RequestMappingHandlerAdapter请求映射处理适配器，在SpringMVC中它是一个非常重要的类，对请求处理方法的调用主要是通过这个类来完成的(这段代码mv = ha.handle(processedRequest, response, mappedHandler.getHandler());)。下面我们就简单的分析一下这个类。首先我们先看一下它的UML类图结构：
画红线的部分是需要我们注意的一些类，如果你对Spring的Bean生命在周期熟悉的话，你会发现这些都是Spring的Bean生命周期相关的一些类（Spring Bean的生命周期小析(一)和Spring Bean的生命周期小析(二)）。我们分析RequestMappingHandlerAdapter的时候也会从这些接口的实现方法中开始。首先我们先看一下它的构造函数：
public RequestMappingHandlerAdapter() {
StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
stringHttpMessageConverter.setWriteAcceptCharset(false); // see SPR-7316
this.messageConverters = new ArrayList<HttpMessageConverter<?>>(4);
this.messageConverters.add(new ByteArrayHttpMessageConverter());
this.messageConverters.add(stringHttpMessageConverter);
this.messageConverters.add(new SourceHttpMessageConverter<Source>());
this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
}从上面的代码中我们可以看到这里默认添加了四种类型的Http数据转换器。其中我们需要关注的时候AllEncompassingFormHttpMessageConverter这个转换器，我们也去它的构造函数中看一下：
public AllEncompassingFormHttpMessageConverter() {
addPartConverter(new SourceHttpMessageConverter<Source>());
//如果Classpath下面有javax.xml.bind.Binder类，
//没有com.fasterxml.jackson.dataformat.xml.XmlMapper类的话
//则添加Jaxb2RootElementHttpMessageConverter转换器
if (jaxb2Present && !jackson2XmlPresent) {
addPartConverter(new Jaxb2RootElementHttpMessageConverter());
}
//如果Classpath下有com.fasterxml.jackson.databind.ObjectMapper
//和com.fasterxml.jackson.core.JsonGenerator的话，则添加
//MappingJackson2HttpMessageConverter转换器
if (jackson2Present) {
addPartConverter(new MappingJackson2HttpMessageConverter());
}
//如果Classpath下面有com.google.gson.Gson类的话，则添加
//GsonHttpMessageConverter转换器
else if (gsonPresent) {
addPartConverter(new GsonHttpMessageConverter());
}
//如果Classpath下有com.fasterxml.jackson.dataformat.xml.XmlMapper类的话，
//则添加MappingJackson2XmlHttpMessageConverter转换器
if (jackson2XmlPresent) {
addPartConverter(new MappingJackson2XmlHttpMessageConverter());
}
}其实在它的父类中还添加了三种类型的Convert：
public FormHttpMessageConverter() {
//application/x-www-form-urlencoded
this.supportedMediaTypes.add(MediaType.APPLICATION_FORM_URLENCODED);
//multipart/form-data
this.supportedMediaTypes.add(MediaType.MULTIPART_FORM_DATA);
//ByteArrayHttpMessageConverter
this.partConverters.add(new ByteArrayHttpMessageConverter());
StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
stringHttpMessageConverter.setWriteAcceptCharset(false);
//StringHttpMessageConverter
this.partConverters.add(stringHttpMessageConverter);
//ResourceHttpMessageConverter
this.partConverters.add(new ResourceHttpMessageConverter());
applyDefaultCharset();
}按照Spring的Bean的生命周期的执行顺序，这里会先调用BeanFactoryAware#setBeanFactory方法、接着调用ApplicationContextAware#setApplicationContext方法，最后调用InitializingBean#afterPropertiesSet方法。我们先看一下重写之后的setBeanFactory方法的源码：
@Override
public void setBeanFactory(BeanFactory beanFactory) {
if (beanFactory instanceof ConfigurableBeanFactory) {
this.beanFactory = (ConfigurableBeanFactory) beanFactory;
}
}这个方法的内容很简单，就是设置一下beanFactory的属性值(其实这里的beanFactory是DefaultListableBeanFactory的实例)。
下面我们看一下setApplicationContext这个方法的源码，这个方法藏的比较深，在它的父类ApplicationObjectSupport#setApplicationContext方法中。
public final void setApplicationContext(ApplicationContext context) throws BeansException {
//isContextRequired()方法返回true
if (context == null && !isContextRequired()) {
// Reset internal context state.
this.applicationContext = null;
this.messageSourceAccessor = null;
}
else if (this.applicationContext == null) {
// Initialize with passed-in context.
//所传入的context如果不能被实例化，则抛出异常
if (!requiredContextClass().isInstance(context)) {
throw new ApplicationContextException(
"Invalid application context: needs to be of type [" + requiredContextClass().getName() + "]");
}
this.applicationContext = context;
//国际化
this.messageSourceAccessor = new MessageSourceAccessor(context);
//初始化ApplicationContext
initApplicationContext(context);
}
else {
// Ignore reinitialization if same context passed in.
if (this.applicationContext != context) {
throw new ApplicationContextException(
"Cannot reinitialize with different application context: current one is [" +
this.applicationContext + "], passed-in one is [" + context + "]");
}
}
}我们主要看一下WebApplicationObjectSupport#initApplicationContext这个方法的内容：
@Override
protected void initApplicationContext(ApplicationContext context) {
//调用父类中的initApplicationContext方法
super.initApplicationContext(context);
if (this.servletContext == null && context instanceof WebApplicationContext) {
this.servletContext = ((WebApplicationContext) context).getServletContext();
if (this.servletContext != null) {
//初始化ServletContext
initServletContext(this.servletContext);
}
}
}这里先调用父类中的initApplicationContext方法，然后会初始ServletContext。先看一下ApplicationObjectSupport#initApplicationContext方法：
protected void initApplicationContext(ApplicationContext context) throws BeansException {
initApplicationContext();
}在我们的这个类的继承体系中,initApplicationContext();是一个空实现。同样initServletContext也是一个空实现。接下来我们要分析的一个重点InitializingBean#afterPropertiesSet的方法，它的源码内容如下，下面我们一点一点的分析这个类：
public void afterPropertiesSet() {
// Do this first, it may add ResponseBody advice beans
initControllerAdviceCache();
if (this.argumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
if (this.initBinderArgumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
if (this.returnValueHandlers == null) {
List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
}
}我们先看initControllerAdviceCache这个方法：
private void initControllerAdviceCache() {
//通过上面的分析我们知道，这个方法的内容不为null
if (getApplicationContext() == null) {
return;
}
//获取所有带ControllerAdvice注解的类
List<ControllerAdviceBean> beans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
//对获取到的ControllerAdvice注解的类进行排序，排序的规则是基于实现PriorityOrdered接口或者带有Order注解
AnnotationAwareOrderComparator.sort(beans);
List<Object> requestResponseBodyAdviceBeans = new ArrayList<Object>();
//循环获取到的ControllerAdviceBean
for (ControllerAdviceBean bean : beans) {
//获取所有ModelAttribute注解的方法，并且没有RequestMapping注解的方法
Set<Method> attrMethods = MethodIntrospector.selectMethods(bean.getBeanType(), MODEL_ATTRIBUTE_METHODS);
if (!attrMethods.isEmpty()) {
this.modelAttributeAdviceCache.put(bean, attrMethods);
if (logger.isInfoEnabled()) {
logger.info("Detected @ModelAttribute methods in " + bean);
}
}
//获取所有带InitBinder注解的方法
Set<Method> binderMethods = MethodIntrospector.selectMethods(bean.getBeanType(), INIT_BINDER_METHODS);
if (!binderMethods.isEmpty()) {
this.initBinderAdviceCache.put(bean, binderMethods);
if (logger.isInfoEnabled()) {
logger.info("Detected @InitBinder methods in " + bean);
}
}
//如果实现了RequestBodyAdvice接口
if (RequestBodyAdvice.class.isAssignableFrom(bean.getBeanType())) {
requestResponseBodyAdviceBeans.add(bean);
if (logger.isInfoEnabled()) {
logger.info("Detected RequestBodyAdvice bean in " + bean);
}
}
//如果实现了ResponseBodyAdvice接口
if (ResponseBodyAdvice.class.isAssignableFrom(bean.getBeanType())) {
requestResponseBodyAdviceBeans.add(bean);
if (logger.isInfoEnabled()) {
logger.info("Detected ResponseBodyAdvice bean in " + bean);
}
}
}
//添加到requestResponseBodyAdvice集合中
if (!requestResponseBodyAdviceBeans.isEmpty()) {
this.requestResponseBodyAdvice.addAll(0, requestResponseBodyAdviceBeans);
}
}在这个方法里主要是获取了带ControllerAdvice注解的类，并从这些类中查找实现了RequestBodyAdvice或者ResponseBodyAdvice接口的类，添加到requestResponseBodyAdvice集合中，另外获取所有带ModelAttribute注解且没有RequestMapping注解的方法，放到modelAttributeAdviceCache集合中，获取所有带InitBinder注解的方法放到initBinderAdviceCache的集合中。下面我们再看这一段代码：
if (this.argumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
private HandlerMethodArgumentResolverComposite argumentResolvers;argumentResolvers这个属性是HandlerMethodArgumentResolverComposite 类型的，如果我们在配置RequestMappingHandlerAdapter的时候设置了一系列HandlerMethodArgumentResolver的实现类的话，如下所示，
public void setArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {
if (argumentResolvers == null) {
this.argumentResolvers = null;
}
else {
this.argumentResolvers = new HandlerMethodArgumentResolverComposite();
this.argumentResolvers.addResolvers(argumentResolvers);
}
}它会先创建HandlerMethodArgumentResolverComposite对象，然后把配置的HandlerMethodArgumentResolver的实现类添加到HandlerMethodArgumentResolverComposite的对象中。这里我们是没有手工配置RequestMappingHandlerAdapter的，所以，会先调用getDefaultArgumentResolvers方法，获取一系列默认的HandlerMethodArgumentResolver的实现类，代码如下：
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
List<HandlerMethodArgumentResolver> resolvers = new ArrayList<HandlerMethodArgumentResolver>();
// Annotation-based argument resolution
resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
resolvers.add(new RequestParamMapMethodArgumentResolver());
resolvers.add(new PathVariableMethodArgumentResolver());
resolvers.add(new PathVariableMapMethodArgumentResolver());
resolvers.add(new MatrixVariableMethodArgumentResolver());
resolvers.add(new MatrixVariableMapMethodArgumentResolver());
resolvers.add(new ServletModelAttributeMethodProcessor(false));
resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
resolvers.add(new RequestHeaderMapMethodArgumentResolver());
resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
resolvers.add(new SessionAttributeMethodArgumentResolver());
resolvers.add(new RequestAttributeMethodArgumentResolver());
// Type-based argument resolution
resolvers.add(new ServletRequestMethodArgumentResolver());
resolvers.add(new ServletResponseMethodArgumentResolver());
resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
resolvers.add(new RedirectAttributesMethodArgumentResolver());
resolvers.add(new ModelMethodProcessor());
resolvers.add(new MapMethodProcessor());
resolvers.add(new ErrorsMethodArgumentResolver());
resolvers.add(new SessionStatusMethodArgumentResolver());
resolvers.add(new UriComponentsBuilderMethodArgumentResolver());
// Custom arguments
if (getCustomArgumentResolvers() != null) {
resolvers.addAll(getCustomArgumentResolvers());
}
// Catch-all
resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
resolvers.add(new ServletModelAttributeMethodProcessor(true));
return resolvers;
}SpringMVC为我们自动添加了大概26个HandlerMethodArgumentResolver的实现类。这里有一个方法需要我们注意一下：getCustomArgumentResolvers() 方法。这个方法是用来获取自定义的HandlerMethodArgumentResolver的实现类，也就是说如果我们有自己写的HandlerMethodArgumentResolver的方法的话，我们配置的属性是customArgumentResolvers，最好不要配置argumentResolvers这个属性。然后我们注意的一点是我们所添加的所以的HandlerMethodArgumentResolver的实现类，都是添加到了HandlerMethodArgumentResolverComposite这个类中，HandlerMethodArgumentResolverComposite也是HandlerMethodArgumentResolver的一个实现类。还有一点需要注意的是，这里创建了两个RequestParamMethodArgumentResolver的实例，一个传了true参数，一个传了false参数，这是用来解析不同的请求参数的，详情请看这里：
SpringMVC之分析请求对应处理器方法参数的解析过程(一)
