七、SXSSFWorkbook生成大excle，避免内存溢出

1.SXSSFWorkbook理解：
　　SXSSFWorkbook是用来生成海量excel数据文件，主要原理是借助临时存储空间生成excel，SXSSFWorkbook专门处理大数据，对于大型excel的创建且不会内存溢出的，就只有SXSSFWorkbook了。它的原理很简单，用硬盘空间换内存（就像hashmap用空间换时间一样）。 SXSSFWorkbook是streaming版本的XSSFWorkbook,它只会保存最新的excel rows在内存里供查看，在此之前的excel rows都会被写入到硬盘里（Windows电脑的话，是写入到C盘根目录下的temp文件夹）。被写入到硬盘里的rows是不可见的/不可访问的。只有还保存在内存里的才可以被访问到。
注:HSSFWorkbook和XSSFWorkbook的Excel Sheet导出条数上限(<=2003版)是65535行、256列,(>=2007版)是1048576行,16384列,如果数据量超过了此上限,那么可以使用SXSSFWorkbook来导出。实际上上万条数据，甚至上千条数据就可以考虑使用SXSSFWorkbook了。
注意：首先需要引入依赖：注意：4.0.0版本的JDK需要1.8以上，如果JDK是1.7的，那么就使用3.9版本的依赖

2.数据过多使用SXSSFWorkbook也是会出现内存溢出的问题，主要出现的两个地方：
a.从数据库读取数据到内存时溢出。
    优化角度1：取数据时用分页的方法分批取数据，然后写入sheet中。这样就可以避免取数据时内存溢出;
    java.lang.OutOfMemoryError:GC overhead limit exceeded
b.FileOutputStream os = new FileOutputStream(path); wb.write(os);
    优化角度2：    创建Workbook时设置工作簿保存在内存中数据的条数，这样一旦这个Workbook中数据量超过1000就会写入到磁盘中，减少内存的使用量来提高速度和避免溢出。
    Workbook wb = new SXSSFWorkbook(1000);

3.通过分页查询实现海量excle数据的导出，避免发生OOM。代码如下：

@RequestMapping("/exportPurchaseInfo.do")
@ResponseBody
public void exportPurchaseGoodsInfo(HttpServletRequest request, HttpServletResponse response,QueryParamDto reqDto) {
    try {
            int pageSize = 1000;//分页的大小,即每次分页查询多少条记录开关
            String[] headers = {"序号","商品编号","商品名称","品类","品牌","采购金额（元）","采购数量（件）"};
            SXSSFWorkbook workbook = ExportExcelByPageUtil.makeSXSSFWorkbook(purchseReportExportExcelByPageService,reqDto, headers, pageSize);
            ExportExcelByPageUtil.exportExcel(request, response, workbook, "商品销售报表.xlsx");

        } catch (Exception e) {
            logger.error("导出商品销售报表异常",e);
        }

}

ExportExcelByPageUtil.java

public static <T> SXSSFWorkbook makeSXSSFWorkbook(ReportExportExcelByPageService<T> exportExcelByPageTyService, T queryDataBo, String[] headers, int pageSize){
        int rowAccessWindowSize = 100;
        //这样表示SXSSFWorkbook只会保留100条数据在内存中，其它的数据都会写到磁盘里，这样的话占用的内存就会很少
        SXSSFWorkbook workbook = new SXSSFWorkbook(rowAccessWindowSize);
        SXSSFSheet sheet = (SXSSFSheet) workbook.createSheet("sheet1");
        Map<String,CellStyle> cellStyles=getCellStyle(workbook);
        CellStyle cs = cellStyles.get("cs");
        CellStyle cs2 = cellStyles.get("cs2");

        //设置列名
        Row row = sheet.createRow((short) 0);
        for (int i = 0; i < headers.length; i++) {
            Cell cell = row.createCell(i);
            cell.setCellValue(headers[i]);
            cell.setCellStyle(cs);
        }

        String pattern = "yyyy-MM-dd HH:mm:ss";
        String beginTime = DateUtil.formatDate(new Date(), pattern);
        int allCount = exportExcelByPageTyService.queryAllCount(queryDataBo);
        if(allCount <= 0){
            return workbook;
        }

        //处理数据量超过一个sheet页最大量时异常，支持的最大导出数据量为10000
        int maxExportCount = 10000;
        allCount = allCount > maxExportCount ? maxExportCount : allCount;
        //page：查询总数据量为 allCount条记录时，每次生成pageSize条记录  需要page 次进行导出
        int page = (int) Math.ceil((double) allCount / (double) pageSize);
        //101条记录 aliquotFlag=false
        boolean aliquotFlag = allCount % pageSize == 0;

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        for(int i = 0; i < page; i++){
            logger.error("第"+(i+1)+"次循环开始："+ DateUtil.formatDate(new Date(), pattern));
            int startIndex = i*pageSize;
            int maxCount = pageSize;
            //最后一次的最大记录数
            if(!aliquotFlag && i==(page-1)){
                maxCount = allCount - startIndex;
            }
            logger.error("导出分页：startIndex==>"+startIndex+", pageSize==>"+maxCount);
            List<String> ids = exportExcelByPageTyService.queryIdsInfo(queryDataBo, startIndex, maxCount);
            List<LinkedHashMap<String, Object>> dataList = exportExcelByPageTyService.queryDatas(ids);
            if(CollectionUtils.isNotEmpty(dataList)){
                Set<Map.Entry<String, Object>> set = null;
                Iterator<Map.Entry<String, Object>> iterator = null;
                int j = 0;
                for (int rowNum = 1, len = dataList.size() + 1; rowNum < len; rowNum++) {
                    Row row1 = sheet.createRow(startIndex+rowNum);
                    set = dataList.get(rowNum - 1).entrySet();
                    iterator = set.iterator();
                    j = 0;
                    while (iterator.hasNext()) {
                        Map.Entry<String, Object> entry = (Map.Entry<String, Object>) iterator.next();
                        String cellValue = "";
                        if(entry.getValue()==null){
                            cellValue = "";
                        }else if(entry.getValue() instanceof Timestamp){
                            cellValue = sdf.format((Timestamp) entry.getValue());
                        } else {
                            cellValue = String.valueOf(entry.getValue());
                        }
                        Cell cell = row1.createCell(j);
                        cell.setCellValue(cellValue);
                        cell.setCellStyle(cs2);
                        j++;
                    }
                    if (rowNum % rowAccessWindowSize == 0) {
                        try {
                            sheet.flushRows();
                        } catch (IOException e) {
                            logger.error("sheet从内存写入本地硬盘失败", e);
                        }
                    }
                }
            }
            dataList = null;
            logger.error("第"+(i+1)+"次循环结束："+formatDate(new Date(), pattern));
        }
        logger.error("导出分页开始："+beginTime);
        logger.error("导出分页结束："+ formatDate(new Date(), pattern));
        return workbook;
    }

public static boolean exportExcel(HttpServletRequest request, HttpServletResponse response,SXSSFWorkbook workbook, String fileName){
    OutputStream out = null;
    try {
        String userAgent = request.getHeader("USER-AGENT");
        fileName = dealChineseFileName(userAgent, fileName);
        out = response.getOutputStream();
        response.setContentType("application/x-download");
        response.setHeader("Pragma", "public");
        response.setHeader("Cache-Control", "max-age=30");
        response.setHeader("Content-disposition", "attachment;filename=" + fileName);
        workbook.write(out);
        out.flush();
    } catch (Exception e) {
        logger.error("导出Excel文件失败", e);
        return false;
    }finally {
        if (out != null) {
            try {
                out.close();
            } catch (IOException e) {
                logger.error("导出Excel文件关闭输出流失败", e);
            } finally {
                out = null;
            }
        }
        if(workbook!=null){
            try {
                workbook.dispose();
                workbook.close();
            } catch (IOException e) {
                logger.error("导出Excel文件关闭输出流失败", e);
            } finally {
                workbook = null;
            }
        }
    }
    return true;
}    

ReportExportExcelByPageService.java

public interface ReportExportExcelByPageService<T> {
    /** 查满足条件记录数 */
    @Method(description = "查满足条件count")
    public int queryAllCount(T queryDataBo);
    /** 分页查满足条件Ids  */
    @Method(idempotent = true, retryTimes = 3, timeout = 30000, description = "分页查满足条件Ids")
    public List<String> queryIdsInfo(T queryDataBo, int startIndex, int pageSize);
    /** 根据Ids查数据 */
    @Method(idempotent = true, retryTimes = 3, timeout = 30000, description = "根据Ids查数据")
    public List<LinkedHashMap<String, Object>> queryDatas(List<String> ids);
}

ReportExportExcelByPageServiceImpl.java

public class ReportExportExcelByPageServiceImpl implements ReportExportExcelByPageService<QueryParamDto> {
    protected Logger logger = LoggerFactory.getLogger(this.getClass());

    @Override
    public int queryAllCount(QueryParamDto queryDataBo) {
        //查询记录数业务代码
    }

    @Override
    public List<String> queryIdsInfo(QueryParamDto queryDataBo, int startIndex, int pageSize) {
        List<String> list = new ArrayList<String>();
        list.add(queryDataBo.getBeginTime());
        list.add(queryDataBo.getEndTime());
        list.add(queryDataBo.getUserCode());
        list.add(queryDataBo.getType());
        list.add(queryDataBo.getAllType());
        list.add(String.valueOf(startIndex));
        list.add(String.valueOf(pageSize));
        return list;
    }

    @Override
    public List<LinkedHashMap<String, Object>> queryDatas(final List<String> ids) {
        List<LinkedHashMap<String, Object>> result = new ArrayList<>();
        if (CollectionUtils.isEmpty(ids)) {
            return result;
        }
        QueryParamDto queryDataBo=new QueryParamDto();
        queryDataBo.setBeginTime( ids.get(0));
        queryDataBo.setEndTime(ids.get(1));
        queryDataBo.setUserCode(ids.get(2));
        queryDataBo.setType(ids.get(3));
        queryDataBo.setAllType(ids.get(4));
        queryDataBo.setStartIndex(Integer.parseInt(ids.get(5)));
        queryDataBo.setPageSize(Integer.parseInt(ids.get(6)));

        //根据分页的入参进行分页查询:返回结果为list
        List<QueryResultData>  commodityListInfo = ...;
        /*
         SELECT
                zp.commodity_code commodityCode,
                zp.commodity_name commodityName,
                zp.category_name categoryName,
                zp.brand brand,
                sum(zp.purchase_money) purchaseMoney,
                sum(zp.purchase_num) purchaseNum
         FROM
                tableName zp
         WHERE
            zp.purchase_day BETWEEN :beginTime AND :endTime
            and user_code =:userCode
            group by zp.commodity_code
            order by purchaseMoney desc
            <#if startIndex !=null && startIndex !="" && pageSize!=null && pageSize !="">
            LIMIT :startIndex ,:pageSize
            </#if>
        */

        //排名","商品编号","商品名称","品类","品牌","采购金额（元）","采购数量（件）
        if(org.apache.commons.collections.CollectionUtils.isNotEmpty(commodityListInfo)){
            for (int i = 0; i < commodityListInfo.size(); i++) {
                LinkedHashMap map = new LinkedHashMap();
                QueryResultData queryResultData = commodityListInfo.get(i);
                map.put("xuHao", (i+1) + "");
                map.put("commodityCode", queryResultData.getCommodityCode());
                map.put("commodityName", queryResultData.getCommodityName());
                map.put("categoryName", queryResultData.getCategoryName());
                map.put("brand", queryResultData.getBrand());
                map.put("purchaseMoney", queryResultData.getPurchaseMoney());
                 map.put("purchaseNum", queryResultData.getPurchaseNum());
                result.add(map);
            }
        }
            return result;
    }
}

DateUtil.java

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class DateUtil {
    public static final String YYYYMMDD = "yyyy-MM-dd";
    public static final String YYYYMMDDHHMMSS = "yyyy-MM-dd HH:mm:ss";
    public static final String YYYYMMDDHHmmss = "yyyy-MM-dd HH:mm:ss";

    private DateUtil() {

    }

    public static Date parseDate(String time, String pattern) {
        if (time == null) {
            return null;
        } else {
            SimpleDateFormat df = new SimpleDateFormat(pattern);

            try {
                return df.parse(time);
            } catch (ParseException var4) {
                return null;
            }
        }
    }

    public static String formatDate(Date time, String pattern) {
        if (time == null) {
            return "";
        } else {
            String result = null;
            DateFormat df = new SimpleDateFormat(pattern);
            result = df.format(time);
            return result;
        }
    }
}

getCellStyle方法

public static Map<String,CellStyle> getCellStyle(SXSSFWorkbook workbook){
        Map<String,CellStyle> cellStyles=new HashMap<String,CellStyle>();

        // 创建两种单元格格式
        CellStyle cs = workbook.createCellStyle();
        CellStyle cs2 = workbook.createCellStyle();

        // 创建两种字体
        Font f = workbook.createFont();
        Font f2 = workbook.createFont();

        // 创建第一种字体样式（用于列名）
        f.setFontHeightInPoints((short) 10);
        f.setColor(IndexedColors.BLACK.getIndex());
        f.setBoldweight(Font.BOLDWEIGHT_BOLD);

        // 创建第二种字体样式（用于值）
        f2.setFontHeightInPoints((short) 10);
        f2.setColor(IndexedColors.BLACK.getIndex());

        // 设置第一种单元格的样式（用于列名）
        cs.setFont(f);
        cs.setBorderLeft(CellStyle.BORDER_THIN);
        cs.setBorderRight(CellStyle.BORDER_THIN);
        cs.setBorderTop(CellStyle.BORDER_THIN);
        cs.setBorderBottom(CellStyle.BORDER_THIN);
        cs.setAlignment(CellStyle.ALIGN_CENTER);

        // 设置第二种单元格的样式（用于值）
        cs2.setFont(f2);
        cs2.setBorderLeft(CellStyle.BORDER_THIN);
        cs2.setBorderRight(CellStyle.BORDER_THIN);
        cs2.setBorderTop(CellStyle.BORDER_THIN);
        cs2.setBorderBottom(CellStyle.BORDER_THIN);
        cs2.setAlignment(CellStyle.ALIGN_CENTER);
        cellStyles.put("cs",cs);
        cellStyles.put("cs2",cs2);
        return cellStyles;
    }