SpringMVC 肯定pojo值的三步

1，若是隐含模型中有这个key（标了modelAttribute 注解就是注解指定的value，
没标就是参数类型的首字母小写）指定的值；若是有将这个值赋值给bindObject
2，若是是sessionAttribute标注的属性，就从session中拿，拿不到就抛异常
3，若是都不是，就利用反射建立对象

源码以下：类名HandlerMethodInvoker

private WebDataBinder resolveModelAttribute(String attrName, MethodParameter methodParam,
			ExtendedModelMap implicitModel, NativeWebRequest webRequest, Object handler) throws Exception {

		// Bind request parameter onto object...
		String name = attrName;
		if ("".equals(name)) {
			name = Conventions.getVariableNameForParameter(methodParam);
		}
		Class<?> paramType = methodParam.getParameterType();
		Object bindObject;
		if (implicitModel.containsKey(name)) {
			bindObject = implicitModel.get(name);
		}
		else if (this.methodResolver.isSessionAttribute(name, paramType)) {
			bindObject = this.sessionAttributeStore.retrieveAttribute(webRequest, name);
			if (bindObject == null) {
				raiseSessionRequiredException("Session attribute '" + name + "' required - not found in session");
			}
		}
		else {
			bindObject = BeanUtils.instantiateClass(paramType);
		}
		WebDataBinder binder = createBinder(webRequest, bindObject, name);
		initBinder(handler, name, binder, webRequest);
		return binder;
	}