mybatis四大接口之 Executor

【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);
        }
 
    }
}