[TOC]
date [2019-8-15]
XmlBeanDefinitionReader
Spring中在外部看来，是通过XMLBeanDefinitionReader来完成Bean定义的加载、解析、以及Bean定义的注册，但是XMLBeanDefinitionReader其底层维护了多个组件来完成上面的每一件事情，这很好地体现了单一职责的设计。而DefaultDocumentLoader这个组件，就是完成Bean定义的加载功能。
XmlBeanDefinitionReader.png
XmlBeanDefinitionReader 完成Bean加载
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
documentReader.setEnvironment(this.getEnvironment());
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}
解析xml生成BeanDefinitionDoucmentReader的类
DefinitionDocumentReader 只有一个实现类DefaultBeanDefinitionDocumentReader
BeanDefinitionDocumentReader.png
package org.springframework.beans.factory.xml;
import java.io.IOException;
import java.net.URISyntaxException;
import java.util.LinkedHashSet;
import java.util.Set;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.springframework.beans.factory.BeanDefinitionStoreException;
import org.springframework.beans.factory.config.BeanDefinitionHolder;
import org.springframework.beans.factory.parsing.BeanComponentDefinition;
import org.springframework.beans.factory.support.BeanDefinitionReaderUtils;
import org.springframework.core.env.Environment;
import org.springframework.core.io.Resource;
import org.springframework.core.io.support.ResourcePatternUtils;
import org.springframework.util.Assert;
import org.springframework.util.ResourceUtils;
import org.springframework.util.StringUtils;
public class DefaultBeanDefinitionDocumentReader implements BeanDefinitionDocumentReader {
public static final String BEAN_ELEMENT = BeanDefinitionParserDelegate.BEAN_ELEMENT;
public static final String NESTED_BEANS_ELEMENT = "beans";
public static final String ALIAS_ELEMENT = "alias";
public static final String NAME_ATTRIBUTE = "name";
public static final String ALIAS_ATTRIBUTE = "alias";
public static final String IMPORT_ELEMENT = "import";
public static final String RESOURCE_ATTRIBUTE = "resource";
public static final String PROFILE_ATTRIBUTE = "profile";
protected final Log logger = LogFactory.getLog(getClass());
private Environment environment;
private XmlReaderContext readerContext;
private BeanDefinitionParserDelegate delegate;
/**
* {@inheritDoc}
* <p>This implementation parses bean definitions according to the "spring-beans" XSD
* (or DTD, historically).
* <p>Opens a DOM Document; then initializes the default settings
* specified at the {@code <beans/>} level; then parses the contained bean definitions.
*/
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
doRegisterBeanDefinitions(root);
}
/**
* Register each bean definition within the given root {@code <beans/>} element.
* @throws IllegalStateException if {@code <beans profile="..."} attribute is present
* and Environment property has not been set
* @see #setEnvironment
*/
protected void doRegisterBeanDefinitions(Element root) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
Assert.state(this.environment != null, "Environment must be set for evaluating profiles");
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!this.environment.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;
}
protected BeanDefinitionParserDelegate createDelegate(
XmlReaderContext readerContext, Element root, BeanDefinitionParserDelegate parentDelegate) {
BeanDefinitionParserDelegate delegate = createHelper(readerContext, root, parentDelegate);
if (delegate == null) {
delegate = new BeanDefinitionParserDelegate(readerContext, this.environment);
delegate.initDefaults(root, parentDelegate);
}
return delegate;
}
@Deprecated
protected BeanDefinitionParserDelegate createHelper(
XmlReaderContext readerContext, Element root, BeanDefinitionParserDelegate parentDelegate) {
return null;
}
/**
* Return the descriptor for the XML resource that this parser works on.
*/
protected final XmlReaderContext getReaderContext() {
return this.readerContext;
}
/**
* Invoke the {@link org.springframework.beans.factory.parsing.SourceExtractor} to pull the
* source metadata from the supplied {@link Element}.
*/
protected Object extractSource(Element ele) {
return this.readerContext.extractSource(ele);
}
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);
}
}
protected void importBeanDefinitionResource(Element ele) {
String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
if (!StringUtils.hasText(location)) {
getReaderContext().error("Resource location must not be empty", ele);
return;
}
// Resolve system properties: e.g. "${user.dir}"
location = environment.resolveRequiredPlaceholders(location);
Set<Resource> actualResources = new LinkedHashSet<Resource>(4);
// Discover whether the location is an absolute or relative URI
boolean absoluteLocation = false;
try {
absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
}
catch (URISyntaxException ex) {
// cannot convert to an URI, considering the location relative
// unless it is the well-known Spring prefix "classpath*:"
}
// Absolute or relative?
if (absoluteLocation) {
try {
int importCount = getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
if (logger.isDebugEnabled()) {
logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
}
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error(
"Failed to import bean definitions from URL location [" + location + "]", ele, ex);
}
}
else {
// No URL -> considering resource location as relative to the current file.
try {
int importCount;
Resource relativeResource = getReaderContext().getResource().createRelative(location);
if (relativeResource.exists()) {
importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
actualResources.add(relativeResource);
}
else {
String baseLocation = getReaderContext().getResource().getURL().toString();
importCount = getReaderContext().getReader().loadBeanDefinitions(
StringUtils.applyRelativePath(baseLocation, location), actualResources);
}
if (logger.isDebugEnabled()) {
logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
}
}
catch (IOException ex) {
getReaderContext().error("Failed to resolve current resource location", ele, ex);
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]",
ele, ex);
}
}
Resource[] actResArray = actualResources.toArray(new Resource[actualResources.size()]);
getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
}
protected void processAliasRegistration(Element ele) {
String name = ele.getAttribute(NAME_ATTRIBUTE);
String alias = ele.getAttribute(ALIAS_ATTRIBUTE);
boolean valid = true;
if (!StringUtils.hasText(name)) {
getReaderContext().error("Name must not be empty", ele);
valid = false;
}
if (!StringUtils.hasText(alias)) {
getReaderContext().error("Alias must not be empty", ele);
valid = false;
}
if (valid) {
try {
getReaderContext().getRegistry().registerAlias(name, alias);
}
catch (Exception ex) {
getReaderContext().error("Failed to register alias '" + alias +
"' for bean with name '" + name + "'", ele, ex);
}
getReaderContext().fireAliasRegistered(name, alias, extractSource(ele));
}
}
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));
}
}
protected void preProcessXml(Element root) {
}
protected void postProcessXml(Element root) {
}
}
EntityResolver
在loadDocument方法中涉及一个参数EntityResolver。
EntityResolver的作用是项目本身就可以提供一个如何寻找DTD声明的方法，由程序来实现寻找DTD声明的过程，比如将DTD文件放到
项目某处，在实现时直接将此文档读取并返回给SAX。避免通过网络来寻找相应的声明。
EntityResolver接口声明如下：
public interface EntityResolver {
public abstract InputSource resolveEntity (String publicId,
String systemId)
throws SAXException, IOException;
}
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