这篇“Spring是如何扩展解析xml接口的”文章的知识点大部分人都不太理解，所以小编给大家总结了以下内容，内容详细，步骤清晰，具有一定的借鉴价值，希望大家阅读完这篇文章能有所收获，下面我们一起来看看这篇“Spring是如何扩展解析xml接口的”文章吧。
自定义Spring配置
Java自定义Spring配置标签
BeanDefinitionParserDelegate
在DefaultBeanDefinitionDocumentReader处理Document元素时,将Document文档内元素具体解析工作委托给BeanDefinitionParserDelegate类来处理,默认BeanDefinitionParserDelegate会处理”
public class BeanDefinitionParserDelegate {
public static final String BEANS_NAMESPACE_URI = "http://www.springframework.org/schema/beans";
public static final String MULTI_VALUE_ATTRIBUTE_DELIMITERS = ",; ";
private final XmlReaderContext readerContext;
private final DocumentDefaultsDefinition defaults = new DocumentDefaultsDefinition();
/**
*初始化 default lazy-init, autowire, dependency check settings,
* init-method, destroy-method and merge settings等属性值
* 并且填充到属性 DocumentDefaultsDefinition defaults;
* 如果 当前Xml文件没有配置默认属性，则查父类delegate有没有设置属性；如果没有则 使用默认配置
*/
protected void populateDefaults(DocumentDefaultsDefinition defaults, DocumentDefaultsDefinition parentDefaults, Element root) {
String lazyInit = root.getAttribute(DEFAULT_LAZY_INIT_ATTRIBUTE);
if (DEFAULT_VALUE.equals(lazyInit)) {
// Potentially inherited from outer <beans> sections, otherwise falling back to false.
lazyInit = (parentDefaults != null ? parentDefaults.getLazyInit() : FALSE_VALUE);
}
defaults.setLazyInit(lazyInit);
String merge = root.getAttribute(DEFAULT_MERGE_ATTRIBUTE);
if (DEFAULT_VALUE.equals(merge)) {
// Potentially inherited from outer <beans> sections, otherwise falling back to false.
merge = (parentDefaults != null ? parentDefaults.getMerge() : FALSE_VALUE);
}
defaults.setMerge(merge);
String autowire = root.getAttribute(DEFAULT_AUTOWIRE_ATTRIBUTE);
if (DEFAULT_VALUE.equals(autowire)) {
// Potentially inherited from outer <beans> sections, otherwise falling back to 'no'.
autowire = (parentDefaults != null ? parentDefaults.getAutowire() : AUTOWIRE_NO_VALUE);
}
defaults.setAutowire(autowire);
// Don't fall back to parentDefaults for dependency-check as it's no longer supported in
// <beans> as of 3.0. Therefore, no nested <beans> would ever need to fall back to it.
defaults.setDependencyCheck(root.getAttribute(DEFAULT_DEPENDENCY_CHECK_ATTRIBUTE));
if (root.hasAttribute(DEFAULT_AUTOWIRE_CANDIDATES_ATTRIBUTE)) {
defaults.setAutowireCandidates(root.getAttribute(DEFAULT_AUTOWIRE_CANDIDATES_ATTRIBUTE));
}
else if (parentDefaults != null) {
defaults.setAutowireCandidates(parentDefaults.getAutowireCandidates());
}
if (root.hasAttribute(DEFAULT_INIT_METHOD_ATTRIBUTE)) {
defaults.setInitMethod(root.getAttribute(DEFAULT_INIT_METHOD_ATTRIBUTE));
}
else if (parentDefaults != null) {
defaults.setInitMethod(parentDefaults.getInitMethod());
}
if (root.hasAttribute(DEFAULT_DESTROY_METHOD_ATTRIBUTE)) {
defaults.setDestroyMethod(root.getAttribute(DEFAULT_DESTROY_METHOD_ATTRIBUTE));
}
else if (parentDefaults != null) {
defaults.setDestroyMethod(parentDefaults.getDestroyMethod());
}
defaults.setSource(this.readerContext.extractSource(root));
}
//解析自定义元素
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
//获取当前Element的命名空间
String namespaceUri = getNamespaceURI(ele);
//获取namespaceHandlerResolver然后根据命名空间得到具体的 NamespaceHandler处理类
NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
if (handler == null) {
error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
return null;
}
//具体的NamespaceHandler处理类去解析Element；返回 BeanDefinition 对象
return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}
//返回defaults属性值
public BeanDefinitionDefaults getBeanDefinitionDefaults() {
BeanDefinitionDefaults bdd = new BeanDefinitionDefaults();
bdd.setLazyInit("TRUE".equalsIgnoreCase(this.defaults.getLazyInit()));
bdd.setDependencyCheck(this.getDependencyCheck(DEFAULT_VALUE));
bdd.setAutowireMode(this.getAutowireMode(DEFAULT_VALUE));
bdd.setInitMethodName(this.defaults.getInitMethod());
bdd.setDestroyMethodName(this.defaults.getDestroyMethod());
return bdd;
}DefaultNamespaceHandlerResolver
DefaultNamespaceHandlerResolver implements NamespaceHandlerResolver； 默认的 命名空间解析对象选择器； 主要作用就是 根据不同的命名空间来选择对应的解析器；
/**
* 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
*/
@Override
public NamespaceHandler resolve(String namespaceUri) {
Map<String, Object> handlerMappings = getHandlerMappings();
Object handlerOrClassName = handlerMappings.get(namespaceUri);
if (handlerOrClassName == null) {
return null;
}
//如果已经把全类名路径转换成具体的实例了 就直接返回
else if (handlerOrClassName instanceof NamespaceHandler) {
return (NamespaceHandler) handlerOrClassName;
}
else {
String className = (String) handlerOrClassName;
try {
//根据全类名路径 来加载这个类
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");
}
//加载完类之后，通过反射生成具体实例对象;例如：ContextNamespaceHandler
NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
//调用具体对象的init()方法，注册对应的解析类，例如annotation-config 对应的解析类是 AnnotationConfigBeanDefinitionParser
namespaceHandler.init();
//将实例对象覆盖 全类名路径
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);
}
}
}
/**
* 第一次加载的时候 Map里面的对象 是String类型，全类名路径
*/
private Map<String, Object> getHandlerMappings() {
if (this.handlerMappings == null) {
synchronized (this) {
if (this.handlerMappings == null) {
//加载META-INF/spring.handlers的所有属性
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());
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;
}NamespaceHandlerSupport implements NamespaceHandler
实现了NamespaceHandler中的parse()和decorate方法,并且提供一些属性来维护 具体的解析元素的对象BeanDefinitionParser； 例如:www.springframework.org/schema/cont… 命名空间对应的操作类是 ContextNamespaceHandler; 这个操作类会调用init()方法
@Override
public void init() {
registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser());
registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser());
registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser());
registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser());
}根据不同的属性，注册不同的解析类 BeanDefinitionParser
我们以ContextNamespaceHandler看下, 实现类只要实现init方法就行了; 对应的解析类 通过registerBeanDefinitionParser方法，存放到NamespaceHandlerSupport中的Map parsers 中
public abstract class NamespaceHandlerSupport implements NamespaceHandler {
//将元素头 与对应的解析器对应起来
protected final void registerBeanDefinitionParser(String elementName, BeanDefinitionParser parser) {
this.parsers.put(elementName, parser);
}
/**
*具体的NamespaceHandler处理类去解析Element；返回 BeanDefinition 对象
* BeanDefinitionParserDelegate.parseCustomElement 中 在获取到了对应的handler后，会执行handler.parse(ele, new ParserContext(this.readerContext, this, containingBd))方法；
* 例如：获取到的操作类是 ContextNamespaceHandler,调用parse的方法其实调用了NamespaceHandlerSupportde parse方法;也就是下面的
* 例如annotation-config得到的是AnnotationConfigBeanDefinitionParser实例， 再调用AnnotationConfigBeanDefinitionParser的parse方法；这样就实现了根据不同元素来解析成 Spring中的 定义Bean BeanDefinition了；
*/
@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
return findParserForElement(element, parserContext).parse(element, parserContext);
}
/**
* 根据元素节点的名称 获取对应的解析类实例；例如annotation-config得到的是AnnotationConfigBeanDefinitionParser实例，
*/
private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {
String localName = parserContext.getDelegate().getLocalName(element);
BeanDefinitionParser parser = this.parsers.get(localName);
if (parser == null) {
parserContext.getReaderContext().fatal(
"Cannot locate BeanDefinitionParser for element [" + localName + "]", element);
}
return parser;
}
}public class ContextNamespaceHandler extends NamespaceHandlerSupport {
@Override
public void init() {
registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser());
registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser());
registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser());
registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser());
}
}BeanDefinitionParser
Element元素的解析类；只有一个方法BeanDefinition parse(Element element, ParserContext parserContext); 实现类去实现这个方法就行了；
我们看到parse方法中传入了ParserContext 参数；我们了解一下这个；
ParserContext
解析类上下文；这类hold住了 XmlReaderContext和 BeanDefinitionParserDelegate；可以最大限度的让用户自已去解析
public ParserContext(XmlReaderContext readerContext, BeanDefinitionParserDelegate delegate,
BeanDefinition containingBeanDefinition) {
this.readerContext = readerContext;
this.delegate = delegate;
this.containingBeanDefinition = containingBeanDefinition;
}