Mybatis3源码笔记（三）Configuration

1. XMLConfigBuilder

上一篇大致介绍了SqlSession的生成。在DefaultSqlSessionFactory的构造函数中就提到了Configuration这个对象。现在我们来看看Configuration的生成流程。

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

代码比较简单，就是根据XMLConfigBuilder根据配置文件XML来parse生成的。其实不用看代码，我们脑海中应该也有一个大致的Configuration的构成细节，肯定是根据mybatis-config.xml具体生成对应的组成属性。一般的mybatis-config.xml文件如下：

<configuration>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC">
                <property name="" value="" />
            </transactionManager>
            <dataSource type="UNPOOLED">
                <property name="driver" value="org.hsqldb.jdbcDriver" />
                <property name="url" value="jdbc:hsqldb:mem:stringlist" />
                <property name="username" value="sa" />
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="org/apache/ibatis/submitted/stringlist/Mapper.xml" />
    </mappers>

</configuration>

但是我们都知道XML的解析都有DTD文件来约束和验证，那我们常用的mybatis-config.xml是DTD肯定也是有的。在哪呢？秘密就在XMLConfigBuilder构造函数中的XMLMapperEntityResolver。

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

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";
  
  ...

上面很明显就用到mybatis-3-config.dtd和mybatis-3-mapper.dtd，同时为了兼容旧版本的ibatis,还用到了ibatis-3-config.dtd和ibatis-3-mapper.dtd。

之前说过mybatis-3-mapper.dtd，那mybatis-3-mapper.dtd看名字，我们也能猜到是专门用来解析mapper的xml文件的，一般的样例如下：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="org.apache.ibatis.submitted.stringlist.Mapper">

    <select id="getUsersAndGroups" resultMap="results">
        select * from users where id = #{id}
    </select>

    <resultMap type="org.apache.ibatis.submitted.stringlist.User" id="results">
        <id column="id" property="id"/>
        <collection property="groups" ofType="string">
            <result column="group_id"/>
        </collection>
        <collection property="roles" ofType="string">
            <result column="rol_id"/>
        </collection>
    </resultMap>

    <select id="getUsersAndGroupsMap" resultMap="mapResults">
        select * from users where id = #{id}
    </select>

    <resultMap type="map" id="mapResults">
        <id column="id" property="id" />
        <collection property="groups" ofType="string" javaType="list">
            <result column="group_id" />
        </collection>
        <collection property="roles" ofType="string" javaType="list">
            <result column="rol_id"/>
        </collection>
  </resultMap>

</mapper>

具体的dtd文件就不贴出来了，免的有凑字数的嫌疑。继续看代码：

public XPathParser(InputStream inputStream, boolean validation, Properties variables, EntityResolver entityResolver) {
    commonConstructor(validation, variables, entityResolver);
   //最终把XML文件生成document对象用来后面的解析工作
    this.document = createDocument(new InputSource(inputStream));
  }  
  private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
    super(new Configuration());
    //本地异常日志记录（ThreadLocal）单例模式来记录每一次的执行过程，用来详细定位异常信息。(为了防止内存泄露，在finally里有reset的操作)
    ErrorContext.instance().resource("SQL Mapper Configuration");
    //记录入参props
    this.configuration.setVariables(props);
    this.parsed = false;
    //记录入参environment 
    this.environment = environment;
    this.parser = parser;
  }

  public Configuration parse() {
    //相同的XMLConfigBuilder对象只允许parse一次
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //开始解析configuration根节点
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

下面就到了重头戏Configuration的详细解析过程。其实前几项的解析相对来说比较简单，就是最后的mapper的解析比较复杂。

private void parseConfiguration(XNode root) {
    try {
      //解析properties节点（variables）
      propertiesElement(root.evalNode("properties"));
      //解析settings节点
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      //根据settings内容解析VFS(虚拟文件系统vfsImpl)
      loadCustomVfs(settings);
      //根据settings内容解析log日志具体实现类（logImpl）
      loadCustomLogImpl(settings);
      //解析typeAliases节点（TypeAliasRegistry里注册对应的别名）
      typeAliasesElement(root.evalNode("typeAliases"));
      //解析plugins节点（注册interceptorChain里记录对应的拦截器）
      pluginElement(root.evalNode("plugins"));
      //解析objectFactory节点（自定义objectFactory）
      objectFactoryElement(root.evalNode("objectFactory"));
      //解析objectWrapperFactory节点（自定义objectWrapperFactory）
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //解析reflectorFactory节点（自定义reflectorFactory）
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      //设置其它的setting参数
      settingsElement(settings);
      //解析environments节点(environment)解析放到objectFactory and objectWrapperFactory解析之后，具体原因参见issue117
      environmentsElement(root.evalNode("environments"));
      //解析databaseIdProvider节点（databaseId）
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //解析typeHandlers节点，注册类型转换器（typeHandlerRegistry）
      typeHandlerElement(root.evalNode("typeHandlers"));
      //解析mappers节点（重中之重）
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

下面逐一解析下各个节点

1. propertiesElement(root.evalNode("properties"))

  private void propertiesElement(XNode context) throws Exception {
    if (context != null) {
      //解析所有子节点property
      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.");
      }
      //根据url或者resource生成对应的property集合
      if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        defaults.putAll(Resources.getUrlAsProperties(url));
      }
      //入参中的variables如果也存在的话，一并放入defaults
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      parser.setVariables(defaults);
      //设置variables
      configuration.setVariables(defaults);
    }
  }

  <properties resource="org/apache/ibatis/builder/jdbc.properties">
    <property name="prop1" value="aaaa"/>
    <property name="jdbcTypeForNull" value="NULL" />
  </properties>

  <properties url="file:./src/test/java/org/apache/ibatis/builder/jdbc.properties">
    <property name="prop1" value="bbbb"/>
  </properties>

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

  private Properties settingsAsProperties(XNode context) {
    if (context == null) {
      return new Properties();
    }
    Properties props = context.getChildrenAsProperties();
    // 利用MetaClass来检查Configuration
    MetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);
    //检查下对应的setting的key值在configuration里存不存在
    for (Object key : props.keySet()) {
      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;
  }

  <settings>
    <setting name="autoMappingBehavior" value="NONE"/>
    <setting name="autoMappingUnknownColumnBehavior" value="WARNING"/>
    <setting name="cacheEnabled" value="false"/>
    <setting name="proxyFactory" value="CGLIB"/>
    <setting name="lazyLoadingEnabled" value="true"/>
    <setting name="aggressiveLazyLoading" value="true"/>
    <setting name="multipleResultSetsEnabled" value="false"/>
    <setting name="useColumnLabel" value="false"/>
    <setting name="useGeneratedKeys" value="true"/>
    <setting name="defaultExecutorType" value="BATCH"/>
    <setting name="defaultStatementTimeout" value="10"/>
    <setting name="defaultFetchSize" value="100"/>
    <setting name="defaultResultSetType" value="SCROLL_INSENSITIVE"/>
    <setting name="mapUnderscoreToCamelCase" value="true"/>
    <setting name="safeRowBoundsEnabled" value="true"/>
    <setting name="localCacheScope" value="STATEMENT"/>
    <setting name="jdbcTypeForNull" value="${jdbcTypeForNull}"/>
    <setting name="lazyLoadTriggerMethods" value="equals,clone,hashCode,toString,xxx"/>
    <setting name="safeResultHandlerEnabled" value="false"/>
    <setting name="defaultScriptingLanguage" value="org.apache.ibatis.scripting.defaults.RawLanguageDriver"/>
    <setting name="callSettersOnNulls" value="true"/>
    <setting name="logPrefix" value="mybatis_"/>
    <setting name="logImpl" value="SLF4J"/>
    <setting name="vfsImpl" value="org.apache.ibatis.io.JBoss6VFS"/>
    <setting name="configurationFactory" value="java.lang.String"/>
    <setting name="defaultEnumTypeHandler" value="org.apache.ibatis.type.EnumOrdinalTypeHandler"/>
    <setting name="shrinkWhitespacesInSql" value="true"/>
    <setting name="defaultSqlProviderType" value="org.apache.ibatis.builder.XmlConfigBuilderTest$MySqlProvider"/>
  </settings>

3. loadCustomVfs(settings)根据setting解析Vfs（代码比较简单，就不注释了）

  private void loadCustomVfs(Properties props) throws ClassNotFoundException {
    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);
        }
      }
    }
  }

4. loadCustomLogImpl(settings)根据setting解析log实现

private void loadCustomLogImpl(Properties props) {
  Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
  configuration.setLogImpl(logImpl);
}

这个resolveClass方法会经常用到，我们跟一下看看。

public <T> Class<T> resolveAlias(String string) {
  try {
  if (string == null) {
  return null;
  }
  // issue #748
  String key = string.toLowerCase(Locale.ENGLISH);
  Class<T> value;
  //根据typeAliases先去捞一波class，如果没有的话用Resources走classpath生成class
  if (typeAliases.containsKey(key)) {
  value = (Class<T>) typeAliases.get(key);
  } else {
  value = (Class<T>) Resources.classForName(string);
  }
  return value;
  } catch (ClassNotFoundException e) {
  throw new TypeException("Could not resolve type alias '" + string + "'. Cause: " + e, e);
  }
}

public class TypeAliasRegistry {

private final Map<String, Class<?>> typeAliases = new HashMap<>();
//内置了一系列的类型别名
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);
...

5. typeAliasesElement(root.evalNode("typeAliases"))解析typeAliases节点（TypeAliasRegistry里注册对应的别名），正好跟上前的resolveClass联动起来。

  private void typeAliasesElement(XNode parent) {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        //注册package包下面所有的Class,key:getSimpleName(),value:Class（不包括接口，内部类，匿名类）
        if ("package".equals(child.getName())) {
          String typeAliasPackage = child.getStringAttribute("name");
          configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
        } else {
         //普通加载
          String alias = child.getStringAttribute("alias");
          String type = child.getStringAttribute("type");
          try {
            Class<?> clazz = Resources.classForName(type);
            if (alias == null) {
              typeAliasRegistry.registerAlias(clazz);
            } else {
              typeAliasRegistry.registerAlias(alias, clazz);
            }
          } catch (ClassNotFoundException e) {
            throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
          }
        }
      }
    }
  }

  <typeAliases>
    <typeAlias alias="BlogAuthor" type="org.apache.ibatis.domain.blog.Author"/>
    <typeAlias type="org.apache.ibatis.domain.blog.Blog"/>
    <typeAlias type="org.apache.ibatis.domain.blog.Post"/>
    <package name="org.apache.ibatis.domain.jpetstore"/>
  </typeAliases>