name)) {
//从ModelMap中获取参数值
attribute = mavContainer.getModel().get(name);
}
else {
// Create attribute instance
try {
attribute = createAttribute(name, parameter, binderFactory, webRequest);
}
catch (BindException ex) {
if (isBindExceptionRequired(parameter)) {
// No BindingResult parameter -> fail with BindException
throw ex;
}
// Otherwise, expose null/empty value and associated BindingResult
if (parameter.getParameterType() == Optional.class) {
attribute = Optional.empty();
}
else {
attribute = ex.getTarget();
}
bindingResult = ex.getBindingResult();
}
}
if (bindingResult == null) {
// Bean property binding and validation;
// skipped in case of binding failure on construction.
//创建绑定器
WebDataBinder binder = binderFactory.createBinder(webRequest, attribute, name);
if (binder.getTarget() != null) {
if (!mavContainer.isBindingDisabled(name)) {
bindRequestParameters(binder, webRequest);
}
//参数校验
validateIfApplicable(binder, parameter);
if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {
throw new BindException(binder.getBindingResult());
}
}
// Value type adaptation, also covering java.util.Optional
//参数类型不一致时进行转换
if (!parameter.getParameterType().isInstance(attribute)) {
attribute = binder.convertIfNecessary(binder.getTarget(), parameter.getParameterType(), parameter);
}
bindingResult = binder.getBindingResult();
}
// Add resolved attribute and BindingResult at the end of the model
Map<String, Object> bindingResultModel = bindingResult.getModel();
mavContainer.removeAttributes(bindingResultModel);
mavContainer.addAllAttributes(bindingResultModel);
return attribute;
}