mybatis，Spring等工具对xml文件正确性的验证

　　我们知道mybatis或者spring都是使用xml文件作为配置文件，配置文件的格式都是定义在叫做.dtd或者.xsd文件中的，当工具在解析用户自己定义的xml文件的时候，如何才能知道用户自定义的文件是否正确的呢？我们不能在xml文件中乱写一些框架不认识的标签，比如在spring的xml文件中写如下<user>标签，毫无疑问会报错。那么框架是怎么来验证我们所写的标签是否正确的呢？

<user>
  <id>100</id>
</user>

　　由于mybatis使用的是dom解析，利用JDK的dom解析API，如下：

    @Test
    public void docFactoryTest() throws Exception {
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        factory.setValidating(true);
        factory.setNamespaceAware(false);
        factory.setIgnoringComments(true);
        factory.setIgnoringElementContentWhitespace(false);
        factory.setCoalescing(false);
        factory.setExpandEntityReferences(true);
        DocumentBuilder builder = factory.newDocumentBuilder();
        builder.setEntityResolver(new XMLMapperEntityResolver());
　　　　　　...(此处省略一部分代码)
        Document doc = builder.parse(Resources.getResourceAsStream("mybatis-config.xml"));
        System.err.println(doc);

　　在第11行中，用到了XMLMapperEntityResolver对象，这个对象定义如下：

public class XMLMapperEntityResolver implements EntityResolver {

  private static final Map<String, String> doctypeMap = new HashMap<String, String>();

  private static final String IBATIS_CONFIG_PUBLIC = "-//ibatis.apache.org//DTD Config 3.0//EN".toUpperCase(Locale.ENGLISH);
  private static final String IBATIS_CONFIG_SYSTEM = "http://ibatis.apache.org/dtd/ibatis-3-config.dtd".toUpperCase(Locale.ENGLISH);

  private static final String IBATIS_MAPPER_PUBLIC = "-//ibatis.apache.org//DTD Mapper 3.0//EN".toUpperCase(Locale.ENGLISH);
  private static final String IBATIS_MAPPER_SYSTEM = "http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd".toUpperCase(Locale.ENGLISH);

  private static final String MYBATIS_CONFIG_PUBLIC = "-//mybatis.org//DTD Config 3.0//EN".toUpperCase(Locale.ENGLISH);
  private static final String MYBATIS_CONFIG_SYSTEM = "http://mybatis.org/dtd/mybatis-3-config.dtd".toUpperCase(Locale.ENGLISH);

  private static final String MYBATIS_MAPPER_PUBLIC = "-//mybatis.org//DTD Mapper 3.0//EN".toUpperCase(Locale.ENGLISH);
  private static final String MYBATIS_MAPPER_SYSTEM = "http://mybatis.org/dtd/mybatis-3-mapper.dtd".toUpperCase(Locale.ENGLISH);

  private static final String MYBATIS_CONFIG_DTD = "org/apache/ibatis/builder/xml/mybatis-3-config.dtd";
  private static final String MYBATIS_MAPPER_DTD = "org/apache/ibatis/builder/xml/mybatis-3-mapper.dtd";

  static {
    doctypeMap.put(IBATIS_CONFIG_SYSTEM, MYBATIS_CONFIG_DTD);
    doctypeMap.put(IBATIS_CONFIG_PUBLIC, MYBATIS_CONFIG_DTD);

    doctypeMap.put(IBATIS_MAPPER_SYSTEM, MYBATIS_MAPPER_DTD);
    doctypeMap.put(IBATIS_MAPPER_PUBLIC, MYBATIS_MAPPER_DTD);

    doctypeMap.put(MYBATIS_CONFIG_SYSTEM, MYBATIS_CONFIG_DTD);
    doctypeMap.put(MYBATIS_CONFIG_PUBLIC, MYBATIS_CONFIG_DTD);

    doctypeMap.put(MYBATIS_MAPPER_SYSTEM, MYBATIS_MAPPER_DTD);
    doctypeMap.put(MYBATIS_MAPPER_PUBLIC, MYBATIS_MAPPER_DTD);
  }

  /*
   * Converts a public DTD into a local one
   * 
   * @param publicId The public id that is what comes after "PUBLIC"
   * @param systemId The system id that is what comes after the public id.
   * @return The InputSource for the DTD
   * 
   * @throws org.xml.sax.SAXException If anything goes wrong
   */
  @Override
  public InputSource resolveEntity(String publicId, String systemId) throws SAXException {

    if (publicId != null) {
      publicId = publicId.toUpperCase(Locale.ENGLISH);
    }
    if (systemId != null) {
      systemId = systemId.toUpperCase(Locale.ENGLISH);
    }

    InputSource source = null;
    try {
      String path = doctypeMap.get(publicId);
      source = getInputSource(path, source);
      if (source == null) {
        path = doctypeMap.get(systemId);
        source = getInputSource(path, source);
      }
    } catch (Exception e) {
      throw new SAXException(e.toString());
    }
    return source;
  }

  private InputSource getInputSource(String path, InputSource source) {
    if (path != null) {
      InputStream in;
      try {
        in = Resources.getResourceAsStream(path);
        source = new InputSource(in);
      } catch (IOException e) {
        // ignore, null is ok
      }
    }
    return source;
  }

}

　　可以看到，当框架在解析xml文件的时候，会把xml文件开头的publicId和systemId传给EntityResolver，而EntityResolver对象就是从classpath中去寻找.dtd文件(文件就在org/apache/ibatis/builder/xml/目录下)，在spring中还会存在.xsd文件，原理都是一样的，然后利用classpath中的.dtd文件进行验证。如果不指定这个.dtd文件，那么会从互联网上面下载.dtd文件，性能不好。