MyBatis(2)SqlSession的工作原理

SqlSession对象的生成

SqlSession session = factory.openSession();

//=>
@Override
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);
//生成一个事务
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//生成一个执行器，并将事务传给他
final Executor executor = configuration.newExecutor(tx, execType);
//生成一个默认SqlSession,将配置，执行器传给他
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
//关闭事务
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}

使用SqlSession的selectList()和getMapper()

@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//根据statement,在Configuration找到对应的MappedStatement
MappedStatement ms = configuration.getMappedStatement(statement);
//使用执行器来查询结果
//NO_RESULT_HANDLER是null
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 <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}

使用executor.query()方法

@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//得到绑定的SQL,
BoundSql boundSql = ms.getBoundSql(parameter);
//创建缓存Key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//查询
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
//先清缓存，再查询。
//仅当查询堆栈为0才清缓存，为了处理递归调用
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
//这样递归调用到上面就不会清局部缓存了
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
//若查到localCache缓存，则处理
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//从数据库查询
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
//清堆栈
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
//放进缓存
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}

@Override
public <E> List<E> 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);
//准备查询语句
stmt = prepareStatement(handler, ms.getStatementLog());
//用handler去处理查询语句，将结果用ResultHandler来处理。
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}