public class Test {
public static void main(String[] args) throws Exception {
BeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("bean.xml"));
Person person = beanFactory.getBean("person",Person.class);
person.info();
}
}
<?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"> <bean id="person" class="com.wh.Person">
<property name="age" value="25"></property>
<property name="name" value="wanghong"></property>
</bean>
</beans>
从资源文件得到DOM对象
先看看XmlBeanFactory所处的地位。
XmlBeanFactory构造函数中调用了XmlBeanDefinitionReader类型的reader属性提供的方法this.reader.loadBeanDefinitions（resource）。这句代码是整个资源加载的切入点。
public class XmlBeanFactory extends DefaultListableBeanFactory {
private final XmlBeanDefinitionReader reader; 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);
//利用XmlBeanDefinitionReader类开始资源加载
this.reader.loadBeanDefinitions(resource);
}
}
XmlBeanFactory扩展了DefaultListableBeanFactory，使用XmlBeanDefinitionReader从XML配置文件中读取bean的定义。忽略其他的细节，我们先来看看这个配置文件（是一种Resource）是如何被加载的。跟踪进去，进入XmlBeanDefinitionReader#loadBeanDefinitions方法，然后扑面而来的是下面这个重要的方法。
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
//1.封装资源文件。当进入XmlBeanDefinitionReader后首先对参数Resource使用EncodedResource类进行封装
//EncodedResource类主要用于对资源文件的编码进行处理。其中主要逻辑体现在getReader（）方法中。
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 {
//2获取输入流，从Resource中获取对应的InputStream并构造InputResource
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
//3通过构造的InputResource实例和Resource实例继续调用函数doLoadBeanDefinitions
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();
}
}
}
成员变量resourcesCurrentlyBeingLoaded是一个ThreadLocal，所以资源文件多线程加载是安全的，同时使用HashSet判断资源文件的循环加载，接下来就是读取我们的资源文件了，
其中InputSource表征一个XML实体的输入源，包装了几个字段。
从输入源InputSource中获得表示该XML文件的Document对象，后续就是操纵这个document。
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);
}
}
在上面冗长的代码中假如不考虑异常类的代码，其实只做了三件事，这三件事每一件都必不可少。
1.获取对XML文件的验证模式
2.加载XML文件，并得到对应的Document。
3.根据返回的Document注册bean信息。
然后就是根据Spring的”spring-beans” DTD（就是我们在配置文件开始制定的命名空间和location），注册该DOM对象里面的bean。
protected void doRegisterBeanDefinitions(Element root) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!getEnvironment().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 = createDelegate(this.readerContext, root, parent); preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root); this.delegate = parent;
}
通过上面代码我们看到了处理流程，首先是对profile的处理，然后开始进行解析，可是当我们跟进preProcessXml（root）或者postProcessXml（root）发现
代码是空的，就像面向对象设计方法学上常说的一句话，一个类要么是面向继承的设计的，要么就用final修饰。在DefaultBeanDefinitionDocumentReader中没有用final修饰，
所以是面向继承的。这两个方法是为子类而设计的，这是模板方法模式。如果子类需要在Bean解析前后做一些处理的话，那么只需要重写这两个方法就可以了。
后续的工作就是解析这里的DOM对象，从根节点开始，处理每个node，每个元素可以是默认命名空间有的(如”import”, “alias”, “bean”)，也可以是自定义的。
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);
}
}
重点是看bean的定义是如何实现的，其他的先不关注。
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);
}
}
接下来就是具体处理一个bean定义的过程。
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));
}
}
接下来委派给BeanDefinitionParserDelegate来专门解析XML里的bean定义，如果深究下去太多了，暂时先略过其中的细节，解析成功后返回一个 BeanDefinitionHolder.
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
String id = ele.getAttribute(ID_ATTRIBUTE);
String nameAttr = ele.getAttribute(NAME_ATTRIBUTE); List<String> aliases = new ArrayList<String>();
if (StringUtils.hasLength(nameAttr)) {
String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, BEAN_NAME_DELIMITERS);
aliases.addAll(Arrays.asList(nameArr));
} String beanName = id;
if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
beanName = aliases.remove(0);
if (logger.isDebugEnabled()) {
logger.debug("No XML 'id' specified - using '" + beanName +
"' as bean name and " + aliases + " as aliases");
}
} if (containingBean == null) {
checkNameUniqueness(beanName, aliases, ele);
} AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
if (beanDefinition != null) {
if (!StringUtils.hasText(beanName)) {
try {
if (containingBean != null) {
beanName = BeanDefinitionReaderUtils.generateBeanName(
beanDefinition, this.readerContext.getRegistry(), true);
}
else {
beanName = this.readerContext.generateBeanName(beanDefinition);
// Register an alias for the plain bean class name, if still possible,
// if the generator returned the class name plus a suffix.
// This is expected for Spring 1.2/2.0 backwards compatibility.
String beanClassName = beanDefinition.getBeanClassName();
if (beanClassName != null &&
beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
!this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
aliases.add(beanClassName);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Neither XML 'id' nor 'name' specified - " +
"using generated bean name [" + beanName + "]");
}
}
catch (Exception ex) {
error(ex.getMessage(), ele);
return null;
}
}
String[] aliasesArray = StringUtils.toStringArray(aliases);
return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
} return null;
}
当我们bean definition解析之后，就要进行注册，注册到bean factory中。
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);
}
}
}
这项工作由实现了BeanDefinitionRegistry接口的DefaultListableBeanFactory类实现的。
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException { Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null"); if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
} synchronized (this.beanDefinitionMap) {
Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!this.allowBeanDefinitionOverriding) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
}
else {
this.beanDefinitionNames.add(beanName);
this.frozenBeanDefinitionNames = null;
}
this.beanDefinitionMap.put(beanName, beanDefinition); resetBeanDefinition(beanName);
}
}
可以看到就是把bean的定义存储在一个ConcurrentHashMap中,同时对于重复定义的bean也会报错，同时也把 bean definition name 有序的存在beanDefinitionNames数组中。
