[Mybatis source code analysis] 08-SimpleExecutor's query analysis

The last article analyzed that if the secondary cache is not hit, the query method of an Executor (the default is SimpleExecutor) will be called. SimpleExecutor inherits BaseExecutor and directly calls the query method of BaseExecutor. BaseExecutor has a PerpetualCache instance localCache (first-level cache, because the life cycle is the same as executor) for caching query results.

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();
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      // issue #482
clearLocalCache();
}              
  }
  return list;
}

First, find the corresponding List<E> object according to the key from the local cache. If it does not exist, call the queryFromDatabase method to query the result from the database.

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);
  }
  localCache.putObject(key, list);
  if (ms.getStatementType() == StatementType.CALLABLE) {
    localOutputParameterCache.putObject(key, parameter);
  }
  return list;
}

The doQuery method is protected abstract and the concrete implementation is done by subclasses.

//SimpleExecutor

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);
  }
}

configuration创建了一个StatementHandler,通过这个对象获取Statement对象，最后调用query完成查询。

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
  statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
  return statementHandler;
}

public class RoutingStatementHandler implements StatementHandler {
  private final StatementHandler delegate;
  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }
  }

RoutingStatementHandler is to create a corresponding StatementHandler instance object according to the statementType of ms. Here we take PreparedStatementHandler as an example.

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;
}

调用PreparedStatementHandler的prepare方法(BaseStatementHandler)创建一个Statement对象。

public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
  ErrorContext.instance().sql(boundSql.getSql());
  Statement statement = null;
  try {
    statement = instantiateStatement(connection);
    setStatementTimeout(statement, transactionTimeout);
    setFetchSize(statement);
    return statement;
  } catch (SQLException e) {
    closeStatement(statement);
    throw e;
  } catch (Exception e) {
    closeStatement(statement);
    throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
  }
}

protected Statement instantiateStatement(Connection connection) throws SQLException {//PreparedStatementHandler
  String sql = boundSql.getSql();
  if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
    String[] keyColumnNames = mappedStatement.getKeyColumns();
    if (keyColumnNames == null) {
      return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
    } else {
      return connection.prepareStatement(sql, keyColumnNames);
    }
  } else if (mappedStatement.getResultSetType() != null) {
    return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
  } else {
    return connection.prepareStatement(sql);
  }
}

这个方法就是通过Jdbc的Connection对象创建出一个PrepareStatement对象。

protected void setStatementTimeout(Statement stmt, Integer transactionTimeout) throws SQLException {
  Integer queryTimeout = null;
  if (mappedStatement.getTimeout() != null) {
    queryTimeout = mappedStatement.getTimeout();
  } else if (configuration.getDefaultStatementTimeout() != null) {
    queryTimeout = configuration.getDefaultStatementTimeout();
  }
  if (queryTimeout != null) {
    stmt.setQueryTimeout(queryTimeout);
  }
  StatementUtil.applyTransactionTimeout(stmt, queryTimeout, transactionTimeout);
}

数据库查询超时参数优先级:如果在Mapper映射文件timeout参数>Enviroment的defaultStatementTimeout属性配置(没配置为null)

public static void applyTransactionTimeout(Statement statement, Integer queryTimeout, Integer transactionTimeout) throws SQLException {
  if (transactionTimeout == null){
    return;
  }
  Integer timeToLiveOfQuery = null;
  if (queryTimeout == null || queryTimeout == 0) {
    timeToLiveOfQuery = transactionTimeout;
  } else if (transactionTimeout < queryTimeout) {
    timeToLiveOfQuery = transactionTimeout;
  }
  if (timeToLiveOfQuery != null) {
    statement.setQueryTimeout(timeToLiveOfQuery);
  }
}

可以看到如果事物的超时时间小于查询超时时间，以事物超时时间为查询超时时间。我在这里配置的事物类型为jdbc，所有这里的transactionTimeout为JdbcTransaction的getTimeout方法返回为null。

PreparedStatementHandler的parameterize方法是设置sql的位置参数。使用了parameterHandler的setParameters方法。parameterHandler在StatementHandler构造函数中创建的。

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  this.configuration = mappedStatement.getConfiguration();
  this.executor = executor;
  this.mappedStatement = mappedStatement;
  this.rowBounds = rowBounds;
  this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
  this.objectFactory = configuration.getObjectFactory();
  if (boundSql == null) { // issue #435, get the key before calculating the statement
    generateKeys(parameterObject);
    boundSql = mappedStatement.getBoundSql(parameterObject);
  }
  this.boundSql = boundSql;
  this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
  this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
  ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
  parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
  return parameterHandler;
}

public ParameterHandler createParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
  return new DefaultParameterHandler(mappedStatement, parameterObject, boundSql);
}

可以看到默认使用的ParameterHandler为DefaultParameterHandler。

public void setParameters(PreparedStatement ps) {
  ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
  List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
  if (parameterMappings != null) {
    for (int i = 0; i < parameterMappings.size(); i++) {
      ParameterMapping parameterMapping = parameterMappings.get(i);
      if (parameterMapping.getMode() != ParameterMode.OUT) {
        Object value;
        String propertyName = parameterMapping.getProperty();
        if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
          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);
        }
        TypeHandler typeHandler = parameterMapping.getTypeHandler();
        JdbcType jdbcType = parameterMapping.getJdbcType();
        if (value == null && jdbcType == null) {
          jdbcType = configuration.getJdbcTypeForNull();
        }
        try {
          typeHandler.setParameter(ps, i + 1, value, jdbcType);
        } catch (TypeException e) {
          throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
        } catch (SQLException e) {
          throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
        }
      }
    }
  }
}

从boundSql获取parameterMappings遍历每个参数，从参数对通过参数名取出相应的值，调用typeHandler的setParameter方法对PreparedStatement设置对应的参数值。

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
  ps.execute();
  return resultSetHandler.<E> handleResultSets(ps);
}

ResultSetHandler的handleResultSets方法解析结果，下一篇文章介绍。