Java将List对象导入Excel文件

通过poi依赖将java对象写入excel之中，核心理念是通过反射获取Java对象的getter方法和属性，使用getter方法获取要写入excel中的值，再通过属性上的自定义注解获取excel标题行，然后以文件流的方式写入excel

代码用到poi依赖如下:

 <dependency><groupId>org.apache.poi</groupId><artifactId>poi-ooxml</artifactId><version>3.17</version></dependency>

首先我们定义注解@ExcelTitle，用来初始化excel第一行作为标题行

/*** excel标题头注解，当Java属性没有使用此注解，则默认使用Java属性作为标题。*/
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface ExcelTitle {String value();
}

然后定义对象转excel工具类ExcelWriterUtil

import com.sc.ops.annotations.ExcelTitle;
import com.sc.ops.annotations.Order;
import lombok.extern.slf4j.Slf4j;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.ss.usermodel.*;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.*;
import java.util.stream.Collectors;
/*** 对象转excel工具类*/
@Slf4j
public class ExcelWriterUtil{private static ThreadLocal<ExcelValueFormatter> valueFormatter = ThreadLocal.withInitial(() -> new DateValueFormatter("yyyy-MM-dd"));public static void setExcelValueFormatter(ExcelValueFormatter excelValueFormatter) {valueFormatter.set(excelValueFormatter);}public static <E> void writeToExcel(List<E> list, Class<E> clazz, String fileName)throws InvocationTargetException, IllegalAccessException {@SuppressWarnings("MismatchedQueryAndUpdateOfCollection")List<Object[]> dataList = new ArrayList<>();Field[] fields = getAllFields(clazz);Map<String, Method> fieldMethodMap = buildFieldMethodMap(clazz);sortMethodMap(fields, fieldMethodMap);Map<String, String> fieldTitleMap = buildFieldTitleMap(clazz, fieldMethodMap);List<Map.Entry<String, Method>> methodEntrySet = new ArrayList<>(fieldMethodMap.entrySet());int addMark = 0;int itemSize = fieldTitleMap.size();String[] titleArr = new String[itemSize];for (E obj : list) {Object[] item = new Object[itemSize];for (int i = 0; i < methodEntrySet.size(); i++) {Map.Entry<String, Method> methodEntry = methodEntrySet.get(i);String field = methodEntry.getKey();if (addMark < itemSize) {titleArr[addMark] = fieldTitleMap.get(field);addMark++;}Method method = methodEntry.getValue();Object value = formatValue(method, obj, valueFormatter.get());if (value != null) {item[i] = value;}}dataList.add(item);}writeObjectToExcel(dataList, titleArr, fileName);}private static Object formatValue(Method method, Object obj,ExcelValueFormatter excelValueFormatter)throws InvocationTargetException, IllegalAccessException {Object value = method.invoke(obj);if (value == null) {return null;}if(excelValueFormatter == null) {return value;}Class<?> returnType = method.getReturnType();return excelValueFormatter.formatValue(returnType, value);}private static <E> Map<String, Method> buildFieldMethodMap(Class<E> clazz) {List<Method> getMethods = Arrays.stream(clazz.getMethods()).filter(method -> method.getName().startsWith("get") && !method.getName().equals("getClass")).collect(Collectors.toList());Map<String, Method> fieldMethodMap = new LinkedHashMap<>();for (Method getMethod : getMethods) {String m = getMethod.getName().replace("get", "");String field = m.substring(0, 1).toLowerCase() + m.substring(1);fieldMethodMap.put(field, getMethod);}return fieldMethodMap;}public static <E> Field[] getAllFields(Class<E> clazz){List<Field> fieldList = new ArrayList<>();while (clazz != null){fieldList.addAll(new ArrayList<>(Arrays.asList(clazz.getDeclaredFields())));clazz = (Class<E>) clazz.getSuperclass();}Field[] fields = new Field[fieldList.size()];fieldList.toArray(fields);return fields;}private static <E> Map<String, String> buildFieldTitleMap(Class<E> clazz,Map<String, Method> fieldMethodMap) {Map<String, String> fieldTitleMap = new LinkedHashMap<>();Field[] fields = getAllFields(clazz);Arrays.stream(fields).forEach(field -> {if (fieldMethodMap.containsKey(field.getName())) {ExcelTitle excelTitle = field.getAnnotation(ExcelTitle.class);String title = excelTitle == null ? field.getName() : excelTitle.value();fieldTitleMap.put(field.getName(), title);}});return fieldTitleMap;}private static void writeObjectToExcel(List<Object[]> list, String[]excelTitle, String fileName) {//在内存中创建Excel文件Workbook workbook;if (fileName.endsWith("xls")) {workbook = new HSSFWorkbook();} else if (fileName.endsWith("xlsx")) {workbook = new XSSFWorkbook();} else {throw new IllegalArgumentException("fileName not legal");}Sheet sheet = workbook.createSheet();//标题行Row titleRow = sheet.createRow(0);for (int i = 0; i < excelTitle.length; i++) {titleRow.createCell(i).setCellValue(excelTitle[i]);}//创建数据行并写入值for (Object[] dataArr : list) {int lastRowNum = sheet.getLastRowNum();Row dataRow = sheet.createRow(lastRowNum + 1);for (int i = 0; i < dataArr.length; i++) {Cell cell = dataRow.createCell(i);Object cellValue = dataArr[i];if(cellValue != null) {setCellValue(cellValue, cell);}}}//创建输出流对象FileOutputStream outputStream = null;try {outputStream = new FileOutputStream(new File(fileName));} catch (FileNotFoundException e) {log.error("file not found", e);}try {workbook.write(outputStream);} catch (IOException e) {log.error("write to file failed", e);} finally {if (outputStream != null) {try {outputStream.close();} catch (IOException ignore) {}}}}private static void setCellValue(Object cellValue, Cell cell) {if (cellValue instanceof Boolean) {cell.setCellValue((boolean) cellValue);} else if (cellValue instanceof String) {cell.setCellValue(cellValue.toString());} else if (cellValue instanceof Double || cellValue instanceof Integer|| cellValue instanceof Long) {cell.setCellValue(Double.valueOf(cellValue.toString()));} else if (cellValue instanceof Date) {cell.setCellValue((Date) cellValue);} else if (cellValue instanceof Calendar) {cell.setCellValue((Calendar) cellValue);} else if (cellValue instanceof RichTextString) {cell.setCellValue((RichTextString) cellValue);} else {cell.setCellValue(cellValue.toString());}}private static void sortMethodMap(Field[] fields, Map<String, Method> fieldMethodMap) {Set<String> fieldSet = fieldMethodMap.keySet();List<Field> fieldList = Arrays.stream(fields).filter(e -> fieldSet.contains(e.getName())).collect(Collectors.toList());fields = fieldList.toArray(new Field[]{});Arrays.sort(fields, (o1, o2) -> {Order order1 = o1.getAnnotation(Order.class);Order order2 = o2.getAnnotation(Order.class);if (order1 == null && order2 == null) { //均不含注解时不排序return 0;}if (order1 == null) { //order1 == null && order2 != null 仅有一个含有注解时，默认排到不含注解的后面return -1;}if (order2 == null) { //order1 != null && order2 == null 仅有一个含有注解时，默认排到不含注解的后面return 1;}return order1.value() - order2.value();//均含有注解时，按照注解值从小到大排序});Map<String, Method> sortedMethodMap = new LinkedHashMap<>();Arrays.stream(fields).forEach(e -> {String key = e.getName();sortedMethodMap.put(key, fieldMethodMap.get(key));});fieldMethodMap.clear();fieldMethodMap.putAll(sortedMethodMap);}
}

这个工具类涉及到一个自定义的接口ExcelValueFormatter，它用来实现将不同类型的java属性映射到不同的excel单元格格式。由于ExcelValueFormatter是个接口，所以你可以实现它，自定义不同的映射策略。

public interface ExcelValueFormatter {Object formatValue(Class<?> returnType, Object value);
}

本案例提供一个默认的实现类DateValueFormatter，将Date类型的属性转换为格式为yyyy-MM-dd的文本。

@Data
@AllArgsConstructor
public class DateValueFormatter implements ExcelValueFormatter {private String dateFormat;@Overridepublic Object formatValue(Class<?> returnType, Object value) {if (returnType.equals(Date.class)) {return DateTimeFormatter.ofPattern(dateFormat).format(toLocalDateTime((Date) value));} else {return value;}}private static LocalDateTime toLocalDateTime(Date date) {Instant instant = date.toInstant();ZoneId zoneId = ZoneId.systemDefault();return instant.atZone(zoneId).toLocalDateTime();}
}

为了能让列顺序不被打乱，我们最直接的方法就是给成员指定它的位置是几，也就是定义顺序编号，首先声明一个注解@Order，它只能用于类成员上：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Order {int value();
}

然后通过反射获取每个成员上的该注解，并进行排序：

  private static void sortMethodMap(Field[] fields, Map<String, Method> fieldMethodMap) {Set<String> fieldSet = fieldMethodMap.keySet();List<Field> fieldList = Arrays.stream(fields).filter(e -> fieldSet.contains(e.getName())).collect(Collectors.toList());fields = fieldList.toArray(new Field[]{});Arrays.sort(fields, (o1, o2) -> {Order order1 = o1.getAnnotation(Order.class);Order order2 = o2.getAnnotation(Order.class);if (order1 == null && order2 == null) { //均不含注解时不排序return 0;}if (order1 == null) { //order1 == null && order2 != null 仅有一个含有注解时，默认排到不含注解的后面return -1;}if (order2 == null) { //order1 != null && order2 == null 仅有一个含有注解时，默认排到不含注解的后面return 1;}return order1.value() - order2.value();//均含有注解时，按照注解值从小到大排序});Map<String, Method> sortedMethodMap = new LinkedHashMap<>();Arrays.stream(fields).forEach(e -> {String key = e.getName();sortedMethodMap.put(key, fieldMethodMap.get(key));});fieldMethodMap.clear();fieldMethodMap.putAll(sortedMethodMap);}

最后做个简单的测试。

在Student的成员上添加@Order指定相对位置：

    @Data@AllArgsConstructorpublic static class Student {@ExcelTitle("id")@Order(1)private Integer id;@ExcelTitle("姓名")@Order(2)private String name;@ExcelTitle("薪水")@Order(4)private Double salary;@ExcelTitle("生日")@Order(3)private Date birthDay;}

测试代码：

    public static void main(String[] args) {try {List<Student> students = new ArrayList<>();for (int i = 0; i < 100; i++) {students.add(new Student(i, "member" + i, i * 55D, new Date()));}ExcelWriter.writeToExcel(students, Student.class, "/Users/Downloads/test.xlsx");} catch (Exception ex) {ex.printStackTrace();}}@Data@AllArgsConstructorpublic static class Student {@ExcelTitle("id")private Integer id;@ExcelTitle("姓名")private String name;@ExcelTitle("薪水")private Double salary;@ExcelTitle("生日")private Date birthDay;}

结果如下：

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