IOC容器初始化之学习抽象类AbstractApplicationContext（一）

Spring整个应用的骨架流程就是AbstractApplicationContext中的refresh，Spring中的各个功能都是围绕这个refresh向外扩展的。
我们知道，自从Servlet03.0去web.xml配置以来，启动Spring应用的方式，又新增了一种通过Java Config来实现。因此，Spring为了实现这种无XM了的启动，又新增了另一种实现方式，AnnotationConfigWebApplicationContext。
如果，这个关键流程，被放在XmlWebApplicationContext,再去实现AnnotationConfigWebApplicationContext,代码就会变的复杂臃肿。
因此，流程要放在抽象类中，这样，一个应用才会更容易通过实现子类，来定制自己的特性功能，亦会使得应用更易扩展功能。
AbstractApplicationContext的
通过web.xml方式来loadBeanDefinition的行为在XmlWebApplicationContext中实现。
通过Servlet3.0方式来loadBeanDefinition的行为在AnnotationConfigWebApplicationContext中实现。而它们的父类，AbstractApplicationContext仅仅是定义了lOC容器初始化的流程。AbstractApplicationContext的子类AbstractRefreshableApplicationContext则是具体实现了freshBeanFactory。在这里，创建BeanFactory并加载beanDefinition。同样，加载beanDefinition这个动作交由子类去实现，为了实现Spring两种启动方式。
AbstractApplicationContext交由其直接子类创建BeanFactory：

/**
	 * This implementation performs an actual refresh of this context's underlying
	 * bean factory, shutting down the previous bean factory (if any) and
	 * initializing a fresh bean factory for the next phase of the context's lifecycle.
	 */
	@Override
	protected final void refreshBeanFactory() throws BeansException {
		if (hasBeanFactory()) {
			destroyBeans();
			closeBeanFactory();
		}
		try {
			DefaultListableBeanFactory beanFactory = createBeanFactory();
			beanFactory.setSerializationId(getId());
			customizeBeanFactory(beanFactory);
			loadBeanDefinitions(beanFactory);
			synchronized (this.beanFactoryMonitor) {
				this.beanFactory = beanFactory;
			}
		}
		catch (IOException ex) {
			throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
		}
	}
	/**
	 * Load bean definitions into the given bean factory, typically through
	 * delegating to one or more bean definition readers.
	 * @param beanFactory the bean factory to load bean definitions into
	 * @throws BeansException if parsing of the bean definitions failed
	 * @throws IOException if loading of bean definition files failed
	 * @see org.springframework.beans.factory.support.PropertiesBeanDefinitionReader
	 * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
	 */
	protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)
			throws BeansException, IOException;

Web.xml方式实现loadBeanDefinition：

/**
	 * Loads the bean definitions via an XmlBeanDefinitionReader.
	 * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
	 * @see #initBeanDefinitionReader
	 * @see #loadBeanDefinitions
	 */
	@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		beanDefinitionReader.setEnvironment(getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		initBeanDefinitionReader(beanDefinitionReader);
		loadBeanDefinitions(beanDefinitionReader);
	}

Servlet3.0方式实现loadBeanDefinition

/**
	 * Register a {@link org.springframework.beans.factory.config.BeanDefinition} for
	 * any classes specified by {@link #register(Class...)} and scan any packages
	 * specified by {@link #scan(String...)}.
	 * <p>For any values specified by {@link #setConfigLocation(String)} or
	 * {@link #setConfigLocations(String[])}, attempt first to load each location as a
	 * class, registering a {@code BeanDefinition} if class loading is successful,
	 * and if class loading fails (i.e. a {@code ClassNotFoundException} is raised),
	 * assume the value is a package and attempt to scan it for annotated classes.
	 * <p>Enables the default set of annotation configuration post processors, such that
	 * {@code @Autowired}, {@code @Required}, and associated annotations can be used.
	 * <p>Configuration class bean definitions are registered with generated bean
	 * definition names unless the {@code value} attribute is provided to the stereotype
	 * annotation.
	 * @param beanFactory the bean factory to load bean definitions into
	 * @see #register(Class...)
	 * @see #scan(String...)
	 * @see #setConfigLocation(String)
	 * @see #setConfigLocations(String[])
	 * @see AnnotatedBeanDefinitionReader
	 * @see ClassPathBeanDefinitionScanner
	 */
	@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) {
		AnnotatedBeanDefinitionReader reader = getAnnotatedBeanDefinitionReader(beanFactory);
		ClassPathBeanDefinitionScanner scanner = getClassPathBeanDefinitionScanner(beanFactory);

		BeanNameGenerator beanNameGenerator = getBeanNameGenerator();
		if (beanNameGenerator != null) {
			reader.setBeanNameGenerator(beanNameGenerator);
			scanner.setBeanNameGenerator(beanNameGenerator);
			beanFactory.registerSingleton(AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR, beanNameGenerator);
		}

		ScopeMetadataResolver scopeMetadataResolver = getScopeMetadataResolver();
		if (scopeMetadataResolver != null) {
			reader.setScopeMetadataResolver(scopeMetadataResolver);
			scanner.setScopeMetadataResolver(scopeMetadataResolver);
		}

		if (!this.annotatedClasses.isEmpty()) {
			if (logger.isInfoEnabled()) {
				logger.info("Registering annotated classes: [" +
						StringUtils.collectionToCommaDelimitedString(this.annotatedClasses) + "]");
			}
			reader.register(this.annotatedClasses.toArray(new Class<?>[this.annotatedClasses.size()]));
		}

		if (!this.basePackages.isEmpty()) {
			if (logger.isInfoEnabled()) {
				logger.info("Scanning base packages: [" +
						StringUtils.collectionToCommaDelimitedString(this.basePackages) + "]");
			}
			scanner.scan(this.basePackages.toArray(new String[this.basePackages.size()]));
		}

		String[] configLocations = getConfigLocations();
		if (configLocations != null) {
			for (String configLocation : configLocations) {
				try {
					Class<?> clazz = ClassUtils.forName(configLocation, getClassLoader());
					if (logger.isInfoEnabled()) {
						logger.info("Successfully resolved class for [" + configLocation + "]");
					}
					reader.register(clazz);
				}
				catch (ClassNotFoundException ex) {
					if (logger.isDebugEnabled()) {
						logger.debug("Could not load class for config location [" + configLocation +
								"] - trying package scan. " + ex);
					}
					int count = scanner.scan(configLocation);
					if (logger.isInfoEnabled()) {
						if (count == 0) {
							logger.info("No annotated classes found for specified class/package [" + configLocation + "]");
						}
						else {
							logger.info("Found " + count + " annotated classes in package [" + configLocation + "]");
						}
					}
				}
			}
		}
	}