解决大批量数据导出Excel产生内存溢出的方案

POI或者JXL在导出大量数据的时候，由于它们将每一个单元格生都成一个Cell对象，所以很容易导致内存溢出。解决这个问题，唯一的办法是弄清楚Excel的二进制格式（汗），并且用流的方式读写Excel。POI和JXL其实提供了二进制方式读写Excel的API，只是因为缺少文档和实例，所以使用的人不多。我编写了这个简单的合并Excel的类，它只适合合并结构相同的多个Excel文件。好在这个功能已经可以解决数据导出产生OOM的问题：将数据分批导出然后合并。

下面的代码使用POI3.1，合并11个3000多行的文档用时约6秒，我实在找不到更多的测试用的文档了。

Java代码


@SuppressWarnings("unchecked")
public class XlsMergeUtil {
private static Logger logger = LoggerFactory.getLogger(XlsMergeUtil.class);

/**
* 将多个Xls文件合并为一个，适用于只有一个sheet，并且格式相同的文档
* @param inputs 输入的Xls文件
* @param out 输出文件
*/
public static void merge(InputStream[] inputs, OutputStream out) {
if (inputs == null || inputs.length <= 1) {
throw new IllegalArgumentException("没有传入输入流数组,或只有一个输入流.");
}

List<Record> rootRecords = getRecords(inputs[0]);
Workbook workbook = Workbook.createWorkbook(rootRecords);
List<Sheet> sheets = getSheets(workbook, rootRecords);
if(sheets == null || sheets.size() == 0) {
throw new IllegalArgumentException("第一篇文档的格式错误，必须有至少一个sheet");
}
//以第一篇文档的最后一个sheet为根，以后的数据都追加在这个sheet后面
Sheet rootSheet = sheets.get(sheets.size() - 1);
int rootRows = getRowsOfSheet(rootSheet); //记录第一篇文档的行数，以后的行数在此基础上增加
rootSheet.setLoc(rootSheet.getDimsLoc());
Map<Integer, Integer> map = new HashMap(10000);

for (int i = 1; i < inputs.length; i++) { //从第二篇开始遍历
List<Record> records = getRecords(inputs[i]);
int rowsOfCurXls = 0;
//遍历当前文档的每一个record
for (Iterator itr = records.iterator(); itr.hasNext();) {
Record record = (Record) itr.next();
if (record.getSid() == RowRecord.sid) { //如果是RowRecord
RowRecord rowRecord = (RowRecord) record;
//调整行号
rowRecord.setRowNumber(rootRows + rowRecord.getRowNumber());
rootSheet.addRow(rowRecord); //追加Row
rowsOfCurXls++; //记录当前文档的行数
}
//SST记录，SST保存xls文件中唯一的String，各个String都是对应着SST记录的索引
else if (record.getSid() == SSTRecord.sid) {
SSTRecord sstRecord = (SSTRecord) record;
for (int j = 0; j < sstRecord.getNumUniqueStrings(); j++) {
int index = workbook.addSSTString(sstRecord.getString(j));
//记录原来的索引和现在的索引的对应关系
map.put(Integer.valueOf(j), Integer.valueOf(index));
}
} else if (record.getSid() == LabelSSTRecord.sid) {
LabelSSTRecord label = (LabelSSTRecord) record;
//调整SST索引的对应关系
label.setSSTIndex(map.get(Integer.valueOf(label.getSSTIndex())));
}
//追加ValueCell
if (record instanceof CellValueRecordInterface) {
CellValueRecordInterface cell = (CellValueRecordInterface) record;
int cellRow = cell.getRow() + rootRows;
cell.setRow(cellRow);
rootSheet.addValueRecord(cellRow, cell);
}
}
rootRows += rowsOfCurXls;
}
byte[] data = getBytes(workbook, sheets.toArray(new Sheet[0]));
write(out, data);
}

static void write(OutputStream out, byte[] data) {
POIFSFileSystem fs = new POIFSFileSystem();
// Write out the Workbook stream
try {
fs.createDocument(new ByteArrayInputStream(data), "Workbook");
fs.writeFilesystem(out);
out.flush();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

static List<Sheet> getSheets(Workbook workbook, List records) {
int recOffset = workbook.getNumRecords();
int sheetNum = 0;

// convert all LabelRecord records to LabelSSTRecord
convertLabelRecords(records, recOffset, workbook);
List<Sheet> sheets = new ArrayList();
while (recOffset < records.size()) {
Sheet sh = Sheet.createSheet(records, sheetNum++, recOffset);

recOffset = sh.getEofLoc() + 1;
if (recOffset == 1) {
break;
}
sheets.add(sh);
}
return sheets;
}

static int getRows(List<Record> records) {
int row = 0;
for (Iterator itr = records.iterator(); itr.hasNext();) {
Record record = (Record) itr.next();
if (record.getSid() == RowRecord.sid) {
row++;
}
}
return row;
}

static int getRowsOfSheet(Sheet sheet) {
int rows = 0;
sheet.setLoc(0);
while(sheet.getNextRow() != null) {
rows++;
}
return rows;
}

@SuppressWarnings("deprecation")
static List<Record> getRecords(InputStream input) {
try {
POIFSFileSystem poifs = new POIFSFileSystem(input);
InputStream stream = poifs.getRoot().createDocumentInputStream("Workbook");
return org.apache.poi.hssf.record.RecordFactory.createRecords(stream);
} catch (IOException e) {
logger.error("IO异常：{}", e.getMessage());
e.printStackTrace();
}
return Collections.EMPTY_LIST;
}

static void convertLabelRecords(List records, int offset, Workbook workbook) {

for (int k = offset; k < records.size(); k++) {
Record rec = (Record) records.get(k);

if (rec.getSid() == LabelRecord.sid) {
LabelRecord oldrec = (LabelRecord) rec;

records.remove(k);
LabelSSTRecord newrec = new LabelSSTRecord();
int stringid = workbook.addSSTString(new UnicodeString(oldrec.getValue()));

newrec.setRow(oldrec.getRow());
newrec.setColumn(oldrec.getColumn());
newrec.setXFIndex(oldrec.getXFIndex());
newrec.setSSTIndex(stringid);
records.add(k, newrec);
}
}
}

public static byte[] getBytes(Workbook workbook, Sheet[] sheets) {
// HSSFSheet[] sheets = getSheets();
int nSheets = sheets.length;

// before getting the workbook size we must tell the sheets that
// serialization is about to occur.
for (int i = 0; i < nSheets; i++) {
sheets[i].preSerialize();
}

int totalsize = workbook.getSize();

// pre-calculate all the sheet sizes and set BOF indexes
int[] estimatedSheetSizes = new int[nSheets];
for (int k = 0; k < nSheets; k++) {
workbook.setSheetBof(k, totalsize);
int sheetSize = sheets[k].getSize();
estimatedSheetSizes[k] = sheetSize;
totalsize += sheetSize;
}

byte[] retval = new byte[totalsize];
int pos = workbook.serialize(0, retval);

for (int k = 0; k < nSheets; k++) {
int serializedSize = sheets[k].serialize(pos, retval);
if (serializedSize != estimatedSheetSizes[k]) {
throw new IllegalStateException("Actual serialized sheet size (" + serializedSize
+ ") differs from pre-calculated size (" + estimatedSheetSizes[k] + ") for sheet (" + k
+ ")");
Sheet.serializeIndexRecord() does not
}
pos += serializedSize;
}
return retval;
}

public static void main(String[] args) throws Exception {
final String PATH = "E:\\projects\\java\\ws_0\\export\\data\\";
InputStream[] inputs = new InputStream[10];
inputs[0] = new java.io.FileInputStream(PATH + "07_10.xls");
for(int i = 1; i <= 9; i++) {
inputs[i] = new java.io.FileInputStream(PATH + "07_0" + i + ".xls");
}
OutputStream out = new FileOutputStream(PATH + "xx.xls");
long t1 = System.currentTimeMillis();
merge(inputs, out);
System.out.println(System.currentTimeMillis() - t1);//简陋的测试一下时间
}

}