1、添加依赖

    <dependency>
      <groupId>org.apache.poi</groupId>
      <artifactId>poi</artifactId>
      <version>4.1.2</version>
    </dependency>
    <dependency>
      <groupId>org.apache.poi</groupId>
      <artifactId>poi-ooxml</artifactId>
      <version>4.1.2</version>
    </dependency>

2、xls和xlsx的区别介绍

1. xls是Excel03版本，最大支持65536行、256列，poi 操作xls，使用HSSFWorkbook
2. xlsx是Excel007版本，最大支持1048576行、16384列，poi-ooml操作xlsx，使用XSSFWorkbook

3、代码示例，读取Excel

/**
 * 读.xlsx文件
 */
private static List<List<String>> readXlsx(String path) throws Exception {
    InputStream is = Files.newInputStream(Paths.get(path));
    XSSFWorkbook xssfWorkbook = new XSSFWorkbook(is);
    List<List<String>> result = new ArrayList<List<String>>();
    // 循环每一页，并处理当前循环页 (sheet 页)
    int numberOfSheets = xssfWorkbook.getNumberOfSheets();
    for (int i = 0; i < numberOfSheets; i++) {
        XSSFSheet sheetAt = xssfWorkbook.getSheetAt(i);
        if (sheetAt == null) {
            continue;
        }
        // 从第一行一直循环到当前sheet的最后一行
        for (int rowNum = 1; rowNum <= sheetAt.getLastRowNum(); rowNum++) {
            // 获取行数据，然后在获取列数据
            XSSFRow xssfRow = sheetAt.getRow(rowNum);
            int minColIx = xssfRow.getFirstCellNum();
            int maxColIx = xssfRow.getLastCellNum();
            List<String> rowList = new ArrayList<String>();
            for (int colIx = minColIx; colIx < maxColIx; colIx++) {
                XSSFCell cell = xssfRow.getCell(colIx);
                if (cell == null) {
                    continue;
                }
                rowList.add(cell.toString());
            }
            result.add(rowList);
        }
    }
    return result;
}

/**
 * 读.xls文件
 */
private static List<List<String>> readXls(String path) throws IOException {
    InputStream is = Files.newInputStream(Paths.get(path));
    HSSFWorkbook hssfWorkbook = new HSSFWorkbook(is);
    List<List<String>> result = new ArrayList<List<String>>();
    int numberOfSheets = hssfWorkbook.getNumberOfSheets();
    for (int i = 0; i < numberOfSheets; i++) {
        HSSFSheet hssfSheet = hssfWorkbook.getSheetAt(i);
        if (hssfSheet == null) {
            continue;
        }
        int firstRowNum = hssfSheet.getFirstRowNum();
        int lastRowNum = hssfSheet.getLastRowNum();
        for (int rowIx = firstRowNum; rowIx < lastRowNum; rowIx++) {
            HSSFRow row = hssfSheet.getRow(rowIx);
            int minColIx = row.getFirstCellNum();
            int maxColIx = row.getLastCellNum();
            List<String> rowList = new ArrayList<String>();
            for (int colIx = minColIx; colIx < maxColIx; colIx++) {
                HSSFCell cell = row.getCell(colIx);
                if (cell == null) {
                    continue;
                }
                rowList.add(cell.toString());
            }
            result.add(rowList);
        }
    }
    return result;
}

4、代码示例，写Excel （HSSFWorkBook类似）

// 将上面读取的数据，在重新写到一个新的文件中
private static void writeXlsx(List<List<String>> dataList, String destPath) throws IOException {
    // 创建一个工作簿
    XSSFWorkbook xssfWorkbook = new XSSFWorkbook();

    Sheet sheet001 = xssfWorkbook.createSheet("sheet001");
    for (int i = 0; i < dataList.size(); i++) {
        Row row = sheet001.createRow(i);
        List<String> rowData = dataList.get(i);
        for (int j = 0; j < rowData.size(); j++) {
            Cell cell = row.createCell(j);
            cell.setCellValue(rowData.get(j));
        }
    }

    FileOutputStream fileOutputStream = new FileOutputStream(destPath);
    xssfWorkbook.write(fileOutputStream);
    fileOutputStream.close();
}

5、合并单元格 --- addMergedRegion

private static void mergeWithXSSF(String destPath) throws IOException {
    XSSFWorkbook xssfWorkbook = new XSSFWorkbook();
    XSSFSheet sheet = xssfWorkbook.createSheet("new Sheet");
    XSSFRow row = sheet.createRow(1);
    XSSFCell cell = row.createCell(1);
    cell.setCellValue("测试合并单元格");
    // 按照范围合并单元格
    sheet.addMergedRegion(new CellRangeAddress(1, 1, 1, 2));

    FileOutputStream fileOutputStream = new FileOutputStream(destPath);
    xssfWorkbook.write(fileOutputStream);
    fileOutputStream.close();
}

合并效果：

6.合并单元格的优化 -----  addMergedRegionUnsafe

当我们还使用addMergedRegion方法的时候，比如循环10000次合并操作，可以计算一下耗时

private static void mergeWithXSSF1(String destPath) throws IOException {
    XSSFWorkbook xssfWorkbook = new XSSFWorkbook();
    XSSFSheet sheet = xssfWorkbook.createSheet("new Sheet");

    long startTime = System.currentTimeMillis();
    for (int i = 0; i < 10000; i++) {
        XSSFRow row = sheet.createRow(i);
        XSSFCell cell = row.createCell(1);
        cell.setCellValue("测试合并单元格");
        sheet.addMergedRegion(new CellRangeAddress(i, i, 1, 2));
    }
    long endTime = System.currentTimeMillis();
    System.out.println("耗费时间： " + (endTime - startTime));
    FileOutputStream fileOutputStream = new FileOutputStream(destPath);
    xssfWorkbook.write(fileOutputStream);
    fileOutputStream.close();
}

耗费时间： 22918

如果换成addMergedRegionUnsafe方法，同样循环10000次合并操作，计算了一下耗时

private static void mergeWithXSSF1(String destPath) throws IOException {
    XSSFWorkbook xssfWorkbook = new XSSFWorkbook();
    XSSFSheet sheet = xssfWorkbook.createSheet("new Sheet");

    long startTime = System.currentTimeMillis();
    for (int i = 0; i < 10000; i++) {
        XSSFRow row = sheet.createRow(i);
        XSSFCell cell = row.createCell(1);
        cell.setCellValue("测试合并单元格");
        // 改成不校验
        sheet.addMergedRegionUnsafe(new CellRangeAddress(i, i, 1, 2));
    }
    long endTime = System.currentTimeMillis();
    System.out.println("耗费时间： " + (endTime - startTime));
    FileOutputStream fileOutputStream = new FileOutputStream(destPath);
    xssfWorkbook.write(fileOutputStream);
    fileOutputStream.close();
}

耗费时间： 926

可以看到，消耗的时间是大大减少的。