mybatis 与 xml

mybatis的两大重要组件：配置和映射文件，都是可以通过xml配置的（新版本新增了注解的方式配置Mapper），下面来解析下mybatis是怎么做的

其中，关于配置文件解析的主要是在这个类XMLConfigBuilder里面的parseConfiguration方法，XMLConfigBuilder在初始化的时候会新建一个XPathParser对象用于XML配置文件的解析

public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
    this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
  }

public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

这面有个重要的类XPathParser和重要的方法evalNode，所以我们引出了这两个类：

org.apache.ibatis.parsing.XNode

org.apache.ibatis.parsing.XPathParser

Mybatis在xml解析这一块用的是java dom API 

XNode类是对Node的简单封装，用于表示一个xml 节点，是配置的模型类。其中variables中存放的是变量配置的键值对，用于解析${xx}的字符串（见下文的PropertyParser）xpathParser是一个用于解析节点的解析器，XNode中的大部分方法都是委托给xpathParser对象来进行的。

XPathParser是整个配置的上下文类，document对象表示这个xml的整个文档，variables和XNode一样。

document建立过程：

初始化xpath 

XPathFactory factory = XPathFactory.newInstance();
this.xpath = factory.newXPath();

创建document

private Document createDocument(InputSource inputSource) {
    // important: this must only be called AFTER common constructor
    try {
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      factory.setValidating(validation);

      factory.setNamespaceAware(false);
      factory.setIgnoringComments(true);
      factory.setIgnoringElementContentWhitespace(false);
      factory.setCoalescing(false);
      factory.setExpandEntityReferences(true);

      DocumentBuilder builder = factory.newDocumentBuilder();
      builder.setEntityResolver(entityResolver);
      builder.setErrorHandler(new ErrorHandler() {
        @Override
        public void error(SAXParseException exception) throws SAXException {
          throw exception;
        }

        @Override
        public void fatalError(SAXParseException exception) throws SAXException {
          throw exception;
        }

        @Override
        public void warning(SAXParseException exception) throws SAXException {
        }
      });
      return builder.parse(inputSource);
    } catch (Exception e) {
      throw new BuilderException("Error creating document instance.  Cause: " + e, e);
    }
  }

XPathParser中实现了大量的解析节点的方法和重载：

private Object evaluate(String expression, Object root, QName returnType) {
    try {
      return xpath.evaluate(expression, root, returnType);
    } catch (Exception e) {
      throw new BuilderException("Error evaluating XPath.  Cause: " + e, e);
    }
  }

public XNode evalNode(Object root, String expression) {
    Node node = (Node) evaluate(expression, root, XPathConstants.NODE);
    if (node == null) {
      return null;
    }
    return new XNode(this, node, variables);
  }

PropertyParser类

在XNode和XPathParser中我们都见过对PropertyParser的使用，主要是根据variables解析字符串中的变量用的，

  private String getBodyData(Node child) {
    if (child.getNodeType() == Node.CDATA_SECTION_NODE
        || child.getNodeType() == Node.TEXT_NODE) {
      String data = ((CharacterData) child).getData();
      data = PropertyParser.parse(data, variables);
      return data;
    }
    return null;
  }

  public String evalString(Object root, String expression) {
    String result = (String) evaluate(expression, root, XPathConstants.STRING);
    result = PropertyParser.parse(result, variables);
    return result;
  }

下面是这个方法的实现，它借助于GenericTokenParser来进行解析，

  public static String parse(String string, Properties variables) {
    VariableTokenHandler handler = new VariableTokenHandler(variables);
    GenericTokenParser parser = new GenericTokenParser("${", "}", handler);
    return parser.parse(string);
  }

这里引入了一个新的东西：TokenHandler和GenericTokenParser

GenericTokenParser的parse方法用来解析文本（替换参数），openToken是开始标记，这里是"${"，closeToken是结束标记，这里是"}"

  public String parse(String text) {
    final StringBuilder builder = new StringBuilder();
    final StringBuilder expression = new StringBuilder();
    if (text != null && text.length() > 0) {
      char[] src = text.toCharArray();
      int offset = 0;
      // search open token
      int start = text.indexOf(openToken, offset);
      while (start > -1) {
        if (start > 0 && src[start - 1] == '\') {
          // this open token is escaped. remove the backslash and continue.
          builder.append(src, offset, start - offset - 1).append(openToken);
          offset = start + openToken.length();
        } else {
          // found open token. let's search close token.
          expression.setLength(0);
          builder.append(src, offset, start - offset);
          offset = start + openToken.length();
          int end = text.indexOf(closeToken, offset);
          while (end > -1) {
            if (end > offset && src[end - 1] == '\') {
              // this close token is escaped. remove the backslash and continue.
              expression.append(src, offset, end - offset - 1).append(closeToken);
              offset = end + closeToken.length();
              end = text.indexOf(closeToken, offset);
            } else {
              expression.append(src, offset, end - offset);
              offset = end + closeToken.length();
              break;
            }
          }
          if (end == -1) {
            // close token was not found.
            builder.append(src, start, src.length - start);
            offset = src.length;
          } else {
            builder.append(handler.handleToken(expression.toString()));
            offset = end + closeToken.length();
          }
        }
        start = text.indexOf(openToken, offset);
      }
      if (offset < src.length) {
        builder.append(src, offset, src.length - offset);
      }
    }
    return builder.toString();
  }

上面标红的代码比较重要，其他的代码只是清除噪声，根据openToken和closeToken定位到这个位置，解析出expression，也就是${xxx}的那段字符串，之后交给TokenHandler处理，这里是VariableTokenHandler

    @Override
    public String handleToken(String content) {
      if (variables != null) {
        String key = content;
        if (enableDefaultValue) {
          final int separatorIndex = content.indexOf(defaultValueSeparator);
          String defaultValue = null;
          if (separatorIndex >= 0) {
            key = content.substring(0, separatorIndex);
            defaultValue = content.substring(separatorIndex + defaultValueSeparator.length());
          }
          if (defaultValue != null) {
            return variables.getProperty(key, defaultValue);
          }
        }
        if (variables.containsKey(key)) {
          return variables.getProperty(key);
        }
      }
      return "${" + content + "}";
    }

然后就会得到了解析后的字符串。

最后，我们来看看创建XPathParser时候指定的XMLMapperEntityResolver类，这个是用于验证xml是否符合dtd的（根据doctype）

/**
 * Offline entity resolver for the MyBatis DTDs
 * 
 * @author Clinton Begin
 * @author Eduardo Macarron
 */
public class XMLMapperEntityResolver implements EntityResolver {

  private static final String IBATIS_CONFIG_SYSTEM = "ibatis-3-config.dtd";
  private static final String IBATIS_MAPPER_SYSTEM = "ibatis-3-mapper.dtd";
  private static final String MYBATIS_CONFIG_SYSTEM = "mybatis-3-config.dtd";
  private static final String MYBATIS_MAPPER_SYSTEM = "mybatis-3-mapper.dtd";

  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";

  /*
   * 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 {
    try {
      if (systemId != null) {
        String lowerCaseSystemId = systemId.toLowerCase(Locale.ENGLISH);
        if (lowerCaseSystemId.contains(MYBATIS_CONFIG_SYSTEM) || lowerCaseSystemId.contains(IBATIS_CONFIG_SYSTEM)) {
          return getInputSource(MYBATIS_CONFIG_DTD, publicId, systemId);
        } else if (lowerCaseSystemId.contains(MYBATIS_MAPPER_SYSTEM) || lowerCaseSystemId.contains(IBATIS_MAPPER_SYSTEM)) {
          return getInputSource(MYBATIS_MAPPER_DTD, publicId, systemId);
        }
      }
      return null;
    } catch (Exception e) {
      throw new SAXException(e.toString());
    }
  }

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

}