<dependency>
 <groupId>org.springframework</groupId>
 <artifactId>spring-context</artifactId>
 <version>5.0.2.RELEASE</version>
</dependency>

public class Application {
 public static void main(String[] args) {
 BeanDefinitionRegistry beanFactory = new DefaultListableBeanFactory();
 XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
 ClassPathResource resource = new ClassPathResource("bean.xml");
 //整個資源加載的切入點。
 reader.loadBeanDefinitions(resource);
 }
}

public AbstractAutowireCapableBeanFactory() {
 super();
 ignoreDependencyInterface(BeanNameAware.class);
 ignoreDependencyInterface(BeanFactoryAware.class);
 ignoreDependencyInterface(BeanClassLoaderAware.class);
}

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
 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();
 }
 }
}

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
 throws BeanDefinitionStoreException {
 try {
 Document doc = doLoadDocument(inputSource, resource);
 return registerBeanDefinitions(doc, resource);
 }
}

protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
 return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
 getValidationModeForResource(resource), isNamespaceAware());
}

public class XmlValidationModeDetector {
 /**
 * Indicates that DTD validation should be used (we found a "DOCTYPE" declaration).
 */
 public static final int VALIDATION_DTD = 2;
 /**
 * Indicates that XSD validation should be used (found no "DOCTYPE" declaration).
 */
 public static final int VALIDATION_XSD = 3;
 public int detectValidationMode(InputStream inputStream) throws IOException {
 }
}

public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
 ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {
}

public abstract InputSource resolveEntity (String publicId,String systemId)
 throws SAXException, IOException;

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
</beans>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN"
 "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
</beans>

public class DelegatingEntityResolver {
 @Override
 @Nullable
 public InputSource resolveEntity(String publicId, @Nullable String systemId) throws SAXException, IOException {
 if (systemId != null) {
 if (systemId.endsWith(DTD_SUFFIX)) {
 return this.dtdResolver.resolveEntity(publicId, systemId);
 }
 else if (systemId.endsWith(XSD_SUFFIX)) {
 return this.schemaResolver.resolveEntity(publicId, systemId);
 }
 }
 return null;
 }
}

public class XmlBeanDefinitionReader {
 public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
 // 創(chuàng)建DocumentReader
 BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
 // 記錄統(tǒng)計前的 BeanDefinition 數(shù)
 int countBefore = getRegistry().getBeanDefinitionCount();
 // 注冊 BeanDefinition
 documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
 // 記錄本次加載 BeanDefinition 的個數(shù)
 return getRegistry().getBeanDefinitionCount() - countBefore;
 }
}

public class BeanDefinitionParserDelegate {
 public static final String BEANS_NAMESPACE_URI = "http://www.springframework.org/schema/beans";
 public boolean isDefaultNamespace(@Nullable String namespaceUri) {
 return (!StringUtils.hasLength(namespaceUri) || BEANS_NAMESPACE_URI.equals(namespaceUri));
 }
}

public class DefaultBeanDefinitionDocumentReader {
 @Override
 public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
 this.readerContext = readerContext;
 logger.debug("Loading bean definitions");
 Element root = doc.getDocumentElement();
 doRegisterBeanDefinitions(root);
 }
}

protected void doRegisterBeanDefinitions(Element root) {
 // ...
 String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
 // ... 
 // 空實現(xiàn)
 preProcessXml(root);
 parseBeanDefinitions(root, this.delegate);
 // 空實現(xiàn)
 postProcessXml(root);
 this.delegate = parent;
}

public class DefaultBeanDefinitionDocumentReader {
 /**
 * 解析 root 節(jié)點下的其它節(jié)點 import", "alias", "bean".
 * @param root節(jié)點名稱
 */
 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);
 }
 }
 private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
 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);
 }
 }
 /**
 * 處理 Bean 標簽，然后將其注冊到注冊表中去
 */
 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));
 }
 }
}