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 <T> 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> 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 <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.
* 根据指定的sqlId获取多条记录
* @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.
* 获取多条记录，这个方法容许我们可以传递一些参数
* @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.
* 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 <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")
* 将查询到的结果列表转换为Map类型。
* @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. 这个参数会作为结果map的key
* @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.
* 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 <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.
* 获取多条记录,加上分页,并存入Map
* @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);
/**
* 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<BatchResult> 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 <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
* 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
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
//查询，带分页
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
//刷新批处理语句
List<BatchResult> 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<DeferredLoad> 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<Statement> statementList = new ArrayList<Statement>();
private final List<BatchResult> batchResultList = new ArrayList<BatchResult>();
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<String, Statement> statementMap = new HashMap<String, Statement>();
}

• SimpleExecutor

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