Spring mybatis源码篇章-SqlSessionFactoryBean

通过实例结合源码的方式解读，其中涉及到的文件来自于笔者的Github毕设项目，引用的jar包为mybatis-spring-1.3.0.jar

Mybatis

Mybatis是基于ORM(Object relation mapping)思想而开发的框架插件，本质原理用一句笔者的话便是使用了JAVA连接数据库的方式来执行相应的SQL(PreparedStatement)，并在此基础上提供了丰富的动态SQL配置以及缓存等概念。(后续文章会提及)

其基本上是企业与传统数据库(Oracle/Mysql等)交互常用的数据库持久层框架，以JAVA语言来说，其与Spring的搭配使用也是广为流传。

本文将从源码的角度浅析Mybatis与Spring的亲密搭配

范例

笔者本文只关注SqlSessionFactoryBean的配置，样例如下

    <!--数据源，引用common-pool包的数据源类-->
    <bean id="dataSource-mysql" class="org.apache.commons.dbcp.BasicDataSource">
	    <property name="driverClassName" value="${jdbc.driverClassName}" />
	    <property name="url" value="${jdbc.url}" />
	    <property name="username" value="${jdbc.username}" />
	    <property name="password" value="${jdbc.password}" />
	    <property name="initialSize" value="1" />
	    <property name="maxActive" value="20" />
	</bean> 

    <!--sqlSessionFactoryBean创建-->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
	    <property name="dataSource" ref="dataSource-mysql" />
	    <property name="typeAliasesPackage" value="com.du.wx.model" />
	    <property name="mapperLocations" value="classpath:com/du/wx/resources/mapper/*.xml"/>
	</bean> 

    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
	    <!-- joggle代表接口 -->
	    <property name="basePackage" value="com.du.wx.mapper.joggle" />
    </bean>

由上述的配置可知，在配置SqlSessionFactory数据库会话工厂Bean对象时必须依赖相应的数据源属性dataSource

SqlSessionFactoryBean

笔者分层次来解析此类

基础内部属性

优先观察SqlSessionFactoryBean的内部属性，代码片段如下

    private static final Log LOGGER = LogFactory.getLog(SqlSessionFactoryBean.class);

    private Resource configLocation;

    private Configuration configuration;

    private Resource[] mapperLocations;

    private DataSource dataSource;

    private TransactionFactory transactionFactory;

    private Properties configurationProperties;

    private SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();

    private SqlSessionFactory sqlSessionFactory;

    //EnvironmentAware requires spring 3.1
    private String environment = SqlSessionFactoryBean.class.getSimpleName();

    private boolean failFast;

    private Interceptor[] plugins;

    private TypeHandler<?>[] typeHandlers;

    private String typeHandlersPackage;

    private Class<?>[] typeAliases;

    private String typeAliasesPackage;

    private Class<?> typeAliasesSuperType;

    //issue #19. No default provider.
    private DatabaseIdProvider databaseIdProvider;

    private Class<? extends VFS> vfs;

    private Cache cache;

    private ObjectFactory objectFactory;

    private ObjectWrapperFactory objectWrapperFactory;

常用的属性如下：

configLocation mybatis主配置文件路径，支持classpath语法

mapperLocations 指定mybatis的mapper配置文件，支持classpath语法

dataSource 数据源

typeAliasesPackage 指定model层类名的别名扫描包，这与mapper配置中的parameterType和resultType搭配使用

InitializingBean复写方法

紧接着看下其复写的afterPropertiesSet()方法，代码如下

  @Override
  public void afterPropertiesSet() throws Exception {
    //datasource not allowed to be null.
    notNull(dataSource, "Property 'dataSource' is required");
    notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
    state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),
              "Property 'configuration' and 'configLocation' can not specified with together");

    this.sqlSessionFactory = buildSqlSessionFactory();
  }

与前文提及的对应，必须确保dataSource属性已经得到配置。

实例化步骤

最后查看关键实例方法buildSqlSessionFactory()方法，由于片段过长，笔者截取几段来进行讲解

---

Mybatis主文件加载

	Configuration configuration;

    XMLConfigBuilder xmlConfigBuilder = null;
    if (this.configuration != null) {
      configuration = this.configuration;
      if (configuration.getVariables() == null) {
        configuration.setVariables(this.configurationProperties);
      } else if (this.configurationProperties != null) {
        configuration.getVariables().putAll(this.configurationProperties);
      }
    } else if (this.configLocation != null) {
      //如果spring配置中configLocation属性不为空，则加载指定的Mybatis配置
      xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
      configuration = xmlConfigBuilder.getConfiguration();
    } else {
      //否则则采用默认的Mybatis配置，@see Configuration
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Property `configuration` or 'configLocation' not specified, using default MyBatis Configuration");
      }
      configuration = new Configuration();
      configuration.setVariables(this.configurationProperties);
    }

通过上述配置表明，内部属性configLocation是非必须配置项

---

model别名映射扫描

	if (hasLength(this.typeAliasesPackage)) {
      //截取typeAliasesPackage属性的包，支持,或;
      String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
      for (String packageToScan : typeAliasPackageArray) {
		//对应方法的目的是通过扫描包得到其包以及子包目录下的所有Class，然后为每个class注册别名
        configuration.getTypeAliasRegistry().registerAliases(packageToScan,
        typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
        if (LOGGER.isDebugEnabled()) {
          LOGGER.debug("Scanned package: '" + packageToScan + "' for aliases");
        }
      }
    }

即对内部属性typeAliasesPackage支持,;作为分隔符以加载多个目录，主要是为了方便编程人员使用parameterType/resultType等属性

---

mapper配置文件加载

	if (!isEmpty(this.mapperLocations)) {
      //mapperLocations
      for (Resource mapperLocation : this.mapperLocations) {
        if (mapperLocation == null) {
          continue;
        }

        try {
          //对扫描的每个配置文件进行解析，并保存其中的必要参数，后续会分析
          XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
              configuration, mapperLocation.toString(), configuration.getSqlFragments());
          xmlMapperBuilder.parse();
        } catch (Exception e) {
          throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
        } finally {
          ErrorContext.instance().reset();
        }

        if (LOGGER.isDebugEnabled()) {
          LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
        }
      }
    } else {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
      }
    }

关键处理XML配置的mapper文件，用到了mybatis包中的org.apache.ibatis.builder.xml.XMLMapperBuilder类进行解析。这个后续分析

---

Configuration-所有关于Mybatis的信息均存储在Mybatis包中的org.apache.ibatis.session.Configuration类上

return this.sqlSessionFactoryBuilder.build(configuration);

Spring呢，就通过SqlSessionFactoryBuilder类来创建相应的SqlSessionFactory对象。具体的由于篇幅过长，就放在后文分析。

总结

从上文便可以看出来Spring框架是如何整合Mybatis的，就是这个Mybatis的关键类Configuration。而相应的开关数据库等一系列的功能都是通过Spring的SqlSessionFactory对象来操控的。这也是两者结合最核心的地方