获取SqlSession后，下一步就是执行sql.
User user=sqlSession.selectOne("last.soul.mapper.UserMapper.selectById",map);DefaultSqlSession的次要性能就是实现增删改查性能，以及它们的重载办法。就查问来说，最初都会调用select办法，而后改装成selectOne,selectMap等，代码如下：
/** * * @param statement sql语句ID=xxxMapper.xml文件中的namespace+sql标签的id. * 如：last.soul.mapper.UserMapper.selectById * @param parameter sql的参数 * @param rowBounds 分页信息 * @param <E> * @return */ @Override public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { //sql相干音讯的包装对象，比方：sql语句，返回类型，是否应用缓存等 MappedStatement ms = configuration.getMappedStatement(statement); //执行sql return executor.query(ms, wrapCollection(parameter)/*包装汇合类型参数*/, rowBounds, Executor.NO_RESULT_HANDLER); } catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }由上文得悉executor是CachingExecutor,执行的是CachingExecutor的query办法。
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, boundSql); @SuppressWarnings("unchecked") List<E> list = (List<E>) tcm.getObject(cache, key); if (list == null) { list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578 and #116 } return list; } } //delegate是SimpleExecutor对象 return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }query办法中次要逻辑是判断是否有缓存，有缓存返回缓存值，否则执行delegate对象的query办法，即SimpleExecutor对象的query办法，然而你会发现该类中没有些办法，那它执行的就是他的父类BaseExecutor的query办法。query办法又会调用它的公有办法queryFromDatabase。
queryFromDatabase源代码如下：
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List<E> list; localCache.putObject(key, EXECUTION_PLACEHOLDER); try { //调用子类的实现 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; }其中最要害的代码就是调用了子类(SimpleExecutor类，CachingExecutor类没有实现该办法)的doQuery办法。源代码如下：
@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); stmt = prepareStatement(handler, ms.getStatementLog()); return handler.query(stmt, resultHandler); } finally { closeStatement(stmt); } }这个办法的次要逻辑是创立StatementHandler，而后调用它的query办法。关上configuration.newStatementHandler,咱们发现生成的是RoutingStatementHandler对象。源代码如下：
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql); statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler); return statementHandler; }所以SimpleExecutor类doQuery()办法最初一行执行的query办法是执行的咱们发现生成的是RoutingStatementHandler看的query办法。它的源代码如下：
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { return delegate.query(statement, resultHandler); }delegate代理的是哪个StatementHandler对象呢?这个对象在RoutingStatementHandler构造方法中赋值的。
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { switch (ms.getStatementType()) { case STATEMENT: delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql); break; case PREPARED: delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql); break; case CALLABLE: delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql); break; default: throw new ExecutorException("Unknown statement type: " + ms.getStatementType()); } }ms.getStatementType()在本例中应用的是默认值。MappedStatement的外部类Builder源代码中。
mappedStatement.statementType = StatementType.PREPARED;所以delegate为PreparedStatementHandler对象，最终调用的也是PreparedStatementHandler下的query办法，源代码如下：
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { PreparedStatement ps = (PreparedStatement) statement; ps.execute(); return resultSetHandler.handleResultSets(ps); }逻辑比拟清晰，1、执行sql。2、解决返回后果。