POI大量数据读取内存溢出分析及解决方案

Java
在使用POI进行excel操作时，当数据量较大时经常会产生内存溢出异常。下面通过分析如何解决该问题

一、POI结构图

二、内存溢出问题

在项目中遇到二十万行数据要写入到excel中时会内存溢出，一般方法是调大tomcat的内存，但是调到2048M还是会内存溢出报错。因此分析其原因。
通过分析其源码，得出其实现步骤为通过InputStream一行行读取到TreeMap类型的HSSFRow结构体中，因此当数据量大时就会造成内存溢出。

public HSSFWorkbook(DirectoryNode directory, boolean preserveNodes)
        throws IOException
{
    super(directory);
    String workbookName = getWorkbookDirEntryName(directory);
    this.preserveNodes = preserveNodes;
    // If we're not preserving nodes, don't track the
    //  POIFS any more
    if(! preserveNodes) {
        clearDirectory();
    }
    _sheets = new ArrayList<HSSFSheet>(INITIAL_CAPACITY);
    names  = new ArrayList<HSSFName>(INITIAL_CAPACITY);
    // Grab the data from the workbook stream, however
    //  it happens to be spelled.
    InputStream stream = directory.createDocumentInputStream(workbookName);
    List<Record> records = RecordFactory.createRecords(stream);
    workbook = InternalWorkbook.createWorkbook(records);
    setPropertiesFromWorkbook(workbook);
    int recOffset = workbook.getNumRecords();
    // convert all LabelRecord records to LabelSSTRecord
    convertLabelRecords(records, recOffset);
    RecordStream rs = new RecordStream(records, recOffset);
    while (rs.hasNext()) {
        try {
            InternalSheet sheet = InternalSheet.createSheet(rs);
            _sheets.add(new HSSFSheet(this, sheet));
        } catch (UnsupportedBOFType eb) {
            // Hopefully there's a supported one after this!
            log.log(POILogger.WARN, "Unsupported BOF found of type " + eb.getType());
        }
    }
    for (int i = 0 ; i < workbook.getNumNames() ; ++i){
        NameRecord nameRecord = workbook.getNameRecord(i);
        HSSFName name = new HSSFName(this, nameRecord, workbook.getNameCommentRecord(nameRecord));
        names.add(name);
    }
}
/**
 * add a row to the sheet
 *
 * @param addLow whether to add the row to the low level model - false if its already there
 */
private void addRow(HSSFRow row, boolean addLow) {
_rows.put(Integer.valueOf(row.getRowNum()), row);
if (addLow) {
    _sheet.addRow(row.getRowRecord());
}
boolean firstRow = _rows.size() == 1;
if (row.getRowNum() > getLastRowNum() || firstRow) {
    _lastrow = row.getRowNum();
}
if (row.getRowNum() < getFirstRowNum() || firstRow) {
    _firstrow = row.getRowNum();
}
}

excel数据行读取到内存的存储结构如下：

三、解决方案

poi官网给了一种大批量数据写入的方法，使用SXXFWorkbook类进行大批量写入操作解决了这个问题，可以监控该样例，会发现整体内存呈现锯齿状，能够及时回收，内存相对比较平稳。

package org.bird.poi;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.URL;
import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.util.CellReference;
import org.apache.poi.xssf.streaming.SXSSFWorkbook;
import org.junit.Assert;
public class XSSFWriter {
    private static SXSSFWorkbook wb;
    public static void main(String[] args) throws IOException {
        wb = new SXSSFWorkbook(10000);
        Sheet sh = wb.createSheet();  
        for(int rownum = 0; rownum < 100000; rownum++){  
            Row row = sh.createRow(rownum);  
            for(int cellnum = 0; cellnum < 10; cellnum++){  
                Cell cell = row.createCell(cellnum);  
                String address = new CellReference(cell).formatAsString();  
                cell.setCellValue(address);  
            }  
        }  
        // Rows with rownum < 900 are flushed and not accessible  
        for(int rownum = 0; rownum < 90000; rownum++){  
            Assert.assertNull(sh.getRow(rownum));  
        }  
        // ther last 100 rows are still in memory  
        for(int rownum = 90000; rownum < 100000; rownum++){  
            Assert.assertNotNull(sh.getRow(rownum));  
        }  
        URL url = XSSFWriter.class.getClassLoader().getResource("");
        FileOutputStream out = new FileOutputStream(url.getPath() + File.separator + "wirter.xlsx");  
        wb.write(out);  
        out.close();  
        // dispose of temporary files backing this workbook on disk  
        wb.dispose();  
    }
}