解决POI大数据导出Excel内存溢出、应用假死

最近公司一个06年统计项目在导出Excel时造成应用服务器内存溢出、假死现象；查看代码发现问题一次查询一整年的数据导致堆内存被撑爆（假死），随后改用批量查询往Excel中写数据，同样的问题又出现了！！！随后在网上查阅了部分资料只是在POI大数据导出API的基础上写的demo示例无任何参考价值…

解决内存溢出常用方法就是打开GC日志

{Heap before GC invocations=29 (full 14):
 par new generation   total 306688K, used 306687K [0x0000000080000000, 0x0000000094cc0000, 0x0000000094cc0000)
  eden space 272640K, 100% used [0x0000000080000000, 0x0000000090a40000, 0x0000000090a40000)
  from space 34048K,  99% used [0x0000000090a40000, 0x0000000092b7ffe0, 0x0000000092b80000)
  to   space 34048K,   0% used [0x0000000092b80000, 0x0000000092b80000, 0x0000000094cc0000)
 concurrent mark-sweep generation total 1756416K, used 1756415K [0x0000000094cc0000, 0x0000000100000000, 0x0000000100000000)
 Metaspace       used 43496K, capacity 44680K, committed 45056K, reserved 1089536K
  class space    used 5254K, capacity 5515K, committed 5632K, reserved 1048576K
2017-09-12T21:55:02.954+0800: 239.209: [Full GC (Allocation Failure) 2017-09-12T21:55:02.954+0800: 239.209: [CMS: 1756415K->1756415K(1756416K), 5.4136680 secs] 2063103K->1971243K(2063104K), [Metaspace: 43496K->43496K(1089536K)], 5.4138690 secs] [Times: user=5.41 sys=0.00, real=5.41 secs] 
Heap after GC invocations=30 (full 15):
 par new generation   total 306688K, used 214827K [0x0000000080000000, 0x0000000094cc0000, 0x0000000094cc0000)
  eden space 272640K,  78% used [0x0000000080000000, 0x000000008d1cacb0, 0x0000000090a40000)
  from space 34048K,   0% used [0x0000000090a40000, 0x0000000090a40000, 0x0000000092b80000)
  to   space 34048K,   0% used [0x0000000092b80000, 0x0000000092b80000, 0x0000000094cc0000)
 concurrent mark-sweep generation total 1756416K, used 1756415K [0x0000000094cc0000, 0x0000000100000000, 0x0000000100000000)
 Metaspace       used 43238K, capacity 44256K, committed 45056K, reserved 1089536K
  class space    used 5213K, capacity 5441K, committed 5632K, reserved 1048576K
}

主要信息：
2017-09-12T21:55:02.954+0800: 239.209: [Full GC (Allocation Failure) 2017-09-12T21:55:02.954+0800: 239.209: [CMS: 1756415K->1756415K(1756416K), 5.4136680 secs] 2063103K->1971243K(2063104K), [Metaspace: 43496K->43496K(1089536K)], 5.4138690 secs] [Times: user=5.41 sys=0.00, real=5.41 secs]

通过查看GC日志发现堆空间、元空间不能被回收(对象强引用导致)

解决方法：

查看业务代码：

SXSSFWorkbook sxssfWorkbook = new SXSSFWorkbook(1000);
    for(int i=1;i<=pageCount;i++){
        int tableNum = i;
        int pageIndex = i;
        //分页数据查询
        ListObject>> maps = dbFactory.getJdbcTemplate().queryForList(finalSql,(pageIndex-1)*pageSize,pageIndex*pageSize);
        SXSSFSheet sheet = sxssfWorkbook.createSheet("sheet"+tableNum);
        SXSSFRow sxssfRow = sheet.createRow(0);
        for(int a=0;afor(int a=1;a<=maps.size();a++){
            SXSSFRow sxssfRow = sheet.createRow(a);
            MapObject> data = maps.get(a-1);
            Set keySet = data.keySet();
            Iterator iterator = keySet.iterator();
            int cell = 0;
            while(iterator.hasNext()){
                String key = iterator.next();
                Object valueObject = data.get(key);
                SXSSFCell sxssfCell =  sxssfRow.createCell(cell);
                sxssfCell.setCellValue(valueObject==null?"":valueObject.toString()); 
                cell++;
            }
        }
        //数据清理
        maps.clear();
        //设置空引用
        maps = null;
    }

    FileOutputStream fos = new FileOutputStream(tempPath+fileName);
    sxssfWorkbook.write(fos);
    fos.close();
    sxssfWorkbook.dispose();

代码中数据清理、设置空引用都做了，为什么还是不能被回收呢？？？

通过JVM自带检测工具jmap查看活跃对象

jmap使用说明：论持久战之Java性能监控工具（jmap）

重大发现原来是org.apache.poi.xssf.streaming.SXSSFCell、org.apache.poi.xssf.streaming.SXSSFCell$PlainStringValue、org.apache.poi.xssf.streaming.SXSSFRow这三个鬼把内存占完了

优化代码

SXSSFWorkbook sxssfWorkbook = new SXSSFWorkbook(1000);
    SXSSFCell sxssfCell = null;
    SXSSFRow sxssfRow = null;
    for(int i=1;i<=pageCount;i++){
        int tableNum = i;
        int pageIndex = i;
        ListObject>> maps = dbFactory.getJdbcTemplate().queryForList(finalSql,(pageIndex-1)*pageSize,pageIndex*pageSize);
        SXSSFSheet sheet = sxssfWorkbook.createSheet("sheet"+tableNum);
        sxssfRow = sheet.createRow(0);
        for(int a=0;afor(int a=1;a<=maps.size();a++){
            sxssfRow = sheet.createRow(a);
            MapObject> data = maps.get(a-1);
            Set keySet = data.keySet();
            Iterator iterator = keySet.iterator();
            int cell = 0;
            while(iterator.hasNext()){
                String key = iterator.next();
                Object valueObject = data.get(key);
                sxssfCell =  sxssfRow.createCell(cell);
                sxssfCell.setCellValue(valueObject==null?"":valueObject.toString()); 
                cell++;
            }
            //map数据清理
            data.clear();
        }
        //数据清理
        maps.clear();
        //设置空引用
        maps = null;
    }

    FileOutputStream fos = new FileOutputStream(tempPath+fileName);
    sxssfWorkbook.write(fos);
    fos.close();
    sxssfWorkbook.dispose();

程序SXSSFRow、SXSSFCell这两个对象持有一个引用，每当新创建一个对象时候原来引用失效jvm会自动回收

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