• 场景：需要生成excel文件，并且存储到文件服务器

转为字节数组（生成excel）

public static byte[] ListToExcelOutPutStream(List<?> lst, Class clazz) {
    ByteArrayOutputStream arrayOutputStream = new ByteArrayOutputStream();
    ExcelWriter excelWriter  = EasyExcel.write(arrayOutputStream,clazz).build();
    WriteSheet writeSheet = EasyExcel.writerSheet().build();
    excelWriter.write(lst, writeSheet);
    excelWriter.finish();
    return arrayOutputStream.toByteArray();
}

• 转为字节数组(模板填充)

        String templateFileName = this.getClass().getClassLoader().getResource("test.xlsx").getPath();
        List<Map> mapList = new ArrayList<>();
        Map map = new HashMap<>();
        map.put("name","zhanghao");
        map.put("number","18");
        mapList.add(map);
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        ExcelWriter excelWriter = EasyExcel.write().withTemplate(templateFileName).file(os).build();
        WriteSheet writeSheet = EasyExcel.writerSheet().build();
        excelWriter.fill(mapList, writeSheet);
        excelWriter.finish();
        byte[] buffer = os.toByteArray();

• 转为outputstream（模板填充）

        String templateFileName = this.getClass().getClassLoader().getResource("test.xlsx").getPath();
        List<Map> mapList = new ArrayList<>();
        Map map = new HashMap<>();
        map.put("name","zhanghao");
        map.put("number","18");
        mapList.add(map);
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        ExcelWriter excelWriter = EasyExcel.write().withTemplate(templateFileName).file(os).build();
        WriteSheet writeSheet = EasyExcel.writerSheet().build();
        excelWriter.fill(mapList, writeSheet);
        excelWriter.finish();
        OutputStream outputStream =  excelWriter.writeContext().writeWorkbookHolder().getOutputStream();

• 转为inputStream(模板填充)

        String templateFileName = this.getClass().getClassLoader().getResource("test.xlsx").getPath();
        List<Map> mapList = new ArrayList<>();
        Map map = new HashMap<>();
        map.put("name","zhanghao");
        map.put("number","18");
        mapList.add(map);
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        ExcelWriter excelWriter = EasyExcel.write().withTemplate(templateFileName).file(os).build();
        WriteSheet writeSheet = EasyExcel.writerSheet().build();
        excelWriter.fill(mapList, writeSheet);
        excelWriter.finish();
        byte[] buffer = os.toByteArray();
        InputStream inputStream = new ByteArrayInputStream(buffer);

• 总结

根据官方文档，正常情况下EasyExcel是将文件生成到本地，如果需要再操作，需要将文件转化为inputStream。
我们利用ByteArrayOutputStream为缓存，先将文件写入其中，然后将其转为字节数组，最后利用ByteArrayInputStream转为输入流，这样做省略了存储文件到本地的流程，省略了2个IO过程。
也可以转为输出流，做后续其他的流操作。