spring源码剖析（四）自定义标签解析流程

解析总流程

自定义标签解析入口

 让我们先看BeanDefinitionParserDelegate类的parseCustomElement方法:

public BeanDefinition parseCustomElement(Element ele) {
return parseCustomElement(ele, null);
}
//containingBd为父类bean,对顶层元素的解析应设置为null
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
String namespaceUri = getNamespaceURI(ele);
//根据命名空间找到对应的NamespaceHandler进行解析
NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
if (handler == null) {
error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
return null;
}
//调用自定义的NamespaceHandler进行解析
return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}

提取自定义标签处理器

跟踪this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
进入DefaultNamespaceHandlerResolver中的resolve方法

/**
* Locate the {@link NamespaceHandler} for the supplied namespace URI
* from the configured mappings.
* @param namespaceUri the relevant namespace URI
* @return the located {@link NamespaceHandler}, or {@code null} if none found
*/
public NamespaceHandler resolve(String namespaceUri) {
//获取所有已经配置的handler映射
Map<String, Object> handlerMappings = getHandlerMappings();
//根据命名空间找打对应信息
Object handlerOrClassName = handlerMappings.get(namespaceUri);
if (handlerOrClassName == null) {
return null;
}
else if (handlerOrClassName instanceof NamespaceHandler) {
//已做过解析的情况,直接从缓存读取
return (NamespaceHandler) handlerOrClassName;
}
else {
//没有做过解析,则返回的是类路径
String className = (String) handlerOrClassName;
try {
//使用反射将类路径转换成类
Class<?> handlerClass = ClassUtils.forName(className, this.classLoader);
if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {
throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +
"] does not implement the [" + NamespaceHandler.class.getName() + "] interface");
}
//初始化类
NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
//调用自定义的NamespaceHandler的初始化方法
namespaceHandler.init();
//记录在缓存
handlerMappings.put(namespaceUri, namespaceHandler);
return namespaceHandler;
}
catch (ClassNotFoundException ex) {
throw new FatalBeanException("NamespaceHandler class [" + className + "] for namespace [" +
namespaceUri + "] not found", ex);
}
catch (LinkageError err) {
throw new FatalBeanException("Invalid NamespaceHandler class [" + className + "] for namespace [" +
namespaceUri + "]: problem with handler class file or dependent class", err);
}
}
}

回忆上一篇的自定义标签使用中的MyNamespaceHandler便可以发现,其实上面的方法就是初始化了MyNamespaceHandler,然后调用了他的init()方法

/**
*
* @author Administrator
*/
public class MyNamespaceHandler extends NamespaceHandlerSupport {

/* (non-Javadoc)
* @see org.springframework.beans.factory.xml.NamespaceHandler#init()
*/
@Override
public void init() {
registerBeanDefinitionParser("user", new UserBeanDefinitionParser());
}

}

让我们在看看spring是如何获取handler映射的,查看getHandlerMappings()方法

/**
* Load the specified NamespaceHandler mappings lazily.
*/
private Map<String, Object> getHandlerMappings() {
//如果没有被缓存,则开始进入缓存
if (this.handlerMappings == null) {
synchronized (this) {
if (this.handlerMappings == null) {
try {
//this.handlerMappings在构造函数已被初始化为META-INF/Spring.handlers
Properties mappings =
PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
if (logger.isDebugEnabled()) {
logger.debug("Loaded NamespaceHandler mappings: " + mappings);
}
Map<String, Object> handlerMappings = new ConcurrentHashMap<String, Object>(mappings.size());
//将properties文件合并到Map格式的handlerMappings中
CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
this.handlerMappings = handlerMappings;
}
catch (IOException ex) {
throw new IllegalStateException(
"Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
}
}
}
}
return this.handlerMappings;
}

标签解析

得到了解析器和分析的元素侯,Spring就可以将解析工作委托给自定义解析器去解析了

让我们回到BeanDefinitionParserDelegate的handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));中

进入NamespaceHandlerSupport中的parse方法

/**
* Parses the supplied {@link Element} by delegating to the {@link BeanDefinitionParser} that is
* registered for that {@link Element}.
*/
public BeanDefinition parse(Element element, ParserContext parserContext) {
//寻找解析器,并进行解析操作
return findParserForElement(element, parserContext).parse(element, parserContext);
}

/**
* Locates the {@link BeanDefinitionParser} from the register implementations using
* the local name of the supplied {@link Element}.
*/
private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {
//获取元素名称,也就是<myname:user 中的user,此时localname为user
String localName = parserContext.getDelegate().getLocalName(element);
//根据user找到对应的解析器也就是在
//registerBeanDefinitionParser("user", new UserBeanDefinitionParser());注册的解析器
BeanDefinitionParser parser = this.parsers.get(localName);
if (parser == null) {
parserContext.getReaderContext().fatal(
"Cannot locate BeanDefinitionParser for element [" + localName + "]", element);
}
return parser;
}

让我们再跟踪到AbstractBeanDefinitionParser 的parse()方法

public final BeanDefinition parse(Element element, ParserContext parserContext) {
AbstractBeanDefinition definition = parseInternal(element, parserContext);
if (definition != null && !parserContext.isNested()) {
try {
String id = resolveId(element, definition, parserContext);
if (!StringUtils.hasText(id)) {
parserContext.getReaderContext().error(
"Id is required for element '" + parserContext.getDelegate().getLocalName(element)
+ "' when used as a top-level tag", element);
}
String[] aliases = new String[0];
String name = element.getAttribute(NAME_ATTRIBUTE);
if (StringUtils.hasLength(name)) {
aliases = StringUtils.trimArrayElements(StringUtils.commaDelimitedListToStringArray(name));
}
//将AbstractBeanDefinition转换成BeanDefinitionHolder
BeanDefinitionHolder holder = new BeanDefinitionHolder(definition, id, aliases);
registerBeanDefinition(holder, parserContext.getRegistry());
if (shouldFireEvents()) {
//需要通知监听器进行处理
BeanComponentDefinition componentDefinition = new BeanComponentDefinition(holder);
postProcessComponentDefinition(componentDefinition);
parserContext.registerComponent(componentDefinition);
}
}
catch (BeanDefinitionStoreException ex) {
parserContext.getReaderContext().error(ex.getMessage(), element);
return null;
}
}
return definition;
}

虽然说是对自定义配置文件的解析,但是我们可以看到,在这个函数中大部分的大马士用来处理将解析后的AbstractBeanDefinition转换为BeanDefinitionHolder并注册的功能,而真正去做解析的事情委托了给parseInternal,真是这句代码调用了我们的自定义解析函数。在parseInternal中，并不是直接调用自定义的doParse函数，而是惊醒了一些列的数据准备，包括对beanClass，scope，lazyInit等属性的准备。

接下来，让我们看看parseInternal方法，先跟踪到AbstractSingleBeanDefinitionParser

/**
* Creates a {@link BeanDefinitionBuilder} instance for the
* {@link #getBeanClass bean Class} and passes it to the
* {@link #doParse} strategy method.
* @param element the element that is to be parsed into a single BeanDefinition
* @param parserContext the object encapsulating the current state of the parsing process
* @return the BeanDefinition resulting from the parsing of the supplied {@link Element}
* @throws IllegalStateException if the bean {@link Class} returned from
* {@link #getBeanClass(org.w3c.dom.Element)} is {@code null}
* @see #doParse
*/
@Override
protected final AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) {
BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition();
String parentName = getParentName(element);
if (parentName != null) {
builder.getRawBeanDefinition().setParentName(parentName);
}
//获取自定义标签中的class,此时会调用自定义解析器,如UserBeanDefinitionParser中的getBeanClass方法
Class<?> beanClass = getBeanClass(element);
if (beanClass != null) {
builder.getRawBeanDefinition().setBeanClass(beanClass);
}
else {
//若子类没有重写getBeanClass方法则尝试检查子类是否重写getBeanClassName方法
String beanClassName = getBeanClassName(element);
if (beanClassName != null) {
builder.getRawBeanDefinition().setBeanClassName(beanClassName);
}
}
builder.getRawBeanDefinition().setSource(parserContext.extractSource(element));
if (parserContext.isNested()) {
// Inner bean definition must receive same scope as containing bean.
builder.setScope(parserContext.getContainingBeanDefinition().getScope());
}
if (parserContext.isDefaultLazyInit()) {
// Default-lazy-init applies to custom bean definitions as well.
builder.setLazyInit(true);
}
doParse(element, parserContext, builder);
return builder.getBeanDefinition();
}