【Spring源码这样读】-再次走近容器Spring IOC 二

继上文的问题，我们本章，继续分析我们的容器到底在做了一些什么事情。

上文我们说到我们的ClassPathResource到底做了什么，但是我们阅读源码之后发现只是做了一些初始化。那我们推测的获取配置文件，然后去配置文件里面获取内容的推测还能成立吗？不需要急，我们一步一步往下看就能看到结果。既然我们看了ClassPathResource，我们在来看看new XmlBeanFactory(classPathResource);到底做了什么。

构造bean工厂类

/**
 * Create a new XmlBeanFactory with the given resource,
 * which must be parsable using DOM.
 * @param resource the XML resource to load bean definitions from
 * @throws BeansException in case of loading or parsing errors
 */
public XmlBeanFactory(Resource resource) throws BeansException {
	this(resource, null);
}

/**
 * Create a new XmlBeanFactory with the given input stream,
 * which must be parsable using DOM.
 * @param resource the XML resource to load bean definitions from
 * @param parentBeanFactory parent bean factory
 * @throws BeansException in case of loading or parsing errors
 */
public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
	super(parentBeanFactory);
	this.reader.loadBeanDefinitions(resource);
}

我们使用的就是第一个构造，但是第一个构造调用了第二个，第二个构造只有两步。我们继续往下看

再来看XmlBeanFactory初始化的源码

• super(parentBeanFactory)：用来初始化父类
• this.reader.loadBeanDefinitions(resource)：spring初始化资源加载的真正实现

我们先来看看“super(parentBeanFactory)：用来初始化父类”这一步做了什么吧

/**
 * 先是调用DefaultListableBeanFactory(@Nullable BeanFactory parentBeanFactory)带参构造方法
 */
public DefaultListableBeanFactory(@Nullable BeanFactory parentBeanFactory) {
	super(parentBeanFactory);
}

/**
 * 调用父类AbstractAutowireCapableBeanFactory
 */
public AbstractAutowireCapableBeanFactory(@Nullable BeanFactory parentBeanFactory) {
	this();
	setParentBeanFactory(parentBeanFactory);
}

/**
 * 调用AbstractAutowireCapableBeanFactory()无参构造方法
 */
public AbstractAutowireCapableBeanFactory() {
	super();
	ignoreDependencyInterface(BeanNameAware.class);
	ignoreDependencyInterface(BeanFactoryAware.class);
	ignoreDependencyInterface(BeanClassLoaderAware.class);
}

/** 
 * 在初始化XmlBeanFactory容器
 */
public void setParentBeanFactory(@Nullable BeanFactory parentBeanFactory) {
	if (this.parentBeanFactory != null && this.parentBeanFactory != parentBeanFactory) {
		throw new IllegalStateException("Already associated with parent BeanFactory: " + this.parentBeanFactory);
	}
	this.parentBeanFactory = parentBeanFactory;
}

到这里是不是比较明了的知道了这一步具体做了什么？其实看完super(parentBeanFactory)的源码发现就做了两件事：获取添加需要忽略指定接口实现类的自动装配和初始化。那我们的第二步具体做了什么呢？

this.reader.loadBeanDefinitions(resource)分析

之前我们说了我们应该是根据配置文件找到对应的类，那源码是这样吗？看看这句代码的实现就知道了。

/**
 * 先是调用loadBeanDefinitions(Resource resource)
 */
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
	return loadBeanDefinitions(new EncodedResource(resource));
}

/**
 * 这个方法正是我们之前推测的实现
 * 从xml配置文件中加载bean
 */
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
	Assert.notNull(encodedResource, "EncodedResource must not be null");
	if (logger.isTraceEnabled()) {
		logger.trace("Loading XML bean definitions from " + encodedResource);
	}

	Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
	if (currentResources == null) {
		currentResources = new HashSet<>(4);
		this.resourcesCurrentlyBeingLoaded.set(currentResources);
	}
	if (!currentResources.add(encodedResource)) {
		throw new BeanDefinitionStoreException(
				"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
	}
	try {
		InputStream inputStream = encodedResource.getResource().getInputStream();
		try {
			InputSource inputSource = new InputSource(inputStream);
			if (encodedResource.getEncoding() != null) {
				inputSource.setEncoding(encodedResource.getEncoding());
			}
			return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
		}
		finally {
			inputStream.close();
		}
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException(
				"IOException parsing XML document from " + encodedResource.getResource(), ex);
	}
	finally {
		currentResources.remove(encodedResource);
		if (currentResources.isEmpty()) {
			this.resourcesCurrentlyBeingLoaded.remove();
		}
	}
}

当我们贴完代码之后，是不是可以明显的看到我们之前推测的实际证据？

ClassPathResource和InputStream

我们从上面的源码看到，我们定义的类在xml中，源码里面有InputStream。但是传入的只有一个ClassPathResource。那他是怎么封装进入ClassPathResource的呢？

其实这里的关联比较简单。我们不妨来看看这些类的类结构图

到这里是不是就已经很明了了我们的InputStream是怎么将内容传递进入ClassPathResource

真正获取bean

获取bean才是我们这一段操作的最关键的地方，那bean又是怎么获取出来的呢？这里贴一下主代码吧

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
		@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		if (logger.isTraceEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
						"' that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

	else {
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

		// Check if bean definition exists in this factory.
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
			// Not found -> check parent.
			String nameToLookup = originalBeanName(name);
			if (parentBeanFactory instanceof AbstractBeanFactory) {
				return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
						nameToLookup, requiredType, args, typeCheckOnly);
			}
			else if (args != null) {
				// Delegation to parent with explicit args.
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else if (requiredType != null) {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
			else {
				return (T) parentBeanFactory.getBean(nameToLookup);
			}
		}

		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}

		try {
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

			// Guarantee initialization of beans that the current bean depends on.
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}
					registerDependentBean(dep, beanName);
					try {
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}

			// Create bean instance.
			if (mbd.isSingleton()) {
				sharedInstance = getSingleton(beanName, () -> {
					try {
						return createBean(beanName, mbd, args);
					}
					catch (BeansException ex) {
						// Explicitly remove instance from singleton cache: It might have been put there
						// eagerly by the creation process, to allow for circular reference resolution.
						// Also remove any beans that received a temporary reference to the bean.
						destroySingleton(beanName);
						throw ex;
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}

			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}

			else {
				String scopeName = mbd.getScope();
				final Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
				}
				try {
					Object scopedInstance = scope.get(beanName, () -> {
						beforePrototypeCreation(beanName);
						try {
							return createBean(beanName, mbd, args);
						}
						finally {
							afterPrototypeCreation(beanName);
						}
					});
					bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName,
							"Scope '" + scopeName + "' is not active for the current thread; consider " +
							"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
							ex);
				}
			}
		}
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}

	// Check if required type matches the type of the actual bean instance.
	if (requiredType != null && !requiredType.isInstance(bean)) {
		try {
			T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
			if (convertedBean == null) {
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
			return convertedBean;
		}
		catch (TypeMismatchException ex) {
			if (logger.isTraceEnabled()) {
				logger.trace("Failed to convert bean '" + name + "' to required type '" +
						ClassUtils.getQualifiedName(requiredType) + "'", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}
	return (T) bean;
}

仅从代码量上就能看出来bean的加载经历了一个相当复杂的过程，其中涉及各种各样的考虑。

我们将上面的内容大致总结一下：

• 转换对应beanName。
• 尝试从缓存中加载单例。
• bean的实例化。
• 原型模式的依赖检查。
• 检测parentBeanFactory。
• 将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition。
• 寻找依赖。
• 针对不同的scope进行bean的创建。
• 类型转换。（强转）

其实读到这里，我们不难看出，大致跟我们之前的推测出入并不是很大，只是源码的开发人员们对代码的逻辑做的更为严谨，并且做了很多的必要的校验。