Spring源码：IOC原理解析（二）

版权声明：本文为博主原创文章，转载请注明出处，欢迎交流学习！

接着上一章节的内容，我们来分析当new一个FileSystemXmlApplicationContext对象的时候，spring到底做了那些事。FileSystemXmlApplicationContext类的内容主要是定义了若干重载的构造方法，核心构造方法如下：

/**
* Create a new FileSystemXmlApplicationContext with the given parent,
* loading the definitions from the given XML files.
*
* loading all bean definitions and creating all singletons.
* Alternatively, call refresh manually after further configuring the context.
*
*/
public FileSystemXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {

super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}

从方法说明可以看出，在这个构造方法里加载所有bean定义并创建bean单例实例。其中的refresh()方法就是IOC容器初始化的入口，refresh()方法位AbstractApplicationContext类中，这是一个抽象类，它实现了ApplicationContext的基础功能，这里使用了模版方法模式，给实现它的子类提供了统一的模板：

@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();

// Tell the subclass to refresh the internal bean factory.告诉子类刷新内部bean工厂
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);

try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);

// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);

// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);

// Initialize message source for this context.
initMessageSource();

// Initialize event multicaster for this context.
initApplicationEventMulticaster();

// Initialize other special beans in specific context subclasses.
onRefresh();

// Check for listener beans and register them.
registerListeners();

// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);

// Last step: publish corresponding event.
finishRefresh();
}

catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}

// Destroy already created singletons to avoid dangling resources.
destroyBeans();

// Reset 'active' flag.
cancelRefresh(ex);

// Propagate exception to caller.
throw ex;
}

finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}

refresh()方法里列出了IOC容器初始化的步骤，第一个方法是初始化准备，这里只是设置启动日期和活动标识以及执行属性源的初始化。我们重点看第二个方法obtainFreshBeanFactory()，它告诉子类刷新内部bean工厂，返回了一个ConfigurableListableBeanFactory，跟踪这个方法：

/**
* Tell the subclass to refresh the internal bean factory.
* @return the fresh BeanFactory instance
* @see #refreshBeanFactory()
* @see #getBeanFactory()
*/
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}

protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;

/**
* Return the internal bean factory of this application context.
* Can be used to access specific functionality of the underlying factory.
*
*/
ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;

obtainFreshBeanFactory()方法的第一行调用了refreshBeanFactory()方法，这是一个抽象方法，由它的子类来实现，方法的第二行调用了getBeanFactory()，这是在其父接口中定义的一个空方法。抽象方法refreshBeanFactory()在其子类子类AbstractRefreshableApplicationContext中实现：

/**
* 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.
*
* 此实现执行该上下文的底层bean工厂的实际刷新，关闭以前的bean工厂（如果有的话），
* 并为上下文生命周期的下一阶段初始化一个新的bean工厂
*/
@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);
}
}

这个方法被final关键字修饰，也就是说不可以被重写，IOC容器的初始化就是在这个方法中完成的。第一步先判断有没有现有的工厂，有的话就销毁掉，然后创建一个默认的工厂，也就是DefaultListableBeanFactory ，接下来两行代码是设置bean工厂的一些属性，注意看loadBeanDefinitions(beanFactory)这行，当创建了一个默认的bean工厂后，加载bean定义，这跟我们上一章节使用原始方式初始化bean工厂类似。从这里不难看出，FileSystemXmlApplicationContext的构造方法中其实已经包含了我们上一章节中原始的初始化过程。接下来我们跟踪一下loadBeanDefinitions(beanFactory)的实现，这个方法是由AbstractXmlApplicationContext抽象类实现的：

/**
* Loads the bean definitions via an XmlBeanDefinitionReader.装载bean定义通过XmlBeanDefinitionReader
*
*/
@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(this.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);
}

方法的第一行首先定义了一个Reader，这个Reader就是用来读取xml配置文件的，最后一行就是真正载入bean定义的实现过程，代码如下：

/**
* Load the bean definitions with the given XmlBeanDefinitionReader.
*
*/
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
Resource[] configResources = getConfigResources();
if (configResources != null) {
reader.loadBeanDefinitions(configResources);
}
String[] configLocations = getConfigLocations();
if (configLocations != null) {
reader.loadBeanDefinitions(configLocations);
}
}

上面的方法调用了XmlBeanDefinitionReader类的loadBeanDefinitions(EncodedResource encodedResource)方法：

/**
* Load bean definitions from the specified XML file.
* rows BeanDefinitionStoreException in case of loading or parsing errors
*/
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isInfoEnabled()) {
logger.info("Loading XML bean definitions from " + encodedResource.getResource());
}

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();
}
}
}

从方法说明可以看出，这个方法是从指定的xml文件中加载bean定义，try块中的代码才是载入bean定义的过程。spring将资源返回的输入流包装以后传给了doLoadBeanDefinitions()方法，我们进入这个方法，代码如下：

/**
* Actually load bean definitions from the specified XML file.
*
*/
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
Document doc = doLoadDocument(inputSource, resource);
return registerBeanDefinitions(doc, resource);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (SAXParseException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
}
catch (SAXException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"XML document from " + resource + " is invalid", ex);
}
catch (ParserConfigurationException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Parser configuration exception parsing XML from " + resource, ex);
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"IOException parsing XML document from " + resource, ex);
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Unexpected exception parsing XML document from " + resource, ex);
}
}

/**
* Actually load the specified document using the configured DocumentLoader.
*
*/
protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
getValidationModeForResource(resource), isNamespaceAware());
}

从try块中的代码可以看出，spring使用documentLoader将资源转换成了Document资源，spring使用的documentLoader为DefaultDocumentLoader，loadDocument方法定义在此类中：

/**
* Load the {@link Document} at the supplied {@link InputSource} using the standard JAXP-configured
* XML parser.
*/
@Override
public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {

DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware);
if (logger.isDebugEnabled()) {
logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]");
}
DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler);
return builder.parse(inputSource);
}

从这里不难看出，这就是我们非常熟悉的DOM解析xml了，可以想象spring是根据XSD文件规定的格式解析了xml文件的各节点及属性。我们再来回头看看registerBeanDefinitions(doc, resource)方法，

/**
* Register the bean definitions contained in the given DOM document.
* Called by {@code loadBeanDefinitions}.
*
*/
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}

方法说明很明确的告诉我们，这个方法是注册给定的DOM文档中包含的bean定义。到这里思路就很明确了，spring将包装的输入流解析成DOM文档，然后将DOM中包含的bean定义信息注册到IOC容器持有的Map<String，BeanDefinition>对象中。只要我们的IOC容器持有了bean定义，就能正确的生产bean实例。

通过阅读源码，我们分析了Spring IOC的实现原理。有些实现细节并没有去深究，更重要的是去理解它的核心思想和实现思路。