# 5.1.1.1\. EntityResolver As mentioned previously, Hibernate will first attempt to resolve DTDs in its classpath. The manner in which it does this is by registering a custom `org.xml.sax.EntityResolver` implementation with the SAXReader it uses to read in the xml files. This custom `EntityResolver` recognizes two different systemId namespaces. 如前所述,Hibernate首先在其classpath中查找DTD。其行为是依靠在系统中注册的`org.xml.sax.EntityResolver`的一个具体实现，SAXReader依靠它来读取xml文件。这一 `EntityResolver` 实现能辨认两种不同的 systenId命名空间。 * 若resolver遇到了一个以`http://hibernate.sourceforge.net/`为开头的systemId，它会辨认出是`hibernate namespace`，resolver就试图通过加载Hibernate类的classloader来查找这些实体。 * 若resolver遇到了一个使用`classpath://`URL协议的systemId，它会辨认出这是`user namespace`,resolver试图通过(1)当前线程上下文的classloader和(2)加载Hibernate class的classloader来查找这些实体。 使用user namespace(用户命名空间)的例子： ``` <?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [ <!ENTITY types SYSTEM "classpath://your/domain/types.xml"> ]> <hibernate-mapping package="your.domain"> <class name="MyEntity"> <id name="id" type="my-custom-id-type"> ... </id> <class> &types; </hibernate-mapping> ``` `types.xml`是`your.domain`包中的一个资源，它包含了一个定制的[第 5.2.3 节 “自定义值类型”](../Text/pr01_split_000.html#mapping-types-custom "5.2.3\. 自定义值类型")。