深入理解 Mybatis Executor

MyBatis 是一款优秀的持久层框架，它支持自定义 SQL、存储过程以及高级映射。MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。MyBatis 可以通过简单的 XML 或注解来配置和映射原始类型、接口和 Java POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。

本文介绍了 MyBatis 执行过程和 Executor 的执行体系相关内容。

Mybatis 执行过程

抛开通过 Mapper 接口生成动态代理，MyBatis 的一次执行过程可大致分为以下几个部分：

1. 获取门面会话
2. 执行器逻辑
3. JDBC 处理器逻辑

如图所示：

SqlSession 中提供了基本 API：增、删、改、查

其中 select() 方法有很多，他们的区别主要在参数和返回结果上。

返回结果上，有 void，游标 Cursor<T>，List<E>，Map<K, V>，一条数据 T。参数上，String 就是 StatementId，Mybatis 中所有操作（增删改查）都有一个 Id，通过 Id 找到 SQL 映射，Object 就是 SQL 中的参数，ResultHandler 对结果集进行处理，RowBound 用于设置返回的范围，用于进行分页。多个 select() 方法重载的设计用意是为了方便调用，这种方便调用进行的重载就是「门面模式」。

除了基本 API 之外，还有辅助 API，即提交和关闭会话。会话中可执行多个 SQL，一个会话只能被一个线程调用，线程使用完 SqlSession 后应该立马把它关闭掉，但一个线程可调用多个会话。因此，SqlSession 不能跨线程使用，也即它不是线程安全的。

在 MyBatis 中，DefaultSqlSession 继承了 SqlSession 接口，是 SqlSession 的默认实现，在该实现类中，包含了两个重要对象 Configuration 和 Executor。

public class DefaultSqlSession implements SqlSession {

  private final Configuration configuration;
  private final Executor executor;

  private final boolean autoCommit;
  private boolean dirty;
  private List<Cursor<?>> cursorList;

  public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }

  public DefaultSqlSession(Configuration configuration, Executor executor) {
    this(configuration, executor, false);
  }
  
  ...
  
}

SqlSession 只提供 SQL 会话，具体执行交给 Executor。Executor 也提供了基本功能，其中包括改、查、缓存维护和事务管理。还提供了辅助 API，包括提交、关闭执行器和批处理刷新。

Executor 本身也不会去执行 SQL，执行 SQL 交给了 StatementHandler，StatementHandler 本质上使用了 JDBC 中的 Statement 进行参数处理、执行 SQL 和处理结果。

其中 1 个 SqlSession 对应 1 个 Executor ， 1 个 Executor 对应 n 个 StatementHandler，即执行多少条 SQL 就有多少个 StatementHandler。它们都不能够跨线程调用。

Executor 执行器体系

Executor 的继承体系：

1. BaseExecutor：基础执行器，是一个抽象类，提供了关于 SimpleExecutor、ReuseExecutor、BatchExecutor 共性相关的功能实现，此外，BaseExecutor 还与 MyBatis 的一级缓存和事务管理相关
2. SimpleExecutor：简单执行器，每次都会创建一个新的 Statement（默认是 PreparedStatement）
3. ReuseExecutor：重用执行器，相同的 SQL 语句会使用同一个 Statement
4. BatchExecutor：批处理执行器，批处理提交修改，必须执行 flushStatement 才会生效
5. CachingExecutor：缓存执行器，与 MyBatis 二级缓存相关

BaseExecutor

BaseExecutor 作为 SimpleExecutor、ReuseExecutor、BatchExecutor 的父类，实现了子类中的一些共性功能。

主要方法：

/** 事务管理接口、定义了与JDBC执行相关的功能，如获取连接、提交、回滚、关闭连接等操作 */
protected Transaction transaction;
protected Executor wrapper;
/** 下方三个对象均是与一级缓存相关的属性, 作用域为 BaseExecutor 的生命周期范围 */
protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
protected PerpetualCache localCache;
protected PerpetualCache localOutputParameterCache;
protected Configuration configuration;
/** 与动态sql中的嵌套子查询相关 */
protected int queryStack;
private boolean closed;

protected BaseExecutor(Configuration configuration, Transaction transaction) {
    this.transaction = transaction;
    this.deferredLoads = new ConcurrentLinkedQueue<>();
    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;
}

/**
   * 关闭当前对象、关闭之前判断是否需要回滚
   */
@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()) {
        // 如果不是嵌套查询，并且配置了flushCache为true，清空一级缓存
        clearLocalCache();
    }
    List<E> list;
    try {
        queryStack++;
        // 如果resultHandler为空，尝试从一级缓存中获取数据
        list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        if (list != null) {
            // 与存储过程相关（CallableStatement）
            handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
        } else {
            // 一级缓存中没有对应数据，从数据库查询
            list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    } finally {
        queryStack--;
    }
    if (queryStack == 0) {
        // 延迟装载相关逻辑
        for (DeferredLoad deferredLoad : deferredLoads) {
            deferredLoad.load();
        }
        // issue #601
        deferredLoads.clear();
        if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
            // 如果全局配置的一级缓存作用域为Statement，清除本地缓存，因此一级缓存无法被关闭，只是减少范围至每一个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));
    }
}
/** 创建CacheKey逻辑，主要与mappedStatement的ID，参数，分页参数和sql有关 */
@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();
            }
        }
    }
}
/**
   * 清空本地缓存、在BaseExecutor中，update、query、commit、rollback方法中，均有调用回滚的逻辑
   */
@Override
public void clearLocalCache() {
    if (!closed) {
        localCache.clear();
        localOutputParameterCache.clear();
    }
}
/** 执行更新操作 */
protected abstract int doUpdate(MappedStatement ms, Object parameter) throws SQLException;
/** 执行Statement，填充结果、与批量执行相关 */
protected abstract List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException;
/** 执行查询操作 */
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;
/** 关闭Statement对象 */
protected void closeStatement(Statement statement) {
    if (statement != null) {
        try {
            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);
                }
            }
        }
    }
}
/** 查询数据库 */
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    // 插入key和占位符，为一级缓存做准备
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
        // 调用子类实现的方法查询
        list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
        localCache.removeObject(key);
    }
    // 查询结果保存到缓存中
    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);
    }
}

BaseExecutor 主要为了子类提供了一些共性方法的实现，如事务相关的提交和回滚，一级缓存和获取数据库连接等。同时，定义了 doQuery、doUpdate、doFlushStatement 等抽象方法，由对应的子类去实现具体逻辑。以 query 方法为例，在 queryFormDatabase 中最终调用 doQuery 执行查询。

SimpleExecutor

SimpleExecutor 是简单执行器，MyBatis 默认配置的执行器，如果没有配置指定执行器，使用的都是 SimpleExecutor。

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.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());
        Cursor<E> cursor = handler.queryCursor(stmt);
        stmt.closeOnCompletion();
        return cursor;
    }

    @Override
    public List<BatchResult> doFlushStatements(boolean isRollback) {
        // doFlushStatements 只是给 batch 用的，所以这里返回空
        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;
    }

}

以查询为例，当调用 query 方法时，经过 BaseExecutor 的处理后，调用 queryFormDataBase 时，其内部又调用了doQuery 方法。所以实际上，真正去执行查询的方法为对应执行器的 doQuery 实现。但是在 SimpleExecutor 的 doQuery 方法中，我们可以看到，在 executor 层其实只做了三件事。

1. 通过当前绑定执行的 MappedStatement 去获取一个对应的 StatementHandler 对象
2. 创建 Connection 连接并生成 Statement 对象，并进行参数处理，每次使用 SimpleExecutor 执行查询或更新时，都会创建一个 Statement 对象
3. 交由对应的 StatementHandler 去执行 Statement

ReuseExecutor

ReuseExecutor 名为重用执行器。重用执行器的作用是在一次会话中，重复使用同一个 Statement 对象以提升性能。以查询为例，每次执行查询时，都会去 Statement 缓存中查找，如果已经存在相同 SQL 的 Statement 对象，就会重复使用同一个 Statement 对象。

public class ReuseExecutor extends BaseExecutor {
    /** 可重用的执行器内部用了一个map，用来缓存SQL语句对应的Statement，即key为SQL */
    private final Map<String, Statement> statementMap = new HashMap<>();

    public ReuseExecutor(Configuration configuration, Transaction transaction) {
        super(configuration, transaction);
    }

    @Override
    public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        // 新建一个StatementHandler
        StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
        // 准备语句
        Statement stmt = prepareStatement(handler, ms.getStatementLog());
        return handler.update(stmt);
    }

    @Override
    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.query(stmt, resultHandler);
    }

    @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.queryCursor(stmt);
    }

    @Override
    public List<BatchResult> doFlushStatements(boolean isRollback) {
        for (Statement stmt : statementMap.values()) {
            closeStatement(stmt);
        }
        // flush的时候清除缓存
        statementMap.clear();
        return Collections.emptyList();
    }

    private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
        Statement stmt;
        // 得到绑定的SQL语句
        BoundSql boundSql = handler.getBoundSql();
        String sql = boundSql.getSql();
        // 如果缓存中已经有了Statement，直接获取
        if (hasStatementFor(sql)) {
            stmt = getStatement(sql);
            applyTransactionTimeout(stmt);
        } else {
            // 如果没有，prepare一个放入缓存
            Connection connection = getConnection(statementLog);
            stmt = handler.prepare(connection, transaction.getTimeout());
            putStatement(sql, stmt);
        }
        handler.parameterize(stmt);
        return stmt;
    }

    private boolean hasStatementFor(String sql) {
        try {
            Statement statement = statementMap.get(sql);
            return statement != null && !statement.getConnection().isClosed();
        } catch (SQLException e) {
            return false;
        }
    }

    private Statement getStatement(String s) {
        return statementMap.get(s);
    }

    private void putStatement(String sql, Statement stmt) {
        statementMap.put(sql, stmt);
    }

}

ReuseExecutor 实现 Statement 对象的复用逻辑很简单，在类内部维护一个 Map，使用对应的 SQL 作为 key 来缓存同个会话中的 Statement 对象，每次需要执行时，都先去本地的缓存 Map 中查找，如果存在则直接使用之前的 Statement 对象，如果不存在再去创建 Statement 对象。

BatchExecutor

BatchExecutor 名为批量处理器，用来批量执行 SQL，但是注意，批量处理只对更新（增删改）语句有效，对查询无效。在同一个会话内，增删改 SQL 会一次性处理好 SQL 和参数然后发送给数据库，从而减少与数据库的交互次数，减少 IO 消耗，提升性能。

public class BatchExecutor extends BaseExecutor {

    public static final int BATCH_UPDATE_RETURN_VALUE = Integer.MIN_VALUE + 1002;
    /** 批量执行的Statement对象 **/
    private final List<Statement> statementList = new ArrayList<>();
    /** 批量执行的结果对象 **/
    private final List<BatchResult> batchResultList = new ArrayList<>();
    /** 上一次处理的SQL语句 **/
    private String currentSql;
    /** 上一次处理的MappedStatement对象 **/
    private MappedStatement currentStatement;

    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();
        // 获取SQL语句
        final String sql = boundSql.getSql();
        final Statement stmt;
        // 判断当前要处理的sql语句是否等于上一次执行的sql，MappedStatement也是同理
        // 只有这两个对象都满足时，才能复用上一次的Statement对象
        if (sql.equals(currentSql) && ms.equals(currentStatement)) {
            int last = statementList.size() - 1;
            // 即使满足上述的两个条件，也只能从statement缓存list中获取最后一个对象，相同的sql还必须满足连贯顺序才能复用上一次的Statement对象
            stmt = statementList.get(last);
            applyTransactionTimeout(stmt);
            handler.parameterize(stmt);// fix Issues 322
            BatchResult batchResult = batchResultList.get(last);
            batchResult.addParameterObject(parameterObject);
        } else {
            // 创建和保存新的Statement对象和BatchResult对象
            // 并将当前sql和MappedStatement设置为该次执行的对应对象
            Connection connection = getConnection(ms.getStatementLog());
            stmt = handler.prepare(connection, transaction.getTimeout());
            handler.parameterize(stmt);    // fix Issues 322
            currentSql = sql;
            currentStatement = ms;
            statementList.add(stmt);
            batchResultList.add(new BatchResult(ms, sql, parameterObject));
        }
        handler.batch(stmt);
        return BATCH_UPDATE_RETURN_VALUE;
    }

    @Override
    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, transaction.getTimeout());
            handler.parameterize(stmt);
            return handler.query(stmt, resultHandler);
        } finally {
            closeStatement(stmt);
        }
    }

    @Override
    protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
        flushStatements();
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, null, boundSql);
        Connection connection = getConnection(ms.getStatementLog());
        Statement stmt = handler.prepare(connection, transaction.getTimeout());
        handler.parameterize(stmt);
        Cursor<E> cursor = handler.queryCursor(stmt);
        stmt.closeOnCompletion();
        return cursor;
    }

    @Override
    public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
        try {
            List<BatchResult> results = new ArrayList<>();
            if (isRollback) {
                return Collections.emptyList();
            }
            for (int i = 0, n = statementList.size(); i < n; i++) {
                Statement stmt = statementList.get(i);
                applyTransactionTimeout(stmt);
                BatchResult batchResult = batchResultList.get(i);
                try {
                    // 执行并记录批量执行的数量
                    batchResult.setUpdateCounts(stmt.executeBatch());
                    MappedStatement ms = batchResult.getMappedStatement();
                    List<Object> parameterObjects = batchResult.getParameterObjects();
                    KeyGenerator keyGenerator = ms.getKeyGenerator();
                    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);
                        }
                    }
                    // Close statement to close cursor #1109
                    closeStatement(stmt);
                } catch (BatchUpdateException e) {
                    StringBuilder message = new StringBuilder();
                    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);
            }
            return results;
        } finally {
            for (Statement stmt : statementList) {
                closeStatement(stmt);
            }
            currentSql = null;
            statementList.clear();
            batchResultList.clear();
        }
    }

}

值得注意的时，在使用 BatchExecutor 的时候，如果是相同的 SQL 语句，会进行合并使用同一个 Statement 对象，但是有一个前提是，相同的 SQL 语句必须是相同的 MappedStatement，并且 SQL 必须满足连贯的先后顺序。

CachingExecutor

CachingExecutor是缓存执行器，用于实现MyBatis的二级缓存，直接继承与Executor。

public class CachingExecutor implements Executor {

    /** 装饰者模式内部持有一个Executor */
    private final Executor delegate;
    /** 事务缓存管理器 **/
    private final TransactionalCacheManager tcm = new TransactionalCacheManager();

    public CachingExecutor(Executor delegate) {
        this.delegate = delegate;
        delegate.setExecutorWrapper(this);
    }

    @Override
    public Transaction getTransaction() {
        return delegate.getTransaction();
    }

    @Override
    public void close(boolean forceRollback) {
        try {
            // issues #499, #524 and #573
            if (forceRollback) {
                tcm.rollback();
            } else {
                tcm.commit();
            }
        } finally {
            delegate.close(forceRollback);
        }
    }

    @Override
    public boolean isClosed() {
        return delegate.isClosed();
    }

    @Override
    public int update(MappedStatement ms, Object parameterObject) throws SQLException {
        flushCacheIfRequired(ms);
        return delegate.update(ms, parameterObject);
    }

    @Override
    public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
        flushCacheIfRequired(ms);
        return delegate.queryCursor(ms, parameter, rowBounds);
    }

    @Override
    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);
    }

    @Override
    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) {
            flushCacheIfRequired(ms);
            if (ms.isUseCache() && resultHandler == null) {
                ensureNoOutParams(ms, boundSql);
                @SuppressWarnings("unchecked")
                List<E> list = (List<E>) tcm.getObject(cache, key);
                if (list == null) {
                    list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                    tcm.putObject(cache, key, list); // issue #578 and #116
                }
                return list;
            }
        }
        return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }

    @Override
    public List<BatchResult> flushStatements() throws SQLException {
        return delegate.flushStatements();
    }

    @Override
    public void commit(boolean required) throws SQLException {
        delegate.commit(required);
        tcm.commit();
    }

    @Override
    public void rollback(boolean required) throws SQLException {
        try {
            delegate.rollback(required);
        } finally {
            if (required) {
                tcm.rollback();
            }
        }
    }

    private void ensureNoOutParams(MappedStatement ms, 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.");
                }
            }
        }
    }

    @Override
    public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        return delegate.createCacheKey(ms, parameterObject, rowBounds, boundSql);
    }

    @Override
    public boolean isCached(MappedStatement ms, CacheKey key) {
        return delegate.isCached(ms, key);
    }

    @Override
    public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
        delegate.deferLoad(ms, resultObject, property, key, targetType);
    }

    @Override
    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");
    }

}

CachingExecutor 通过装饰者模式，在内部持有一个 Executor 对象，包装上缓存实现逻辑。同时，CachingExecutor 中还有一个事务缓存管理器来实现事务数据的隔离性。

总结

MyBatis 的 Executor 方法执行栈：

1. 当查询时，通过 SqlSession 的 selectList 方法进行开始执行（不管是执行 selectOne 方法还是 selectList 方法，最终调用的方法都是 selectList）。SqlSession 再调用内部持有的 Executor 的 query 方法执行。

   @Override
   public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
     try {
       // 根据statement id找到对应的MappedStatement
       MappedStatement ms = configuration.getMappedStatement(statement);
       // 转而用执行器来查询结果,注意这里传入的ResultHandler是null
       return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
     } catch (Exception e) {
       throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
     } finally {
       ErrorContext.instance().reset();
     }
   }

2. 在开启二级缓存的时候，SqlSession 内部的 Executor 对象实际上是 CachingExecutor 对象，在执行完相应的缓存逻辑后，调用 CachingExecutor 内部 Executor 对象的 query 方法。

3. 由于在 query 方法的实现只在 BaseExecutor 中，也就是说，当 CachingExecutor 调用 query 方法时，实际上是在调用 BaseExecutor 的 query 方法。

4. 最终执行查询时，会进入SimpleExecutor的doQuery方法，去完成一次真正的数据库查询

最终 Executor 的执行体系如下图：