Executor分成兩大類，一類是CacheExecutor，另一類是普通Executor。

普通類又分為：

ExecutorType.SIMPLE: 這個執行器類型不做特殊的事情。它為每個語句的執行創建一個新的預處理語句。（默認）
ExecutorType.REUSE: 這個執行器類型會復用預處理語句。
ExecutorType.BATCH: 這個執行器會批量執行所有更新語句,如果 SELECT 在它們中間執行還會標定它們是 必須的,來保證一個簡單并易于理解的行為。

分別對應SimpleExecutor，ReuseExecutor，BatchExecutor，他們都繼承于BaseExecutor，BatchExecutor專門用于執行批量sql操作，ReuseExecutor會重用statement執行sql操作，SimpleExecutor只是簡單執行sql沒有什么特別的。

作用范圍：Executor的這些特點，都嚴格限制在SqlSession生命周期范圍內。

Mybatis中如何指定使用哪一種Executor執行器？

在Mybatis配置文件中，可以指定默認的ExecutorType執行器類型，也可以手動給DefaultSqlSessionFactory的創建SqlSession的方法傳遞ExecutorType類型參數。

CacheExecutor有一個重要屬性delegate，它保存的是某類普通的Executor，值在構照時傳入。執行數據庫update操作時，它直接調用delegate的update方法，執行query方法時先嘗試從cache中取值，取不到再調用delegate的查詢方法，并將查詢結果存入cache中。代碼如下：

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);
    }
  }
    //是否關閉了executor 
  public boolean isClosed() {
    return delegate.isClosed();
  }

  public int update(MappedStatement ms, Object parameterObject) throws SQLException {
    flushCacheIfRequired(ms);//是否需要更緩存 
    return delegate.update(ms, parameterObject);//更新數據 
  }

  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);//獲取記錄
  }

  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();//獲取緩存
    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);//從緩存中獲取數據         //如果為空，則查詢數據庫，并把結果保存到緩存中
        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通過類名可以看出，它是一個簡單的執行類，并不會做一些處理就執行sql。（每執行一次update或select，就開啟一個Statement對象，用完立刻關閉Statement對象）源碼及分析如下：

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();//獲得配置      //獲得statementHandler里面有statement，來處理 
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);//最終是一個statement進行處理 
    } 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);//將Statement轉為PrepareStatement 
    return stmt;
  }

}

　　BatchExecutor :通過批量操作來提高性能。（執行update（沒有select，JDBC批處理不支持select），將所有sql都添加到批處理中（addBatch()），等待統一執行（executeBatch()），它緩存了多個Statement對象，每個Statement對象都是addBatch()完畢后，等待逐一執行executeBatch()批處理。與JDBC批處理相同。）

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();//獲得配置信息
    final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
    final BoundSql boundSql = handler.getBoundSql();
    final String sql = boundSql.getSql();//獲得Sql語句
    final Statement stmt;    //如果sql語句等于當前sql MappedStatement 等于當前Map碰到Statement  
    if (sql.equals(currentSql) && ms.equals(currentStatement)) {
      int last = statementList.size() - 1;
      stmt = statementList.get(last);//獲得最后一個      //有相同的MappedStatement和參數
      BatchResult batchResult = batchResultList.get(last);
      batchResult.addParameterObject(parameterObject);
    } else {        //如果不存在就創建一個批處理操作 
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection);
      currentSql = sql;
      currentStatement = ms;
      statementList.add(stmt);//添加批量處理操作
      batchResultList.add(new BatchResult(ms, sql, parameterObject));
    }
    handler.parameterize(stmt);
    handler.batch(stmt);//最終是調用jdbc的批處理操作 
    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，記錄執行次數
  public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
    try {
      List<BatchResult> results = new ArrayList<BatchResult>();
      if (isRollback) {
        return Collections.emptyList();
      } else {        //如果進行了批量處理
        for (int i = 0, n = statementList.size(); i < n; i++) {
          Statement stmt = statementList.get(i);
          BatchResult batchResult = batchResultList.get(i);
          try {            //記錄批量處理執行操作的條數
            batchResult.setUpdateCounts(stmt.executeBatch());
            MappedStatement ms = batchResult.getMappedStatement();
            List<Object> parameterObjects = batchResult.getParameterObjects();//參數對象集合
            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);//記錄操作 
        }
        return results;
      }
    } finally {
      for (Statement stmt : statementList) {
        closeStatement(stmt);
      }
      currentSql = null;
      statementList.clear();
      batchResultList.clear();
    }
  }

}

　　ReuseExecutor: 重復使用執行，其定義了一個Map<String, Statement>，將執行的sql作為key，將執行的Statement作為value保存，這樣執行相同的sql時就可以使用已經存在的Statement，就不需要新創建了。（執行update或select，以sql作為key查找Statement對象，存在就使用，不存在就創建，用完后，不關閉Statement對象，而是放置于Map內，供下一次使用。簡言之，就是重復使用Statement對象。）源碼及分析如下：

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());//轉換為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);//關閉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();//獲得sql語句
    if (hasStatementFor(sql)) {  //查看是否存在Statement  
      stmt = getStatement(sql);//如果存在就獲取Statement  
    } else {
      Connection connection = getConnection(statementLog);
      stmt = handler.prepare(connection);//否則通過連接創建一個Statement 
      putStatement(sql, stmt);//將sql語句及對應的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語句對應的Statement  
      return statementMap.keySet().contains(sql) && !statementMap.get(sql).getConnection().isClosed();
    } catch (SQLException e) {
      return false;
    }
  }
  //獲得Sql語句對應的Statement 
  private Statement getStatement(String s) {
    return statementMap.get(s);
  }
  //將sql語句及對應的Statement保存到map中
  private void putStatement(String sql, Statement stmt) {
    statementMap.put(sql, stmt);
  }

}

　　https://www.cnblogs.com/zsg88/p/7566039.html

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