Mybatis之Executor

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在MyBatis中，执行器(Executor)有着非常重要的作用，它是一个真正执行Java和数据库交互的东西。 在MyBatis配置文件中，可以通过如下方式设置默认执行器

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<settings>  
    <setting name="cacheEnabled" value="true" />  

    <!--- 配置默认执行器 -->
    <setting name="defaultExecutorType" value="SIMPLE" />  
</settings>

defaultExecutorType有3个取值范围，SIMPLE、REUSE、BATCH。
SIMPLE，简易执行器，是默认的执行器。
REUSE，重复使用预处理语句。
BATCH，可以重复执行语句和批量更新，是针对批量专用的执行器。

MyBatis中3种执行器之间的类图关系如下：
Executor class diagram

下面来看看Executor接口, 它主要提供了update、query方法及事物相关的方法接口，源码如下：

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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;

  <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;

  List<BatchResult> flushStatements() throws SQLException;

  void commit(boolean required) throws SQLException;

  void rollback(boolean required) throws SQLException;
  //创建缓存key
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);

  boolean isCached(MappedStatement ms, CacheKey key);

  void clearLocalCache();
  //延迟加载
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);

  Transaction getTransaction();

  void close(boolean forceRollback);

  boolean isClosed();

  void setExecutorWrapper(Executor executor);

}

几种执行器中都提供了query和update的方法，以及Transaction相关的方法。下面我们来看一下MyBatis是如何创建Executor的，主要代码如下：

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public class Configuration {
  ...
  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    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 {// 默认创建SimpleExecutor
      executor = new SimpleExecutor(this, transaction);
    }
    // 如果开启二级缓存则创建CachingExecutor执行器
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    // 生成动态代理对象
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }
  ...

MyBatis会根据配置的Executor类型来确定要创建三种执行器中的哪一种，在创建对象后，调用interceptorChain.pluginAll方法生成代理对象。

CachingExecutor

缓存执行器，query前先查找缓存，有的话直接从缓存取出数据，没有的话就使用delegate调用BaseExecutor的query从数据库取数据，并将查询结果存入缓存中。

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public class CachingExecutor implements Executor {

  private final Executor delegate;
  private final TransactionalCacheManager tcm = new TransactionalCacheManager();

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

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

  @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.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    // 调用BaseExecutor的query方法从数据库取数据
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

  private void flushCacheIfRequired(MappedStatement ms) {
    Cache cache = ms.getCache();
    if (cache != null && ms.isFlushCacheRequired()) {      
      tcm.clear(cache);
    }
  }

  ......

}

BaseExecutor

BaseExecutor是一个抽象类，其实现了接口Executor的部分方法。BaseExecutor使用了模板模式，定义了doUpdate、doFlushStatements和doQuery抽象方法，然后在其子类SimpleExecutor、ReuseExecutor和BatchExecutor中来实现这三个方法。

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public abstract class BaseExecutor implements Executor {

  private static final Log log = LogFactory.getLog(BaseExecutor.class);

  protected Transaction transaction;
  protected Executor wrapper;

  protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
  protected PerpetualCache localCache;
  protected PerpetualCache localOutputParameterCache;
  protected Configuration configuration;

  protected int queryStack;
  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;
  }

  @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.");
    }
    // 执行更新操作会立即清空缓存
    clearLocalCache();
    return doUpdate(ms, parameter);
  }

  @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()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      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();
      // 如果是statement级别的缓存，执行完成后清空缓存
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

  // 从数据库查询数据
  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);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    // 缓存查询结果
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      // 缓存存储过程的输出参数
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

  // 下面三个抽象方法在子类中实现
  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;
  ......
}

SimpleExecutor

SimpleExecutor是一个简单的执行类，通过StatementHandler来执行sql，执行完毕后调用closeStatement方法关闭Statement，源码及分析如下：

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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 {
      // 关闭Statement
      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);
      // 预编译
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      // 关闭Statement
      closeStatement(stmt);
    }
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    // 预编译SQL
    stmt = handler.prepare(connection, transaction.getTimeout());
    // 设置参数
    handler.parameterize(stmt);
    return stmt;
  }

  ......
}

ReuseExecutor

ReuseExecutor可重用执行器，其定义了一个Map<String, Statement>，将执行的sql作为key，将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。与SimpleExecutor相比，每次调用doUpdate、doQuery方法后不再使用closeStatement方法关闭Statement。源码如下：

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public class ReuseExecutor extends BaseExecutor {
  // 保存Statement的map
  private final Map<String, Statement> statementMap = new HashMap<String, Statement>();

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

  @Override
  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());
    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.<E>query(stmt, resultHandler);
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    BoundSql boundSql = handler.getBoundSql();
    String sql = boundSql.getSql();
    // 通过sql判断statementMap中是否存在Statement
    if (hasStatementFor(sql)) {
      stmt = getStatement(sql);
      applyTransactionTimeout(stmt);
    } else {
      Connection connection = getConnection(statementLog);
      // 预编译SQL
      stmt = handler.prepare(connection, transaction.getTimeout());
      // 将sql语句及对应的Statement 保存到statementMap中  
      putStatement(sql, stmt);
    }
    handler.parameterize(stmt);
    return stmt;
  }

  .....
}

BatchExecutor

BatchExecutor可以重复执行语句和批量更新，通过批量操作来提高性能。

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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>();
  private String currentSql;
  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();
    final String sql = boundSql.getSql();
    final Statement stmt;
    // 判断sql和MappedStatement是否和当前的值相等
    if (sql.equals(currentSql) && ms.equals(currentStatement)) {
      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 {// 如果不存在就创建一个批处理操作
      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.parameterize(stmt);
    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();
      // 创建StatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
      Connection connection = getConnection(ms.getStatementLog());
      // 预编译
      stmt = handler.prepare(connection, transaction.getTimeout());
      // 设置参数
      handler.parameterize(stmt);
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  ......
}