Mybatis中SQL语句执行过程详解

我会带着你远行 2022-06-16 12:28 274阅读 0赞

前面的十来篇文章我们对Mybatis中的配置和使用已经进行了比较详细的说明，想了解的朋友可以查看一下我专栏中的其他文章。

但是你对整个SQL语句操作的流程了解吗？如果你还不是很了解，那么可以继续往下看，如果你已经了解了，那么可以跳过啦![大笑][laugh.gif]（因为一大推的源码估计要看的你头晕啊！！！）

所有语句的执行都是通过SqlSession对象来操作的，SqlSession是由SqlSessionFactory类生成的。

首先根据配置文件来创建一个SqlSessionFactory，然后调用openSession来获取一个SqlSession。我们从时序图来看看可能会更加清晰：

![Center][]

（1）生成SqlSessionFactory对象（默认实现是DefaultSqlSessionFactory）的过程

public SqlSessionFactory build(Reader reader, String environment, Properties properties) { try { XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties); return build(parser.parse()); } catch (Exception e) { throw ExceptionFactory.wrapException("Error building SqlSession.", e); } finally { ErrorContext.instance().reset(); try { reader.close(); } catch (IOException e) { // Intentionally ignore. Prefer previous error. } } }

// Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互 private static SqlSessionFactory getSessionFactory() { SqlSessionFactory sessionFactory = null; String resource = "configuration.xml"; try { sessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource)); } catch (IOException e) { e.printStackTrace(); } return sessionFactory; }

（2）获取SqlSession对象

通过调用DefaultSqlSessionFactory的openSession()方法来获取SqlSession对象。

@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); tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); final Executor executor = configuration.newExecutor(tx, execType); return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }

通过代码可以看出，最终返回的是一个DefaultSqlSession实例对象。接下来就是根据这个DefaultSqlSession来获取对应的Mapper对象。

（3）获取MapperProxy对象

UserDao userMapper = sqlSession.getMapper(UserDao.class);

如上所示，通过SqlSession对象调用getMapper()方法来获取相应的Dao接口实现。我们通过代码跟踪一直往下看：

DefaultSqlSession类中：

@Override public <T> T getMapper(Class<T> type) { return configuration.<T>getMapper(type, this); }

Configuration类中：

public <T> T getMapper(Class<T> type, SqlSession sqlSession) { return mapperRegistry.getMapper(type, sqlSession); }

MapperRegistry类中：

@SuppressWarnings("unchecked") public <T> T getMapper(Class<T> type, SqlSession sqlSession) { final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MapperRegistry."); } try { return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } }

在MapperRegistry类中维护着一个Map，这个Map中存储着每个Mapper类型和其对应的代理对象工厂类，如下定义所示：

private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

在mybatis初始化的过程中就根据配置文件<mappers>元素的配置，将相关的映射文件给加载到了内存，同时保存到了这个knownMappers中。这里，在调用getMapper()的时候，就会从这个knownMappers中寻找该Dao接口，如果没有找到，就直接抛出异常，说明没有在配置文件中配置说明，如果获取到了，那么就拿出其对应的代理对象工厂类出来，并从工厂类中通过newInstance()方法来获取一个代理对象。

MapperProxyFactory类中：

@SuppressWarnings("unchecked") protected T newInstance(MapperProxy<T> mapperProxy) { return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); } public T newInstance(SqlSession sqlSession) { final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); }

从代码可以看出来，其实我们调用sqlSession.getMapper(UserDao.class)方法的时候,返回的是一个和UserDao接口对应的MapperProxy代理对象。如下定义所示，MapperProxy类是一个实现了InvocationHandler的代理类：

public class MapperProxy<T> implements InvocationHandler, Serializable

上面的代码，如果整理成时序图，如下所示：

![Center 1][]

拿到了Dao接口的代理对象后，我们应该就可以进行具体的增删改查了，我们继续往下看。

（4）Executor对象

当拿到了UserDao对象（其实是MapperProxy代理对象）后，我们调用Dao接口中定义的方法，如下所示：

User user = userMapper.findUserById(10);

这时候便调用了MapperProxy对象的invoke方法了；

@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { if (Object.class.equals(method.getDeclaringClass())) { try { return method.invoke(this, args); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } final MapperMethod mapperMethod = cachedMapperMethod(method); return mapperMethod.execute(sqlSession, args); }

MapperMethod类中：

public Object execute(SqlSession sqlSession, Object[] args) { Object result; switch (command.getType()) { case INSERT: { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.insert(command.getName(), param)); break; } case UPDATE: { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.update(command.getName(), param)); break; } case DELETE: { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.delete(command.getName(), param)); break; } case SELECT: if (method.returnsVoid() && method.hasResultHandler()) { executeWithResultHandler(sqlSession, args); result = null; } else if (method.returnsMany()) { result = executeForMany(sqlSession, args); } else if (method.returnsMap()) { result = executeForMap(sqlSession, args); } else if (method.returnsCursor()) { result = executeForCursor(sqlSession, args); } else { Object param = method.convertArgsToSqlCommandParam(args); result = sqlSession.selectOne(command.getName(), param); } break; case FLUSH: result = sqlSession.flushStatements(); break; default: throw new BindingException("Unknown execution method for: " + command.getName()); } if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) { throw new BindingException("Mapper method '" + command.getName() + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ")."); } return result; }

从上面这个方法实现上可以看出，已经根据执行方法(CRUD)进行了不同的处理，我们简单看一个方法executeForMany，代码如下所示：

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) { List<E> result; Object param = method.convertArgsToSqlCommandParam(args); if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); result = sqlSession.<E>selectList(command.getName(), param, rowBounds); } else { result = sqlSession.<E>selectList(command.getName(), param); } // issue #510 Collections & arrays support if (!method.getReturnType().isAssignableFrom(result.getClass())) { if (method.getReturnType().isArray()) { return convertToArray(result); } else { return convertToDeclaredCollection(sqlSession.getConfiguration(), result); } } return result; }

通过观察这些代码，发现最终的实现都是通过sqlSession对象来进行操作的。我们继续往里看，看看selectList方法：

@Override public <E> List<E> selectList(String statement) { return this.selectList(statement, null); } @Override public <E> List<E> selectList(String statement, Object parameter) { return this.selectList(statement, parameter, RowBounds.DEFAULT); } @Override public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { MappedStatement ms = configuration.getMappedStatement(statement); 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(); } }

可以看到，内部是把查询操作委托给了一个Executor对象（即executor.query()），Executor是一个接口，mybatis为其实现了一个抽象基类BaseExecutor，我们跟踪上面的代码中的query方法继续往里看：

BaseExecutor类中：

@Override public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { BoundSql boundSql = ms.getBoundSql(parameter); CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql); 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."); } if (queryStack == 0 && ms.isFlushCacheRequired()) { clearLocalCache(); } List<E> list; try { queryStack++; list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } if (queryStack == 0) { for (DeferredLoad deferredLoad : deferredLoads) { deferredLoad.load(); } // issue #601 deferredLoads.clear(); if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { // issue #482 clearLocalCache(); } } return list; }

在上面的方法中，我们看到当list==null的时候会调用queryFromDatabase()方法，这个方法如下：

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 { 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; }

然后会调用doQuery()方法，BaseExecutor中的doQuery方法定义成了抽象方法，由具体的继承类进行个性化的实现。这里，我们拿mybatis中默认使用的SimpleExecutor来看看：

SimpleExecutor类中：

@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.<E>query(stmt, resultHandler); } finally { closeStatement(stmt); } }

从这个方法可以看到，首先根据调用Configuration类的newStatementHandler方法来获取一个sql操作对象：

Configuration类中：

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; }

RoutingStatementHandler类中：

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()); } }

可以看到，这里根据配置来创建Statement、PreparedStatement或者CallableStatement三者之中的一个。然后调用相应的方法，如query（）方法。以SimpleStatementHandler为例，我们看看具体的sql操作：

@Override public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { String sql = boundSql.getSql(); statement.execute(sql); return resultSetHandler.<E>handleResultSets(statement); }

看到这里，我们终于看到了黎明的曙光，因为这里已经看到Jdbc中的数据库操作代码了，即statement.execute(sql)。在查询完之后使用resultSetHandler来进行查询结果集的处理。

上面的代码的整体流程图大概如下所示：

![Center 2][]

至此，一次完整的sql解析和处理过程便讲解完毕了，感兴趣的可以自己对着源码看看。

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![20170430151631454][]![20170430151912379][]

[laugh.gif]: /images/20220616/239d552fed324db29c7e5a46dca9e6f7.png
[Center]: /images/20220616/1f7f3d727a6c47f88101dccc7de1df7f.png
[Center 1]: /images/20220616/b8e253b88f5c4c009b023f29d2131cc1.png
[Center 2]: /images/20220616/af46f0d26cb34bf982e5105d26e2947f.png
[tongue.gif]: /images/20220616/f35861d3eaa34f1d90f388b5c59b673d.png
[20170430151631454]: https://img-blog.csdn.net/20170430151631454
[20170430151912379]: /images/20220616/4b325d0abe9140caada32fdf3e44c1f8.png