【参考文章】:Mybatis-Executor解析

1. Executor的继承结构

   

2. Executor(顶层接口)

  定义了执行器的一些基本操作;

public interface Executor {

  ResultHandler NO_RESULT_HANDLER = null;
// 更新
int update(MappedStatement ms, Object parameter) throws SQLException;
// 查询,先查缓存,再查数据库
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
// 查询
<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;
// 事务提交
void commit(boolean required) throws SQLException;
// 事务回滚
void rollback(boolean required) throws SQLException;
// 创建缓存的键对象
CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
// 缓存中是否有这个查询的结果
boolean isCached(MappedStatement ms, CacheKey key);
// 清空缓存
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); }

3. BaseExecutor

  BaseExecutor是一个抽象类,采用模板方法的设计模式。

  它实现了Executor接口,实现了执行器的基本功能。

  具体使用哪一个Executor则是可以在 mybatis 的 config.xml 中进行配置的。默认为SimpleExecutor;

  配置如下:

<settings>
<!--SIMPLE、REUSE、BATCH-->
<setting name="defaultExecutorType" value="SIMPLE"/>
</settings>

3.1 构造方法

  子类的构造方法会调用 BaseExecutor 的构造方法。

  默认都支持一级缓存;

public abstract class BaseExecutor implements Executor {

  protected BaseExecutor(Configuration configuration, Transaction transaction) {
this.transaction = transaction;
this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>();
// 一级缓存
this.localCache = new PerpetualCache("LocalCache");
this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
this.closed = false;
this.configuration = configuration;
this.wrapper = this;
}
}

3.2 update

  insert,update,delete操作都会调用此方法;

  调用此方法时会清空一级缓存;

   @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.");
}
// 数据变更操作会清空一级缓存
clearLocalCache();
return doUpdate(ms, parameter);
}

3.3 query

  查询操作会先在缓存中查询,缓存命中失败后再去数据中查询

  @Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameter);
// 创建一级缓存的键对象
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
// 调用下面的 query 方法
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++;
// 先在缓存中查询,缓存命中失败再去数据库查询
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;
}

3.4 createCacheKey

  一级缓存通过 HashMap 实现,它的键对象根据SQL的ID,参数,SQL本身,分页参数以及JDBC的参数信息构成。

  @Override
// 创建CacheKey对象
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
CacheKey cacheKey = new CacheKey();
// MappedStatement的id
cacheKey.update(ms.getId());
// 分页参数的offset
cacheKey.update(rowBounds.getOffset());
// 分页参数的limit
cacheKey.update(rowBounds.getLimit());
// SQL语句本身
cacheKey.update(boundSql.getSql());
// 传递给jdbc的参数
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;
}

3.5定义的抽象方法

  // 定义的四个抽象方法,在去掉 do 前缀的相应方法中被调用
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;

4. SimpleExecutor

  最简单的执行器,根据对应的sql直接执行即可,不会做一些额外的操作;

  拼接完SQL之后,直接交给 StatementHandler  去执行。

/**
* 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;
} }

5. BatchExecutor

  通过批量操作来优化性能。通常需要注意的是批量更新操作,由于内部有缓存的实现,使用完成后记得调用flushStatements来清除缓存。

public class BatchExecutor extends BaseExecutor {

  public static final int BATCH_UPDATE_RETURN_VALUE = Integer.MIN_VALUE + 1002;
private final List<Statement> statementList = new ArrayList<Statement>();
private final List<BatchResult> batchResultList = new ArrayList<BatchResult>();
// 上一次的SQL语句
private String currentSql;
// 上一次的MappedStatement 对象
private MappedStatement currentStatement; // 因为调用父类的构造方法,所以 BatchExecutor 自己的私有属性 currentSql和currentStatement 开始都为null
public BatchExecutor(Configuration configuration, Transaction transaction) {
super(configuration, transaction);
} @Override
public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
final Configuration configuration = ms.getConfiguration();
final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
final BoundSql boundSql = handler.getBoundSql();
final String sql = boundSql.getSql();
final Statement stmt;
// 第一次肯定是false,进入else分支,currentSql和currentStatement被初始化,后面进入false分支则进行更新
if (sql.equals(currentSql) && ms.equals(currentStatement)) {
// 取上一次的 Statement 对象
int last = statementList.size() - 1;
stmt = statementList.get(last);
applyTransactionTimeout(stmt);
handler.parameterize(stmt);//fix Issues 322
BatchResult batchResult = batchResultList.get(last);
batchResult.addParameterObject(parameterObject);
} else {
Connection connection = getConnection(ms.getStatementLog());
stmt = handler.prepare(connection, transaction.getTimeout());
handler.parameterize(stmt); //fix Issues 322
// currentSql和currentStatement 更新为此次的对象
currentSql = sql;
currentStatement = ms;
statementList.add(stmt);
batchResultList.add(new BatchResult(ms, sql, parameterObject));
}
// handler.parameterize(stmt);
handler.batch(stmt);
return BATCH_UPDATE_RETURN_VALUE;
} }

6. ReuseExecutor

  可重用的执行器,重用的对象是Statement,也就是说该执行器会缓存同一个sql的Statement,省去Statement的重新创建,优化性能。
  内部的实现是通过一个HashMap来维护Statement对象的。由于当前Map只在该session中有效,所以使用完成后记得调用flushStatements来清除Map。
 
  调用实现的四个抽象方法时会调用 prepareStatement() 
public class ReuseExecutor extends BaseExecutor {

  private final Map<String, Statement> statementMap = new HashMap<String, Statement>();

  // 调用父类构造器
public ReuseExecutor(Configuration configuration, Transaction transaction) {
super(configuration, transaction);
} private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
BoundSql boundSql = handler.getBoundSql();
String sql = boundSql.getSql();
if (hasStatementFor(sql)) {
// 如果缓存了该SQL,则返回其Statement对象
stmt = getStatement(sql);
applyTransactionTimeout(stmt);
} else {
// 如果没有缓存该SQL,则创建SQL的Statement,并加入缓存
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
putStatement(sql, stmt);
}
handler.parameterize(stmt);
return stmt;
} // 是否缓存了这个 sql
private boolean hasStatementFor(String sql) {
try {
return statementMap.keySet().contains(sql) && !statementMap.get(sql).getConnection().isClosed();
} catch (SQLException e) {
return false;
}
} // 返回指定sql的 Statement
private Statement getStatement(String s) {
return statementMap.get(s);
} // 添加SQL和Statement
private void putStatement(String sql, Statement stmt) {
statementMap.put(sql, stmt);
} }

7. CachingExecutor

  启用于二级缓存时的执行器;

  采用静态代理;代理一个 Executor 对象。
 
  执行 update 方法前判断是否清空二级缓存;
  执行 query 方法前先在二级缓存中查询,命中失败再通过被代理类查询。
  

public class CachingExecutor implements Executor {
// 持有的 Executor,最终的操作都由该对象实现
private final Executor delegate;
private final TransactionalCacheManager tcm = new TransactionalCacheManager(); public CachingExecutor(Executor delegate) {
this.delegate = delegate;
delegate.setExecutorWrapper(this);
} public int update(MappedStatement ms, Object parameterObject) throws SQLException {
this.flushCacheIfRequired(ms);
return this.delegate.update(ms, parameterObject);
} public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
this.flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
this.ensureNoOutParams(ms, boundSql);
List<E> list = (List)this.tcm.getObject(cache, key);
if (list == null) {
list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
this.tcm.putObject(cache, key, list);
} return list;
}
} return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
// 是否清空二级缓存
private void flushCacheIfRequired(MappedStatement ms) {
Cache cache = ms.getCache();
if (cache != null && ms.isFlushCacheRequired()) {
this.tcm.clear(cache);
} }
}

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