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MyBatis之SqlSession介绍
鲁春利的工作笔记，好记性不如烂笔头
转载自：深入浅出MyBatis-Sqlsession
SqlSession的创建
Sqlsession对应着一次数据库会话。由于数据库回话不是永久的，因此Sqlsession的生命周期也不应该是永久的，相反，在你每次访问数据库时都需要创建它（当然并不是说在Sqlsession里只能执行一次sql，你可以执行多次，当一旦关闭了Sqlsession就需要重新创建它）。创建Sqlsession的地方只有一个，那就是SqlsessionFactory的openSession方法：
package org.apache.ibatis.session;
import java.sql.Connection;
public interface SqlSessionFactory {
SqlSession openSession();
SqlSession openSession(boolean autoCommit);
SqlSession openSession(Connection connection);
SqlSession openSession(TransactionIsolationLevel level);
SqlSession openSession(ExecutorType execType);
SqlSession openSession(ExecutorType execType, boolean autoCommit);
SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
SqlSession openSession(ExecutorType execType, Connection connection);
Configuration getConfiguration();
}实际创建SqlSession的地方是openSessionFromDataSource，如下：
package org.apache.ibatis.session.defaults;
import java.sql.Connection;
import java.sql.SQLException;
import org.apache.ibatis.exceptions.ExceptionFactory;
import org.apache.ibatis.executor.ErrorContext;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.Environment;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ExecutorType;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.TransactionIsolationLevel;
import org.apache.ibatis.transaction.Transaction;
import org.apache.ibatis.transaction.TransactionFactory;
import org.apache.ibatis.transaction.managed.ManagedTransactionFactory;
public class DefaultSqlSessionFactory implements SqlSessionFactory {
private final Configuration configuration;
public DefaultSqlSessionFactory(Configuration configuration) {
this.configuration = configuration;
}
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final Executor executor = configuration.newExecutor(tx, execType, autoCommit);
return new DefaultSqlSession(configuration, executor);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
}可以看出，创建sqlsession经过了以下几个主要步骤：
1) 从配置中获取Environment；
2) 从Environment中取得DataSource；
3) 从Environment中取得TransactionFactory；
4) 从DataSource里获取数据库连接对象Connection；
5) 在取得的数据库连接上创建事务对象Transaction；
6) 创建Executor对象（该对象非常重要，事实上sqlsession的所有操作都是通过它完成的）；
7) 创建sqlsession对象。
Executor的创建
Sqlsession只是个门面，真正干事的是Executor，Sqlsession对数据库的操作都是通过Executor来完成的。与Sqlsession一样，Executor也是动态创建的：
final Executor executor = configuration.newExecutor(tx, execType, autoCommit);
Configuration类的实现：
public class Configuration {
protected Environment environment;
public Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor, autoCommit);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
}Executor的定义：
package org.apache.ibatis.executor;
import java.sql.SQLException;
import java.util.List;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;
public interface Executor {
ResultHandler NO_RESULT_HANDLER = null;
int update(MappedStatement ms, Object parameter) throws SQLException;
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
List<BatchResult> flushStatements() throws SQLException;
void commit(boolean required) throws SQLException;
void rollback(boolean required) throws SQLException;
CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
boolean isCached(MappedStatement ms, CacheKey key);
void clearLocalCache();
void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
Transaction getTransaction();
void close(boolean forceRollback);
boolean isClosed();
}
可以看出，创建的Executor只是3中基础类型之一：
BatchExecutor专门用于执行批量sql操作
ReuseExecutor会重用statement执行sql操作
SimpleExecutor只是简单执行sql没有什么特别的。
如果开启cache的话（默认是开启的并且没有任何理由去关闭它），就会创建CachingExecutor，它以前面创建的Executor作为唯一参数。CachingExecutor在查询数据库前先查找缓存，若没找到的话调用delegate（就是构造时传入的Executor对象）从数据库查询，并将查询结果存入缓存中。
package org.apache.ibatis.executor;
import java.sql.SQLException;
import java.util.List;
import org.apache.ibatis.cache.Cache;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.cache.TransactionalCacheManager;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.ParameterMapping;
import org.apache.ibatis.mapping.ParameterMode;
import org.apache.ibatis.mapping.StatementType;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;
public class CachingExecutor implements Executor {
private Executor delegate;
private boolean autoCommit; // issue #573. No need to call commit() on autoCommit sessions
private TransactionalCacheManager tcm = new TransactionalCacheManager();
private boolean dirty;
public CachingExecutor(Executor delegate) {
this(delegate, false);
}
public CachingExecutor(Executor delegate, boolean autoCommit) {
this.delegate = delegate;
this.autoCommit = autoCommit;
}
public Transaction getTransaction() {
return delegate.getTransaction();
}
public void close(boolean forceRollback) {
try {
//issue #499. Unresolved session handling
//issue #573. Autocommit sessions should commit
if (dirty && !autoCommit) {
tcm.rollback();
} else {
tcm.commit();
}
} finally {
delegate.close(forceRollback);
}
}
// 其他代码略
}Executor对象是可以被插件拦截的（拦截器Interceptor，如：
@Intercepts({@Signature(type = Executor.class, method = "query",
args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
），如果定义了针对Executor类型的插件，最终生成的Executor对象是被各个插件插入后的代理对象。
Mapper介绍
Mybatis官方手册建议通过mapper对象访问mybatis，因为使用mapper看起来更优雅，就像下面这样：
session = sqlSessionFactory.openSession();
UserMapper userMapper= session.getMapper(UserMapper.class);
UserBo user = new UserBo();
user.setUsername("iMbatis");
user.setPassword("iMbatis");
userMapper.insertUser(user);那么这个mapper到底是什么呢，它是如何创建的呢，它又是怎么与sqlsession等关联起来的呢？下面为你一一解答。
表面上看mapper是在sqlsession里创建的，但实际创建它的地方是MapperRegistry：
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.executor.BatchResult;
/**
* The primary Java interface for working with MyBatis.
* Through this interface you can execute commands, get mappers and manage transactions.
*
*/
public interface SqlSession extends Closeable {
<T> T selectOne(String statement);
<T> T selectOne(String statement, Object parameter);
<E> List<E> selectList(String statement);
<E> List<E> selectList(String statement, Object parameter);
<E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds);
<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);
void select(String statement, Object parameter, ResultHandler handler);
void select(String statement, ResultHandler handler);
void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler);
int insert(String statement);
int insert(String statement, Object parameter);
int update(String statement);
int update(String statement, Object parameter);
int delete(String statement);
int delete(String statement, Object parameter);
void commit();
void commit(boolean force);
void rollback();
void rollback(boolean force);
List<BatchResult> flushStatements();
void close();
void clearCache();
Configuration getConfiguration();
<T> T getMapper(Class<T> type);
Connection getConnection();
}本文出自 “闷葫芦的世界” 博客，请务必保留此出处http://luchunli.blog.51cto.com/2368057/1891463
MyBatis之SqlSession介绍
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