mybatis的sqlSessionFactory的加载过程

使用过SSM的框架的都知道mybatis这个持久层框架，今天小编就来简单说说这个框架的核心工厂类sqlSessionFactory的加载过程，一般的SSM框架我们都会在spring的application.xml中引入如下的配置：

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dataSource"></property>
        <property name="configLocation" value="classpath:config/mybatis-config.xml"></property>
    </bean>

其中的SqlSessionFactoryBean便是加载sqlSessionFactory的入口，首先我们来看看这个类的源代码：

public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {

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

  private Resource configLocation;

 private Resource[] mapperLocations;

  private DataSource dataSource;

 private TransactionFactory transactionFactory;

  private Properties configurationProperties;

  private SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();

  private SqlSessionFactory sqlSessionFactory;

其中标红的两处，就是我们在application.xml中注入的两个属性，从源码中我们可以看出该类实现了InitializingBean接口，实现了其afterPropertiesSet（）方法，

该方法是在当前bean的所有属性被初始化完成之后再执行，也就是其中的datasource和configurationProperties等属性，接下来我们看看这个方法主要干什么了？

 public void afterPropertiesSet() throws Exception {
     notNull(dataSource, "Property 'dataSource' is required");
     notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");

     this.sqlSessionFactory = buildSqlSessionFactory();
   }

其中第5行很明显的buildSqlSessionFactory（）方法初始化了sqlSessionFactory ，接下来我们看看这个方法的主要行为：

 Configuration configuration;

     XMLConfigBuilder xmlConfigBuilder = null;
     if (this.configLocation != null) {
       xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
       configuration = xmlConfigBuilder.getConfiguration();
     } else {
       if (logger.isDebugEnabled()) {
         logger.debug("Property 'configLocation' not specified, using default MyBatis Configuration");
       }
       configuration = new Configuration();
       configuration.setVariables(this.configurationProperties);
     }

     if (this.objectFactory != null) {
       configuration.setObjectFactory(this.objectFactory);
     }

     if (this.objectWrapperFactory != null) {
       configuration.setObjectWrapperFactory(this.objectWrapperFactory);
     }

     if (hasLength(this.typeAliasesPackage)) {
       String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
           ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
       for (String packageToScan : typeAliasPackageArray) {
         configuration.getTypeAliasRegistry().registerAliases(packageToScan,
                 typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
         if (logger.isDebugEnabled()) {
           logger.debug("Scanned package: '" + packageToScan + "' for aliases");
         }
       }
     }

     if (!isEmpty(this.typeAliases)) {
       for (Class<?> typeAlias : this.typeAliases) {
         configuration.getTypeAliasRegistry().registerAlias(typeAlias);
         if (logger.isDebugEnabled()) {
           logger.debug("Registered type alias: '" + typeAlias + "'");
         }
       }
     }

     if (!isEmpty(this.plugins)) {
       for (Interceptor plugin : this.plugins) {
         configuration.addInterceptor(plugin);
         if (logger.isDebugEnabled()) {
           logger.debug("Registered plugin: '" + plugin + "'");
         }
       }
     }

     if (hasLength(this.typeHandlersPackage)) {
       String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
           ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
       for (String packageToScan : typeHandlersPackageArray) {
         configuration.getTypeHandlerRegistry().register(packageToScan);
         if (logger.isDebugEnabled()) {
           logger.debug("Scanned package: '" + packageToScan + "' for type handlers");
         }
       }
     }

     if (!isEmpty(this.typeHandlers)) {
       for (TypeHandler<?> typeHandler : this.typeHandlers) {
         configuration.getTypeHandlerRegistry().register(typeHandler);
         if (logger.isDebugEnabled()) {
           logger.debug("Registered type handler: '" + typeHandler + "'");
         }
       }
     }

     if (xmlConfigBuilder != null) {
       try {
         xmlConfigBuilder.parse();

         if (logger.isDebugEnabled()) {
           logger.debug("Parsed configuration file: '" + this.configLocation + "'");
         }
       } catch (Exception ex) {
         throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
       } finally {
         ErrorContext.instance().reset();
       }
     }

     if (this.transactionFactory == null) {
       this.transactionFactory = new SpringManagedTransactionFactory();
     }

     Environment environment = new Environment(this.environment, this.transactionFactory, this.dataSource);
     configuration.setEnvironment(environment);

     if (this.databaseIdProvider != null) {
       try {
         configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
       } catch (SQLException e) {
         throw new NestedIOException("Failed getting a databaseId", e);
       }
     }

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

         try {
           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");
       }
     }

该方法主要就是设置configuration参数，首先是从mybatis-config.xml中获取对应的配置信息，然后再从spring的注入属性中获取所有的配置信息，从后调用

sqlSessionFactoryBuilder.build(configuration)方法获取sqlSessionFactory的实例。

由于整个加载过程没有过多的难点，所以就介绍到这里，感兴趣的朋友可以自己去源码，了解更多知识。