MyBatis框架的使用及源码分析(四) 解析Mapper接口映射xml文件

在<MyBatis框架中Mapper映射配置的使用及原理解析(二) 配置篇 SqlSessionFactoryBuilder，XMLConfigBuilder> 一文中，我们知道mybatis配置文件是由XMLConfigBuilder来解析的,看以下代码：

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

可以看到解析mapper文件的加载解析i是从 mapperElement(root.evalNode("mappers"));  开始处理的。

我们继续看mapperElement(XNode parent)方法的代码：

  private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {  //解析package扫描指定package下的所有mapper接口
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {  //解析mapper节点
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url != null && mapperClass == null) {
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

可以看到，mybatisConfig.xml配置下的mappers节点有2种子节点：package节点和mapper节点，我这里先讨论xml的模式，先看mapper节点。

mapper节点配置有3个属性：resource,url,class。他们处理的优先级依次是resource,url,class，3个属性只处理一种。resource,url属性映射的是xml的路径，class是mapper接口的类路径。

从源码中我们看到，通过读取resource或url属性得到xml的访问路径后，交给XMLMapperBuilder对象来解析。

我们查看XMLMapperBuilder的parse方法:

  public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
      configurationElement(parser.evalNode("/mapper"));    //从mapper节点开始解析
      configuration.addLoadedResource(resource);  //标记已经加载了次xml资源
      bindMapperForNamespace();  //绑定到命名空间
    }

    parsePendingResultMaps();  //将resultMap映射信息转换成ResultMap对象
    parsePendingChacheRefs();  //将cache映射信息转换成Cache对象
    parsePendingStatements();  //将sql映射转换成MappedStatement
  }

//解析xml
private void configurationElement(XNode context) {

  try {
    String namespace = context.getStringAttribute("namespace");
    if (namespace.equals("")) {
     throw new BuilderException("Mapper's namespace cannot be empty");
    }
    builderAssistant.setCurrentNamespace(namespace);
    cacheRefElement(context.evalNode("cache-ref"));
    cacheElement(context.evalNode("cache"));
    parameterMapElement(context.evalNodes("/mapper/parameterMap"));
    resultMapElements(context.evalNodes("/mapper/resultMap"));
    sqlElement(context.evalNodes("/mapper/sql"));
    buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
  }
}

//绑定到命名空间
private void bindMapperForNamespace() {

  String namespace = builderAssistant.getCurrentNamespace();
  if (namespace != null) {
    Class<?> boundType = null;
    try {
      boundType = Resources.classForName(namespace);
    } catch (ClassNotFoundException e) {
      //ignore, bound type is not required
    }
    if (boundType != null) {
      if (!configuration.hasMapper(boundType)) {
        // Spring may not know the real resource name so we set a flag
        // to prevent loading again this resource from the mapper interface
        // look at MapperAnnotationBuilder#loadXmlResource
        configuration.addLoadedResource("namespace:" + namespace);
        configuration.addMapper(boundType);
      }
    }
  }
}

其中configurationElement(XNode context)负责解析所有的xml元素，bindMapperForNamespace() 绑定到命名空间

而parsePendingResultMaps(), parsePendingChacheRefs(), parsePendingStatements()则分别将对应的xml信息转换成ResultMap对象，Cache对象和MappedStatement对象。

 ResultMap的处理

  private void parsePendingResultMaps() {
    Collection<ResultMapResolver> incompleteResultMaps = configuration.getIncompleteResultMaps();
    synchronized (incompleteResultMaps) {
      Iterator<ResultMapResolver> iter = incompleteResultMaps.iterator();
      while (iter.hasNext()) {
        try {
          iter.next().resolve();  //实际处理的是ResultMapResolver.resolve()方法
          iter.remove();
        } catch (IncompleteElementException e) {
          // ResultMap is still missing a resource...
        }
      }
    }
  }

看代码得知，实际处理的是ResultMapResolver.resolve()方法

package org.apache.ibatis.builder;

import java.util.List;

import org.apache.ibatis.mapping.Discriminator;
import org.apache.ibatis.mapping.ResultMap;
import org.apache.ibatis.mapping.ResultMapping;

/**
 * @author Eduardo Macarron
 */
public class ResultMapResolver {
  private final MapperBuilderAssistant assistant;
  private String id;
  private Class<?> type;
  private String extend;
  private Discriminator discriminator;
  private List<ResultMapping> resultMappings;
  private Boolean autoMapping;

  public ResultMapResolver(MapperBuilderAssistant assistant, String id, Class<?> type, String extend, Discriminator discriminator, List<ResultMapping> resultMappings, Boolean autoMapping) {
    this.assistant = assistant;
    this.id = id;
    this.type = type;
    this.extend = extend;
    this.discriminator = discriminator;
    this.resultMappings = resultMappings;
    this.autoMapping = autoMapping;
  }

  public ResultMap resolve() {
    return assistant.addResultMap(this.id, this.type, this.extend, this.discriminator, this.resultMappings, this.autoMapping);
  }

}

接着发现最终调用的是MapperBuilderAssistant.addResultMap 方法

  public ResultMap addResultMap(
      String id,
      Class<?> type,
      String extend,
      Discriminator discriminator,
      List<ResultMapping> resultMappings,
      Boolean autoMapping) {
    id = applyCurrentNamespace(id, false);
    extend = applyCurrentNamespace(extend, true);

    ResultMap.Builder resultMapBuilder = new ResultMap.Builder(configuration, id, type, resultMappings, autoMapping);
    if (extend != null) {
      if (!configuration.hasResultMap(extend)) {
        throw new IncompleteElementException("Could not find a parent resultmap with id '" + extend + "'");
      }
      ResultMap resultMap = configuration.getResultMap(extend);
      List<ResultMapping> extendedResultMappings = new ArrayList<ResultMapping>(resultMap.getResultMappings());
      extendedResultMappings.removeAll(resultMappings);
      // Remove parent constructor if this resultMap declares a constructor.
      boolean declaresConstructor = false;
      for (ResultMapping resultMapping : resultMappings) {
        if (resultMapping.getFlags().contains(ResultFlag.CONSTRUCTOR)) {
          declaresConstructor = true;
          break;
        }
      }
      if (declaresConstructor) {
        Iterator<ResultMapping> extendedResultMappingsIter = extendedResultMappings.iterator();
        while (extendedResultMappingsIter.hasNext()) {
          if (extendedResultMappingsIter.next().getFlags().contains(ResultFlag.CONSTRUCTOR)) {
            extendedResultMappingsIter.remove();
          }
        }
      }
      resultMappings.addAll(extendedResultMappings);
    }
    resultMapBuilder.discriminator(discriminator);
    ResultMap resultMap = resultMapBuilder.build();
    configuration.addResultMap(resultMap);  //添加到Configuration
    return resultMap;
  }

Cache的处理

  private void parsePendingChacheRefs() {
      Collection<CacheRefResolver> incompleteCacheRefs = configuration.getIncompleteCacheRefs();
      synchronized (incompleteCacheRefs) {
          Iterator<CacheRefResolver> iter = incompleteCacheRefs.iterator();
          while (iter.hasNext()) {
              try {
                  iter.next().resolveCacheRef();
                  iter.remove();
              } catch (IncompleteElementException e) {
                  // Cache ref is still missing a resource...
              }
          }
      }
  }

调用的是org.apache.ibatis.builder.CacheRefResolver.resolveCacheRef()方法：

    public Cache resolveCacheRef() {
        return assistant.useCacheRef(cacheRefNamespace);
    }

最终调用org.apache.ibatis.builder.useCacheRef(String namespace)方法创建Cache对象并添加到Configuration：

  public Cache useCacheRef(String namespace) {
    if (namespace == null) {
      throw new BuilderException("cache-ref element requires a namespace attribute.");
    }
    try {
      unresolvedCacheRef = true;
      Cache cache = configuration.getCache(namespace);
      if (cache == null) {
        throw new IncompleteElementException("No cache for namespace '" + namespace + "' could be found.");
      }
      currentCache = cache;
      unresolvedCacheRef = false;
      return cache;
    } catch (IllegalArgumentException e) {
      throw new IncompleteElementException("No cache for namespace '" + namespace + "' could be found.", e);
    }
  }

MappedStatement的处理

从parsePendingStatements()方法开始跟踪

  private void parsePendingStatements() {
      Collection<XMLStatementBuilder> incompleteStatements = configuration.getIncompleteStatements();
      synchronized (incompleteStatements) {
          Iterator<XMLStatementBuilder> iter = incompleteStatements.iterator();
          while (iter.hasNext()) {
              try {
                  iter.next().parseStatementNode(); //调用XMLStatementBuilder的parseStatementNode方法
                  iter.remove();
              } catch (IncompleteElementException e) {
                  // Statement is still missing a resource...
              }
          }
      }
  }

然后调用org.apache.ibatis.builder.xml.XMLStatementBuilder的parseStatementNode方法

  public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) return;

    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultType = context.getStringAttribute("resultType");
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    Class<?> resultTypeClass = resolveClass(resultType);
    String resultSetType = context.getStringAttribute("resultSetType");
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

    String nodeName = context.getNode().getNodeName();
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    // Include Fragments before parsing
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    // Parse selectKey after includes and remove them.
    processSelectKeyNodes(id, parameterTypeClass, langDriver);

    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    String resultSets = context.getStringAttribute("resultSets");
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    if (configuration.hasKeyGenerator(keyStatementId)) {
      keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? new Jdbc3KeyGenerator() : new NoKeyGenerator();
    }

    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered,
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }

最后MappedStatement由org.apache.ibatis.builder.MapperBuilderAssistant的addMappedStatement方法创建，并加入到Configuration

  public MappedStatement addMappedStatement(
      String id,
      SqlSource sqlSource,
      StatementType statementType,
      SqlCommandType sqlCommandType,
      Integer fetchSize,
      Integer timeout,
      String parameterMap,
      Class<?> parameterType,
      String resultMap,
      Class<?> resultType,
      ResultSetType resultSetType,
      boolean flushCache,
      boolean useCache,
      boolean resultOrdered,
      KeyGenerator keyGenerator,
      String keyProperty,
      String keyColumn,
      String databaseId,
      LanguageDriver lang,
      String resultSets) {

    if (unresolvedCacheRef) throw new IncompleteElementException("Cache-ref not yet resolved");

    id = applyCurrentNamespace(id, false);
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;

    MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType);
    statementBuilder.resource(resource);
    statementBuilder.fetchSize(fetchSize);
    statementBuilder.statementType(statementType);
    statementBuilder.keyGenerator(keyGenerator);
    statementBuilder.keyProperty(keyProperty);
    statementBuilder.keyColumn(keyColumn);
    statementBuilder.databaseId(databaseId);
    statementBuilder.lang(lang);
    statementBuilder.resultOrdered(resultOrdered);
    statementBuilder.resulSets(resultSets);
    setStatementTimeout(timeout, statementBuilder);

    setStatementParameterMap(parameterMap, parameterType, statementBuilder);
    setStatementResultMap(resultMap, resultType, resultSetType, statementBuilder);
    setStatementCache(isSelect, flushCache, useCache, currentCache, statementBuilder);

    MappedStatement statement = statementBuilder.build();
    configuration.addMappedStatement(statement);
    return statement;
  }

Mapper  xml部分的解析，暂时粗略的写这么多，后续我们还将讲解package包的扫描和指定class的mapper的情况。