【小盘子看源码-MyBatis-1】MyBatis配置文件的加载流程

众所周知，Mybatis有一个全局的配置，在程序启动时会加载XML配置文件，将配置信息映射到org.apache.ibatis.session.Configuration类中，例如如下配置文件。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!--resource="db.properties"-->
    <properties  resource="db.properties">
        <property name="test" value="123456"></property>
    </properties>
    <settings>
        <!-- 控制全局缓存（二级缓存）-->
        <setting name="cacheEnabled" value="true"/>
        <!-- 延迟加载的全局开关。当开启时，所有关联对象都会延迟加载。默认 false  -->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!-- 当开启时，任何方法的调用都会加载该对象的所有属性。默认 false，可通过select标签的 fetchType来覆盖-->
        <setting name="aggressiveLazyLoading" value="false"/>
        <setting name="localCacheScope" value="SESSION"/>
    </settings>
    <typeAliases>
        <typeAlias type="com.fanpan26.source.code.mybatis.UserMapper" alias="userMapper"/>
        <package name="com.fanpan26.source.code.mybatis.model" />
    </typeAliases>
    <plugins>
        <plugin interceptor="com.fanpan26.source.code.mybatis.plugin.MyPlugin"></plugin>
    </plugins>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/><!-- 单独使用时配置成MANAGED没有事务 -->
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="UserMapper.xml"/>
    </mappers>
</configuration>

这里我们要注意的是，每个配置项目的顺序不能变,否则在做XML解析的时候会抛异常。

<!ELEMENT configuration (properties?, settings?, typeAliases?, typeHandlers?, objectFactory?, objectWrapperFactory?, reflectorFactory?, plugins?, environments?, databaseIdProvider?, mappers?)>

那么它是如何做到的呢？下面跟着我揭开它的神秘面纱吧。

代码分析

Configuration对象是通过XMLConfigBuilder的parse()方法得到的，示例代码如下：

    XMLConfigBuilder parser = new XMLConfigBuilder(Resources.getResourceAsStream("mybatis-config.xml"));
    Configuration configuration = parser.parse();

XMLConfigBuilder继承自抽象类BaseBuilder.

构造函数没什么好说的：

 public XMLConfigBuilder(InputStream inputStream) {
    this(inputStream, null, null);
  }

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

其中，XPathParser会解析XML中的内容，这里我们就不详细跟进了，我们主要看与MyBatis息息相关的各种属性是如何加载的。

 public Configuration parse() {
    //只能解析一次，否则异常
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //执行具体的解析，从 configuration节点下解析
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

下面主要跟进parseConfiguration方法

private void parseConfiguration(XNode root) {
    try {
      //先读取properties
      propertiesElement(root.evalNode("properties"));
      //将 settings 转化为 Properties
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      //解析typeAliase
      typeAliasesElement(root.evalNode("typeAliases"));
      //解析plugins
      pluginElement(root.evalNode("plugins"));
      //解析 objectFactory
      objectFactoryElement(root.evalNode("objectFactory"));
      //解析objectWrapperFactory
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //解析 reflectorFactory
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      //解析环境信息
      environmentsElement(root.evalNode("environments"));
      //解析databaseIdProvider
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //解析typeHandlers
      typeHandlerElement(root.evalNode("typeHandlers"));
      //解析mappers
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

解析Properties

这一步就是将配置文件中的<Properties> 解析出，然后放到Configuration对象的属性键值对。解析过程分为两个部分：

• 先解析XML中的配置项，例如配置中的 test:123456 信息.

   <properties  resource="db.properties">
          <property name="test" value="123456"></property>
   </properties>
  

• 然后在解析 resource属性或者url属性中的信息，注意它俩不能共存，也就是不能既配置resource又配置url

解析XML中的属性配置项：

 Properties defaults = context.getChildrenAsProperties();
 //XNode 中的方法
 public Properties getChildrenAsProperties() {
    Properties properties = new Properties();
     //遍历子节点信息，读取出name和value属性，赋值给Properties对象
    for (XNode child : getChildren()) {
      //获取name属性
      String name = child.getStringAttribute("name");
      //获取value属性
      String value = child.getStringAttribute("value");
      //name 和value 都不能为 null
      if (name != null && value != null) {
        properties.setProperty(name, value);
      }
    }
    return properties;
  }

然后开始解析resource或者url信息

	  //获取resource 属性
	  String resource = context.getStringAttribute("resource");
	  //获取Java属性
      String url = context.getStringAttribute("url");
	  //它俩不能共存，只能配置一个
      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.");
      }

然后直接调用Resources工具类的方法，将资源转化为Properties的key value值.

 if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        defaults.putAll(Resources.getUrlAsProperties(url));
      }

最后在和Configuration.variables合并.

 Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
	  //XPathParser 保存变量属性留在后边解析备用
      parser.setVariables(defaults);
	  //所有的属性信息就解析完毕，存放到 Configuration 的 变量 variables (Properties类型) 中
      configuration.setVariables(defaults);

将 settings 转化为 Properties

Properties settings = settingsAsProperties(root.evalNode("settings"));

private Properties settingsAsProperties(XNode context) {
    if (context == null) {
      return new Properties();
    }
    //这里就能获取所有的属性键值对了，但是后续要判断键值对中的键是否是Configuration中的属性。
    Properties props = context.getChildrenAsProperties();
    // Check that all settings are known to the configuration class
    MetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);
    for (Object key : props.keySet()) {
      //如果没有该属性的setter方法，就抛出异常，因为 setting中的配置是和Configuration中一一对应的，这里不能配置错
      if (!metaConfig.hasSetter(String.valueOf(key))) {
        throw new BuilderException("The setting " + key + " is not known.  Make sure you spelled it correctly (case sensitive).");
      }
    }
    return props;
  }

赋值 VFS 虚拟文件系统

loadCustomVfs(settings);
private void loadCustomVfs(Properties props) throws ClassNotFoundException {
    //读取 vfsImpl 配置信息
    String value = props.getProperty("vfsImpl");
    if (value != null) {
      String[] clazzes = value.split(",");
      for (String clazz : clazzes) {
        if (!clazz.isEmpty()) {
          @SuppressWarnings("unchecked")
          Class<? extends VFS> vfsImpl = (Class<? extends VFS>)Resources.classForName(clazz);
          configuration.setVfsImpl(vfsImpl);
        }
      }
    }
  }

赋值 Logger

 loadCustomLogImpl(settings);

 private void loadCustomLogImpl(Properties props) {
    //读取 logImpl属性，获取类信息
    Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
    configuration.setLogImpl(logImpl);
  }

解析 typeAlias

<typeAliases>
        <typeAlias type="com.fanpan26.source.code.mybatis.UserMapper" alias="userMapper"/>
        <package name="com.fanpan26.source.code.mybatis.model" />
</typeAliases>

Configuration对象中有一个TypeAliasRegistry对象，TypeAliasRegistry中有一个HashMap<String,Class<?>> 类型的 typeAlias 集合，所有的注册信息都是存放到该HashMap中，MyBatis本身内置了基础类型的映射关系。

public TypeAliasRegistry() {
    registerAlias("string", String.class);

    registerAlias("byte", Byte.class);
    registerAlias("long", Long.class);
    registerAlias("short", Short.class);
    registerAlias("int", Integer.class);
    registerAlias("integer", Integer.class);
    registerAlias("double", Double.class);
    registerAlias("float", Float.class);
    registerAlias("boolean", Boolean.class);

    registerAlias("byte[]", Byte[].class);
    registerAlias("long[]", Long[].class);
    registerAlias("short[]", Short[].class);
    registerAlias("int[]", Integer[].class);
    registerAlias("integer[]", Integer[].class);
    registerAlias("double[]", Double[].class);
    registerAlias("float[]", Float[].class);
    registerAlias("boolean[]", Boolean[].class);

    registerAlias("_byte", byte.class);
    registerAlias("_long", long.class);
    registerAlias("_short", short.class);
    registerAlias("_int", int.class);
    registerAlias("_integer", int.class);
    registerAlias("_double", double.class);
    registerAlias("_float", float.class);
    registerAlias("_boolean", boolean.class);

    registerAlias("_byte[]", byte[].class);
    registerAlias("_long[]", long[].class);
    registerAlias("_short[]", short[].class);
    registerAlias("_int[]", int[].class);
    registerAlias("_integer[]", int[].class);
    registerAlias("_double[]", double[].class);
    registerAlias("_float[]", float[].class);
    registerAlias("_boolean[]", boolean[].class);

    registerAlias("date", Date.class);
    registerAlias("decimal", BigDecimal.class);
    registerAlias("bigdecimal", BigDecimal.class);
    registerAlias("biginteger", BigInteger.class);
    registerAlias("object", Object.class);

    registerAlias("date[]", Date[].class);
    registerAlias("decimal[]", BigDecimal[].class);
    registerAlias("bigdecimal[]", BigDecimal[].class);
    registerAlias("biginteger[]", BigInteger[].class);
    registerAlias("object[]", Object[].class);

    registerAlias("map", Map.class);
    registerAlias("hashmap", HashMap.class);
    registerAlias("list", List.class);
    registerAlias("arraylist", ArrayList.class);
    registerAlias("collection", Collection.class);
    registerAlias("iterator", Iterator.class);

    registerAlias("ResultSet", ResultSet.class);
  }

当配置文件中存在<typeAlias>属性的时候，就将alias注册上。

  typeAliasesElement(root.evalNode("typeAliases"));

其中，它有两种注册方式，一种就是通过package批量注册，毕竟一个类一个类的写太麻烦了。另外就是单个注册。

单个注册方式：

 		  //读取属性中的 alias 值
          String alias = child.getStringAttribute("alias");
		  //读取类型
          String type = child.getStringAttribute("type");
          try {
            //反射获取类
            Class<?> clazz = Resources.classForName(type);
            //如果没有配置 alias，没关系，按照默认 名称注册
            if (alias == null) {
              typeAliasRegistry.registerAlias(clazz);
            } else {
              //否则直接注册
              typeAliasRegistry.registerAlias(alias, clazz);
            }

按照默认名称注册

 public void registerAlias(Class<?> type) {
    //反射获取类名
    String alias = type.getSimpleName();
    //这里是为了支持注解，如果存在Alias注解，那么使用Alias注解中的value值
    Alias aliasAnnotation = type.getAnnotation(Alias.class);
    if (aliasAnnotation != null) {
      alias = aliasAnnotation.value();
    }
    //注册
    registerAlias(alias, type);
  }

按照package注册，思路是这样的，遍历package下所有的类进行注册。

public void registerAliases(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    //获取packages下的所有的类
    Set<Class<? extends Class<?>>> typeSet = resolverUtil.getClasses();
    for (Class<?> type : typeSet) {
      //排除接口，内部类
      if (!type.isAnonymousClass() && !type.isInterface() && !type.isMemberClass()) {
        registerAlias(type);
      }
    }
  }

注册plugins

	<plugins>
        <plugin interceptor="com.fanpan26.source.code.mybatis.plugin.MyPlugin"></plugin>
    </plugins>

pluginElement(root.evalNode("plugins"));

private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        //获取 interceptor 属性
        String interceptor = child.getStringAttribute("interceptor");
        //获取属性信息
        Properties properties = child.getChildrenAsProperties();
        //这里利用反射将 Interceptor 实例化，然后调用 setProperties 方法
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
        interceptorInstance.setProperties(properties);
        //最后将 Interceptor 实例加入到拦截链中，List<Interceptor> 对象
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }

注册 ObjectFactory

其实大多数场景下，使用默认的 DefaultObjectFactory即可。

 	<objectFactory type="com.fanpan26.source.code.mybatis.factory.MyObjectFactory">
        <property name="a" value="b"/>
    </objectFactory>

 objectFactoryElement(root.evalNode("objectFactory"));
 //注册 objectFactory
 private void objectFactoryElement(XNode context) throws Exception {
    if (context != null) {
      //获取类的全路径名称
      String type = context.getStringAttribute("type");
      //获取其中的属性
      Properties properties = context.getChildrenAsProperties();
      //利用反射创建实例
      ObjectFactory factory = (ObjectFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      //调用接口方法，注入自定义属性
      factory.setProperties(properties);
      //调用 setObjectFactory 方法
      // protected ObjectFactory objectFactory = new DefaultObjectFactory();
      configuration.setObjectFactory(factory);
    }
  }

注册 ObjectWrapperFactory

<objectWrapperFactory type="com.fanpan26.source.code.mybatis.factory.MyObjectWrapperFactory"/>

默认是 DefaultObjectWrapperFactory,不过这个类好像没啥用

public class DefaultObjectWrapperFactory implements ObjectWrapperFactory {
    public DefaultObjectWrapperFactory() {
    }

    public boolean hasWrapperFor(Object object) {
        return false;
    }

    public ObjectWrapper getWrapperFor(MetaObject metaObject, Object object) {
        throw new ReflectionException("The DefaultObjectWrapperFactory should never be called to provide an ObjectWrapper.");
    }
}

注册方法如下：

private void objectWrapperFactoryElement(XNode context) throws Exception {
    if (context != null) {
      //获取类全名
      String type = context.getStringAttribute("type");
      //反射创建
      ObjectWrapperFactory factory = (ObjectWrapperFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      //设置属性
      //protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();
      configuration.setObjectWrapperFactory(factory);
    }
  }

注册reflectorFactory

<reflectorFactory type="com.fanpan26.source.code.mybatis.factory.MyReflectorFactory"/>

reflectorFactory 用户创建类的 Reflector 对象。其中带有缓存功能。使用 ConcurrentHashMap<Class<?>,Reflector> 缓存。

reflectorFactoryElement(root.evalNode("reflectorFactory"));

private void reflectorFactoryElement(XNode context) throws Exception {
    if (context != null) {
      //获取类型
      String type = context.getStringAttribute("type");
      //反射生成对象
      ReflectorFactory factory = (ReflectorFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      //设置
      //protected ReflectorFactory reflectorFactory = new DefaultReflectorFactory();
      configuration.setReflectorFactory(factory);
    }
  }

注册 settings

就是将<settings>节点中的所有设置的属性值都设置到Configuration对象中。

settingsElement(settings);

private void settingsElement(Properties props) {
 //自动映射等
  configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL")));
   configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
    configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
    configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")));
    configuration.setLazyLoadingEnabled(booleanValueOf(props.getProperty("lazyLoadingEnabled"), false));
    configuration.setAggressiveLazyLoading(booleanValueOf(props.getProperty("aggressiveLazyLoading"), false));
    configuration.setMultipleResultSetsEnabled(booleanValueOf(props.getProperty("multipleResultSetsEnabled"), true));
    configuration.setUseColumnLabel(booleanValueOf(props.getProperty("useColumnLabel"), true));
    configuration.setUseGeneratedKeys(booleanValueOf(props.getProperty("useGeneratedKeys"), false));
    configuration.setDefaultExecutorType(ExecutorType.valueOf(props.getProperty("defaultExecutorType", "SIMPLE")));
    configuration.setDefaultStatementTimeout(integerValueOf(props.getProperty("defaultStatementTimeout"), null));
    configuration.setDefaultFetchSize(integerValueOf(props.getProperty("defaultFetchSize"), null));
    configuration.setDefaultResultSetType(resolveResultSetType(props.getProperty("defaultResultSetType")));
    configuration.setMapUnderscoreToCamelCase(booleanValueOf(props.getProperty("mapUnderscoreToCamelCase"), false));
    configuration.setSafeRowBoundsEnabled(booleanValueOf(props.getProperty("safeRowBoundsEnabled"), false));
    configuration.setLocalCacheScope(LocalCacheScope.valueOf(props.getProperty("localCacheScope", "SESSION")));
    configuration.setJdbcTypeForNull(JdbcType.valueOf(props.getProperty("jdbcTypeForNull", "OTHER")));
    configuration.setLazyLoadTriggerMethods(stringSetValueOf(props.getProperty("lazyLoadTriggerMethods"), "equals,clone,hashCode,toString"));
    configuration.setSafeResultHandlerEnabled(booleanValueOf(props.getProperty("safeResultHandlerEnabled"), true));
    configuration.setDefaultScriptingLanguage(resolveClass(props.getProperty("defaultScriptingLanguage")));
    configuration.setDefaultEnumTypeHandler(resolveClass(props.getProperty("defaultEnumTypeHandler")));
    configuration.setCallSettersOnNulls(booleanValueOf(props.getProperty("callSettersOnNulls"), false));
    configuration.setUseActualParamName(booleanValueOf(props.getProperty("useActualParamName"), true));
    configuration.setReturnInstanceForEmptyRow(booleanValueOf(props.getProperty("returnInstanceForEmptyRow"), false));
    configuration.setLogPrefix(props.getProperty("logPrefix"));
    configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory")));
  }

注册 environments

environmentsElement(root.evalNode("environments"));
private void environmentsElement(XNode context) throws Exception {
    if (context != null) {
      if (environment == null) {
        //这里如果之前没有传递 environment 参数，则取 default 值
        environment = context.getStringAttribute("default");
      }
      for (XNode child : context.getChildren()) {
        //获取id
        String id = child.getStringAttribute("id");
         //这里会判断id是否为空或者 evnironment是否为空，
        if (isSpecifiedEnvironment(id)) {
          //获取事务相关配置，这里和之前的代码差不多，解析type内容，然后反射创建对象
          TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
          //获取DataSource相关配置
          DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
          //获取DataSource
          DataSource dataSource = dsFactory.getDataSource();
          Environment.Builder environmentBuilder = new Environment.Builder(id)
              .transactionFactory(txFactory)
              .dataSource(dataSource);
          //设置环境
          configuration.setEnvironment(environmentBuilder.build());
        }
      }
    }
  }

这里以 PooledDataSource 举例，看看DataSource如何初始化的。

private DataSourceFactory dataSourceElement(XNode context) throws Exception {
    if (context != null) {
      //先获取DataSourceFactory类型，这里是 POOLED
      String type = context.getStringAttribute("type");
      //获取子属性，相关数据库配置， url username password 等
      Properties props = context.getChildrenAsProperties();
      //反射创建 DataSourceFactory，这里是 PooledDataSourceFactory 实例
      //org.apache.ibatis.datasource.pooled.PooledDataSourceFactory
      DataSourceFactory factory = (DataSourceFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      //这里设置属性的时候，利用了反射，将各个属性值赋值给了 DataSource 对象
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a DataSourceFactory.");
  }

注册 DatabaseIdProvider

	<databaseIdProvider type="com.fanpan26.source.code.mybatis.factory.MyDatabaseIdProvider">
        <property name="dataBaseId" value="db1"/>
    </databaseIdProvider>

databaseIdProviderElement(root.evalNode("databaseIdProvider"));

private void databaseIdProviderElement(XNode context) throws Exception {
    DatabaseIdProvider databaseIdProvider = null;
    if (context != null) {
      //获取类全名
      String type = context.getStringAttribute("type");
      // awful patch to keep backward compatibility
      if ("VENDOR".equals(type)) {
        type = "DB_VENDOR";
      }
      //获取属性信息
      Properties properties = context.getChildrenAsProperties();
      //反射常见类对象
      databaseIdProvider = (DatabaseIdProvider) resolveClass(type).getDeclaredConstructor().newInstance();
      //调用属性赋值
      databaseIdProvider.setProperties(properties);
    }
    Environment environment = configuration.getEnvironment();
    if (environment != null && databaseIdProvider != null) {
      //获取dataBaseId
      String databaseId = databaseIdProvider.getDatabaseId(environment.getDataSource());
      //将dataBaseId设置给environment
      configuration.setDatabaseId(databaseId);
    }
  }

注册 TypeHandlers

 <typeHandlers>
        <typeHandler handler="com.fanpan26.source.code.mybatis.handler.MyTypeHandler"></typeHandler>
        <package name="com.fanpan26.source.code.mybatis.handler"/>
    </typeHandlers>

和 typeAlias 类似，都可以通过单独注册和package批量注册

typeHandlerElement(root.evalNode("typeHandlers"));
private void typeHandlerElement(XNode parent) {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          //获取package name
          String typeHandlerPackage = child.getStringAttribute("name");
          //批量注册
          typeHandlerRegistry.register(typeHandlerPackage);
        } else {
          /*
           private final Map<JdbcType, TypeHandler<?>>  jdbcTypeHandlerMap = new EnumMap<>(JdbcType.class);
  private final Map<Type, Map<JdbcType, TypeHandler<?>>> typeHandlerMap = new ConcurrentHashMap<>();
  private final TypeHandler<Object> unknownTypeHandler = new UnknownTypeHandler(this);
  private final Map<Class<?>, TypeHandler<?>> allTypeHandlersMap = new HashMap<>();
          */
          //获取java类型
          String javaTypeName = child.getStringAttribute("javaType");
          //获取jdbcType类型
          String jdbcTypeName = child.getStringAttribute("jdbcType");
          //获取handler
          String handlerTypeName = child.getStringAttribute("handler");
          //反射获取类型
          Class<?> javaTypeClass = resolveClass(javaTypeName);
          //反射获取JdbcType类型
          JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
          Class<?> typeHandlerClass = resolveClass(handlerTypeName);
          //注册自定义映射处理器，后续源码分析会分析 TypeHandler的作用
          if (javaTypeClass != null) {
            if (jdbcType == null) {
              typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
            } else {
              typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
            }
          } else {
            typeHandlerRegistry.register(typeHandlerClass);
          }
        }
      }
    }
  }

注册 mappers

 <mappers>
        <mapper resource="UserMapper.xml"/>
    </mappers>

注册核心业务处理接口 Mapper

mapperElement(root.evalNode("mappers"));

  private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
         //扫描包注册
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          //内部执行了批量注册，将 package 下的接口都注册到mappers中
          configuration.addMappers(mapperPackage);
        } else {
          //resource 注册，通常是 xml 文件
          String resource = child.getStringAttribute("resource");
          //或者通过url注册，同理url或者resource不能同时使用
          String url = child.getStringAttribute("url");
          //获取 class
          String mapperClass = child.getStringAttribute("class");
          //当只有resource的时候
          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();
            //当使用url的时候
          } 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();
            //最后解析class
          } else if (resource == null && url == null && mapperClass != null) {
            //注册class
            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.");
          }
        }
      }
    }
  }

mapper的解析比较复杂，尤其是解析XML文件，需要解析内部的各种属性元素等。

从xml文件解析Mapper流程

从xml文件解析Mapper和解析Configuration差不多，无非就是解析xml元素，然后找到对应属性赋值即可。

  InputStream inputStream = Resources.getResourceAsStream(resource);
  //读取配置文件
  XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
  //详细解析流程
  mapperParser.parse();

下面看一下parse方法

public void parse() {
    //如果该文件已经加载过，不必重新加载
    if (!configuration.isResourceLoaded(resource)) {
      //解析 mapper 节点下的信息 select insert update delete
      configurationElement(parser.evalNode("/mapper"));
      //将资源加入到已加载资源列表中
      configuration.addLoadedResource(resource);
      //将mapper加入到configuration中
      bindMapperForNamespace();
    }

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

总结

MyBatis的Configuration对象的加载就告一段落，这就意味着MyBatis准备就绪了。接下来就要解析一个重要的对象SqlSessionFactory了，因为它负责创建SqlSession，而SqlSession则负责执行各种SQL和方法。