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。