spring+mybatis无网启动时，java.net.UnknownHostException: mybatis.org 异常

spring+mybatis无网启动报错

提示加载不到xsd或dtd文件

    项目改造时，遇到了项目在无外网环境下无法启动的问题。启动时抛出java.net.UnknownHostException: mybatis.org 异常。

原因是spring在加载mybatis-config.xml 文件时，无法找到
mybatis-3-config.dtd 这个文件。一般遇到这种项目加载不到本地dtd或xsd文件时，只要检查引入的jar包版本是否与xml文件中声明的dtd或xsd文件是否相匹配即可。
于是，我检查了一下引入的mybatis版本3.2.3，xml文件中声明的是mybatis-3-config.dtd，jar包中是存在的。这就有点困惑了，那就看一下spring是怎么加载dtd文件的吧。
这个文件在

mybatis配置文件.png

    
    
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2. spring加载dtd文件
   spring 使用 org.springframework.beans.factory.xml.BeansDtdResolver这个类来加载dtd文件，这个文件的注释是这么写的：

/**
 * EntityResolver implementation for the Spring beans DTD,
 * to load the DTD from the Spring class path (or JAR file).
 *
 * 
Fetches "spring-beans-2.0.dtd" from the class path resource
 * "/org/springframework/beans/factory/xml/spring-beans-2.0.dtd",
 * no matter whether specified as some local URL that includes "spring-beans"
 * in the DTD name or as "http://www.springframework.org/dtd/spring-beans-2.0.dtd".
 *
 * @author Juergen Hoeller
 * @author Colin Sampaleanu
 * @since 04.06.2003
 * @see ResourceEntityResolver
 */
public class BeansDtdResolver implements EntityResolver {

    private static final String DTD_EXTENSION = ".dtd";

    private static final String DTD_FILENAME = "spring-beans-2.0";

    private static final String DTD_NAME = "spring-beans";

    private static final Log logger = LogFactory.getLog(BeansDtdResolver.class);


    @Override
    public InputSource resolveEntity(String publicId, 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 && systemId.endsWith(DTD_EXTENSION)) {
            int lastPathSeparator = systemId.lastIndexOf("/");
            //看这里，该类只加载以spring-beans开头的dtd文件
            int dtdNameStart = systemId.indexOf(DTD_NAME, lastPathSeparator);
            if (dtdNameStart != -1) {
                String dtdFile = DTD_FILENAME + DTD_EXTENSION;
                if (logger.isTraceEnabled()) {
                    logger.trace("Trying to locate [" + dtdFile + "] in Spring jar on classpath");
                }
                try {
                    Resource resource = new ClassPathResource(dtdFile, getClass());
                    InputSource source = new InputSource(resource.getInputStream());
                    source.setPublicId(publicId);
                    source.setSystemId(systemId);
                    if (logger.isDebugEnabled()) {
                        logger.debug("Found beans DTD [" + systemId + "] in classpath: " + dtdFile);
                    }
                    return source;
                }
                catch (IOException ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug("Could not resolve beans DTD [" + systemId + "]: not found in classpath", ex);
                    }
                }

            }
        }

        // Use the default behavior -> download from website or wherever.
        return null;
    }


    @Override
    public String toString() {
        return "EntityResolver for spring-beans DTD";
    }

}

这个类很简单，只有一个resolveEntity方法。我们看注释和源码知道这个方法只加载spring-beans开头的dtd文件，所以mybatis-3-config.dtd文件当然不会从本地加载。

看到这里，我们就知道了mybatis-config.xml这个文件不应该交给spring来解析，应该由mybatis自己解析。所以我们修改一下项目配置，在classpath下单独给mybatis-config.xml 建一个目录，配置spring 只解析config目录下的xml文件

配置修改.png

给mybatis的SqlSessionFactoryBean配置新的mybatis-config.xml文件地址。

mybatis有自己的xml解析器

mybatis-xml解析器.png

这个类也非常简单，也是只实现了EntityResolver接口。有兴趣可以看一下。
改完，断网启动，没有问题。齐活，溜。

/*
 * Offline entity resolver for the iBATIS DTDs
 */
public class XMLMapperEntityResolver implements EntityResolver {

  private static final Map doctypeMap = new HashMap();

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

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

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

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

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

  static {
    doctypeMap.put(IBATIS_CONFIG_URL, IBATIS_CONFIG_DTD);
    doctypeMap.put(IBATIS_CONFIG_DOCTYPE, IBATIS_CONFIG_DTD);

    doctypeMap.put(IBATIS_MAPPER_URL, IBATIS_MAPPER_DTD);
    doctypeMap.put(IBATIS_MAPPER_DOCTYPE, IBATIS_MAPPER_DTD);

    doctypeMap.put(MYBATIS_CONFIG_URL, IBATIS_CONFIG_DTD);
    doctypeMap.put(MYBATIS_CONFIG_DOCTYPE, IBATIS_CONFIG_DTD);

    doctypeMap.put(MYBATIS_MAPPER_URL, IBATIS_MAPPER_DTD);
    doctypeMap.put(MYBATIS_MAPPER_DOCTYPE, IBATIS_MAPPER_DTD);
  }

  /*
   * Converts a public DTD into a local one
   *
   * @param publicId Unused but required by EntityResolver interface
   * @param systemId The DTD that is being requested
   * @return The InputSource for the DTD
   * @throws org.xml.sax.SAXException If anything goes wrong
   */
  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;
  }

}