1. Excel存储格式及解析流程
1.1 存储格式
Excel本质上是以xml存储的，这些xml内容符合office open xml规范。把后缀名改为压缩文件的后缀名（zip，tar等等）可以看到其基本结构：
其主要数据内容保存于sharedString.xml以及worksheets里的xml：
实际上对于Excel的解析就是对于XML的解析，但是各个xml之间存在着关系，解析时更为复杂。
1.2 解析流程
2. 写入性能对比
2.1 测试代码：
见文末
2.2 结果
| 模式 | 10万数据 | 100万数据 |
|---|---|---|
| POI(XSSF) | 10833ms | GC overhead limit exceeded |
| POI(SXSSF) | 1378ms | 9274ms |
| EasyExcel | 1339ms | 9077ms |
结论：
10万级别POI的SXSSF和EasyExcel的速度基本一致。
100万级别POI的XSSF模式直接无法生成，而SXSSF模式和EasyExcel的速度基本一致。
2.3 分析
POI传统模式XSSF会全部写入内存，内存占用很高，然后一次性刷盘；而SXSSF模式基于滑动窗口，部分刷盘，所以避免了大量GC时间及内存占用。
SXSSF (package: org.apache.poi.xssf.streaming) is an API-compatible streaming extension of XSSF to be used when very large spreadsheets have to be produced, and heap space is limited. SXSSF achieves its low memory footprint by limiting access to the rows that are within a sliding window, while XSSF gives access to all rows in the document. Older rows that are no longer in the window become inaccessible, as they are written to the disk.
https://poi.apache.org/components/spreadsheet/how-to.html#sxssf
所以，通过这个问题也能看出，把数据完全放在内存操作在某些场景下并不是最好的，性能反而不如多次IO操作，因为其中可能会产生FULL CG而占用大量时间。
3. 读取性能对比
3.1 测试代码
见文末
3.2 结果
| 数据量 | POI耗时 | EasyExcel耗时 |
|---|---|---|
| 10万 | 4223ms | 2813ms |
10万数据量，EasyExcel比POI快了1.4秒左右，数据量更大时差异更明显。
3.3 分析
读取操作其实主要耗费在了将表格数据加载到内存中（包含解析过程），而加载完成后，实际上读取速度是差不多的。POI采用的是一次性加载到内存中，而EasyExcel则是部分加载，同时POI加载的是包含Excel样式的数据，这部分数据占了很大一部分，EasyExcel就丢弃了样式数据，只加载数据。
4. 多线程解析表格
读取表格数据时，除了使用EasyExcel加快速度外，可以用多线程+POI来提高速度：
| 线程数 | 耗时 |
|---|---|
| 1 | 1807ms |
| 2 | 1425ms |
| 4 | 1311ms |
| 8 | 2335ms |
直接读取使用单线程和多线程花费的时间差别不大，并且，对于大数据量表格，时间多花费在读取文件上。如果在读取过程中需要对每一行进行操作，那么多线程的效果就比较明显了，以下是在读取每行数据时加入耗时操作后的结果(为方便测试，数据量减小为1万)：
| 线程数 | 耗时 |
|---|---|
| 1 | 36356ms |
| 2 | 18064ms |
| 4 | 9082ms |
| 8 | 4532ms |
| 16 | 2662ms |
| 32 | 1287ms |
| 64 | 688ms |
| 128 | 380ms |
| 256 | 335ms |
| 512 | 1253ms |
5. 测试代码
import cn.hutool.core.date.StopWatch;
import cn.hutool.core.util.RandomUtil;
import com.alibaba.excel.EasyExcel;
import com.alibaba.excel.ExcelReader;
import com.alibaba.excel.ExcelWriter;
import com.alibaba.excel.context.AnalysisContext;
import com.alibaba.excel.event.AnalysisEventListener;
import com.alibaba.excel.read.metadata.ReadSheet;
import com.alibaba.excel.write.metadata.WriteSheet;
import org.apache.poi.ss.usermodel.*;
import org.apache.poi.xssf.streaming.SXSSFWorkbook;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
import org.junit.Test;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.*;
public class ExcelUtilTest {
private static final String[] USERNAME = {"赵", "钱", "孙", "李", "周", "吴", "郑", "王", "冯", "陈", "褚", "卫", "蒋", "沈", "韩", "杨", "朱", "秦", "尤", "许",
"何", "吕", "施", "张", "孔", "曹", "严", "华", "金", "魏", "陶", "姜", "戚", "谢", "邹", "喻", "柏", "水", "窦", "章", "云", "苏", "潘", "葛", "奚", "范", "彭", "郎",
"鲁", "韦", "昌", "马", "苗", "凤", "花", "方", "俞", "任", "袁", "柳", "酆", "鲍", "史", "唐", "费", "廉", "岑", "薛", "雷", "贺", "倪", "汤", "滕", "殷",
"罗", "毕", "郝", "邬", "安", "常", "乐", "于", "时", "傅", "皮", "卞", "齐", "康", "伍", "余", "元", "卜", "顾", "孟", "平", "黄", "和",
"穆", "萧", "尹", "姚", "邵", "湛", "汪", "祁", "毛", "禹", "狄", "米", "贝", "明", "臧", "计", "伏", "成", "戴", "谈", "宋", "茅", "庞", "熊", "纪", "舒",
"屈", "项", "祝", "董", "梁", "杜", "阮", "蓝", "闵", "席", "季"};
private static final String GIRL = "秀娟英华慧巧美娜静淑惠珠翠雅芝玉萍红娥玲芬芳燕彩春菊兰凤洁梅琳素云莲真环雪荣爱妹霞香月莺媛艳瑞凡佳嘉琼勤珍贞莉桂娣叶璧璐娅琦晶妍茜秋珊莎锦黛青倩婷姣婉娴瑾颖露瑶怡婵雁蓓纨仪荷丹蓉眉君琴蕊薇菁梦岚苑婕馨瑗琰韵融园艺咏卿聪澜纯毓悦昭冰爽琬茗羽希宁欣飘育滢馥筠柔竹霭凝晓欢霄枫芸菲寒伊亚宜可姬舒影荔枝思丽 ";
private static final String BOY = "伟刚勇毅俊峰强军平保东文辉力明永健世广志义兴良海山仁波宁贵福生龙元全国胜学祥才发武新利清飞彬富顺信子杰涛昌成康星光天达安岩中茂进林有坚和彪博诚先敬震振壮会思群豪心邦承乐绍功松善厚庆磊民友裕河哲江超浩亮政谦亨奇固之轮翰朗伯宏言若鸣朋斌梁栋维启克伦翔旭鹏泽晨辰士以建家致树炎德行时泰盛雄琛钧冠策腾楠榕风航弘";
/**
* 写测试
*/
@Test
public void test() throws IOException {
int number = 100000;
StopWatch sw = new StopWatch();
sw.start();
poiTest(number, "XSSF");
sw.stop();
System.out.println("POI(XSSF)写入" + number + "条数据耗时" + sw.getLastTaskTimeMillis() + "ms");
sw.start();
poiTest(number, "SXSSF");
sw.stop();
System.out.println("POI(SXSSF)写入" + number + "条数据耗时" + sw.getLastTaskTimeMillis() + "ms");
sw.start();
easyExcelTest(number);
sw.stop();
System.out.println("EasyExcel写入" + number + "条数据耗时" + sw.getLastTaskTimeMillis() + "ms");
}
private void poiTest(int number, String type) throws IOException {
String path = "D:\\tmp\\test.xlsx";
try (Workbook wb = "SXSSF".equals(type) ? new SXSSFWorkbook() : new XSSFWorkbook()) {
Sheet sheet = wb.createSheet();
Row row = sheet.createRow(0);
row.createCell(0).setCellValue("ID");
row.createCell(1).setCellValue("姓名");
row.createCell(2).setCellValue("年龄");
row.createCell(3).setCellValue("性别");
row.createCell(4).setCellValue("是否会员");
for (int i = 1; i < number; i++) {
row = sheet.createRow(i);
List<String> randomData = this.getRandomData();
row.createCell(0).setCellValue(i);
for (int col = 0; col < randomData.size(); col++) {
row.createCell(col + 1).setCellValue(randomData.get(col));
}
}
wb.write(new FileOutputStream(path));
}
}
private void easyExcelTest(int number) throws IOException {
List<List> dataList = new LinkedList<>();
List<String> header = new LinkedList<>();
header.add("ID");
header.add("姓名");
header.add("年龄");
header.add("性别");
header.add("是否会员");
dataList.add(header);
for (int i = 1; i < number; i++) {
List<String> randomData = this.getRandomData();
List<String> data = new LinkedList<>();
data.add(String.valueOf(i));
data.addAll(randomData);
dataList.add(data);
}
String path = "D:\\tmp\\test.xlsx";
File file = new File(path);
ExcelWriter excelWriter = EasyExcel.write(file).build();
WriteSheet writeSheet = EasyExcel.writerSheet(0).build();
excelWriter.write(dataList, writeSheet);
excelWriter.finish();
}
private List<String> getRandomData() {
int usernameRandom = RandomUtil.randomInt(0, USERNAME.length - 1);
String name = USERNAME[usernameRandom];
if (usernameRandom % 2 == 0) {
name += GIRL.substring(usernameRandom % GIRL.length(), (usernameRandom + 2) % GIRL.length());
} else {
name += BOY.substring(usernameRandom % BOY.length(), (usernameRandom + 1) % BOY.length());
}
String age = String.valueOf(RandomUtil.randomInt(10, 50));
String sex = usernameRandom % 2 == 0 ? "女" : "男";
String isVip = usernameRandom % 2 == 0 ? "是" : "否";
return Arrays.asList(name, age, sex, isVip);
}
/**
* 读测试
*/
@Test
public void readTest() throws IOException {
StopWatch sw = new StopWatch();
sw.start();
poiReadTest();
sw.stop();
System.out.println("POI读取数据耗时" + sw.getLastTaskTimeMillis() + "ms");
sw.start();
easyExcelReadTest();
sw.stop();
System.out.println("EasyExcel读取数据耗时" + sw.getLastTaskTimeMillis() + "ms");
}
private void poiReadTest() throws IOException {
String path = "D:\\tmp\\test.xlsx";
File file = new File(path);
List<List<String>> result = new LinkedList<>();
Workbook wb = WorkbookFactory.create(file);
Sheet sheet = wb.getSheetAt(0);
Iterator<Row> rowIterator = sheet.rowIterator();
Row row;
while (rowIterator.hasNext()) {
row = rowIterator.next();
Iterator<Cell> cellIterator = row.cellIterator();
List<String> data = new LinkedList<>();
while (cellIterator.hasNext()) {
data.add(cellIterator.next().getStringCellValue());
}
result.add(data);
}
wb.close();
System.out.println("获取到" + result.size() + "条数据");
}
private void easyExcelReadTest() throws IOException {
String path = "D:\\tmp\\test.xlsx";
File file = new File(path);
ExcelReader excelReader = EasyExcel.read(file, new ExcelListener()).build();
ReadSheet readSheet = new ReadSheet(0);
excelReader.read(readSheet);
excelReader.finish();
}
private class ExcelListener extends AnalysisEventListener<LinkedHashMap> {
private List<LinkedHashMap> result = new ArrayList<>();
@Override
public void invoke(LinkedHashMap linkedHashMap, AnalysisContext analysisContext) {
result.add(linkedHashMap);
}
@Override
public void doAfterAllAnalysed(AnalysisContext analysisContext) {
System.out.println("获取到" + result.size() + "条数据");
}
}
}