Spring MVC 源码学习札记（四）HandlerMapping和HandlerAdapter（1）
在分析HandlerMapping和HandlerAdapter以及其子类关系和代码之前，我们先来看看DispatcherServlet对它们的初始化是怎样的，在DispatcherServlet类中，有HandlerMapping和HandlerAdapter的链表。
/** List of HandlerMappings used by this servlet */ private List<HandlerMapping> handlerMappings; /** List of HandlerAdapters used by this servlet */ private List<HandlerAdapter> handlerAdapters;
下面的布尔属性是DispatcherServlet中用来标记探测结果的，true表示探测所有的HandlerMapping，false表示只探测已在***-servlet.xml中声明的handlerMapping bean，Adapter同上。
/** Detect all HandlerMappings or just expect "handlerMapping" bean? */ private boolean detectAllHandlerMappings = true; /** Detect all HandlerAdapters or just expect "handlerAdapter" bean? */ private boolean detectAllHandlerAdapters = true;
下面是HandlerMapping的初始化方法：
/**
* Initialize the HandlerMappings used by this class.
* <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
* we default to BeanNameUrlHandlerMapping.
*/
private void initHandlerMappings(ApplicationContext context) {
this.handlerMappings = null;
if (this.detectAllHandlerMappings) {
// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
Map<String, HandlerMapping> matchingBeans =
BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
// We keep HandlerMappings in sorted order.
OrderComparator.sort(this.handlerMappings);
}
}
else {
try {
HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
this.handlerMappings = Collections.singletonList(hm);
}
catch (NoSuchBeanDefinitionException ex) {
// Ignore, we'll add a default HandlerMapping later.
}
}
// Ensure we have at least one HandlerMapping, by registering
// a default HandlerMapping if no other mappings are found.
if (this.handlerMappings == null) {
this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
if (logger.isDebugEnabled()) {
logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
}
}
}
前面的一个if...else...很好理解，就是根据是否探测上下文中所有bean name不管是handlerMapping还是不是的HandlerMapping实体bean。第二个if，当在上下文中没有找到注册的HandlerMapping时进入该条件，注意方法getDefaultStrategies，它有两个参数，一个是上下文，一个是Class对象。
/**
* Create a List of default strategy objects for the given strategy interface.
* <p>The default implementation uses the "DispatcherServlet.properties" file (in the same
* package as the DispatcherServlet class) to determine the class names. It instantiates
* the strategy objects through the context's BeanFactory.
* @param context the current WebApplicationContext
* @param strategyInterface the strategy interface
* @return the List of corresponding strategy objects
*/
@SuppressWarnings("unchecked")
protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
String key = strategyInterface.getName();
String value = defaultStrategies.getProperty(key);
if (value != null) {
String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
List<T> strategies = new ArrayList<T>(classNames.length);
for (String className : classNames) {
try {
Class clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
Object strategy = createDefaultStrategy(context, clazz);
strategies.add((T) strategy);
}
catch (ClassNotFoundException ex) {
throw new BeanInitializationException(
"Could not find DispatcherServlet's default strategy class [" + className +
"] for interface [" + key + "]", ex);
}
catch (LinkageError err) {
throw new BeanInitializationException(
"Error loading DispatcherServlet's default strategy class [" + className +
"] for interface [" + key + "]: problem with class file or dependent class", err);
}
}
return strategies;
}
else {
return new LinkedList<T>();
}
}
String key这一行就不用解释了，就是得到类名，请看第二行
String value = defaultStrategies.getProperty(key);
这里defaultStrategies是DispatcherSerlvet的属性
private static final Properties defaultStrategies;
static {
// Load default strategy implementations from properties file.
// This is currently strictly internal and not meant to be customized
// by application developers.
try {
ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
}
catch (IOException ex) {
throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
}
}
/** * Name of the class path resource (relative to the DispatcherServlet class) that defines DispatcherServlet's default * strategy names. */ private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";
在DispatcherServlet的静态域，即第一次实例化该类时，运行的区域，读取了DispatcherSerlvet.properties这个配置文件，Properties类就不解释了，打开源码目录的时候，我们可以看到在DispatcherSerlvet.java的同级目录下有这个配置文件K=V。
org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\ org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\ org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\ org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\ org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\ org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
可以看到HandlerMapping的默认类有BeanNameUrlHandlerMapping（bean和Url直直接映射的解析HandlerMapping），DefaultAnnotationHandlerMapping（默认注解支持的HandlerMapping）。
我们继续向下看getDefaultStrategies部分的源码，当得到value的String值之后，将String进行破分，变为String数组，并对BeanNameUrlHandlerMapping和DefaultAnnotationHandlerMapping进行bean的创建，使用语句
Object strategy = createDefaultStrategy(context, clazz);
该方法内部：
protected Object createDefaultStrategy(ApplicationContext context, Class<?> clazz) {
return context.getAutowireCapableBeanFactory().createBean(clazz);
}
将创建后的bean加入到链表strategies中并返回。
这样HandlerMapping的链表就初始化完成了。
HandlerAdapter也是同理。
这一篇已经太长了，所以将重点介绍放在下一节。