JAVA每日小知识（关于excel下载时插入和stream流遍历优化）

1、在windows系统下启动rocketmq操作：
在bin目录下使用cmd
分别输入
start mqnamesrv.cmd
start mqbroker.cmd -n 127.0.0.1:9876 autoCreateTopicEnable=true

2、在stream流中需要new对象时，可能会出现new很多对象堆积在堆中，这是需要用try,finally在finally中将new的对象为null，并用gc收集
如下：

List<ChildModelParamDTO> childModelParamDTOS = childModels.stream().map(childModel -> {ChildModelParamDTO childModelParamDTO=new ChildModelParamDTO();try {LambdaQueryWrapper<PredictionModelParam> eq1 = new QueryWrapper<PredictionModelParam>().lambda().eq(PredictionModelParam::getPredictionModelId, childModel.getId());PredictionModelParam predictionModelParam = predictionModelParamMapper.selectOne(eq1);childModelParamDTO.setCode(childModel.getCode());childModelParamDTO.setName(childModel.getName());childModelParamDTO.setParameterWeight(predictionModelParam.getParameterWeight());return childModelParamDTO;}finally {childModelParamDTO=null;System.gc();}}).collect(Collectors.toList());

3、在使用easyexcel时，由于监听器无法被spring管理，所以无法在监听器内部使用mp，此时可以用重写的方式将
excel中数据添加进数据库中，如：
 

 EasyExcel.read(file, PredictionParamExcelDTO.class, new ReadListener<PredictionParamExcelDTO>() {@Overridepublic void invoke(PredictionParamExcelDTO predictionParamExcelDTO, AnalysisContext analysisContext) {PredictionParam predictionParam = null;try {predictionParam = new PredictionParam();predictionParam.setMaterialCode(predictionParamExcelDTO.getMaterialCode());predictionParam.setMaterialName(predictionParamExcelDTO.getMaterialName());predictionParam.setProductionLine(predictionParamExcelDTO.getProductionLine());predictionParam.setCapacity(Integer.valueOf(predictionParamExcelDTO.getCapacity()));predictionParamService.save(predictionParam);} catch (Exception e) {e.printStackTrace();log.error("模板上传失败");throw e;}}@Overridepublic void doAfterAllAnalysed(AnalysisContext analysisContext) {}}).sheet().doRead();

4、下载excel的同时，将数据写入excel中：
 

try {httpServletResponse.setContentType("application/vnd.ms-excel");String fileName = URLEncoder.encode("D:/idea/idea_projects/mwlc-dcm-requirement-backend/predictionParamExcel.xlsx", "UTF-8");httpServletResponse.setHeader("content-disposition", "attachment;filename="+fileName);ServletOutputStream outputStream;outputStream=httpServletResponse.getOutputStream();EasyExcel.write(outputStream).head(PredictionParamExcelDTO.class).excelType(ExcelTypeEnum.XLSX).sheet("日基准产能表模板").doWrite(这里放需要写入excel的数据);outputStream.close();}catch (IOException o){throw new RuntimeException(o);}