SpringMvc学习心得（五）控制器产生与构建

    在springmvc中，控制器(controller)是一个很重要的概念。在实际项目中，我们一般在控制器里完成具体的业务逻辑。控制器是非常重要，因此讨论控制器的产生和构建就变得很有意义(PS：我们在这里主要讨论基于注解的配置方式)。

  在讨论控制器的相关问题之前，需要考虑的第一个问题是：ApplicationContext的类型是如何确定的？ApplicationContext是spring的IOC机制实现的一个核心，spring的很多功能都是通过ApplicationContext对外输出的。而springmvc的ApplicationContext则是“写死”在FrameworkServlet这个类的field中。

  FrameworkServlet.java

public abstract class FrameworkServlet extends HttpServletBean {/** * Suffix for WebApplicationContext namespaces. If a servlet of this class is * given the name "test" in a context, the namespace used by the servlet will * resolve to "test-servlet". */public static final String DEFAULT_NAMESPACE_SUFFIX = "-servlet";/** * Default context class for FrameworkServlet. * @see org.springframework.web.context.support.XmlWebApplicationContext */public static final Class<?> DEFAULT_CONTEXT_CLASS = XmlWebApplicationContext.class;

        //some other code

  而在XmlWebApplicationContext中，spring“手动”创建了一个ApplicationContextAwareProcessor，并将它注册到beanfactory的beanPostProcessor列表中，具体代码如下：

beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));

public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");this.beanPostProcessors.remove(beanPostProcessor);this.beanPostProcessors.add(beanPostProcessor);if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {this.hasInstantiationAwareBeanPostProcessors = true;}if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {this.hasDestructionAwareBeanPostProcessors = true;}}

  至此，产生和构建控制器的前置步骤就完成了。

   控制器的产生：打开@Controller这一注解的源代码，你会发现该注解会被@Component这一注解所修饰。因此，所有被@Controller所修饰的类都会默认被@Component所修饰。同时这也意味着搭配<context:component-scan base-package="xxx.xxxx.xxxx" />这一标签，所有被@Controller所修饰的类都会被注册成为JavaBean。这个JavaBean与其它自定义的JavaBean没有什么区别。而重要的区别则在于被@Controller所修饰的类，能够被之前注册的BeanPostProcessor所扫描并进行处理。

   控制器的构建：springmvc将构建控制器的这一工作交给了BeanPostProcessor进行处理。在<mvc:annotation-driven/>这一标签里，springmvc创建了一个JavaBean，这个bean是RequestMappingHandlerMapping。当这个JavaBean被反射出来但是还没有被初始化的时候，BeanPostProcessor的postProcessBeforeInitialization会发挥作用。当然，由于会首先对JavaBean进行过滤。具体代码如下：

if (bean instanceof ApplicationContextAware) {((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);}

  RequestMappingHandlerMapping会使用Applicationcontext的getBeanNamesForType函数去查找所有基类是Object的JavaBean。显而易见，所有的JavaBean都会通过该方法被扫描出来。然后对其中那些被@Controller所修饰的类进行进一步探测和处理。具体代码如下：

 RequestMappingHandlerMapping扫描JavaBean并处理的方法：

protected void initHandlerMethods() {if (logger.isDebugEnabled()) {logger.debug("Looking for request mappings in application context: " + getApplicationContext());}String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :getApplicationContext().getBeanNamesForType(Object.class));for (String beanName : beanNames) {if (isHandler(getApplicationContext().getType(beanName))){detectHandlerMethods(beanName);}}handlerMethodsInitialized(getHandlerMethods());}

   isHandler方法(ps：判断是否为控制器)：

protected boolean isHandler(Class<?> beanType) {return AnnotationUtils.findAnnotation(beanType, Controller.class) != null;}

  RequestMappingHandlerMapping中一个很重要的函数是detectHandlerMethods。该函数具体代码如下：

protected void detectHandlerMethods(final Object handler) {Class<?> handlerType = (handler instanceof String) ? getApplicationContext().getType((String) handler) : handler.getClass();final Class<?> userType = ClassUtils.getUserClass(handlerType);Set<Method> methods = HandlerMethodSelector.selectMethods(userType, new MethodFilter() {public boolean matches(Method method) {return getMappingForMethod(method, userType) != null;}});for (Method method : methods) {T mapping = getMappingForMethod(method, userType);registerHandlerMethod(handler, method, mapping);}}

  可以看到，该函数主要工作是查找handler中的所有method，并根据method生成对应的mapping，并将mapping，method和handler进行注册。而其中最重要的函数则是getMappingForMethod。该函数的主要逻辑是根据method产生一个RequestMappingInfo，然后根据handlerType产生另一个RequestMappingInfo。最后将这两个RequestMappingInfo进行合并。该函数代码如下：

protected RequestMappingInfo getMappingForMethod(Method method, Class<?> handlerType) {RequestMappingInfo info = null;RequestMapping methodAnnotation = AnnotationUtils.findAnnotation(method, RequestMapping.class);if (methodAnnotation != null) {RequestCondition<?> methodCondition = getCustomMethodCondition(method);info = createRequestMappingInfo(methodAnnotation, methodCondition);RequestMapping typeAnnotation = AnnotationUtils.findAnnotation(handlerType, RequestMapping.class);if (typeAnnotation != null) {RequestCondition<?> typeCondition = getCustomTypeCondition(handlerType);info = createRequestMappingInfo(typeAnnotation, typeCondition).combine(info);}}return info;}

   RequestMappingHandlerMapping通过registerHandlerMethod函数对mapping，handler和method进行注册，RequestMappingHandlerMapping会生成一个LinkedHashMap，并以mapping为key，以handler和method为value。 同时在注册时会检测mapping的是否重复。至此，controller的构建所示完成了。
  既然是以mapping为可以，那么必然会涉及equals函数和hashcode函数，下面是RequestMappingInfode 的equals函数以及hashcode函数。

public int hashCode() {int result = hash;if (result == 0) {result = patternsCondition.hashCode();result = 31 * result + methodsCondition.hashCode();result = 31 * result + paramsCondition.hashCode();result = 31 * result + headersCondition.hashCode();result = 31 * result + consumesCondition.hashCode();result = 31 * result + producesCondition.hashCode();result = 31 * result + customConditionHolder.hashCode();hash = result;}return result;}

public boolean equals(Object obj) {if (this == obj) {return true;}if (obj != null && obj instanceof RequestMappingInfo) {RequestMappingInfo other = (RequestMappingInfo) obj;return (this.patternsCondition.equals(other.patternsCondition) &&this.methodsCondition.equals(other.methodsCondition) &&this.paramsCondition.equals(other.paramsCondition) &&this.headersCondition.equals(other.headersCondition) &&this.consumesCondition.equals(other.consumesCondition) &&this.producesCondition.equals(other.producesCondition) && this.customConditionHolder.equals(other.customConditionHolder));}return false;}