获取SqlSession后，下一步就是执行sql.
User user=sqlSession.selectOne("last.soul.mapper.UserMapper.selectById",map);
DefaultSqlSession的主要功能就是实现增删改查功能，以及它们的重载方法。就查询来说，最后都会调用select方法，然后改装成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
* @return
*/
@Override
public List selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//sql相关消息的包装对象，比如：sql语句，返回类型，是否使用缓存等
MappedStatement ms = configuration.getMappedStatement(statement);
//执行sql
return executor.query(ms, wrapCollection(parameter)/*包装集合类型参数*/, rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
} 由上文得知executor是CachingExecutor,执行的是CachingExecutor的query方法。
public List 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 list = (List) 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方法中主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方法，即SimpleExecutor对象的query方法，然而你会发现该类中没有些方法，那它执行的就是他的父类BaseExecutor的query方法。query方法又会调用它的私有方法queryFromDatabase。
queryFromDatabase源代码如下：
private List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List 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;
} 其中最关键的代码就是调用了子类(SimpleExecutor类，CachingExecutor类没有实现该方法)的doQuery方法。源代码如下：
@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);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
} 这个方法的主要逻辑是创建StatementHandler，然后调用它的query方法。打开configuration.newStatementHandler,我们发现生成的是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()方法最后一行执行的query方法是执行的我们发现生成的是RoutingStatementHandler看的query方法。它的源代码如下：
public List 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()在本例中使用的是默认值。MappedStatement的内部类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方法，源代码如下：
public List query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.handleResultSets(ps);
} 逻辑比较清晰，1、执行sql。2、处理返回结果。