jvm的xmx已经配置了2g，为什么还是加载不了一个50m的文件

Question

环境：openjdk11，springboot2.3，windows
代码：ExcelUtil.getBigWriter("my.xlsx")
jvm：

Debugger attached successfully.
Server compiler detected.
JVM version is 11+28

using thread-local object allocation.
Garbage-First (G1) GC with 8 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 2147483648 (2048.0MB)
   NewSize                  = 1363144 (1.2999954223632812MB)
   MaxNewSize               = 1287651328 (1228.0MB)
   OldSize                  = 5452592 (5.1999969482421875MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 1048576 (1.0MB)

Heap Usage:
G1 Heap:
   regions  = 2048
   capacity = 2147483648 (2048.0MB)
   used     = 105174440 (100.30216217041016MB)
   free     = 2042309208 (1947.6978378295898MB)
   4.897566512227058% used
G1 Young Generation:
Eden Space:
   regions  = 52
   capacity = 1352663040 (1290.0MB)
   used     = 54525952 (52.0MB)
   free     = 1298137088 (1238.0MB)
   4.0310077519379846% used
Survivor Space:
   regions  = 0
   capacity = 0 (0.0MB)
   used     = 0 (0.0MB)
   free     = 0 (0.0MB)
   0.0% used
G1 Old Generation:
   regions  = 53
   capacity = 794820608 (758.0MB)
   used     = 51697064 (49.302162170410156MB)
   free     = 743123544 (708.6978378295898MB)
   6.504243030397118% used

问题：需求是要加载一个大excel进行处理，这个excel有50m，用的是hutool的excel操作类，但是一执行到这句代码，就会报错。另外好奇一个问题，jvm的OldSize为什么默认只有5m？

cn.hutool.poi.exceptions.POIException: IOException: Java heap space
        at cn.hutool.poi.excel.WorkbookUtil.createBook(WorkbookUtil.java:86)
        at cn.hutool.poi.excel.WorkbookUtil.createSXSSFBook(WorkbookUtil.java:194)
        at cn.hutool.poi.excel.WorkbookUtil.createSXSSFBook(WorkbookUtil.java:182)

Answer 1

excel本身是个压缩文件，你把.xlsx后缀的excel用解压软件是可以直接打开的，主要就是一堆XML文件。

所以，看似50M大，因为文本的压缩率很高，实际可能有500M。

500M不小了吧，加上解析excel要创建很多object，那么OOM也就很正常了。解析excel有技巧，不能一次加载全部单元格，具体的代码网上很多了。