入口
正如我們所知，spring容器啟動會經歷如下幾個步驟：
1、定位，定位到資源文件，并且解析為Resource對象
2、加載，加載xml，將xml文件解析為對應的BeanDefinition
3、注冊，注冊對應的BeanDefinition
下面以AbstractXmlApplicationContext.loadBeanDefinitions為入口進行分析:
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// Create a new XmlBeanDefinitionReader for the given BeanFactory.
//為當前工廠創建xml解析器
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
//通過上下文的資源加載環境配置bean解析器
beanDefinitionReader.setEnvironment(this.getEnvironment());//配置當前環境
beanDefinitionReader.setResourceLoader(this);//配置資源解析器
//配置schemas或者dtd的資源解析器，EntityResolver維護了url->schemalocation的路徑
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
//子類提供自定義的reader的初始化方法
initBeanDefinitionReader(beanDefinitionReader);
//加載bean定義
loadBeanDefinitions(beanDefinitionReader);
}
定位到beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
ResouceEntityResolver,該類往上追述可知父類為EntityResolver。
何為EntityResolver
如果SAX應用程序需要實現自定義處理外部實體,則必須實現此接口,并使用setEntityResolver方法向SAX 驅動器注冊一個實例.也就是說,對于解析一個xml,sax首先會讀取該xml文檔上的聲明,根據聲明去尋找相應的dtd定義,以便對文檔的進行驗證,默認的尋找規則,(即:通過網絡,實現上就是聲明DTD的地址URI地址來下載DTD聲明),并進行認證,下載的過程是一個漫長的過程,而且當網絡不可用時,這里會報錯,就是應為相應的dtd沒找到,
EntityResolver 的作用
就是項目本身就可以提供一個如何尋找DTD/XSD的聲明方法
即:由程序來實現尋找DTD/XSD聲明的過程,比如我們將DTD/XSD放在項目的某處在實現時直接將此文檔讀取并返回個SAX即可,這樣就避免了通過網絡來尋找DTD/XSD的聲明
查看EntityResolver的接口
public interface EntityResolver {
public abstract InputSource resolveEntity (String publicId,
String systemId)
throws SAXException, IOException;
}
傳入的參數為publicId以及systemId:
對于DTD解析的配置文件：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
...
</beans>
publicId=-//SPRING//DTD BEAN//EN
systemId=http://www.springframework.org/dtd/spring-beans.dtd
對于xsd解析的配置文件：
<?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-2.5.xsd">
...
</beans>
publicId=null
systemId=http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
spring獲取xml驗證文件的過程
spring在獲取驗證文件（dtd或者xsd）的時候，先讀取xml的聲明，判斷采用dtd還是xsd的方式進行讀取：
追述ResourceEntityResolver的父類，可得父類為DelegatingEntityResolver，該類維護了兩個屬性：
private final EntityResolver dtdResolver;//dtd解析器
private final EntityResolver schemaResolver;//xsd解析器
public DelegatingEntityResolver(@Nullable ClassLoader classLoader) {
this.dtdResolver = new BeansDtdResolver();//DTD解析器
this.schemaResolver = new PluggableSchemaResolver(classLoader);//schema解析器
}
//讀取xml聲明，如果有.dtd后綴，調用dtd解析器；如果.xsd后綴，調用schema解析器
public InputSource resolveEntity(String publicId, @Nullable String systemId) throws SAXException, IOException {
if (systemId != null) {
if (systemId.endsWith(DTD_SUFFIX)) {
return this.dtdResolver.resolveEntity(publicId, systemId);
}
else if (systemId.endsWith(XSD_SUFFIX)) {
return this.schemaResolver.resolveEntity(publicId, systemId);
}
}
return null;
}
spring根據xml聲明，將解析的任務委托給dtd解析器BeansDtdResolver或者PluggableSchemaResolver;
BeansDtdResolver解析
它會尋找當前classpath路徑下spring-beans.dtd文件:/org/springframework/beans/factory/xml/spring-beans.dtd
PluggableSchemaResolver解析
//讀取路徑，META-INF/spring.schemas
public static final String DEFAULT_SCHEMA_MAPPINGS_LOCATION = "META-INF/spring.schemas";
//將schemas路徑以URL->filepath的形式存儲
private volatile Map<String, String> schemaMappings;
首先，PluggableSchemaResolver會加載META-INF/spring.schemas下所有schemas的信息存儲在schemaMappings
private Map<String, String> getSchemaMappings() {
Map<String, String> schemaMappings = this.schemaMappings;
//雙重鎖檢查
if (schemaMappings == null) {
synchronized (this) {
schemaMappings = this.schemaMappings;
if (schemaMappings == null) {
if (logger.isDebugEnabled()) {
logger.debug("Loading schema mappings from [" + this.schemaMappingsLocation + "]");
}
try {
//加載路徑為META-INF/spring.schemas，以Properties形式存儲
Properties mappings =
PropertiesLoaderUtils.loadAllProperties(this.schemaMappingsLocation, this.classLoader);
if (logger.isDebugEnabled()) {
logger.debug("Loaded schema mappings: " + mappings);
}
//將加載的內容轉成map
Map<String, String> mappingsToUse = new ConcurrentHashMap<>(mappings.size());
CollectionUtils.mergePropertiesIntoMap(mappings, mappingsToUse);
schemaMappings = mappingsToUse;
//使用schemaMappings存儲
this.schemaMappings = schemaMappings;
}
catch (IOException ex) {
throw new IllegalStateException(
"Unable to load schema mappings from location [" + this.schemaMappingsLocation + "]", ex);
}
}
}
}
return schemaMappings;
}
如何解析對應xsd文件
public InputSource resolveEntity(String publicId, @Nullable String systemId) throws IOException {
if (logger.isTraceEnabled()) {
logger.trace("Trying to resolve XML entity with public id [" + publicId +
"] and system id [" + systemId + "]");
}
if (systemId != null) {
//根據URL即systemid，找到本地xsd文件的存儲路徑
String resourceLocation = getSchemaMappings().get(systemId);
if (resourceLocation != null) {
//將對應xsd文件轉為Resource
Resource resource = new ClassPathResource(resourceLocation, this.classLoader);
try {
InputSource source = new InputSource(resource.getInputStream());
source.setPublicId(publicId);
source.setSystemId(systemId);
if (logger.isDebugEnabled()) {
logger.debug("Found XML schema [" + systemId + "] in classpath: " + resourceLocation);
}
return source;
}
catch (FileNotFoundException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Couldn't find XML schema [" + systemId + "]: " + resource, ex);
}
}
}
}
return null;
}
因此，此類一共做了這么幾件事：
1、將spring-schemas的信息以URL->schemalocation的形式存儲在map中,如圖
2、通過xml聲明的systemid，可以獲取到對應xsd文件在當前路徑下的地址，然后進行加載
