MyBatis框架原理1：构建SqlSessionFactory的过程

SqlSessionFactoryBuilder

首先创建了一个SqlSessionFactoryBuilder对象，然后调用该对象的build方法加载全局XML配置的流文件构建出一个SqlSessionFactory对象。

//指定全局配置文件路径
String resource = "org/mybatis/example/mybatis-config.xml";
//加载配置文件
InputStream inputStream = Resources.getResourceAsStream(resource);
//构建者模式创建SqlSessionFactory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

查看一下SqlSessionFactoryBuilder的源码：

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);
  }

SqlSessionFactoryBuilder只有一堆重载的build方法，除了build(Configuration)方法，其他方法的参数都是输入流，最终由build(Configuration)方法生成SqlSessionFactory对象，下面来看如何构建Configuration对象。

XMLConfigBuilder解析配置文件

从XMLConfigBuilder类名就可以看出，这是用来解析XML配置文件的类，其父类为BaseBuilder。
BaseBuilder还包含了MapperBuilderAssistant, SqlSourceBuilder, XMLConfigBuilder, XMLMapperBuilder, XMLScriptBuilder, XMLStatementBuilder等子类，这些子类都是用来解析MyBatis各个配置文件，他们通过BaseBuilder父类共同维护一个全局的Configuration对象，
XMLConfigBuilder的作用就是解析全局配置文件，调用BaseBuilder其他子类解析其他配置文件，生成最终的Configuration对象。

 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;
  }

查看XMLConfigBuilder源码，可以得知XML配置文件最终是由org.apache.ibatis.parsing.XPathParser封装的XPath解析的。通过XpathParser构造方法传入我们读取的XML流文件、Properites流文件和environment等参数得到了一个XpathParser实例对象parser,这里parser已包含全局XML配置文件解析后的所有信息,再将parser作为参数传给XMLConfigBuilder构造方法。XMLConfigBuilder构造方法调用其父类BaseBuilder的构造方法BaseBuilder(Configuration),构造出一个XMLConfigBuilder对象。值得注意的是，这里BaseBuilder构造方法参数是一个初始化的Configuration对象，Configuration对象初始化的时候，内置的别名注册器TypeAliasRegistry注册了默认的别名：

public Configuration() {
    typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
    typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);

    typeAliasRegistry.registerAlias("JNDI", JndiDataSourceFactory.class);
    typeAliasRegistry.registerAlias("POOLED", PooledDataSourceFactory.class);
    typeAliasRegistry.registerAlias("UNPOOLED", UnpooledDataSourceFactory.class);

... 

所以XML配置文件里可以直接用这些别名。

此时我们已经得到了XMLConfigBuilder对象，再看SqlSessionFactoryBuilder的build方法，将XMLConfigBuilder实例对象parser调用parser()方法得到的Configuration实例对象config作为参数，调用SqlSessionFactory接口的实现类DefaultSqlSessionFactory构造出SqlSessionFactory对象。

XMLConfigBuilder对象在调用parser()方法时，会读出所有所有配置文件，将配置文件解析后保存在Configuration对象中。

public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //参数是<configuraton>标签根节点
    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);
    }
  }

XMLConfigBuilder的parseConfiguration(XNode)方法把XML全局配置文件中每一个节点的信息都读取出来，保存在一个Configuration对象中，Configuration分别对以下内容做出了初始化：

• properties 属性
• settings 设置
• typeAliases 类型别名
• typeHandlers 类型处理器
• objectFactory 对象工厂
• plugins 插件
• environments 环境
• databaseIdProvider 数据库厂商标识
• mappers 映射器
  这里对properties和mappers的初始化进行分析：

properties的初始化

 private void propertiesElement(XNode context) throws Exception {
    if (context != null) {
      Properties defaults = context.getChildrenAsProperties();
      String resource = context.getStringAttribute("resource");
      String url = context.getStringAttribute("url");
      if (resource != null && url != null) {
        throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");
      }
      if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        defaults.putAll(Resources.getUrlAsProperties(url));
      }
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      parser.setVariables(defaults);
      configuration.setVariables(defaults);
    }
  }

XMLConfigBuilder的getChildrenAsProperties()方法读取properties标签的子节点，保存到Configuration对象的Properties属性里，这里可以看出只能在resource和url两种方式中二选一来加载外部properties配置文件，如果外部properties文件里面属性名和主配置XML文件properties标签的子元素属性重名，则会覆盖主配置文件的属性值，然后将初始化的Configuration对象中的Properties与解析配置文件后封装好的Properties合并，最后再将Properties保存到Configuration对象中。

mappers的初始化

 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.");
          }
        }
      }
    }
  }

遍历mappers标签下所有子节点

• 如果遍历到package子节点，是以包名引入映射器，则将该包下所有Class注册到Configuration的mapperRegistry中。
• 如果遍历到mapper子节点的class属性，则将制定的Class注册到注册到Configuration的mapperRegistry中。
• 如果遍历到mapper子节点的resource或者url属性，则直接对资源文件进行解析：
  首先构建一个XMLMapperBuilder对象,构建过程如下

 public XMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
    this(new XPathParser(inputStream, true, configuration.getVariables(), new XMLMapperEntityResolver()),
        configuration, resource, sqlFragments);
  }

private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
    super(configuration);
    this.builderAssistant = new MapperBuilderAssistant(configuration, resource);
    this.parser = parser;
    this.sqlFragments = sqlFragments;
    this.resource = resource;
  }

XPathParser将mapper配置文件解析成Document对象后封装到一个XPathParser对象，再将XPathParser对象作为参数传给XMLMapperBuilder构造方法并构造出一个XMLMapperBuilder对象，XMLMapperBuilder对象的builderAssistant字段是一个MapperBuilderAssistant对象，同样也是BaseBuilder的一个子类，其作用是对MappedStatement对象进行封装。

有了XMLMapperBuilder对象后，就可以进入解析mapper映射文件的过程：

public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
      configurationElement(parser.evalNode("/mapper"));
      configuration.addLoadedResource(resource);
      bindMapperForNamespace();
    }

    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
  }

调用XMLMapperBuilder的configurationElement方法，mapper映射文件进行解析

  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);
    }
  }

mapper映射文件必须有namespace属性值，否则抛出异常，将namespace属性保存到XMLMapperBuilder的MapperBuilderAssistant对象中，以便其他方法调用。
该方法对mapper映射文件每个标签逐一解析并保存到Configuration和MapperBuilderAssistant对象中，最后调用buildStatementFromContext方法解析select、insert、update和delete节点。

private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
      buildStatementFromContext(list, configuration.getDatabaseId());
    }
    buildStatementFromContext(list, null);
  }

  private void buildStatementFromContext(List<XNode> 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);
      }
    }
  }

buildStatementFromContext方法中调用XMLStatementBuilder来完成解析，可以看到SQL语句封装到一个SqlSource对象，SqlSource是个接口，如果是动态SQL就创建DynamicSqlSource实现类，否则创建StaticSqlSource实现类。

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))
          ? 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);
  }

MapperBuilderAssistant通过addMappedStatement方法将MappedStatement对象放入Configuration对象的mappedStatements容器中，得到了最终的Configuration对象后传入SqlSessionFactoryBuilder的构造方法，生成我们需要的DefaultSqlSessionFactory对象。
这里只分析了MyBatis是如何解析resource和url方式指定的mapper配置文件，如果是通过指定Mapper接口的package或者class全限定名配置方式，则Configuration对象会通过addMappers方法将接口注册，再通过Java反射技术和JDK动态代理技术，根据接口class的全限定名找到对应的XML配置文件或者注解进行解析，如果是非注解模式的xml配置文件必须和这个class在同一级目录，且与class同名，这里不再详解。

最后附上SqlSessionFactory构建过程的简易流程图