Mybatis如何创建SqlSession以及Excutor的使用
Mybatis如何创建SqlSession以及Excutor的使用，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。
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今天我们分析下Mybatis如何创建SqlSession（ sessionFactory.openSession()）和Excutor到底做了什么事情
public static void main(String[] args) throws Exception {
SqlSessionFactory sessionFactory = null;
String resource = "configuration.xml";
sessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource));
SqlSession sqlSession = sessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
System.out.println(userMapper.findUserById(1));
}1：openSessionFromDataSource
通过openSession()最终调用的是 openSessionFromDataSource，源码如下：
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);
return new DefaultSqlSession(configuration, executor, autoCommit);
} 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();
}
}说明：ExecutorType（执行器类型）类型有 SIMPLE（默认）,REUSE,BATCH,
TransactionIsolationLevel 隔离级别5种
NONE(Connection.TRANSACTION_NONE), READ_COMMITTED(Connection.TRANSACTION_READ_COMMITTED), READ_UNCOMMITTED(Connection.TRANSACTION_READ_UNCOMMITTED), REPEATABLE_READ(Connection.TRANSACTION_REPEATABLE_READ),
SERIALIZABLE(Connection.TRANSACTION_SERIALIZABLE);
通过源码看出首先是从configuration获取environment，接着创建Transaction，
最后通过 configuration.newExecutor创建Executor。
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
//根据executorType创建不同的Executor对象
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);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}根据executorType创建对应的Executor，从源码可以看出他有BatchExecutor、ReuseExecutor、CachingExecutor、SimpleExecutor
那么Executor是做什么的呢？
2：Executor
Executor是接口，是对于Statement的封装，我们看下Executor,他是真正执行sql的地方。
public interface Executor {
ResultHandler NO_RESULT_HANDLER = null;
int update(MappedStatement ms, Object parameter) throws SQLException;
List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
Cursor queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
List 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();
void setExecutorWrapper(Executor executor);
}上面源码我可以看到Executor接口定义了update 、query、commit、rollback等方法，他的实现类如下图
我们拿其中的SimpleExecutor类里面的doQuery方法看下源码
@Override
public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
//StatementHandler封装了Statement, 让 StatementHandler 去处理
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}我们看看StatementHandler 的一个实现类 PreparedStatementHandler（这也是我们最常用的，封装的是PreparedStatement）, 看看它使怎么去处理的：
@Override
public List query(Statement statement, ResultHandler resultHandler) throws SQLException {
String sql = boundSql.getSql();
//这个和jdbc一样执行sql
statement.execute(sql);
//结果交给了ResultSetHandler 去处理
return resultSetHandler.handleResultSets(statement);
}我们继续回到openSessionFromDataSource上面来，通过上面的executor的创建最终是创建 new DefaultSqlSession(configuration, executor, autoCommit)。
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