总览

初始化过程

xml解析器结构

XMLConfigBuilder:解析Mybatis全局配置文件的Builder;XMLMapperBuiler: 用来解析Mapper映射文件;MapperBuilderAssistant:用来辅助解析mappery映射文件并生成MappedStatemnt对象的助手；XMLStatementBuilder: 用来解析Mapper映射文件中的statement语句;SQLSourceBuilder: BaseBuilder：抽象类，所有XMLxxxBuilder的基类。该类中维护了全局的Configuration对象(封装的是xml配置文件中的内容)，以及全局的TypeAliasRegistry(别名注册器)和TypeHandlerRegistry(类型转换注册器)。

public abstract class BaseBuilder {  protected final Configuration configuration;  protected final TypeAliasRegistry typeAliasRegistry;  protected final TypeHandlerRegistry typeHandlerRegistry;  public BaseBuilder(Configuration configuration) {    this.configuration = configuration;    this.typeAliasRegistry = this.configuration.getTypeAliasRegistry();    this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry();  }...}

创建SqlSessionFactory

通过SqlSessionFactoryBuilder创建SqlSessionFactory(构建器模式):

@Beforepublic void loadConfig() {    try (InputStream inputStream = Resources.getResourceAsStream("mybatis2.xml")) {        factory = new SqlSessionFactoryBuilder().build(inputStream);    } catch (IOException e) {        e.printStackTrace();    }}

进入SqlSessionFactoryBuilder类:

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {    try {      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);      return build(parser.parse());    } catch (Exception e) {      throw ExceptionFactory.wrapException("Error building SqlSession.", e);    } finally {      ErrorContext.instance().reset();      try {        inputStream.close();      } catch (IOException e) {        // Intentionally ignore. Prefer previous error.      }    }}public SqlSessionFactory build(Configuration config) {    return new DefaultSqlSessionFactory(config);}

核心就是通过XMLConfigBuilder解析xml文件构造出Configuration对象，然后生成DefaultSqlSessionFactory对象。 因此重点就是Configuration对象的构造过程。

构造XMLConfigBuilder对象

build方法内创建了XMLConfigBuilder对象。查看XMLConfigBuilder的构造方法：

public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {    this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);}private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {    super(new Configuration());    ErrorContext.instance().resource("SQL Mapper Configuration");    this.configuration.setVariables(props);    this.parsed = false;    this.environment = environment;    this.parser = parser;}

方法内构造了XPathParser对象，用于解析xml文件。并且在XMLConfigBuilder的构造器中，初始化了COnfiguration对象。

解析全局配置文件

再看XMLConfigBuilder的parse方法:

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

该方法内部，通过XPath解析器，解析mybatis全局配置文件的<configuration>根节点，解析为Configuration对象。

再进入parseConfiguration方法，这里面就是在解析<configuration>根节点下面的各个子节点：

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);    }}

解析mappers标签

着重看mapperElement方法，看看是怎么解析mapper文件的(从源码中也可以看出mappers节点的几种不同的配置方式,具体参考官方文档)：

private void mapperElement(XNode parent) throws Exception {    if (parent != null) {      for (XNode child : parent.getChildren()) {        if ("package".equals(child.getName())) {          String mapperPackage = child.getStringAttribute("name");          configuration.addMappers(mapperPackage);        } else {          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mapper文件是通过XMLMapperBuilder来解析的。

解析mapper映射文件

查看XMLMapperBuilder的构造器，同样也是用了XPathParser来解析xml文件。 并且使用了MapperBuilderAssistant 作为构建MappedStatement对象的助手类。

public XMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map
    
      sqlFragments) {    this(new XPathParser(inputStream, true, configuration.getVariables(), new XMLMapperEntityResolver()),        configuration, resource, sqlFragments);}private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map
     
       sqlFragments) {    super(configuration);    this.builderAssistant = new MapperBuilderAssistant(configuration, resource);    this.parser = parser;    this.sqlFragments = sqlFragments;    this.resource = resource;}
     
    

查看XMLMapperBuilder的parse方法:

public void parse() {    if (!configuration.isResourceLoaded(resource)) {      configurationElement(parser.evalNode("/mapper"));      configuration.addLoadedResource(resource);      bindMapperForNamespace();    }    parsePendingResultMaps();    parsePendingCacheRefs();    parsePendingStatements();}private void configurationElement(XNode context) {    try {      String namespace = context.getStringAttribute("namespace");      if (namespace == null || 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. The XML location is '" + resource + "'. Cause: " + e, e);    }}

该方法就是通过xpath解析器解析xml文件中的各个节点，着重看buildStatementFromContext方法，看看是mybatis内部是怎么解析sql语句的：

private void buildStatementFromContext(List
    
      list, String requiredDatabaseId) {    for (XNode context : list) {      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);      try {        statementParser.parseStatementNode();      } catch (IncompleteElementException e) {        configuration.addIncompleteStatement(statementParser);      }    }}
    

可以看到XMLStatementBuilder是用来解析sql语句的。接下来进入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: 
    
      and 
     
       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))          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;    }    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,        resultSetTypeEnum, flushCache, useCache, resultOrdered,         keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);}
     
    

可以看到还是通过xpath解析出各个节点的属性，然后通过builderAssistant 去构造出MapperStatement对象。 查看builderAssistant.addMappedStatement方法:

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)        .resource(resource)        .fetchSize(fetchSize)        .timeout(timeout)        .statementType(statementType)        .keyGenerator(keyGenerator)        .keyProperty(keyProperty)        .keyColumn(keyColumn)        .databaseId(databaseId)        .lang(lang)        .resultOrdered(resultOrdered)        .resultSets(resultSets)        .resultMaps(getStatementResultMaps(resultMap, resultType, id))        .resultSetType(resultSetType)        .flushCacheRequired(valueOrDefault(flushCache, !isSelect))        .useCache(valueOrDefault(useCache, isSelect))        .cache(currentCache);    ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);    if (statementParameterMap != null) {      statementBuilder.parameterMap(statementParameterMap);    }    MappedStatement statement = statementBuilder.build();    configuration.addMappedStatement(statement);    return statement;}

通过构建器模式构造出MappedStatement对象，然后将这个MappedStatement对象放入全局的Configuration对象属性中，key为id,value就是MappedStatement对象。

引申：xpath