Mybatis的Cursor避免OOM异常的方法详解

/**
 * Cursor contract to handle fetching items lazily using an Iterator.
 * Cursors are a perfect fit to handle millions of items queries that would not normally fits in memory.
 * If you use collections in resultMaps then cursor SQL queries must be ordered (resultOrdered="true")
 * using the id columns of the resultMap.
 *
 * @author Guillaume Darmont / guillaume@dropinocean.com
 */

@Select("SELECT * FROM log")
Cursor<Log> selectAll();

public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {
    // 解析方法返回值
    Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
    if (resolvedReturnType instanceof Class<?>) {
        this.returnType = (Class<?>) resolvedReturnType;
    } else if (resolvedReturnType instanceof ParameterizedType) {
        this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();
    } else {
        this.returnType = method.getReturnType();
    }
    this.returnsVoid = void.class.equals(this.returnType);
    this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
    // 方法是否返回Cursor类型
    this.returnsCursor = Cursor.class.equals(this.returnType);
    this.returnsOptional = Optional.class.equals(this.returnType);
    this.mapKey = getMapKey(method);
    this.returnsMap = this.mapKey != null;
    this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
    this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
    this.paramNameResolver = new ParamNameResolver(configuration, method);
}

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
    // ---------- 其他查询----------------
        case SELECT:
            if (method.returnsVoid() && method.hasResultHandler()) {
                executeWithResultHandler(sqlSession, args);
                result = null;
            } else if (method.returnsMany()) {
                result = executeForMany(sqlSession, args);
            } else if (method.returnsMap()) {
                result = executeForMap(sqlSession, args);
            } else if (method.returnsCursor()) {
                // Cursor返回类型
                result = executeForCursor(sqlSession, args);
            } else {
                Object param = method.convertArgsToSqlCommandParam(args);
                result = sqlSession.selectOne(command.getName(), param);
                if (method.returnsOptional() && (result == null || !method.getReturnType().equals(result.getClass()))) {
                    result = Optional.ofNullable(result);
                }
            }
            break;
    // ---------- 其他查询----------------
    return result;
}

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;
}

public <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling cursor results").object(mappedStatement.getId());
    
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    if (resultMapCount != 1) {
        throw new ExecutorException("Cursor results cannot be mapped to multiple resultMaps");
    }
    
    ResultMap resultMap = resultMaps.get(0);
    return new DefaultCursor<>(this, resultMap, rsw, rowBounds);
}

  @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 <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);
    }
  }