日常開發作業中對于檔案的相關操作大家多少都會涉及：上傳決議、資料匯出等，此篇內容主要分享一下作業中常用的Excel檔案的決議和匯出作業類實作，

實踐

1.maven依賴

首先引入POI包依賴

<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.API

大家需要了解一下相關的API類，便于后面理解代碼邏輯和排查問題，重寫相關邏輯實作自己的業務需求，

　　1.import org.apache.poi.ss.usermodel.Workbook,對應Excel檔案；
　　2.import org.apache.poi.hssf.usermodel.HSSFWorkbook，對應xls格式的Excel檔案；
　　3.import org.apache.poi.xssf.usermodel.XSSFWorkbook，對應xlsx格式的Excel檔案；
　　4.import org.apache.poi.ss.usermodel.Sheet，對應Excel檔案中的一個sheet；
　　5.import org.apache.poi.ss.usermodel.Row，對應一個sheet中的一行；
　　6.import org.apache.poi.ss.usermodel.Cell，對應一個單元格，

3.匯入

public class ImportExcelUtil {

    private static final String EXCEL_XLS_SUFFIX = ".xls";

    private static final String EXCEL_XLSX_SUFFIX = ".xlsx";
 /**
     * 讀取Excel資料內容
     *
     * @param rowIndex    指定行號
     * @param columnIndex 指定列號
     * @return Map 包含單元格資料內容的Map物件
     */
    public static List<Map<Integer, Object>> readExcelContent(String filepath, Integer rowIndex, Integer columnIndex) throws Exception {
        List<Map<Integer, Object>> returnList = new LinkedList<>();
        Workbook wb = null;
        Sheet sheet;
        Row row;
        try {
            InputStream is = new FileInputStream(filepath);
            if (filepath.endsWith(EXCEL_XLS_SUFFIX)) {
                wb = new HSSFWorkbook(is);
            } else if (filepath.endsWith(EXCEL_XLSX_SUFFIX)) {
                wb = new XSSFWorkbook(is);
            }

            if (wb == null) {
                throw new Exception("Workbook物件為空！");
            }

            sheet = wb.getSheetAt(0);
            //決議檔案總行數、總列數
            int rowNum = rowIndex != null ? rowIndex : sheet.getLastRowNum();
            row = sheet.getRow(0);
            int colNum = columnIndex != null ? columnIndex : row.getLastCellNum();

            //回圈列
            for (int colIndex = colNum; colIndex > 0; colIndex--) {
                Cell cell = row.getCell(colIndex);
                if (cell != null && !"".equals(cell.toString())) {
                    colNum = colIndex;
                    break;
                }
            }
            logger.info("have data col:{}", colNum);

            // 正文內容應該從第二行開始,第一行為表頭的標題
            for (int i = 0; i <= rowNum; i++) {
                row = sheet.getRow(i);
                int j = 0;

                int size = (int) (colNum / .75f) + 1;
                //存盤單元格資料
                Map<Integer, Object> cellValue = new LinkedHashMap<>(size);

                if (row == null) {
                    continue;
                }

                while (j <= colNum) {
                    Cell cell = row.getCell(j);
                    String value = "";
                    //日期單元格需格式化日期
                    if (cell != null) {
                        if (cell.getCellType() == CellType.NUMERIC) {
                            if (HSSFDateUtil.isCellDateFormatted(cell)) {
                                Date d = cell.getDateCellValue();
                                SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
                                value = formatter.format(d);
                            } else if (cell.toString().contains("E")) {
                                DecimalFormat nf = new DecimalFormat("0");
                                value = nf.format(cell.getNumericCellValue());
                            } else {
                                value = cell.toString().endsWith(".0") ? cell.toString().replace(".0", "") : cell.toString().trim();
                            }
                        } else if (cell.getCellType() == CellType.FORMULA) {
                            value = String.valueOf(cell.getNumericCellValue());
                        } else {
                            value = cell.toString().trim();
                        }
                    }
                    cellValue.put(j, value);
                    j++;
                }
              returnList.add(cellValueMap);
            }
            wb.close();
        } catch (FileNotFoundException e) {
            logger.error("FileNotFoundException", e);
        } catch (IOException e) {
            logger.error("IOException", e);
        } finally {
            if (wb != null) {
                wb.close();
            }
        }
        return returnList;
    }

}

** 解釋： **
此處方法只是提供一個思路，后期大家可以根據自己的業務需求改寫，比如指定sheet，行號，列號等等，著重說明一下回傳值資料結構List，主要存盤結構為Map，key=列號，value=單元格內容；這種操作便于后期驗證必須列是否缺失、以及可以動態設計上傳檔案的列結構不必固定列位置等等，

4.匯出

public class ExportExcelUtil {
	/**
	 * 匯出excel檔案，表頭為一維陣串列示不用合并單元格
	 * @param sheetName
	 * @param excelTitle
	 * @param dataCollection
	 * @param <T>
	 * @return
	 */
	public static<T> HSSFWorkbook exportExcel(String sheetName, String[] excelTitle, Collection<T> dataCollection) {
		//創建一個Excel檔案
		HSSFWorkbook workbook = new HSSFWorkbook();
		//創建一個Sheet表格作業空間
		HSSFSheet sheet = workbook.createSheet(sheetName);
		HSSFCellStyle style = workbook.createCellStyle();
		//設定表格默認寬度
		sheet.setDefaultColumnWidth(20);
		//設定表格的表頭
		HSSFCell cellHeader;
		HSSFRow row = sheet.createRow(0);
		for (int i = 0; i < excelTitle.length; i++) {
			//創建單元格表頭
			cellHeader = row.createCell(i);
			cellHeader.setCellValue(new HSSFRichTextString(excelTitle[i]));
		}
		//匹配表頭設定單元格的值
		setWorkBookValue(sheet, dataCollection,0, style);

		return workbook;
	}

/**
     * （根據自定義）把具體資料寫入到excel中
     * @param sheet
     * @param dataCollection
     * @param index
     * @param style
     * @param <T>
     */
    @SuppressWarnings("unchecked")
	private static<T> void setWorkBookValue(HSSFSheet sheet,Collection<T> dataCollection, int index,HSSFCellStyle style){
		T t;
		Object[] fields;
		String fieldName;
		String getMethodName;
		HSSFCell cell;
		HSSFRow row;
		Class tClass;
		Method getMethod;
		Object value;
		//遍歷集合設定單元格值
		Iterator<T> it = dataCollection.iterator();
		while(it.hasNext()){
			//創建一行單元格
			index ++;
			row = sheet.createRow(index);
			//獲取資料
			t = it.next();
			//利用反射，根據JavaBean屬性的先后順序，動態呼叫getXxx()方法得到屬性值
			fields = t.getClass().getDeclaredFields();
			for(int i = 0; i < fields.length; i++){
				cell = row.createCell(i);
				style.setAlignment(HorizontalAlignment.LEFT);
				cell.setCellStyle(style);
				//利用反射，根據JavaBean屬性的先后順序，動態呼叫getXxx()方法得到屬性值
				Field[] newFields = t.getClass().getDeclaredFields();
				fieldName = newFields[i].getName();
				getMethodName = "get" + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1);
				try {
					tClass = t.getClass();
					getMethod = tClass.getMethod(getMethodName, new Class[]{});
					value = getMethod.invoke(t, new Object[]{});
					setCellValue(value,cell);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		}
	}
/**
	 * value格式校驗
	 */
	private static void setCellValue(Object value,HSSFCell cell){
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
		String textValue = null;
		Pattern pattern = Pattern.compile(RULE);
		Matcher matcher;
		HSSFRichTextString richTextString;
		if (!StringUtils.isEmpty(value)){
			//value進行型別轉換
			if (value instanceof Integer) {
				cell.setCellValue((Integer) value);
			} else if (value instanceof Float) {
				textValue = String.valueOf(value);
				cell.setCellValue(textValue);
			} else if (value instanceof Double) {
				textValue = String.valueOf(value);
				cell.setCellValue(textValue);
			} else if (value instanceof Long) {
				cell.setCellValue((Long) value);
			} else if (value instanceof Boolean) {
				textValue = "是";
				if (!(Boolean) value) {
					textValue = "否";
				}
			} else if (value instanceof Date) {
				textValue = sdf.format((Date) value);
			} else {
				// 其它資料型別都當作字串簡單處理
				textValue = value.toString();
			}
			if (textValue != null) {
				matcher = pattern.matcher(textValue);
				if (matcher.matches()) {
					// 是數字當作double處理
					cell.setCellValue(Double.parseDouble(textValue));
				} else {
					richTextString = new HSSFRichTextString(textValue);
					cell.setCellValue(richTextString);
				}
			}
		}
	}

/**
	 * excel 匯出檔案
	 * @param response
	 * @param workbook
	 * @param fileName
	 * @throws IOException
	 */
	public static void exportExcelFile(HttpServletResponse response, HSSFWorkbook workbook, String fileName) throws IOException {
		if (workbook != null) {
			response.reset();
			//指定下載的檔案名
			SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMddHHmmss");
			String filePrefix = sdf.format(new Date());
			response.setContentType("application/vnd.ms-excel");
            response.setCharacterEncoding("utf-8");
			fileName = URLEncoder.encode(filePrefix + "_" + fileName, "UTF-8").replaceAll("\\+", "%20");
            response.setHeader("Content-disposition", "attachment;filename*=utf-8''" + fileName + ".xlsx");
			BufferedOutputStream bufferedOutput = null;
			try {
				bufferedOutput = new BufferedOutputStream(response.getOutputStream());
				workbook.write(bufferedOutput);
				bufferedOutput.flush();
			} catch (IOException e) {
			    e.printStackTrace();
				throw e;
			} finally {
				if (bufferedOutput != null) {
					try {
						bufferedOutput.close();
					} catch (IOException e) {
						e.printStackTrace();
					}
				}
			}
		}
	}
}

外部訪問匯出檔案示例：

public void exportFile(String objectJson, HttpServletResponse response) throws Exception {
		....
		//省略業務代碼
        HSSFWorkbook workbook = ExportExcelUtil.exportExcel(SHEET_NAME, TITLE_LINE, xxxList);
        ExportExcelUtil.exportExcelFile(response, workbook, FILE_NAME);
    }

特殊說明：匯出操作實際上利用反射機制實作，則需保證Excel標題列順序需與匯出定義的物體物件屬性順序保持一致，否則會出現錯位現象，