一. MybatisProperties 

在使用 mybatis 時, 還需要對mapper進行配置:

mybatis:
    mapper-locations: classpath:mapper/**Mapper.xml

這些配置其實是映射到 mybatis-spring-boot-autoconfigure 包下的  MybatisProperties 文件中的.

@ConfigurationProperties(prefix = MybatisProperties.MYBATIS_PREFIX)
public class MybatisProperties {

  public static final String MYBATIS_PREFIX = "mybatis";

  /**
   * Location of MyBatis xml config file.
   */
  private String configLocation;

  /**
   * Locations of MyBatis mapper files.
   */
  private String[] mapperLocations;

  /**
   * Packages to search type aliases. (Package delimiters are ",; \t\n")
   */
  private String typeAliasesPackage;

  /**
   * Packages to search for type handlers. (Package delimiters are ",; \t\n")
   */
  private String typeHandlersPackage;

  /**
   * Indicates whether perform presence check of the MyBatis xml config file.
   */
  private boolean checkConfigLocation = false;

  /**
   * Execution mode for {@link org.mybatis.spring.SqlSessionTemplate}.
   */
  private ExecutorType executorType;

  /**
   * Externalized properties for MyBatis configuration.
   */
  private Properties configurationProperties;

  /**
   * A Configuration object for customize default settings. If {@link #configLocation}
   * is specified, this property is not used.
   */
  @NestedConfigurationProperty
  private Configuration configuration;
......
}

二. MybatisAutoConfiguration 

MybatisAutoConfiguration 是 mybatis-spring-boot-autoconfigure 中的一個配置類.

在這個配置類中, 主要加入了兩個組件: SqlSessionFactory 和 SqlSessionTemplate

1. SqlSessionFactory

  @Bean
  @ConditionalOnMissingBean
  public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
    SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
    factory.setDataSource(dataSource);
    factory.setVfs(SpringBootVFS.class);
    if (StringUtils.hasText(this.properties.getConfigLocation())) {
      factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
    }
    Configuration configuration = this.properties.getConfiguration();
    if (configuration == null && !StringUtils.hasText(this.properties.getConfigLocation())) {
      configuration = new Configuration();
    }
    if (configuration != null && !CollectionUtils.isEmpty(this.configurationCustomizers)) {
      for (ConfigurationCustomizer customizer : this.configurationCustomizers) {
        customizer.customize(configuration);
      }
    }
    factory.setConfiguration(configuration);
    if (this.properties.getConfigurationProperties() != null) {
      factory.setConfigurationProperties(this.properties.getConfigurationProperties());
    }
    if (!ObjectUtils.isEmpty(this.interceptors)) {
      factory.setPlugins(this.interceptors);
    }
    if (this.databaseIdProvider != null) {
      factory.setDatabaseIdProvider(this.databaseIdProvider);
    }
    if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {
      factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());
    }
    if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {
      factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());
    }
    if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
      factory.setMapperLocations(this.properties.resolveMapperLocations());
    }

    return factory.getObject();
  }

(從名字可以看出來, SqlSessionFactory 是 SqlSession的一個工廠類. SqlSession 這個類很重要, 裡面定義了 資料庫的一套增刪改查操作的規範.)

MybatisProperties 中的配置會在這個方法中進行解析.

1. 創建了 Configuration 配置類(後面會用到, 這個類也很重要)的實例

2. 通過 factory.getObject() 方法創建了 SqlSessionFactory 類的實例

創建的過程中, 又調用了 protected SqlSessionFactory buildSqlSessionFactory() 方法進行創建, 創建過程比較多, 留給後面在解析.

2. SqlSessionTemplate 

  @Bean
  @ConditionalOnMissingBean
  public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
    ExecutorType executorType = this.properties.getExecutorType();
    if (executorType != null) {
      return new SqlSessionTemplate(sqlSessionFactory, executorType);
    } else {
      return new SqlSessionTemplate(sqlSessionFactory);
    }
  }

SqlSessionTemplate 這個類 實現了 SqlSession 介面,  也就是說, 他也有一套 資料庫的 增刪改查的方法. 先看一下構造過程:

  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
    this(sqlSessionFactory, sqlSessionFactory.getConfiguration().getDefaultExecutorType());
  }

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  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType) {
    this(sqlSessionFactory, executorType,
        new MyBatisExceptionTranslator(
            sqlSessionFactory.getConfiguration().getEnvironment().getDataSource(), true));
  }

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  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator) {

    notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
    notNull(executorType, "Property 'executorType' is required");

    this.sqlSessionFactory = sqlSessionFactory;
    this.executorType = executorType;
    this.exceptionTranslator = exceptionTranslator;
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
  }

這裡初始化了一個很重要的屬性:  sqlSessionProxy. 

sqlSessionProxy 是為 SqlSession 創建了一個代理類, 且代理方法為:  SqlSessionInterceptor

看一個 實現了 SqlSession 介面中的  selectOne 方法:

  @Override
  public <T> T selectOne(String statement, Object parameter) {
    return this.sqlSessionProxy.<T> selectOne(statement, parameter);
  }

可以看到, 在調用 selectOne 方法的時候, 其實就是調用了 SqlSessionInterceptor 的 invoke 方法.