package bean;
public class TestBean {
  private String beanName = "beanName";
  public String getBeanName() {
    return beanName;
  }
  public void setBeanName(String beanName) {
    this.beanName = beanName;
  }
}

<?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-4.1.xsd">

  <bean id="testBean" class="bean.TestBean">
</beans>

public class BeanTest {

  private static final java.util.logging.Logger logger = LoggerFactory.getLogger(BeanTest.class);

  @Test
  public void getBeanTest() {
    BeanFactory factory = new XmlBeanFactory(new ClassPathResource("root.xml"));
    TestBean bean = factory.getBean("testBean");
    logger.info(bean.getBeanName);
  }
}

public class XmlBeanFactory extends DefaultListableBeanFactory {

  private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);

  public XmlBeanFactory(Resource resource) throws BeansException {
    this(resource, null);
  }

  public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
    super(parentBeanFactory);
    this.reader.loadBeanDefinitions(resource);
  }
}

public class XmlBeanDefinitionReader extends AbstractBeanDefinitionReader {

  private Class<?> documentReaderClass = DefaultBeanDefinitionDocumentReader.class;

  private ProblemReporter problemReporter = new FailFastProblemReporter();

  private ReaderEventListener eventListener = new EmptyReaderEventListener();

  private SourceExtractor sourceExtractor = new NullSourceExtractor();

  private NamespaceHandlerResolver namespaceHandlerResolver;

  private DocumentLoader documentLoader = new DefaultDocumentLoader();

  private EntityResolver entityResolver;

  private ErrorHandler errorHandler = new SimpleSaxErrorHandler(logger);
}

public abstract class AbstractBeanDefinitionReader implements EnvironmentCapable, BeanDefinitionReader {

  protected final Log logger = LogFactory.getLog(getClass());

  private final BeanDefinitionRegistry registry;

  private ResourceLoader resourceLoader;

  private ClassLoader beanClassLoader;

  private Environment environment;

  private BeanNameGenerator beanNameGenerator = new DefaultBeanNameGenerator();
}

BeanFactory factory = new XmlBeanFactory(new ClassPathResource("root.xml"));

public XmlBeanFactory(Resource resource) throws BeansException {
   this(resource, null);
}

public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
   super(parentBeanFactory);
   this.reader.loadBeanDefinitions(resource);
}

public class XmlBeanDefinitionReader extends AbstractBeanDefinitionReader {

  @Override
  public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
    return loadBeanDefinitions(new EncodedResource(resource));
  }
}

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());
  }

  // 通过属性记录已加载的资源
  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 {
    // 从resource中获取对应的InputStream，用于下面构造InputSource
    InputStream inputStream = encodedResource.getResource().getInputStream();
    try {
      InputSource inputSource = new InputSource(inputStream);
      if (encodedResource.getEncoding() != null) {
        inputSource.setEncoding(encodedResource.getEncoding());
      }
      // 调用doLoadBeanDefinitions方法
      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();
    }
  }
}

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
    throws BeanDefinitionStoreException {
  try {
    Document doc = doLoadDocument(inputSource, resource);
    return registerBeanDefinitions(doc, resource);
  }
  /* 省略一堆catch */
}

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

public class DefaultDocumentLoader implements DocumentLoader {

  @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);
  }
}

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
  BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
  documentReader.setEnvironment(getEnvironment());
  // 还没注册bean前的BeanDefinition加载个数
  int countBefore = getRegistry().getBeanDefinitionCount();
  // 加载注册bean
  documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
  // 本次加载注册的BeanDefinition个数
  return getRegistry().getBeanDefinitionCount() - countBefore;
}

@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) {
  BeanDefinitionParserDelegate parent = this.delegate;
  this.delegate = createDelegate(getReaderContext(), root, parent);

  if (this.delegate.isDefaultNamespace(root)) {
    String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
    if (StringUtils.hasText(profileSpec)) {
      String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
          profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
      if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
        return;
      }
    }
  }

  preProcessXml(root);
  parseBeanDefinitions(root, this.delegate);
  postProcessXml(root);

  this.delegate = parent;
}

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;
        // 下面对bean进行处理
        if (delegate.isDefaultNamespace(ele)) {
          parseDefaultElement(ele, delegate);
        }
        else {
          delegate.parseCustomElement(ele);
        }
      }
    }
  }
  else {
    delegate.parseCustomElement(root);
  }
}

<bean id="testBean" class="bean.TestBean">

<tx:annotation-driven />