mybatis源码学习(二):SQL的执行过程
从上一篇文章中,我们了解到MapperMethod将SQL的执行交给了sqlsession处理。今天我们继续往下看处理的过程。
SqlSession接口除了提供获取Configuration,Mapper的方法之外,主要的作用就是提供增删该查的方法。
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/** * Copyright 2009-2016 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.ibatis.session; import java.io.Closeable; import java.sql.Connection; import java.util.List; import java.util.Map; import org.apache.ibatis.cursor.Cursor; import org.apache.ibatis.executor.BatchResult; /** * The primary Java interface for working with MyBatis. * Through this interface you can execute commands, get mappers and manage transactions. * * @author Clinton Begin */ public interface SqlSession extends Closeable { /** * Retrieve a single row mapped from the statement key * @param <T> the returned object type * @param statement * @return Mapped object */ <T> T selectOne(String statement); /** * Retrieve a single row mapped from the statement key and parameter. * @param <T> the returned object type * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @return Mapped object */ <T> T selectOne(String statement, Object parameter); /** * Retrieve a list of mapped objects from the statement key and parameter. * @param <E> the returned list element type * @param statement Unique identifier matching the statement to use. * @return List of mapped object */ <E> List<E> selectList(String statement); /** * Retrieve a list of mapped objects from the statement key and parameter. * @param <E> the returned list element type * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @return List of mapped object */ <E> List<E> selectList(String statement, Object parameter); /** * Retrieve a list of mapped objects from the statement key and parameter, * within the specified row bounds. * @param <E> the returned list element type * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @param rowBounds Bounds to limit object retrieval * @return List of mapped object */ <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds); /** * The selectMap is a special case in that it is designed to convert a list * of results into a Map based on one of the properties in the resulting * objects. * Eg. Return a of Map[Integer,Author] for selectMap("selectAuthors","id") * @param <K> the returned Map keys type * @param <V> the returned Map values type * @param statement Unique identifier matching the statement to use. * @param mapKey The property to use as key for each value in the list. * @return Map containing key pair data. */ <K, V> Map<K, V> selectMap(String statement, String mapKey); /** * The selectMap is a special case in that it is designed to convert a list * of results into a Map based on one of the properties in the resulting * objects. * @param <K> the returned Map keys type * @param <V> the returned Map values type * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @param mapKey The property to use as key for each value in the list. * @return Map containing key pair data. */ <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey); /** * The selectMap is a special case in that it is designed to convert a list * of results into a Map based on one of the properties in the resulting * objects. * @param <K> the returned Map keys type * @param <V> the returned Map values type * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @param mapKey The property to use as key for each value in the list. * @param rowBounds Bounds to limit object retrieval * @return Map containing key pair data. */ <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds); /** * A Cursor offers the same results as a List, except it fetches data lazily using an Iterator. * @param <T> the returned cursor element type. * @param statement Unique identifier matching the statement to use. * @return Cursor of mapped objects */ <T> Cursor<T> selectCursor(String statement); /** * A Cursor offers the same results as a List, except it fetches data lazily using an Iterator. * @param <T> the returned cursor element type. * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @return Cursor of mapped objects */ <T> Cursor<T> selectCursor(String statement, Object parameter); /** * A Cursor offers the same results as a List, except it fetches data lazily using an Iterator. * @param <T> the returned cursor element type. * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @param rowBounds Bounds to limit object retrieval * @return Cursor of mapped objects */ <T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds); /** * Retrieve a single row mapped from the statement key and parameter * using a {@code ResultHandler}. * @param statement Unique identifier matching the statement to use. * @param parameter A parameter object to pass to the statement. * @param handler ResultHandler that will handle each retrieved row */ void select(String statement, Object parameter, ResultHandler handler); /** * Retrieve a single row mapped from the statement * using a {@code ResultHandler}. * @param statement Unique identifier matching the statement to use. * @param handler ResultHandler that will handle each retrieved row */ void select(String statement, ResultHandler handler); /** * Retrieve a single row mapped from the statement key and parameter * using a {@code ResultHandler} and {@code RowBounds} * @param statement Unique identifier matching the statement to use. * @param rowBounds RowBound instance to limit the query results * @param handler ResultHandler that will handle each retrieved row */ void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler); /** * Execute an insert statement. * @param statement Unique identifier matching the statement to execute. * @return int The number of rows affected by the insert. */ int insert(String statement); /** * Execute an insert statement with the given parameter object. Any generated * autoincrement values or selectKey entries will modify the given parameter * object properties. Only the number of rows affected will be returned. * @param statement Unique identifier matching the statement to execute. * @param parameter A parameter object to pass to the statement. * @return int The number of rows affected by the insert. */ int insert(String statement, Object parameter); /** * Execute an update statement. The number of rows affected will be returned. * @param statement Unique identifier matching the statement to execute. * @return int The number of rows affected by the update. */ int update(String statement); /** * Execute an update statement. The number of rows affected will be returned. * @param statement Unique identifier matching the statement to execute. * @param parameter A parameter object to pass to the statement. * @return int The number of rows affected by the update. */ int update(String statement, Object parameter); /** * Execute a delete statement. The number of rows affected will be returned. * @param statement Unique identifier matching the statement to execute. * @return int The number of rows affected by the delete. */ int delete(String statement); /** * Execute a delete statement. The number of rows affected will be returned. * @param statement Unique identifier matching the statement to execute. * @param parameter A parameter object to pass to the statement. * @return int The number of rows affected by the delete. */ int delete(String statement, Object parameter); /** * Flushes batch statements and commits database connection. * Note that database connection will not be committed if no updates/deletes/inserts were called. * To force the commit call {@link SqlSession#commit(boolean)} */ void commit(); /** * Flushes batch statements and commits database connection. * @param force forces connection commit */ void commit(boolean force); /** * Discards pending batch statements and rolls database connection back. * Note that database connection will not be rolled back if no updates/deletes/inserts were called. * To force the rollback call {@link SqlSession#rollback(boolean)} */ void rollback(); /** * Discards pending batch statements and rolls database connection back. * Note that database connection will not be rolled back if no updates/deletes/inserts were called. * @param force forces connection rollback */ void rollback(boolean force); /** * Flushes batch statements. * @return BatchResult list of updated records * @since 3.0.6 */ List<BatchResult> flushStatements(); /** * Closes the session */ @Override void close(); /** * Clears local session cache */ void clearCache(); /** * Retrieves current configuration * @return Configuration */ Configuration getConfiguration(); /** * Retrieves a mapper. * @param <T> the mapper type * @param type Mapper interface class * @return a mapper bound to this SqlSession */ <T> T getMapper(Class<T> type); /** * Retrieves inner database connection * @return Connection */ Connection getConnection(); }
mybatis默认的sqlsession的实现是DefaultSqlSession,如果使用Spring进行事务管理的话则可能是其他的类型。这里我们就从最简单的实现了解。
增删改查的方法比较类似,就以select方法为例,作具体了解。
@Override public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) { try { MappedStatement ms = configuration.getMappedStatement(statement); executor.query(ms, wrapCollection(parameter), rowBounds, handler); } catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }
首先是从configuration中根据传入的statement获取MappedStatement对象,这里的statement就是 Mapper的namespace + SQL的id 构成的一个id。configuration会缓存这个id和对应的MappedStatement对象。
MappedStatement对象是Mybatis中重要的一个对象,用来表示一个XML中的一个SQL节点。我们会在之后再去具体了解及configuration解析XML的过程。
我们可以看到sqlsession是将查询的相关动作交给Executor。Executor(执行器)同样是Mybatis中一个重要的对象。
public interface Executor { ResultHandler NO_RESULT_HANDLER = null; int update(MappedStatement ms, Object parameter) throws SQLException; <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException; <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException; <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException; List<BatchResult> flushStatements() throws SQLException; void commit(boolean required) throws SQLException; void rollback(boolean required) throws SQLException; CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql); boolean isCached(MappedStatement ms, CacheKey key); void clearLocalCache(); void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType); Transaction getTransaction(); void close(boolean forceRollback); boolean isClosed(); void setExecutorWrapper(Executor executor); }
可以看到Executor的主要功能都是和操作数据库相关的,执行查询,关闭链接,获取事务等等。
BaseExecutor是一个抽象类,提供了Executor的基本实现,并将具体的查询过程作为继续保留为抽象方法。这应用了模版方法。贴出具体代码,注意看四个抽象方法:
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/** * Copyright 2009-2017 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.ibatis.executor; import static org.apache.ibatis.executor.ExecutionPlaceholder.EXECUTION_PLACEHOLDER; import java.sql.Connection; import java.sql.SQLException; import java.sql.Statement; import java.util.List; import java.util.concurrent.ConcurrentLinkedQueue; import org.apache.ibatis.cache.CacheKey; import org.apache.ibatis.cache.impl.PerpetualCache; import org.apache.ibatis.cursor.Cursor; import org.apache.ibatis.executor.statement.StatementUtil; import org.apache.ibatis.logging.Log; import org.apache.ibatis.logging.LogFactory; import org.apache.ibatis.logging.jdbc.ConnectionLogger; import org.apache.ibatis.mapping.BoundSql; import org.apache.ibatis.mapping.MappedStatement; import org.apache.ibatis.mapping.ParameterMapping; import org.apache.ibatis.mapping.ParameterMode; import org.apache.ibatis.mapping.StatementType; import org.apache.ibatis.reflection.MetaObject; import org.apache.ibatis.reflection.factory.ObjectFactory; import org.apache.ibatis.session.Configuration; import org.apache.ibatis.session.LocalCacheScope; import org.apache.ibatis.session.ResultHandler; import org.apache.ibatis.session.RowBounds; import org.apache.ibatis.transaction.Transaction; import org.apache.ibatis.type.TypeHandlerRegistry; /** * @author Clinton Begin */ public abstract class BaseExecutor implements Executor { private static final Log log = LogFactory.getLog(BaseExecutor.class); protected Transaction transaction; protected Executor wrapper; protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads; protected PerpetualCache localCache; protected PerpetualCache localOutputParameterCache; protected Configuration configuration; protected int queryStack; private boolean closed; protected BaseExecutor(Configuration configuration, Transaction transaction) { this.transaction = transaction; this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>(); this.localCache = new PerpetualCache("LocalCache"); this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache"); this.closed = false; this.configuration = configuration; this.wrapper = this; } @Override public Transaction getTransaction() { if (closed) { throw new ExecutorException("Executor was closed."); } return transaction; } @Override public void close(boolean forceRollback) { try { try { rollback(forceRollback); } finally { if (transaction != null) { transaction.close(); } } } catch (SQLException e) { // Ignore. There's nothing that can be done at this point. log.warn("Unexpected exception on closing transaction. Cause: " + e); } finally { transaction = null; deferredLoads = null; localCache = null; localOutputParameterCache = null; closed = true; } } @Override public boolean isClosed() { return closed; } @Override public int update(MappedStatement ms, Object parameter) throws SQLException { ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId()); if (closed) { throw new ExecutorException("Executor was closed."); } clearLocalCache(); return doUpdate(ms, parameter); } @Override public List<BatchResult> flushStatements() throws SQLException { return flushStatements(false); } public List<BatchResult> flushStatements(boolean isRollBack) throws SQLException { if (closed) { throw new ExecutorException("Executor was closed."); } return doFlushStatements(isRollBack); } @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; } @Override public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException { BoundSql boundSql = ms.getBoundSql(parameter); return doQueryCursor(ms, parameter, rowBounds, boundSql); } @Override public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) { if (closed) { throw new ExecutorException("Executor was closed."); } DeferredLoad deferredLoad = new DeferredLoad(resultObject, property, key, localCache, configuration, targetType); if (deferredLoad.canLoad()) { deferredLoad.load(); } else { deferredLoads.add(new DeferredLoad(resultObject, property, key, localCache, configuration, targetType)); } } @Override public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) { if (closed) { throw new ExecutorException("Executor was closed."); } CacheKey cacheKey = new CacheKey(); cacheKey.update(ms.getId()); cacheKey.update(rowBounds.getOffset()); cacheKey.update(rowBounds.getLimit()); cacheKey.update(boundSql.getSql()); List<ParameterMapping> parameterMappings = boundSql.getParameterMappings(); TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry(); // mimic DefaultParameterHandler logic for (ParameterMapping parameterMapping : parameterMappings) { if (parameterMapping.getMode() != ParameterMode.OUT) { Object value; String propertyName = parameterMapping.getProperty(); if (boundSql.hasAdditionalParameter(propertyName)) { value = boundSql.getAdditionalParameter(propertyName); } else if (parameterObject == null) { value = null; } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) { value = parameterObject; } else { MetaObject metaObject = configuration.newMetaObject(parameterObject); value = metaObject.getValue(propertyName); } cacheKey.update(value); } } if (configuration.getEnvironment() != null) { // issue #176 cacheKey.update(configuration.getEnvironment().getId()); } return cacheKey; } @Override public boolean isCached(MappedStatement ms, CacheKey key) { return localCache.getObject(key) != null; } @Override public void commit(boolean required) throws SQLException { if (closed) { throw new ExecutorException("Cannot commit, transaction is already closed"); } clearLocalCache(); flushStatements(); if (required) { transaction.commit(); } } @Override public void rollback(boolean required) throws SQLException { if (!closed) { try { clearLocalCache(); flushStatements(true); } finally { if (required) { transaction.rollback(); } } } } @Override public void clearLocalCache() { if (!closed) { localCache.clear(); localOutputParameterCache.clear(); } } protected abstract int doUpdate(MappedStatement ms, Object parameter) throws SQLException; protected abstract List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException; protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException; protected abstract <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException; protected void closeStatement(Statement statement) { if (statement != null) { try { if (!statement.isClosed()) { statement.close(); } } catch (SQLException e) { // ignore } } } /** * Apply a transaction timeout. * @param statement a current statement * @throws SQLException if a database access error occurs, this method is called on a closed <code>Statement</code> * @since 3.4.0 * @see StatementUtil#applyTransactionTimeout(Statement, Integer, Integer) */ protected void applyTransactionTimeout(Statement statement) throws SQLException { StatementUtil.applyTransactionTimeout(statement, statement.getQueryTimeout(), transaction.getTimeout()); } private void handleLocallyCachedOutputParameters(MappedStatement ms, CacheKey key, Object parameter, BoundSql boundSql) { if (ms.getStatementType() == StatementType.CALLABLE) { final Object cachedParameter = localOutputParameterCache.getObject(key); if (cachedParameter != null && parameter != null) { final MetaObject metaCachedParameter = configuration.newMetaObject(cachedParameter); final MetaObject metaParameter = configuration.newMetaObject(parameter); for (ParameterMapping parameterMapping : boundSql.getParameterMappings()) { if (parameterMapping.getMode() != ParameterMode.IN) { final String parameterName = parameterMapping.getProperty(); final Object cachedValue = metaCachedParameter.getValue(parameterName); metaParameter.setValue(parameterName, cachedValue); } } } } } 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; } protected Connection getConnection(Log statementLog) throws SQLException { Connection connection = transaction.getConnection(); if (statementLog.isDebugEnabled()) { return ConnectionLogger.newInstance(connection, statementLog, queryStack); } else { return connection; } } @Override public void setExecutorWrapper(Executor wrapper) { this.wrapper = wrapper; } private static class DeferredLoad { private final MetaObject resultObject; private final String property; private final Class<?> targetType; private final CacheKey key; private final PerpetualCache localCache; private final ObjectFactory objectFactory; private final ResultExtractor resultExtractor; // issue #781 public DeferredLoad(MetaObject resultObject, String property, CacheKey key, PerpetualCache localCache, Configuration configuration, Class<?> targetType) { this.resultObject = resultObject; this.property = property; this.key = key; this.localCache = localCache; this.objectFactory = configuration.getObjectFactory(); this.resultExtractor = new ResultExtractor(configuration, objectFactory); this.targetType = targetType; } public boolean canLoad() { return localCache.getObject(key) != null && localCache.getObject(key) != EXECUTION_PLACEHOLDER; } public void load() { @SuppressWarnings( "unchecked" ) // we suppose we get back a List List<Object> list = (List<Object>) localCache.getObject(key); Object value = resultExtractor.extractObjectFromList(list, targetType); resultObject.setValue(property, value); } } }
除了应用模版模式之外,Executor还应用了包装器模式,SimpleExecutor是执行器最基本的实现,其他Executor包装了Executor,将核心的方法委托给内部的executor执行,并在方法执行前,增加了对应特性的功能。比如mybatis的二级缓存就是通过CachingExecutor的实现的,它会在查询前先判断时候有缓存命中,如果没有则继续将查询委托给内部的executor,并在执行后将结果进行缓存并返回结果。
我们先来看下最简单的实现:
/** * Copyright 2009-2016 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.ibatis.executor; import org.apache.ibatis.cursor.Cursor; import org.apache.ibatis.executor.statement.StatementHandler; import org.apache.ibatis.logging.Log; import org.apache.ibatis.mapping.BoundSql; import org.apache.ibatis.mapping.MappedStatement; import org.apache.ibatis.session.Configuration; import org.apache.ibatis.session.ResultHandler; import org.apache.ibatis.session.RowBounds; import org.apache.ibatis.transaction.Transaction; import java.sql.Connection; import java.sql.SQLException; import java.sql.Statement; import java.util.Collections; import java.util.List; /** * @author Clinton Begin */ public class SimpleExecutor extends BaseExecutor { public SimpleExecutor(Configuration configuration, Transaction transaction) { super(configuration, transaction); } @Override public int doUpdate(MappedStatement ms, Object parameter) throws SQLException { Statement stmt = null; try { Configuration configuration = ms.getConfiguration(); StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); stmt = prepareStatement(handler, ms.getStatementLog()); return handler.update(stmt); } finally { closeStatement(stmt); } } @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); } } @Override protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException { Configuration configuration = ms.getConfiguration(); StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, null, boundSql); Statement stmt = prepareStatement(handler, ms.getStatementLog()); return handler.<E>queryCursor(stmt); } @Override public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException { return Collections.emptyList(); } private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Statement stmt; Connection connection = getConnection(statementLog); stmt = handler.prepare(connection, transaction.getTimeout()); handler.parameterize(stmt); return stmt; } }
我们以查询方法为例,来看一下doQuery方法。
这里用到了mybatis中另一个重要的对象,StatementHandler。顾名思义这个对象是用来处理Statement的,而Statement又是我们SQL执行的主角。
看一下StatementHandler接口:
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public interface StatementHandler { //提供了获取Statement方法 Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException; //设置参数的方法 void parameterize(Statement statement) throws SQLException; //批处理 void batch(Statement statement) throws SQLException; //更新操作 int update(Statement statement) throws SQLException; //查询操作 <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException; <E> Cursor<E> queryCursor(Statement statement) throws SQLException; //获取对应的BoundSql对象,boundsql对象表示一个sql语句,但是还未设置参数 BoundSql getBoundSql(); //获取参数处理对象 ParameterHandler getParameterHandler(); }
StatementHandler也有一个抽象类的实现,BaseStatementHandler,提供了大部分的实现,并留有几个抽象方法(同样是模板模式)。
StatementHandler之后再做更多了解,先回到doQuery中,用configuration对象的newStatementHandler对象创建出StatementHandler对象。
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; }
可以看到我们创建的是一个RoutingStatementHandler对象。它实现了StatementHandler接口,并根据MappedStatement的类型,确定不同的委托对象(又是一个代理模式的应用)。
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { //根据不同的类型,确定委托对象的类型 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()); } }
回到SimpleExecutor的doQuery中来,获得到StatementHandler之后,又通过prepareStatement方法获得Statement对象。该方法首先获取connection对象,然后调用statementHandler的prepare方法获取statement对象,并通过parameterize方法设置参数。
正如我们上文提到的BaseStatementHandler提供了StatementHandler大部分的实现,并提供了抽象方法让子类定义自己的过程。
instantiateStatement方法就是子类自己定义获取statement对象的过程。
SimpleStatementHandler直接通过connection的createStatement方法创建Statement对象,而PrepareStatementHandler需要预编译SQL,因此是通过connection的prepareStatement方法获得PrepareStatement对象,CallableStatementHandler需要对过程进行转储,因此是通过connection的prepareCall方法。
预编译的prepareStatement需要设置参数,因此还要通过parameterize设置参数。
最后Statemthandler会通过query方法执行查询。而这个方法就是通过statement.execute()方法执行SQL,获得ResultSet。最后再通过ResultSetHandler将结果集转换成需要的Java对象。
结果转换的过程会在下下篇文章中介绍,下篇文章先介绍MappedStatement的解析。
最后我们在梳理下SQL执行过程都通过了哪些对象:
MapperProxy(Mapper的代理对象)->MapperMethod(对应一个Mapper方法)->SqlSession->executor(执行器)->StatementHandler(Statement处理器)->Statement(最终执行SQL的对象)