import net.sf.jsqlparser.statement.select.SelectItemVisitor; //导入依赖的package包/类
@Override
public Result execute() {
    final MongoColumnsExpressionVisitor columnsExprVisitor = new MongoColumnsExpressionVisitor();
    final ListIterator<SelectItem> iterator = select.getSelectItems().listIterator();

    while (iterator.hasNext()) {
        iterator.next().accept(new SelectItemVisitor() {
            @Override
            public void visit(AllColumns allColumns) {
                selectAll = true;
            }

            @Override
            public void visit(AllTableColumns allTableColumns) {
            }

            @Override
            public void visit(SelectExpressionItem selectExpressionItem) {
                selectExpressionItem.getExpression().accept(columnsExprVisitor);
            }
        });
    }

    BasicDBObject columnsExpr = columnsExprVisitor.getExpression();
    if(!selectAll && !columnsExpr.containsField("_id")) {
        columnsExpr.append("_id", 0);
    }

    MongoWhereExpressionVisitor whereExprVisitor = new MongoWhereExpressionVisitor();
    Expression updateWhere = select.getWhere();
    if (updateWhere != null) {
        updateWhere.accept(whereExprVisitor);
    }

    DBCollection mongoCollection = MongoClient.getDatastore().getCollection(collection);
    DBCursor cursor = mongoCollection.find(whereExprVisitor.getExpression(), columnsExpr);

    List<DBObject> resultList = new ArrayList<DBObject>();
    try {
        while (cursor.hasNext()) {
            resultList.add(cursor.next());
        }
    } finally {
        cursor.close();
    }
    return new MongoResult<String>(JSON.serialize(resultList));
}