接着上一章節的內容，我們來分析當new一個FileSystemXmlApplicationContext對象的時候，spring到底做了那些事。FileSystemXmlApplicationContext類的內容主要是定義了若干重載的構造方法，核心構造方法如下：
/**
* 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();
}
}
從方法說明可以看出，在這個構造方法里加載所有bean定義並創建bean單例實例。其中的refresh()方法就是IOC容器初始化的入口，refresh()方法位AbstractApplicationContext類中，這是一個抽象類，它實現了ApplicationContext的基礎功能，這裏使用了模版方法模式，給實現它的子類提供了統一的模板：
@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.告訴子類刷新內部bean工廠
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()方法裏列出了IOC容器初始化的步驟，第一個方法是初始化準備，這裏只是設置啓動日期和活動標識以及執行屬性源的初始化。我們重點看第二個方法obtainFreshBeanFactory()，它告訴子類刷新內部bean工廠，返回了一個ConfigurableListableBeanFactory，跟蹤這個方法：
/**
* 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()方法的第一行調用了refreshBeanFactory()方法，這是一個抽象方法，由它的子類來實現，方法的第二行調用了getBeanFactory()，這是在其父接口中定義的一個空方法。抽象方法refreshBeanFactory()在其子類子類AbstractRefreshableApplicationContext中實現：
/**
* 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.
*
* 此實現執行該上下文的底層bean工廠的實際刷新，關閉以前的bean工廠（如果有的話），
* 併爲上下文生命週期的下一階段初始化一個新的bean工廠
*/
@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);
}
}
這個方法被final關鍵字修飾，也就是說不可以被重寫，IOC容器的初始化就是在這個方法中完成的。第一步先判斷有沒有現有的工廠，有的話就銷燬掉，然後創建一個默認的工廠，也就是DefaultListableBeanFactory ，接下來兩行代碼是設置bean工廠的一些屬性，注意看loadBeanDefinitions(beanFactory)這行，當創建了一個默認的bean工廠後，加載bean定義，這跟我們上一章節使用原始方式初始化bean工廠類似。從這裏不難看出，FileSystemXmlApplicationContext的構造方法中其實已經包含了我們上一章節中原始的初始化過程。接下來我們跟蹤一下loadBeanDefinitions(beanFactory)的實現，這個方法是由AbstractXmlApplicationContext抽象類實現的：
/**
* Loads the bean definitions via an XmlBeanDefinitionReader.裝載bean定義通過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義了一個Reader，這個Reader就是用來讀取xml配置文件的，最後一行就是真正載入bean定義的實現過程，代碼如下：
/**
* 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);
}
}
上面的方法調用了XmlBeanDefinitionReader類的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();
}
}
}
從方法說明可以看出，這個方法是從指定的xml文件中加載bean定義，try塊中的代碼纔是載入bean定義的過程。spring將資源返回的輸入流包裝以後傳給了doLoadBeanDefinitions()方法，我們進入這個方法，代碼如下：
/**
* 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());
}
從try塊中的代碼可以看出，spring使用documentLoader將資源轉換成了Document資源，spring使用的documentLoader爲DefaultDocumentLoader，loadDocument方法定義在此類中：
/**
* 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);
}
從這裏不難看出，這就是我們非常熟悉的DOM解析xml了，可以想象spring是根據XSD文件規定的格式解析了xml文件的各節點及屬性。我們再來回頭看看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;
}
方法說明很明確的告訴我們，這個方法是註冊給定的DOM文檔中包含的bean定義。到這裏思路就很明確了，spring將包裝的輸入流解析成DOM文檔，然後將DOM中包含的bean定義信息註冊到IOC容器持有的Map<String，BeanDefinition>對象中。只要我們的IOC容器持有了bean定義，就能正確的生產bean實例。
通過閱讀源碼，我們分析了Spring IOC的實現原理。有些實現細節並沒有去深究，更重要的是去理解它的核心思想和實現思路。
作者：風中程序猿
出處：https://www.cnblogs.com/fangfuhai/p/7131434.html