概览
要了解spring的bean加载，可以从下面这段代码入手
public static void main(String[] args) {
BeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("beanFactoryTest.xml"));
MyBeanTest beanTest = (MyBeanTest) beanFactory.getBean("myBeanTest");
System.out.println(beanTest.getTestString());
}
代码的逻辑很清楚
1.读取beanFactoryTest.xml的内容并创建bean实例存入beanFactory。
2.从beanFactory中获取到myBeanTest。
3.调用myBeanTest的方法并打印参数。
下面根据代码逐步讨论详细过程
一、ClassPathResource
···
public ClassPathResource(String path) {
this(path, (ClassLoader)null);
}
public ClassPathResource(String path, ClassLoader classLoader) {
Assert.notNull(path, "Path must not be null");
String pathToUse = StringUtils.cleanPath(path);
if (pathToUse.startsWith("/")) {
pathToUse = pathToUse.substring(1);
}
this.path = pathToUse;
this.classLoader = classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader();
}
···
进入new ClassPathResource方法可以看到是这样的，ClassPathResource类中包含很多构造方法，不过这里用到的是这两个。首先我们传入了一个path参数，调用了第一个构造方法，而第一个构造方法会设置classLoader参数为null，进而调用第二个构造方法。第二个构造方法实现的逻辑大致是，处理path参数，使其标准化；验证是否有自定义的classLoader，如果有则使用自定义classLoader，如果没有，则使用默认的classLoader。这一步封装了一个resource对象，提供给XmlBeanFactory进行解析。（关于类加载器可以参考：java classLoader学习笔记）
二、XmlBeanFactory
...
public XmlBeanFactory(Resource resource) throws BeansException {
this(resource, (BeanFactory)null);
}
public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
super(parentBeanFactory);
this.reader = new XmlBeanDefinitionReader(this);
this.reader.loadBeanDefinitions(resource);
}
...
核心是XmlBeanDefinitionReader的loadBeanDefinitions方法
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
return loadBeanDefinitions(new EncodedResource(resource));
}
这里会对resource进行再封装，封装为EncodedResource对象，EncodedResource对象主要是用于对资源文件的编码进行处理。设置了编码属性的时候，Spring会使用相应的编码作为输入流的编码。封装之后再次转入了loadBeanDefinitions方法。
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<EncodedResource>(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();
}
}
}
以上代码太多，核心的加载方法推测是doLoadBeanDefinitions
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
int validationMode = getValidationModeForResource(resource);
Document doc = this.documentLoader.loadDocument(
inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
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);
}
}
上面的代码，在try代码块里只有三个步骤：
1.获取对XML文件的验证模式（验证模式是为了保证XML文件的正确性）
2.加载XML文件，得到对应的Docunment
3.根据返回的Document注册Bean信息
三、Bean的解析与注册
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
documentReader.setEnvironment(this.getEnvironment());
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}
在registerBeanDefinitions方法中，传入了转化得到的Document和Resource，然后通过documentReader去解析得到的document，这里的documentReader.registerBeanDefinitions方法，实际使用的是DefaultBeanDefinitionDocumentReader的registerBeanDefinitions方法（createBeanDefinitionDocumentReader()这个方法返回的对象为DefaultBeanDefinitionDocumentReader）
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
doRegisterBeanDefinitions(root);
}
在这里提取root，提取到的root再次传入doRegisterBeanDefinitions
protected void doRegisterBeanDefinitions(Element root) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
Assert.state(this.environment != null, "environment property must not be null");
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!this.environment.acceptsProfiles(specifiedProfiles)) {
return;
}
}
// any nested <beans> elements will cause recursion in this method. In
// order to propagate and preserve <beans> default-* attributes correctly,
// keep track of the current (parent) delegate, which may be null. Create
// the new (child) delegate with a reference to the parent for fallback purposes,
// then ultimately reset this.delegate back to its original (parent) reference.
// this behavior emulates a stack of delegates without actually necessitating one.
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createHelper(readerContext, root, parent);
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}
进入真正的解析方法 parseBeanDefinitions
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
parseDefaultElement(ele, delegate);
}
else {
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
大致是循环取到每一个标签元素，并进行解析。进入parseDefaultElement方法
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
再进入对bean标签的解析
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}
可以看到，解析完成后会调用BeanDefinitionReaderUtils.registerBeanDefinition方法进行bean的注册。
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String aliase : aliases) {
registry.registerAlias(beanName, aliase);
}
}
}
载入Bean
载入bean过程
当main方法在getBean时，首先会去获取bean，获取不到再调用creatrBean()，核心方法在AbstractAutowireCapableBeanFactory中，创建bean时，先创建bean实例，再根据从beanFactory拿到的beanDefination填充bean的属性，在填充时，如果有依赖其他bean，则先创建被依赖的bean。依次创建完成，最后返回需要的bean实例。
