mybatis源码解析5---SqlSession解析

由之前解析可知，mybatis启动的时候会加载XML配置文件解析生成全局配置对象Configuration对象，SqlSessionFactoryBuilder类会根据Configuration对象创建一个DefaultSqlSessionFactory对象，而DefaultSqlSessionFactory对象实现了SqlSessionFactory中的创建SqlSession的方法，最终新建了一个SqlSession接口的默认实现类DefaultSqlSession，现在先来了解下SqlSession以及它的实现类

SqlSession解析

SqlSession位于mybatis包的org.apache.ibatis.session目录下，字面意思就是sql的会话，用于程序和数据库直接的sql会话，程序执行一次数据库操作就需要创建一个sqlSession，操作结束即关闭sqlSession；

既然是程序和数据库之间的会话，那么sqlSession接口的方法应该是程序和数据库都容易理解的，sqlSession中定义的方法都是关于数据库操作的方法，源码如下：

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;

public interface SqlSession extends Closeable {

    //根据Sql语句查询单条记录
  <T> T selectOne(String statement);
  <T> T selectOne(String statement, Object parameter);//根据Sql语句和参数查询单条记录

  //根据Sql语句查询多条记录
  <E> List<E> selectList(String statement);
  <E> List<E> selectList(String statement, Object parameter);//根据Sql语句和参数查询多条记录
  <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds);//根据Sql语句和参数以及分页参数查询多条记录

   //selectMap和selectList原理一样,只是将结果集映射成Map对象返回
  <K, V> Map<K, V> selectMap(String statement, String mapKey);
  <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey);
  <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds);

  //返回游标对象
  <T> Cursor<T> selectCursor(String statement);
  <T> Cursor<T> selectCursor(String statement, Object parameter);
  <T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds);

  //查询的结果对象由指定的ResultHandler处理
  void select(String statement, Object parameter, ResultHandler handler);
  void select(String statement, ResultHandler handler);
  void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler);

  //执行insert语句
  int insert(String statement);
  int insert(String statement, Object parameter);

  //执行update语句
  int update(String statement);
  int update(String statement, Object parameter);

  //执行delete语句
  int delete(String statement);
  int delete(String statement, Object parameter);

  //提交事务
  void commit();
  void commit(boolean force);

  //事务回滚
  void rollback();
  void rollback(boolean force);

  //将请求刷新到数据库
  List<BatchResult> flushStatements();

  //关闭sqlSession
  @Override
  void close();

  //清除缓存  
  void clearCache();
  
  //获取Configuration对象
  Configuration getConfiguration();

  //获取Type对象的Mapper对象
  <T> T getMapper(Class<T> type);

  //获取sqlSession对象的数据库连接
  Connection getConnection();
}

SqlSession中的方法全是和数据库相关的增删改查以及事务的提交方法。

SqlSession有三个实现类，除了默认的DefaultSqlSession之外，还有SqlSessionManager和SqlSessionTemplate

接下来先看下SqlSession默认的实现类DefaultSqlSession是如何实现的。

DefaultSqlSession类解析

DefaultSqlSession有5个属性和2个构造方法如下：

  private Configuration configuration;//全局配置
  private Executor executor;//执行器

  private boolean autoCommit;//自动提交标识
  private boolean dirty;
  private List<Cursor<?>> cursorList;//游标列表

  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);
  }

下面以select方法为例

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

  @Override
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.<T>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;
    }
  }

很明显selectOne的方法最终都是调用了selectList方法，然后取唯一的一条数据返回。那在看看selectList相关的代码

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

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

  @Override
  public <E> List<E> 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();
    }
  }

共有三个selectList方法，最终都是调用了最后一个selectList方法，这里有三个参数：statement是sql语句；parameter是传入的参数；RowBounds是和分页相关的参数

RowBounds源码如下：

package org.apache.ibatis.session;

/**
 * @author Clinton Begin
 */
public class RowBounds {

  public static final int NO_ROW_OFFSET = 0;
  public static final int NO_ROW_LIMIT = Integer.MAX_VALUE; //int的最大值
  public static final RowBounds DEFAULT = new RowBounds();

  private int offset;
  private int limit;

  public RowBounds() {
    this.offset = NO_ROW_OFFSET;
    this.limit = NO_ROW_LIMIT;
  }

  public RowBounds(int offset, int limit) {
    this.offset = offset;
    this.limit = limit;
  }

  public int getOffset() {
    return offset;
  }

  public int getLimit() {
    return limit;
  }

}

RowBounds的两个属性:offSet是指查询数据时从多少位置开始查询，limit是指返回数据的调试，默认是从0位置开始查询到Integer的最大值，相关于默认是不做分页处理；

回到正题，selectList最后执行的方法中执行了两行代码

MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);

调用configuration的getMapped的Statement方法获取MappedStatement对象，然后调用执行器executor的query方法获取查询结果。

这是select方法，再来看看其他的insert、update、delete方法

@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);
  }

可以看出SqlSession的insert和delete的方法最终都是调用了update方法，而update方法最终也是调用了Executor的update

由此可得出结论sqlSession虽然叫程序和数据库之间的SQL会话，但是它并没有具体去执行sql语句，最终的sql语句的执行是由执行器Executor执行的，而SqlSession的作用只是创建了MappedStatement对象以及调用执行器去执行SQL

其他的commit、rollback方法同样最终都是调用的执行器Executor的对应的方法，那么接下来就去了解下执行器Executor是干嘛的，以及SqlSession创建的MappedStatement又是什么？

DefaultSqlSession完整源码如下：

/**
 *    Copyright 2009-2015 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.defaults;

import java.io.IOException;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.apache.ibatis.binding.BindingException;
import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.exceptions.ExceptionFactory;
import org.apache.ibatis.exceptions.TooManyResultsException;
import org.apache.ibatis.executor.BatchResult;
import org.apache.ibatis.executor.ErrorContext;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.executor.result.DefaultMapResultHandler;
import org.apache.ibatis.executor.result.DefaultResultContext;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.session.SqlSession;

/**
 *
 * The default implementation for {@link SqlSession}.
 * Note that this class is not Thread-Safe.
 *
 * @author Clinton Begin
 */
public class DefaultSqlSession implements SqlSession {

  private Configuration configuration;
  private Executor executor;

  private boolean autoCommit;
  private boolean dirty;
  private List<Cursor<?>> cursorList;

  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> T selectOne(String statement) {
    return this.<T>selectOne(statement, null);
  }

  @Override
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.<T>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 <K, V> Map<K, V> selectMap(String statement, String mapKey) {
    return this.selectMap(statement, null, mapKey, RowBounds.DEFAULT);
  }

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

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

  @Override
  public <T> Cursor<T> selectCursor(String statement) {
    return selectCursor(statement, null);
  }

  @Override
  public <T> Cursor<T> selectCursor(String statement, Object parameter) {
    return selectCursor(statement, parameter, RowBounds.DEFAULT);
  }

  @Override
  public <T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      Cursor<T> cursor = executor.queryCursor(ms, wrapCollection(parameter), rowBounds);
      registerCursor(cursor);
      return cursor;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

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

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

  @Override
  public <E> List<E> 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<BatchResult> 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));
      closeCursors();
      dirty = false;
    } finally {
      ErrorContext.instance().reset();
    }
  }

  private void closeCursors() {
    if (cursorList != null && cursorList.size() != 0) {
      for (Cursor<?> cursor : cursorList) {
        try {
          cursor.close();
        } catch (IOException e) {
          throw ExceptionFactory.wrapException("Error closing cursor.  Cause: " + e, e);
        }
      }
      cursorList.clear();
    }
  }

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

  @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>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 <T> void registerCursor(Cursor<T> cursor) {
    if (cursorList == null) {
      cursorList = new ArrayList<Cursor<?>>();
    }
    cursorList.add(cursor);
  }

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

  private Object wrapCollection(final Object object) {
    if (object instanceof Collection) {
      StrictMap<Object> map = new StrictMap<Object>();
      map.put("collection", object);
      if (object instanceof List) {
        map.put("list", object);
      }
      return map;
    } else if (object != null && object.getClass().isArray()) {
      StrictMap<Object> map = new StrictMap<Object>();
      map.put("array", object);
      return map;
    }
    return object;
  }

  public static class StrictMap<V> extends HashMap<String, V> {

    private static final long serialVersionUID = -5741767162221585340L;

    @Override
    public V get(Object key) {
      if (!super.containsKey(key)) {
        throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + this.keySet());
      }
      return super.get(key);
    }

  }

}