spring加载xml验证文件(dtd,xsd)分析

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入口

正如我们所知,spring容器启动会经历如下几个步骤: 1、定位,定位到资源文件,并且解析为Resource对象 2、加载,加载xml,将xml文件解析为对应的BeanDefinition 3、注册,注册对应的BeanDefinition 下面以AbstractXmlApplicationContext.loadBeanDefinitions为入口进行分析:

	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		//为当前工厂创建xml解析器
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		//通过上下文的资源加载环境配置bean解析器
		beanDefinitionReader.setEnvironment(this.getEnvironment());//配置当前环境
		beanDefinitionReader.setResourceLoader(this);//配置资源解析器
		//配置schemas或者dtd的资源解析器,EntityResolver维护了url->schemalocation的路径
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		//子类提供自定义的reader的初始化方法
		initBeanDefinitionReader(beanDefinitionReader);
		//加载bean定义
		loadBeanDefinitions(beanDefinitionReader);
	}

定位到beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this)); ResouceEntityResolver,该类往上追述可知父类为EntityResolver。

何为EntityResolver

如果SAX应用程序需要实现自定义处理外部实体,则必须实现此接口,并使用setEntityResolver方法向SAX 驱动器注册一个实例.也就是说,对于解析一个xml,sax首先会读取该xml文档上的声明,根据声明去寻找相应的dtd定义,以便对文档的进行验证,默认的寻找规则,(即:通过网络,实现上就是声明DTD的地址URI地址来下载DTD声明),并进行认证,下载的过程是一个漫长的过程,而且当网络不可用时,这里会报错,就是应为相应的dtd没找到,

EntityResolver 的作用

就是项目本身就可以提供一个如何寻找DTD/XSD的声明方法 即:由程序来实现寻找DTD/XSD声明的过程,比如我们将DTD/XSD放在项目的某处在实现时直接将此文档读取并返回个SAX即可,这样就避免了通过网络来寻找DTD/XSD的声明 查看EntityResolver的接口

public interface EntityResolver {  
public abstract InputSource resolveEntity (String publicId,
                                               String systemId)
        throws SAXException, IOException;

}

传入的参数为publicId以及systemId:

对于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

对于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获取xml验证文件的过程

spring在获取验证文件(dtd或者xsd)的时候,先读取xml的声明,判断采用dtd还是xsd的方式进行读取: 追述ResourceEntityResolver的父类,可得父类为DelegatingEntityResolver,该类维护了两个属性:

	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
	}
	//读取xml声明,如果有.dtd后缀,调用dtd解析器;如果.xsd后缀,调用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根据xml声明,将解析的任务委托给dtd解析器BeansDtdResolver或者PluggableSchemaResolver;

BeansDtdResolver解析

1538990352061.png

它会寻找当前classpath路径下spring-beans.dtd文件:/org/springframework/beans/factory/xml/spring-beans.dtd 1538990352066.png

PluggableSchemaResolver解析

//读取路径,META-INF/spring.schemas
public static final String DEFAULT_SCHEMA_MAPPINGS_LOCATION = "META-INF/spring.schemas";
//将schemas路径以URL->filepath的形式存储
private volatile Map<String, String> schemaMappings;

首先,PluggableSchemaResolver会加载META-INF/spring.schemas下所有schemas的信息存储在schemaMappings

	private Map<String, String> getSchemaMappings() {
		Map<String, String> schemaMappings = this.schemaMappings;
		//双重锁检查
		if (schemaMappings == null) {
			synchronized (this) {
				schemaMappings = this.schemaMappings;
				if (schemaMappings == null) {
					if (logger.isDebugEnabled()) {
						logger.debug("Loading schema mappings from [" + this.schemaMappingsLocation + "]");
					}
					try {
						//加载路径为META-INF/spring.schemas,以Properties形式存储
						Properties mappings =
								PropertiesLoaderUtils.loadAllProperties(this.schemaMappingsLocation, this.classLoader);
						if (logger.isDebugEnabled()) {
							logger.debug("Loaded schema mappings: " + mappings);
						}
						//将加载的内容转成map
						Map<String, String> mappingsToUse = new ConcurrentHashMap<>(mappings.size());
						CollectionUtils.mergePropertiesIntoMap(mappings, mappingsToUse);
						schemaMappings = mappingsToUse;
						//使用schemaMappings存储
						this.schemaMappings = schemaMappings;
					}
					catch (IOException ex) {
						throw new IllegalStateException(
								"Unable to load schema mappings from location [" + this.schemaMappingsLocation + "]", ex);
					}
				}
			}
		}
		return schemaMappings;
	}

如何解析对应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) {
			//根据URL即systemid,找到本地xsd文件的存储路径
			String resourceLocation = getSchemaMappings().get(systemId);
			if (resourceLocation != null) {
				//将对应xsd文件转为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,此类一共做了这么几件事: 1、将spring-schemas的信息以URL->schemalocation的形式存储在map中,如图 1538991356375.png 1538991475107.png

2、通过xml声明的systemid,可以获取到对应xsd文件在当前路径下的地址,然后进行加载