从数据库导出List＜Map＞数据动态生成sheet（数据量少可以参考看看）

有个问题要注意，数据库导出Map数据如果值为空，这个字段和值不会查询出来
因为我上面配置没成功----execl列名（我用的是sql中文别名截取，就不用创建实体类了，如果你上面没问题就忽略）
第一步POM文件

 <properties>
        <!-- 版本管理 -->
        <poi.version>3.17</poi.version>
    </properties>
    
 <dependency>
            <groupId>org.apache.poi</groupId>
            <artifactId>poi</artifactId>
            <version>${poi.version}</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.apache.poi/poi-ooxml -->
        <dependency>
            <groupId>org.apache.poi</groupId>
            <artifactId>poi-ooxml</artifactId>
            <version>${poi.version}</version>
        </dependency>

第2步JAVA文件（注释最好看一下）

public class ExportExcel
{
    public static List<String> keyList=new ArrayList<>();
    public static int num;
**//fileName execl地址
//title   execlsheet名
//list   你的列名（我的）
//dataset   你的Map数据
//index  目前的行数**
    public static Workbook exportExcel(String fileName, String title,List<String> list, 
    List<LinkedHashMap<String, Object>> dataset, int index)
    {
        Sheet sheet=null;
        Workbook workbook=null;
        File file = new File(fileName);
        if(index==0&&file.exists())file.delete();
        if(!file.exists()){
            if (fileName.endsWith("xlsx"))
            {
                workbook = new XSSFWorkbook();
            } else if (fileName.endsWith("xls"))
            {
                workbook = new HSSFWorkbook();
            } else
            {
                try
                {
                    throw new Exception("invalid file name, should be xls or xlsx");
                } catch (Exception e)
                {
                    e.printStackTrace();
                }

            }
            sheet=workbook.createSheet(title);
        }else {
            try(BufferedInputStream bufferedInputStream=new BufferedInputStream(new FileInputStream(fileName))
            ) {
                workbook=new XSSFWorkbook(bufferedInputStream);
            } catch (Exception e) {
                e.printStackTrace();
            }
           sheet=workbook.getSheetAt(0);
        }
        keyList=new ArrayList<>(list);
        **//列名转成数组**
        String[] headers=keyList.toArray(new String[keyList.size()]);
        // 列名
        Row row;
        if(index==0){
        //添加第一行
            row = sheet.createRow(0);
            for (int i = 0; i < headers.length; i++)
            {
                Cell cell = row.createCell(i);
                sheet.setColumnWidth(i, 5000);
                cell.setCellValue(headers[i]);
            }
        }else {
            index= sheet.getLastRowNum();
        }
        Iterator<LinkedHashMap<String,Object>> it = dataset.iterator();

        while (it.hasNext())
        {
            index++;
            row = sheet.createRow(index);
            Map map = it.next();
            Set<String> mapKey = (Set<String>)map.keySet();
            Iterator<String> iterator = mapKey.iterator();
            num  = 0;
            while(iterator.hasNext()){
                String key = iterator.next();
                if(key.equals("ROW_ID")) {
                    continue;
                }
                if(num==5) {
                    System.out.println("aaa");
                }
                Cell cell = row.createCell(num);
                **//空的值填空**
                Cell cell1 = addNum(cell, key, row);
                Object obj = map.get(key);
                if(cell1!=null){
                    cell1 = row.createCell(num);
                    add(cell1,obj);
                    num++;
                    continue;
                }
                add(cell,obj);
                num++;
            }
        }

        try(FileOutputStream fos=new FileOutputStream(fileName);
            BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(fos);)
        {
            workbook.write(bufferedOutputStream);

        } catch (IOException e)
        {
            e.printStackTrace();

        }
        return workbook;
    }
    public static Cell addNum(Cell cell,String key,Row row){
        while (true){
            if(num!=keyList.indexOf(key)){
                cell.setCellValue("");
                num++;
                Cell cell1 = row.createCell(num);
                addNum(cell1,key,row);
                return cell1;
            }else {
                break;
            }
        }
        return null;
    }

    public static void add(Cell cell,Object obj){
    //对数字的处理（不然会导出文本格式）
        if(obj instanceof BigDecimal){
            cell.setCellValue(Integer.valueOf(String.valueOf(obj)));
        }
        else if (obj instanceof Double) {
            cell.setCellValue(Double.parseDouble(String.valueOf(obj)));
        } else {
             cell.setCellValue(String.valueOf(obj));
        }
    }
}