mybatis源码分析——SqlSession的作用

 

sqlSession，顾名思义，是一次sql的回话，它起到了承上启下的作用，这个类既维护了Configuration对象，又包含Executor对象，可以

执行jdbc操作，在mybatis体系中非常重要，下面我们来看一下这个类的源码：

 

1：sqlSession类的源码

SqlSession接口类定义了增删改查的操作：

public interface SqlSession extends Closeable {

  /**
   * Retrieve a single row mapped from the statement key
   * @param  the returned object type
   * @param statement
   * @return Mapped object
   */
   T selectOne(String statement);

  /**
   * Retrieve a single row mapped from the statement key and parameter.
   * @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.
   * @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.
   * @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.
   * @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")
   * @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.
   * @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.
   * @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.
   * @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}.
   * @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}
   * @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.
   * @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.
   * @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.
   */
  int insert(String statement, Object parameter);

  /**
   * Execute an update statement. The number of rows affected will be returned.
   * @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.
   * @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.
   */
  int update(String statement, Object parameter);

  /**
   * Execute a delete statement. The number of rows affected will be returned.
   * @param statement Unique identifier matching the statement to execute.
   * @return int The number of rows affected by the delete.
   */
  int delete(String statement);

  /**
   * Execute a delete statement. The number of rows affected will be returned.
   * @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.
   */
  int delete(String statement, Object parameter);

  /**
   * 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.
   * @return BatchResult list of updated records
   * @since 3.0.6
   */
  List flushStatements();

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

  /**
   * Clears local session cache
   */
  void clearCache();

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

  /**
   * Retrieves a mapper.
   * @param  the mapper type
   * @param type Mapper interface class
   * @return a mapper bound to this SqlSession
   */
   T getMapper(Class type);

  /**
   * Retrieves inner database connection
   * @return Connection
   */
  Connection getConnection();
}

　　

实现类DefaultSqlSession维护了Configuration对象，可以直接从Configuration对象中拿到代理对象，也可以直接从Configuration对象中拿到MapperStatement对象

然后就是对SqlSession接口类的具体实现

public class DefaultSqlSession implements SqlSession {

  private Configuration configuration;
  private Executor executor;

  private boolean autoCommit;
  private boolean dirty;

  public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }

  public DefaultSqlSession(Configuration configuration, Executor executor) {
    this(configuration, executor, false);
  }

  @Override
  public  T selectOne(String statement) {
    return this.selectOne(statement, null);
  }

  @Override
  public  T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List list = this.selectList(statement, parameter);
    if (list.size() == 1) {
      return list.get(0);
    } else if (list.size() > 1) {
      throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
      return null;
    }
  }

  @Override
  public  Map selectMap(String statement, String mapKey) {
    return this.selectMap(statement, null, mapKey, RowBounds.DEFAULT);
  }

  @Override
  public  Map selectMap(String statement, Object parameter, String mapKey) {
    return this.selectMap(statement, parameter, mapKey, RowBounds.DEFAULT);
  }

  @Override
  public  Map selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds) {
    final List list = selectList(statement, parameter, rowBounds);
    final DefaultMapResultHandler mapResultHandler = new DefaultMapResultHandler(mapKey,
        configuration.getObjectFactory(), configuration.getObjectWrapperFactory(), configuration.getReflectorFactory());
    final DefaultResultContext context = new DefaultResultContext();
    for (V o : list) {
      context.nextResultObject(o);
      mapResultHandler.handleResult(context);
    }
    return mapResultHandler.getMappedResults();
  }

  @Override
  public  List selectList(String statement) {
    return this.selectList(statement, null);
  }

  @Override
  public  List selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }

  @Override
  public  List selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public void select(String statement, Object parameter, ResultHandler handler) {
    select(statement, parameter, RowBounds.DEFAULT, handler);
  }

  @Override
  public void select(String statement, ResultHandler handler) {
    select(statement, null, RowBounds.DEFAULT, handler);
  }

  @Override
  public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      executor.query(ms, wrapCollection(parameter), rowBounds, handler);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public int insert(String statement) {
    return insert(statement, null);
  }

  @Override
  public int insert(String statement, Object parameter) {
    return update(statement, parameter);
  }

  @Override
  public int update(String statement) {
    return update(statement, null);
  }

  @Override
  public int update(String statement, Object parameter) {
    try {
      dirty = true;
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.update(ms, wrapCollection(parameter));
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public int delete(String statement) {
    return update(statement, null);
  }

  @Override
  public int delete(String statement, Object parameter) {
    return update(statement, parameter);
  }

  @Override
  public void commit() {
    commit(false);
  }

  @Override
  public void commit(boolean force) {
    try {
      executor.commit(isCommitOrRollbackRequired(force));
      dirty = false;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error committing transaction.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public void rollback() {
    rollback(false);
  }

  @Override
  public void rollback(boolean force) {
    try {
      executor.rollback(isCommitOrRollbackRequired(force));
      dirty = false;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error rolling back transaction.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public List flushStatements() {
    try {
      return executor.flushStatements();
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error flushing statements.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public void close() {
    try {
      executor.close(isCommitOrRollbackRequired(false));
      dirty = false;
    } finally {
      ErrorContext.instance().reset();
    }
  }

  @Override
  public Configuration getConfiguration() {
    return configuration;
  }

  @Override
  public  T getMapper(Class type) {
    return configuration.getMapper(type, this);
  }

  @Override
  public Connection getConnection() {
    try {
      return executor.getTransaction().getConnection();
    } catch (SQLException e) {
      throw ExceptionFactory.wrapException("Error getting a new connection.  Cause: " + e, e);
    }
  }

  @Override
  public void clearCache() {
    executor.clearLocalCache();
  }

  private boolean isCommitOrRollbackRequired(boolean force) {
    return (!autoCommit && dirty) || force;
  }

  private Object wrapCollection(final Object object) {
    if (object instanceof Collection) {
      StrictMap