mybatis，Spring等工具對xml文件正確性的驗證

　　我們知道mybatis或者spring都是使用xml文件作為配置文件，配置文件的格式都是定義在叫做.dtd或者.xsd文件中的，當工具在解析用戶自己定義的xml文件的時候，如何才能知道用戶自定義的文件是否正確的呢？我們不能在xml文件中亂寫一些框架不認識的標簽，比如在spring的xml文件中寫如下<user>標簽，毫無疑問會報錯。那么框架是怎么來驗證我們所寫的標簽是否正確的呢？

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

　　由於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());
　　　　　　...(此處省略一部分代碼)
        Document doc = builder.parse(Resources.getResourceAsStream("mybatis-config.xml"));
        System.err.println(doc);

　　在第11行中，用到了XMLMapperEntityResolver對象，這個對象定義如下：

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，當框架在解析xml文件的時候，會把xml文件開頭的publicId和systemId傳給EntityResolver，而EntityResolver對象就是從classpath中去尋找.dtd文件(文件就在org/apache/ibatis/builder/xml/目錄下)，在spring中還會存在.xsd文件，原理都是一樣的，然后利用classpath中的.dtd文件進行驗證。如果不指定這個.dtd文件，那么會從互聯網上面下載.dtd文件，性能不好。