Mybatis源码分析[04.SqlSession]

/**
 * 这是MyBatis用来执行SQL的类，获取映射器，管理事务
 *
 */
public interface SqlSession extends Closeable {

  /**
   * Retrieve a single row mapped from the statement key
   * 根据指定的SqlID获取一条记录的封装对象
   * @param  the returned object type 封装之后的对象类型
   * @param statement sqlID
   * @return Mapped object 封装之后的对象
   */
   T selectOne(String statement);

  /**
   * Retrieve a single row mapped from the statement key and parameter.
   * 根据指定的SqlID获取一条记录的封装对象，只不过这个方法容许我们可以给sql传递一些参数
   * 一般在实际使用中，这个参数传递的是pojo，或者Map或者ImmutableMap
   * @param  the returned object type
   * @param statement Unique identifier matching the statement to use.
   * @param parameter A parameter object to pass to the statement.
   * @return Mapped object
   */
   T selectOne(String statement, Object parameter);

  /**
   * Retrieve a list of mapped objects from the statement key and parameter.
   * 根据指定的sqlId获取多条记录
   * @param  the returned list element type
   * @param statement Unique identifier matching the statement to use.
   * @return List of mapped object
   */
   List selectList(String statement);

  /**
   * Retrieve a list of mapped objects from the statement key and parameter.
   * 获取多条记录，这个方法容许我们可以传递一些参数
   * @param  the returned list element type
   * @param statement Unique identifier matching the statement to use.
   * @param parameter A parameter object to pass to the statement.
   * @return List of mapped object
   */
   List selectList(String statement, Object parameter);

  /**
   * Retrieve a list of mapped objects from the statement key and parameter,
   * within the specified row bounds.
   * 获取多条记录，这个方法容许我们可以传递一些参数，不过这个方法容许我们进行
   * 分页查询。
   *
   * 需要注意的是默认情况下，Mybatis为了扩展性，仅仅支持内存分页。也就是会先把
   * 所有的数据查询出来以后，然后在内存中进行分页。因此在实际的情况中，需要注意
   * 这一点。
   *
   * @param  the returned list element type
   * @param statement Unique identifier matching the statement to use.
   * @param parameter A parameter object to pass to the statement.
   * @param rowBounds  Bounds to limit object retrieval
   * @return List of mapped object
   */
   List selectList(String statement, Object parameter, RowBounds rowBounds);

  /**
   * The selectMap is a special case in that it is designed to convert a list
   * of results into a Map based on one of the properties in the resulting
   * objects.
   * Eg. Return a of Map[Integer,Author] for selectMap("selectAuthors","id")
   * 将查询到的结果列表转换为Map类型。
   * @param  the returned Map keys type
   * @param  the returned Map values type
   * @param statement Unique identifier matching the statement to use.
   * @param mapKey The property to use as key for each value in the list. 这个参数会作为结果map的key
   * @return Map containing key pair data.
   */
   Map selectMap(String statement, String mapKey);

  /**
   * The selectMap is a special case in that it is designed to convert a list
   * of results into a Map based on one of the properties in the resulting
   * objects.
   * 将查询到的结果列表转换为Map类型。这个方法容许我们传入需要的参数
   * @param  the returned Map keys type
   * @param  the returned Map values type
   * @param statement Unique identifier matching the statement to use.
   * @param parameter A parameter object to pass to the statement.
   * @param mapKey The property to use as key for each value in the list.
   * @return Map containing key pair data.
   */
   Map selectMap(String statement, Object parameter, String mapKey);

  /**
   * The selectMap is a special case in that it is designed to convert a list
   * of results into a Map based on one of the properties in the resulting
   * objects.
   * 获取多条记录,加上分页,并存入Map
   * @param  the returned Map keys type
   * @param  the returned Map values type
   * @param statement Unique identifier matching the statement to use.
   * @param parameter A parameter object to pass to the statement.
   * @param mapKey The property to use as key for each value in the list.
   * @param rowBounds  Bounds to limit object retrieval
   * @return Map containing key pair data.
   */
   Map selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds);

  /**
   * Retrieve a single row mapped from the statement key and parameter
   * using a {@code ResultHandler}.
   *
   * @param statement Unique identifier matching the statement to use.
   * @param parameter A parameter object to pass to the statement.
   * @param handler ResultHandler that will handle each retrieved row
   * @return Mapped object
   */
  void select(String statement, Object parameter, ResultHandler handler);

  /**
   * Retrieve a single row mapped from the statement
   * using a {@code ResultHandler}.
   * 获取一条记录,并转交给ResultHandler处理。这个方法容许我们自己定义对
   * 查询到的行的处理方式。
   * @param statement Unique identifier matching the statement to use.
   * @param handler ResultHandler that will handle each retrieved row
   * @return Mapped object
   */
  void select(String statement, ResultHandler handler);

  /**
   * Retrieve a single row mapped from the statement key and parameter
   * using a {@code ResultHandler} and {@code RowBounds}
   * 获取一条记录,加上分页,并转交给ResultHandler处理
   * @param statement Unique identifier matching the statement to use.
   * @param rowBounds RowBound instance to limit the query results
   * @param handler ResultHandler that will handle each retrieved row
   * @return Mapped object
   */
  void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler);

  /**
   * Execute an insert statement.
   * 插入记录。
   * @param statement Unique identifier matching the statement to execute.
   * @return int The number of rows affected by the insert.
   */
  int insert(String statement);

  /**
   * Execute an insert statement with the given parameter object. Any generated
   * autoincrement values or selectKey entries will modify the given parameter
   * object properties. Only the number of rows affected will be returned.
   * 插入记录，容许传入参数。
   * @param statement Unique identifier matching the statement to execute.
   * @param parameter A parameter object to pass to the statement.
   * @return int The number of rows affected by the insert. 注意返回的是受影响的行数
   */
  int insert(String statement, Object parameter);

  /**
   * Execute an update statement. The number of rows affected will be returned.
   * 更新记录。返回的是受影响的行数
   * @param statement Unique identifier matching the statement to execute.
   * @return int The number of rows affected by the update.
   */
  int update(String statement);

  /**
   * Execute an update statement. The number of rows affected will be returned.
   * 更新记录
   * @param statement Unique identifier matching the statement to execute.
   * @param parameter A parameter object to pass to the statement.
   * @return int The number of rows affected by the update. 返回的是受影响的行数
   */
  int update(String statement, Object parameter);

  /**
   * Execute a delete statement. The number of rows affected will be returned.
   * 删除记录
   * @param statement Unique identifier matching the statement to execute.
   * @return int The number of rows affected by the delete. 返回的是受影响的行数
   */
  int delete(String statement);

  /**
   * Execute a delete statement. The number of rows affected will be returned.
   * 删除记录
   * @param statement Unique identifier matching the statement to execute.
   * @param parameter A parameter object to pass to the statement.
   * @return int The number of rows affected by the delete. 返回的是受影响的行数
   */
  int delete(String statement, Object parameter);

  //以下是事务控制方法,commit,rollback
  /**
   * Flushes batch statements and commits database connection.
   * Note that database connection will not be committed if no updates/deletes/inserts were called.
   * To force the commit call {@link SqlSession#commit(boolean)}
   */
  void commit();

  /**
   * Flushes batch statements and commits database connection.
   * @param force forces connection commit
   */
  void commit(boolean force);

  /**
   * Discards pending batch statements and rolls database connection back.
   * Note that database connection will not be rolled back if no updates/deletes/inserts were called.
   * To force the rollback call {@link SqlSession#rollback(boolean)}
   */
  void rollback();

  /**
   * Discards pending batch statements and rolls database connection back.
   * Note that database connection will not be rolled back if no updates/deletes/inserts were called.
   * @param force forces connection rollback
   */
  void rollback(boolean force);

  /**
   * Flushes batch statements.
   * 刷新批处理语句,返回批处理结果
   * @return BatchResult list of updated records
   * @since 3.0.6
   */
  List flushStatements();

  /**
   * Closes the session
   * 关闭Session
   */
  @Override
  void close();

  /**
   * Clears local session cache
   * 清理Session缓存
   */
  void clearCache();

  /**
   * Retrieves current configuration
   * 得到配置
   * @return Configuration
   */
  Configuration getConfiguration();

  /**
   * Retrieves a mapper.
   * 得到映射器
   * 这个巧妙的使用了泛型，使得类型安全
   * 到了MyBatis 3，还可以用注解,这样xml都不用写了
   * @param  the mapper type
   * @param type Mapper interface class
   * @return a mapper bound to this SqlSession
   */
   T getMapper(Class type);

  /**
   * Retrieves inner database connection
   * 得到数据库连接
   * @return Connection
   */
  Connection getConnection();
}

SqlSession是一个接口类,其实际实现类为DefaultSqlSession

// DefaultSqlSession属性
public class DefaultSqlSession implements SqlSession {

  private Configuration configuration;
  private Executor executor;

  /**
   * 是否自动提交
   */
  private boolean autoCommit;
  private boolean dirty;
}

DefaultSqlSession最为关键的是Executor executor,Executor为一个接口类,DefaultSqlSession对外提供的接口,内部均调用Executor executor.

public interface Executor {

  //不需要ResultHandler
  ResultHandler NO_RESULT_HANDLER = null;

  //更新
  int update(MappedStatement ms, Object parameter) throws SQLException;

  //查询，带分页，带缓存，BoundSql
   List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;

  //查询，带分页
   List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

  //刷新批处理语句
  List flushStatements() throws SQLException;

  //提交和回滚，参数是是否要强制
  void commit(boolean required) throws SQLException;

  void rollback(boolean required) throws SQLException;

  //创建CacheKey
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);

  //判断是否缓存了
  boolean isCached(MappedStatement ms, CacheKey key);

  //清理Session缓存
  void clearLocalCache();

  //延迟加载
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class targetType);

  Transaction getTransaction();

  void close(boolean forceRollback);

  boolean isClosed();

  void setExecutorWrapper(Executor executor);

}

在mybatis源码中,Executor的子类为BaseExecutor,BaseExecutor是执行器的基类

public abstract class BaseExecutor implements Executor {

  private static final Log log = LogFactory.getLog(BaseExecutor.class);

  protected Transaction transaction;
  protected Executor wrapper;

  //延迟加载队列（线程安全）
  protected ConcurrentLinkedQueue deferredLoads;
  //本地缓存机制（Local Cache）防止循环引用（circular references）和加速重复嵌套查询(一级缓存)
  //本地缓存
  protected PerpetualCache localCache;
  //本地输出参数缓存
  protected PerpetualCache localOutputParameterCache;
  protected Configuration configuration;

  //查询堆栈
  protected int queryStack = 0;
  private boolean closed;
}

BatchExecutor、CachingExecutor、ReuseExecutor、SimpleExecutor都是BaseExecutor的子类

• BatchExecutor

// 批量操作执行器
public class BatchExecutor extends BaseExecutor {

  public static final int BATCH_UPDATE_RETURN_VALUE = Integer.MIN_VALUE + 1002;

  private final List statementList = new ArrayList();
  private final List batchResultList = new ArrayList();
  private String currentSql;
  private MappedStatement currentStatement;
}

• CachingExecutor

// 缓存执行器
public class CachingExecutor implements Executor {

  private Executor delegate;
  private TransactionalCacheManager tcm = new TransactionalCacheManager();
}

• ReuseExecutor

// 可重用执行器
public class ReuseExecutor extends BaseExecutor {

  //可重用的执行器内部用了一个map，key为执行的sql,value为sql对应的Statement
  private final Map statementMap = new HashMap();
}

• SimpleExecutor

// 简单执行器
public class SimpleExecutor extends BaseExecutor {
}