【MyBatis源码分析】select源码分析及小结

示例代码

之前的文章说过，对于MyBatis来说insert、update、delete是一组的，因为对于MyBatis来说它们都是update；select是一组的，因为对于MyBatis来说它就是select。

本文研究一下select的实现流程，示例代码为：

 1 public void testSelectOne() {
 2     System.out.println(mailDao.selectMailById(8));
 3 }

selectMailById方法的实现为：

1 public Mail selectMailById(long id) {
2     SqlSession ss = ssf.openSession();
3     try {
4         return ss.selectOne(NAME_SPACE + "selectMailById", id);
5     } finally {
6         ss.close();
7     }
8 }

我们知道MyBatis提供的select有selectList和selectOne两个方法，但是本文只分析且只需要分析selectOne方法，原因后面说。

 

selectOne方法流程

先看一下SqlSession的selectOne方法流程，方法位于DefaultSqlSession中：

 1 public  T selectOne(String statement, Object parameter) {
 2     // Popular vote was to return null on 0 results and throw exception on too many.
 3     List list = this.selectList(statement, parameter);
 4     if (list.size() == 1) {
 5       return list.get(0);
 6     } else if (list.size() > 1) {
 7       throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
 8     } else {
 9       return null;
10     }
11 }

这里就是为什么我说selectOne与selectList两个方法只需要分析selectList方法就可以了的原因，因为在MyBatis中所有selectOne操作最后都会转换为selectList操作，无非就是操作完毕之后判断一下结果集的个数，如果结果集个数超过一个就报错。

接着看下第3行的selectList的代码实现，方法同样位于DefaultSqlSession中：

 1 public  List selectList(String statement, Object parameter, RowBounds rowBounds) {
 2     try {
 3       MappedStatement ms = configuration.getMappedStatement(statement);
 4       return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
 5     } catch (Exception e) {
 6       throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
 7     } finally {
 8       ErrorContext.instance().reset();
 9     }
10 }

第3行获取MappedStatement就不说了，跟一下第4行Executor的query方法实现，这里使用了一个装饰器模式，给SimpleExecutor加上了缓存功能，代码位于CachingExecutor中：

1 public  List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
2     BoundSql boundSql = ms.getBoundSql(parameterObject);
3     CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
4     return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
5 }

第2行的代码获取BoundSql，BoundSql中的内容上文已经说过了，最后也会有总结。

第3行的代码根据输入参数构建缓存Key。

第4行的代码执行查询操作，看下代码实现，代码同样位于CachingExecutor中：

 1 public  List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
 2       throws SQLException {
 3     Cache cache = ms.getCache();
 4     if (cache != null) {
 5       flushCacheIfRequired(ms);
 6       if (ms.isUseCache() && resultHandler == null) {
 7         ensureNoOutParams(ms, parameterObject, boundSql);
 8         @SuppressWarnings("unchecked")
 9         List list = (List) tcm.getObject(cache, key);
10         if (list == null) {
11           list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
12           tcm.putObject(cache, key, list); // issue #578 and #116
13         }
14         return list;
15       }
16     }
17     return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
18 }

这里并没有配置且引用Cache，因此不执行第4行的判断，执行第17行的代码，代码位于SimpleExecutor的父类BaseExecutor中，源码实现为：

 1 public  List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
 2     ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
 3     if (closed) {
 4       throw new ExecutorException("Executor was closed.");
 5     }
 6     if (queryStack == 0 && ms.isFlushCacheRequired()) {
 7       clearLocalCache();
 8     }
 9     List list;
10     try {
11       queryStack++;
12       list = resultHandler == null ? (List) localCache.getObject(key) : null;
13       if (list != null) {
14         handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
15       } else {
16         list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
17       }
18     } finally {
19       queryStack--;
20     }
21     if (queryStack == 0) {
22       for (DeferredLoad deferredLoad : deferredLoads) {
23         deferredLoad.load();
24       }
25       // issue #601
26       deferredLoads.clear();
27       if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
28         // issue #482
29         clearLocalCache();
30       }
31     }
32     return list;
33 }

这里执行第16行的代码，queryFromDatabase方法实现为：

 1 private  List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
 2     List list;
 3     localCache.putObject(key, EXECUTION_PLACEHOLDER);
 4     try {
 5       list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
 6     } finally {
 7       localCache.removeObject(key);
 8     }
 9     localCache.putObject(key, list);
10     if (ms.getStatementType() == StatementType.CALLABLE) {
11       localOutputParameterCache.putObject(key, parameter);
12     }
13     return list;
14 }

代码走到第5行，最终执行duQuery方法，方法的实现为：

 1 public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
 2     Statement stmt = null;
 3     try {
 4       Configuration configuration = ms.getConfiguration();
 5       StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
 6       stmt = prepareStatement(handler, ms.getStatementLog());
 7       return handler.query(stmt, resultHandler);
 8     } finally {
 9       closeStatement(stmt);
10     }
11 }

看到第4行~第6行的代码都和前文update是一样的，就不说了，handler有印象的朋友应该记得是PreparedStatementHandler，下一部分就分析一下和update的区别，PreparedStatementHandler的query方法是如何实现的。

 

PreparedStatementHandler的query方法实现

跟一下PreparedStatementHandler的query方法跟到底，其最终实现为：

1 public  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
2     PreparedStatement ps = (PreparedStatement) statement;
3     ps.execute();
4     return resultSetHandler. handleResultSets(ps);
5 }

看到第3行执行查询操作，第4行的代码处理结果集，将结果集转换为List，handleResultSets方法实现为：

 1 public List