select源碼分析及小結

知識 06-13

示例代碼

之前的文章說過，對於MyBatis來說insert、update、delete是一組的，因為對於MyBatis來說它們都是update；select是一組的，因為對於MyBatis來說它就是select。

本文研究一下select的實現流程，示例代碼為：

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

selectMailById方法的實現為：

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 }

我們知道MyBatis提供的select有selectList和selectOne兩個方法，但是本文只分析且只需要分析selectOne方法，原因後面說。

selectOne方法流程

先看一下SqlSession的selectOne方法流程，方法位於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 }

這裡就是為什麼我說selectOne與selectList兩個方法只需要分析selectList方法就可以了的原因，因為在MyBatis中所有selectOne操作最後都會轉換為selectList操作，無非就是操作完畢之後判斷一下結果集的個數，如果結果集個數超過一個就報錯。

接著看下第3行的selectList的代碼實現，方法同樣位於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行獲取MappedStatement就不說了，跟一下第4行Executor的query方法實現，這裡使用了一個裝飾器模式，給SimpleExecutor加上了緩存功能，代碼位於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行的代碼獲取BoundSql，BoundSql中的內容上文已經說過了，最後也會有總結。

第3行的代碼根據輸入參數構建緩存Key。

第4行的代碼執行查詢操作，看下代碼實現，代碼同樣位於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 }

這裡並沒有配置且引用Cache，因此不執行第4行的判斷，執行第17行的代碼，代碼位於SimpleExecutor的父類BaseExecutor中，源碼實現為：

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 }

這裡執行第16行的代碼，queryFromDatabase方法實現為：

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 }

代碼走到第5行，最終執行duQuery方法，方法的實現為：

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行的代碼都和前文update是一樣的，就不說了，handler有印象的朋友應該記得是PreparedStatementHandler，下一部分就分析一下和update的區別，PreparedStatementHandler的query方法是如何實現的。

PreparedStatementHandler的query方法實現

跟一下PreparedStatementHandler的query方法跟到底，其最終實現為：

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行執行查詢操作，第4行的代碼處理結果集，將結果集轉換為List，handleResultSets方法實現為：

1 public List