Executor分成两大类，一类是CacheExecutor，另一类是普通Executor。
普通类又分为：
ExecutorType.SIMPLE: 这个执行器类型不做特殊的事情。它为每个语句的执行创建一个新的预处理语句。（默认）
ExecutorType.REUSE: 这个执行器类型会复用预处理语句。
ExecutorType.BATCH: 这个执行器会批量执行所有更新语句,如果 SELECT 在它们中间执行还会标定它们是 必须的,来保证一个简单并易于理解的行为。
分别对应SimpleExecutor，ReuseExecutor，BatchExecutor，他们都继承于BaseExecutor，BatchExecutor专门用于执行批量sql操作，ReuseExecutor会重用statement执行sql操作，SimpleExecutor只是简单执行sql没有什么特别的。
作用范围：Executor的这些特点，都严格限制在SqlSession生命周期范围内。
Mybatis中如何指定使用哪一种Executor执行器？
在Mybatis配置文件中，可以指定默认的ExecutorType执行器类型，也可以手动给DefaultSqlSessionFactory的创建SqlSession的方法传递ExecutorType类型参数。
CacheExecutor有一个重要属性delegate，它保存的是某类普通的Executor，值在构照时传入。执行数据库update操作时，它直接调用delegate的update方法，执行query方法时先尝试从cache中取值，取不到再调用delegate的查询方法，并将查询结果存入cache中。代码如下：
package org.apache.ibatis.executor;import java.sql.SQLException;
import java.util.List;import org.apache.ibatis.cache.Cache;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.cache.TransactionalCacheManager;
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.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;/**
* @author Clinton Begin
* @author Eduardo Macarron
*/
public class CachingExecutor implements Executor { private Executor delegate;
private TransactionalCacheManager tcm = new TransactionalCacheManager(); public CachingExecutor(Executor delegate) {
this.delegate = delegate;
delegate.setExecutorWrapper(this);
} public Transaction getTransaction() {
return delegate.getTransaction();
} public void close(boolean forceRollback) {
try {
//issues #499, #524 and #573
if (forceRollback) {
tcm.rollback();
} else {
tcm.commit();
}
} finally {
delegate.close(forceRollback);
}
}
//是否关闭了executor
public boolean isClosed() {
return delegate.isClosed();
} public int update(MappedStatement ms, Object parameterObject) throws SQLException {
flushCacheIfRequired(ms);//是否需要更缓存
return delegate.update(ms, parameterObject);//更新数据
} public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);//获取记录
} public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();//获取缓存
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);//从缓存中获取数据 //如果为空，则查询数据库，并把结果保存到缓存中
if (list == null) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578. Query must be not synchronized to prevent deadlocks
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
} public List<BatchResult> flushStatements() throws SQLException {
return delegate.flushStatements();
} public void commit(boolean required) throws SQLException {
delegate.commit(required);
tcm.commit();
} public void rollback(boolean required) throws SQLException {
try {
delegate.rollback(required);
} finally {
if (required) {
tcm.rollback();
}
}
} private void ensureNoOutParams(MappedStatement ms, Object parameter, BoundSql boundSql) {
if (ms.getStatementType() == StatementType.CALLABLE) {
for (ParameterMapping parameterMapping : boundSql.getParameterMappings()) {
if (parameterMapping.getMode() != ParameterMode.IN) {
throw new ExecutorException("Caching stored procedures with OUT params is not supported. Please configure useCache=false in " + ms.getId() + " statement.");
}
}
}
} public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
return delegate.createCacheKey(ms, parameterObject, rowBounds, boundSql);
} public boolean isCached(MappedStatement ms, CacheKey key) {
return delegate.isCached(ms, key);
} public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
delegate.deferLoad(ms, resultObject, property, key, targetType);
} public void clearLocalCache() {
delegate.clearLocalCache();
} private void flushCacheIfRequired(MappedStatement ms) {
Cache cache = ms.getCache();
if (cache != null && ms.isFlushCacheRequired()) {
tcm.clear(cache);
}
} @Override
public void setExecutorWrapper(Executor executor) {
throw new UnsupportedOperationException("This method should not be called");
}}
SimpleExecutor通过类名可以看出，它是一个简单的执行类，并不会做一些处理就执行sql。（每执行一次update或select，就开启一个Statement对象，用完立刻关闭Statement对象）源码及分析如下：
package org.apache.ibatis.executor;import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Collections;
import java.util.List;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;/**
* @author Clinton Begin
*/
public class SimpleExecutor extends BaseExecutor { public SimpleExecutor(Configuration configuration, Transaction transaction) {
super(configuration, transaction);
} public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();//获得配置 //获得statementHandler里面有statement，来处理
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.update(stmt);//最终是一个statement进行处理
} finally {
closeStatement(stmt);
}
} 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);
}
} 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);
handler.parameterize(stmt);//将Statement转为PrepareStatement
return stmt;
}}
BatchExecutor : 通过批量操作来提高性能。（执行update（没有select，JDBC批处理不支持select），将所有sql都添加到批处理中（addBatch()），等待统一执行（executeBatch()），它缓存了多个Statement对象，每个Statement对象都是addBatch()完毕后，等待逐一执行executeBatch()批处理。与JDBC批处理相同。）
package org.apache.ibatis.executor;import java.sql.BatchUpdateException;
import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;import org.apache.ibatis.executor.keygen.Jdbc3KeyGenerator;
import org.apache.ibatis.executor.keygen.KeyGenerator;
import org.apache.ibatis.executor.keygen.NoKeyGenerator;
import org.apache.ibatis.executor.statement.StatementHandler;
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;/**
* @author Jeff Butler
*/
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>();//Statement list
private final List<BatchResult> batchResultList = new ArrayList<BatchResult>();//batch list
private String currentSql;
private MappedStatement currentStatement; public BatchExecutor(Configuration configuration, Transaction transaction) {
super(configuration, transaction);
} public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
final Configuration configuration = ms.getConfiguration();//获得配置信息
final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
final BoundSql boundSql = handler.getBoundSql();
final String sql = boundSql.getSql();//获得Sql语句
final Statement stmt; //如果sql语句等于当前sql MappedStatement 等于当前Map碰到Statement
if (sql.equals(currentSql) && ms.equals(currentStatement)) {
int last = statementList.size() - 1;
stmt = statementList.get(last);//获得最后一个 //有相同的MappedStatement和参数
BatchResult batchResult = batchResultList.get(last);
batchResult.addParameterObject(parameterObject);
} else { //如果不存在就创建一个批处理操作
Connection connection = getConnection(ms.getStatementLog());
stmt = handler.prepare(connection);
currentSql = sql;
currentStatement = ms;
statementList.add(stmt);//添加批量处理操作
batchResultList.add(new BatchResult(ms, sql, parameterObject));
}
handler.parameterize(stmt);
handler.batch(stmt);//最终是调用jdbc的批处理操作
return BATCH_UPDATE_RETURN_VALUE;
} public <E> List<E> doQuery(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
throws SQLException {
Statement stmt = null;
try {
flushStatements();
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
Connection connection = getConnection(ms.getStatementLog());
stmt = handler.prepare(connection);
handler.parameterize(stmt);
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
//刷新Statement，记录执行次数
public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
try {
List<BatchResult> results = new ArrayList<BatchResult>();
if (isRollback) {
return Collections.emptyList();
} else { //如果进行了批量处理
for (int i = 0, n = statementList.size(); i < n; i++) {
Statement stmt = statementList.get(i);
BatchResult batchResult = batchResultList.get(i);
try { //记录批量处理执行操作的条数
batchResult.setUpdateCounts(stmt.executeBatch());
MappedStatement ms = batchResult.getMappedStatement();
List<Object> parameterObjects = batchResult.getParameterObjects();//参数对象集合
KeyGenerator keyGenerator = ms.getKeyGenerator();//生成key
if (Jdbc3KeyGenerator.class.equals(keyGenerator.getClass())) {
Jdbc3KeyGenerator jdbc3KeyGenerator = (Jdbc3KeyGenerator) keyGenerator;
jdbc3KeyGenerator.processBatch(ms, stmt, parameterObjects);
} else if (!NoKeyGenerator.class.equals(keyGenerator.getClass())) { //issue #141
for (Object parameter : parameterObjects) {
keyGenerator.processAfter(this, ms, stmt, parameter);
}
}
} catch (BatchUpdateException e) {
StringBuffer message = new StringBuffer();
message.append(batchResult.getMappedStatement().getId())
.append(" (batch index #")
.append(i + 1)
.append(")")
.append(" failed.");
if (i > 0) {
message.append(" ")
.append(i)
.append(" prior sub executor(s) completed successfully, but will be rolled back.");
}
throw new BatchExecutorException(message.toString(), e, results, batchResult);
}
results.add(batchResult);//记录操作
}
return results;
}
} finally {
for (Statement stmt : statementList) {
closeStatement(stmt);
}
currentSql = null;
statementList.clear();
batchResultList.clear();
}
}}
ReuseExecutor: 重复使用执行，其定义了一个Map<String, Statement>，将执行的sql作为key，将执行的Statement作为value保存，这样执行相同的sql时就可以使用已经存在的Statement，就不需要新创建了。（执行update或select，以sql作为key查找Statement对象，存在就使用，不存在就创建，用完后，不关闭Statement对象，而是放置于Map内，供下一次使用。简言之，就是重复使用Statement对象。）源码及分析如下：
package org.apache.ibatis.executor;import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;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;/**
* @author Clinton Begin
*/
public class ReuseExecutor extends BaseExecutor { private final Map<String, Statement> statementMap = new HashMap<String, Statement>(); public ReuseExecutor(Configuration configuration, Transaction transaction) {
super(configuration, transaction);
} public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
Statement stmt = prepareStatement(handler, ms.getStatementLog());//转换为PrepareStatement
return handler.update(stmt);
} public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
Statement stmt = prepareStatement(handler, ms.getStatementLog());
return handler.<E>query(stmt, resultHandler);
} public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
for (Statement stmt : statementMap.values()) {
closeStatement(stmt);//关闭Statement
}
statementMap.clear();//清空sql
return Collections.emptyList();
} private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
BoundSql boundSql = handler.getBoundSql();
String sql = boundSql.getSql();//获得sql语句
if (hasStatementFor(sql)) { //查看是否存在Statement
stmt = getStatement(sql);//如果存在就获取Statement
} else {
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection);//否则通过连接创建一个Statement
putStatement(sql, stmt);//将sql语句及对应的Statement 保存到map中
}
handler.parameterize(stmt);
return stmt;
} private boolean hasStatementFor(String sql) {
try { //查看map中是否含有sql语句对应的Statement
return statementMap.keySet().contains(sql) && !statementMap.get(sql).getConnection().isClosed();
} catch (SQLException e) {
return false;
}
}
//获得Sql语句对应的Statement
private Statement getStatement(String s) {
return statementMap.get(s);
}
//将sql语句及对应的Statement保存到map中
private void putStatement(String sql, Statement stmt) {
statementMap.put(sql, stmt);
}}
