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。
/**
* 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：
/**
* 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：
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）
原文地址：https://www.cnblogs.com/insaneXs/p/9070398.html
时间： 2024-11-08 09:30:55