MyBatis框架SqlSession浅析

1、SqlSessionFactory作用

MyBatis框架SqlSessionFactory是线程安全的，负责创建SqlSession。

DefaultSqlSessionFactory是线程安全的，属性Final。

2、SqlSessionFactoryBuilder

SqlSessionFactoryBuilder负责创建SqlSessionFactory。SqlSessionFactoryBuilder则可以从XML配置文件或一个预先定制的Configuration的实例构建出SqlSessionFactory的实例。

java">  public SqlSessionFactory build(Configuration config) {return new DefaultSqlSessionFactory(config);}

3、SqlSession

作用：封装数据库操作，线程不安全。

java">/***    Copyright 2009-2016 the original author or authors.**    Licensed under the Apache License, Version 2.0 (the "License");*    you may not use this file except in compliance with the License.*    You may obtain a copy of the License at**       http://www.apache.org/licenses/LICENSE-2.0**    Unless required by applicable law or agreed to in writing, software*    distributed under the License is distributed on an "AS IS" BASIS,*    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.*    See the License for the specific language governing permissions and*    limitations under the License.*/
package org.apache.ibatis.session;import java.io.Closeable;
import java.sql.Connection;
import java.util.List;
import java.util.Map;import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.executor.BatchResult;/*** The primary Java interface for working with MyBatis.* Through this interface you can execute commands, get mappers and manage transactions.** @author Clinton Begin*/
public interface SqlSession extends Closeable {/*** Retrieve a single row mapped from the statement key* @param <T> the returned object type* @param statement* @return Mapped object*/<T> T selectOne(String statement);/*** Retrieve a single row mapped from the statement key and parameter.* @param <T> 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> T selectOne(String statement, Object parameter);/*** Retrieve a list of mapped objects from the statement key and parameter.* @param <E> the returned list element type* @param statement Unique identifier matching the statement to use.* @return List of mapped object*/<E> List<E> selectList(String statement);/*** Retrieve a list of mapped objects from the statement key and parameter.* @param <E> 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*/<E> List<E> selectList(String statement, Object parameter);/*** Retrieve a list of mapped objects from the statement key and parameter,* within the specified row bounds.* @param <E> 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*/<E> List<E> 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 <K> the returned Map keys type* @param <V> 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.*/<K, V> Map<K, V> 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 <K> the returned Map keys type* @param <V> 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.*/<K, V> Map<K, V> 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 <K> the returned Map keys type* @param <V> 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.*/<K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds);/*** A Cursor offers the same results as a List, except it fetches data lazily using an Iterator.* @param <T> the returned cursor element type.* @param statement Unique identifier matching the statement to use.* @return Cursor of mapped objects*/<T> Cursor<T> selectCursor(String statement);/*** A Cursor offers the same results as a List, except it fetches data lazily using an Iterator.* @param <T> the returned cursor element type.* @param statement Unique identifier matching the statement to use.* @param parameter A parameter object to pass to the statement.* @return Cursor of mapped objects*/<T> Cursor<T> selectCursor(String statement, Object parameter);/*** A Cursor offers the same results as a List, except it fetches data lazily using an Iterator.* @param <T> the returned cursor 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 Cursor of mapped objects*/<T> Cursor<T> selectCursor(String statement, Object parameter, 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*/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*/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*/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<BatchResult> flushStatements();/*** Closes the session*/@Overridevoid close();/*** Clears local session cache*/void clearCache();/*** Retrieves current configuration* @return Configuration*/Configuration getConfiguration();/*** Retrieves a mapper.* @param <T> the mapper type* @param type Mapper interface class* @return a mapper bound to this SqlSession*/<T> T getMapper(Class<T> type);/*** Retrieves inner database connection* @return Connection*/Connection getConnection();
}

DefaultSqlSession:SqlSession的默认实现，不是线程安全的.

线程安全性考虑，在使用MyBatis时，通过mapper接口的使用是基于动态代理的，提供了两种情况下的线程安全性：

在不使用事务的情况下，每个SQL操作会新建一个SqlSession对象，这种方式是线程安全的。

在使用事务时，通过ThreadLocal保证每个线程对应一个SqlSession对象，因此也是线程安全的‌。

4、SqlSessionManager

实现SqlSession和SqlSessionFactory，实现基础功能。

java">public class SqlSessionManager implements SqlSessionFactory, SqlSession {private final SqlSessionFactory sqlSessionFactory;private final SqlSession sqlSessionProxy;private final ThreadLocal<SqlSession> localSqlSession = new ThreadLocal<>();private SqlSessionManager(SqlSessionFactory sqlSessionFactory) {this.sqlSessionFactory = sqlSessionFactory;this.sqlSessionProxy = (SqlSession) Proxy.newProxyInstance(SqlSessionFactory.class.getClassLoader(),new Class[]{SqlSession.class},new SqlSessionInterceptor());}

5、SqlSessionTemplate

是一个线程安全的类，它实现了SqlSession接口。通过SqlSessionTemplate可以完成对数据库的CRUD操作，并且保证单例线程安全。是位于mybatis-spring 2.0.0包中，通过SqlSessionTemplate可以完成对数据库的CRUD操作，并且保证单例线程安全。SqlSessionTemplate通过拥有一个SqlSessionProxy的代理对象来实现这一点，这个代理对象在调用时会导向SqlSessionInterceptor的invoke方法触发代理逻辑，从而保证线程安全。

6、三种Executor执行器

```
BaseExecutor
```

```
SimpleExecutor
```

SimpleExecutor 每执行一次update或select，就开启一个Statement对象，用完立刻关闭Statement对象。

PauseExecutor。

PauseExecutor 执行update或select，以sql做为key查找Statement对象，存在就使用，不存在就创建，用完后，不关闭Statement对象，而且放置于Map内，供下一次使用。简言之，就是重复使用Statement对象。

BatchExecutor

BatchExecutor 执行update，将所有sql通过addBatch()都添加到批处理中，等待统一执行executeBatch()，它缓存了多个Statement对象，每个Statement对象都是addBatch()完毕后，等待逐一执行executeBatch()批处理。与JDBC批处理相同。