MyBatis框架的使用及源码分析(九) Executor

从<MyBatis框架的使用及源码分析(八) MapperMethod>文中我们知道执行Mapper的每一个接口方法，最后调用的是MapperMethod.execute方法。而当执行MapperMethod的execute方法的时候，根据当前MapperMethod对应的mapper配置会执行Session的insert, update, delete, select, selectList, selectMap, selectCursor, selectOne或flushStatements方法。

我们来看DefaultSqlSession一个具体的实现方法：

  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
      return result;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

我们看到，真正执行的代码主体在Executor对象里面。

DefaultSqlSession类的成员executor是在构造函数里面给他赋值的。所以我们又要回头去查看一下是在什么时候实例化了DefaultSqlSession类。

DefaultSqlSession在SqlSessionFactory的实现类DefaultSqlSessionFactory中被创建：

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    final Environment environment = configuration.getEnvironment();
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
  } catch (Exception e) {
    closeTransaction(tx); // may have fetched a connection so lets call close()
    throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}    

private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
  try {
    boolean autoCommit;
    try {
      autoCommit = connection.getAutoCommit();
    } catch (SQLException e) {
      // Failover to true, as most poor drivers
      // or databases won't support transactions
      autoCommit = true;
    }
    final Environment environment = configuration.getEnvironment();
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    final Transaction tx = transactionFactory.newTransaction(connection);
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}    

从源码中我们可以看到他是通过Configuration类的newExecutor方法来得到的，代码如下：

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {  
  //根据executorType来选择实现子类 
  executorType = executorType == null ? defaultExecutorType : executorType;
  executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
  Executor executor;
  if (ExecutorType.BATCH == executorType) {
    executor = new BatchExecutor(this, transaction);
  } else if (ExecutorType.REUSE == executorType) {
    executor = new ReuseExecutor(this, transaction);
  } else {
    executor = new SimpleExecutor(this, transaction);
  }
  if (cacheEnabled) {
    executor = new CachingExecutor(executor);
  }
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}  

Mybatis对外统一提供了一个操作接口类Executor，提供的接口方法有update、query、flushStatements、commit、rollback等接口函数，源码如下：

package org.apache.ibatis.executor;

import java.sql.SQLException;
import java.util.List;

import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
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
 */
public interface Executor {

  ResultHandler NO_RESULT_HANDLER = null;

  int update(MappedStatement ms, Object parameter) throws SQLException;

  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;

  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

  List<BatchResult> flushStatements() throws SQLException;
  //事物提交
  void commit(boolean required) throws SQLException;
  //事物回滚
  void rollback(boolean required) throws SQLException;
  //创建CacheKey
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
  //是否开启缓存
  boolean isCached(MappedStatement ms, CacheKey key);
  //清除本地缓存
  void clearLocalCache();
  //延迟加载
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
  //获取事物管理器
  Transaction getTransaction();
  //关闭连接
  void close(boolean forceRollback);
  //Executor是否关闭
  boolean isClosed();

  void setExecutorWrapper(Executor executor);

}

具体实现类有抽象类BaseExecutor、实现类CachingExecutor、实现类BatchExecutor、实现类ReuseExecutor和实现类SimpleExecutor。

具体选用哪个子类SimpleExecutor、ReuseExecutor和BatchExecutor实现，可以在Mybatis的配置文件中进行配，配置如下：

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

配置之后在Configuration类中的newExecutor（）函数会选择具体使用的子类。

抽象类BaseExecutor的实现源码及解析如下如下：

package org.apache.ibatis.executor;

import static org.apache.ibatis.executor.ExecutionPlaceholder.EXECUTION_PLACEHOLDER;

import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.List;
import java.util.concurrent.ConcurrentLinkedQueue;

import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.cache.impl.PerpetualCache;
import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;
import org.apache.ibatis.logging.jdbc.ConnectionLogger;
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.reflection.factory.ObjectFactory;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.LocalCacheScope;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;
import org.apache.ibatis.type.TypeHandlerRegistry;

/**
 * @author Clinton Begin
 */
public abstract class BaseExecutor implements Executor {

  private static final Log log = LogFactory.getLog(BaseExecutor.class);
  //事务
  protected Transaction transaction;
  //执行器的包装对象
  protected Executor wrapper;
  //线程安全队列
  protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
  //本地缓存
  protected PerpetualCache localCache;
  protected PerpetualCache localOutputParameterCache;
  //mybatis的配置信息
  protected Configuration configuration;
  //查询堆栈
  protected int queryStack = 0;
  private boolean closed;

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

  public Transaction getTransaction() {
    if (closed) throw new ExecutorException("Executor was closed.");
    return transaction;
  }

  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;
    }
  }

  public boolean isClosed() {
    return closed;
  }

   //SqlSession的update/insert/delete会调用此方法
  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.");
    //先清局部缓存，再更新，如何更新由子类实现，模板方法模式
    clearLocalCache();
    return doUpdate(ms, parameter);
  }

  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);
  }

  //SqlSession.selectList会调用此方法 
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);//得到绑定sql  
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);//创建缓存key
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);//查询  
 }

  @SuppressWarnings("unchecked")
  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.");
    //先清局部缓存，再查询，但仅仅查询堆栈为0才清，为了处理递归调用 
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;//加一，这样递归调用到上面的时候就不会再清局部缓存了 
      //根据cachekey从localCache去查 
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        //如果查到localCache缓存，处理localOutputParameterCache  
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        //从数据库查
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        //延迟加载队列中所有元素 
        deferredLoad.load();
      }
       //清空延迟加载队列
      deferredLoads.clear(); // issue #601
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        //如果是statement，清本地缓存  
        clearLocalCache(); // issue #482
      }
    }
    return list;
  }

  //延迟加载 
  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()) {//如果能加载则立即加载，否则加入到延迟加载队列中
        deferredLoad.load();
    } else {
        deferredLoads.add(new DeferredLoad(resultObject, property, key, localCache, configuration, targetType));
    }
  }

  //创建缓存key 
  public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
    if (closed) throw new ExecutorException("Executor was closed.");
    CacheKey cacheKey = new CacheKey();
    //MyBatis 对于其 Key 的生成采取规则为：[mappedStementId + offset + limit + SQL + queryParams + environment]生成一个哈希码
    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();
    for (int i = 0; i < parameterMappings.size(); i++) { // mimic DefaultParameterHandler logic
      ParameterMapping parameterMapping = parameterMappings.get(i);
      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);
      }
    }
    return cacheKey;
  }    

  public boolean isCached(MappedStatement ms, CacheKey key) {
    return localCache.getObject(key) != null;
  }

  public void commit(boolean required) throws SQLException {
    if (closed) throw new ExecutorException("Cannot commit, transaction is already closed");
    clearLocalCache();
    flushStatements();
    if (required) {
      transaction.commit();
    }
  }

  public void rollback(boolean required) throws SQLException {
    if (!closed) {
      try {
        clearLocalCache();
        flushStatements(true);
      } finally {
        if (required) {
          transaction.rollback();
        }
      }
    }
  }

  //清空本地缓存，一个map结构  
  public void clearLocalCache() {
    if (!closed) {
      localCache.clear();
      localOutputParameterCache.clear();
    }
  }

  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 void closeStatement(Statement statement) {
    if (statement != null) {
      try {
        statement.close();
      } catch (SQLException e) {
        // ignore
      }
    }
  }

  //处理存储过程的out参数
  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);
          }
        }
      }
    }
  }

  //从数据库中查 
  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);//向缓存中放入占位符  
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);//清除占位符
    }
    localCache.putObject(key, list);//加入缓存
    if (ms.getStatementType() == StatementType.CALLABLE) {//如果是存储过程，OUT参数也加入缓存
      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;
    }
  }

  public void setExecutorWrapper(Executor wrapper) {
    this.wrapper = wrapper;
  }
  
  //延迟加载  
  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;

    public DeferredLoad(MetaObject resultObject,
                        String property,
                        CacheKey key,
                        PerpetualCache localCache,
                        Configuration configuration,
                        Class<?> targetType) { // issue #781
      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;
    }
 
    //缓存中找到，且不为占位符，代表可以加载 
    public boolean canLoad() {
      return localCache.getObject(key) != null && localCache.getObject(key) != EXECUTION_PLACEHOLDER;
    }

    //加载
    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);
    }

  }

}