Export Any Java Object to CSV/Excel with Reflection – A Universal Utility

Introduction

In this article we show how to export objects of any class to a CSV/Excel file without limiting the type of the class, using Java reflection and Apache POI.

Core Idea

The class type is unknown (User, Student, District, etc.). The utility iterates a list of such objects, extracts field names via reflection for the header row, and writes field values to the CSV body.

Implementation Steps

1. Dependencies

<dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>poi-ooxml</artifactId>
    <version>3.15</version>
</dependency>
<dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>poi-scratchpad</artifactId>
    <version>3.15</version>
</dependency>
<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.69</version>
</dependency>
<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.5</version>
</dependency>

2. Utility Class

The MyCsvFileUtil class provides methods to create CSV files, write files, build file names, generate header rows, convert objects to maps, and assemble the CSV body.

import lombok.extern.slf4j.Slf4j;
import org.apache.commons.io.IOUtils;
import java.io.*;
import java.lang.reflect.*;
import java.text.SimpleDateFormat;
import java.util.*;

@Slf4j
public class MyCsvFileUtil {
    public static final String FILE_SUFFIX = ".csv";
    public static final String CSV_DELIMITER = ",";
    public static final String CSV_TAIL = "
";
    protected static final String DATE_STR_FILE_NAME = "yyyyMMddHHmmssSSS";

    public static void createCsvFile(String savePath, String contextStr) throws IOException {
        File file = new File(savePath);
        file.createNewFile();
        FileOutputStream fos = new FileOutputStream(file);
        fos.write(contextStr.getBytes("gbk"));
        fos.flush();
        fos.close();
    }

    public static void writeFile(String fileName, String content) {
        FileOutputStream fos = null;
        OutputStreamWriter writer = null;
        try {
            fos = new FileOutputStream(fileName, true);
            writer = new OutputStreamWriter(fos, "GBK");
            writer.write(content);
            writer.flush();
        } catch (Exception e) {
            log.error("写文件异常|{}", e);
        } finally {
            if (fos != null) IOUtils.closeQuietly(fos);
            if (writer != null) IOUtils.closeQuietly(writer);
        }
    }

    public static String buildCsvFileFileName(List dataList) {
        return dataList.get(0).getClass().getSimpleName() + new SimpleDateFormat(DATE_STR_FILE_NAME).format(new Date()) + FILE_SUFFIX;
    }

    public static String buildCsvFileTableNames(List dataList) {
        Map<String, Object> map = toMap(dataList.get(0));
        StringBuilder tableNames = new StringBuilder();
        for (String key : map.keySet()) {
            tableNames.append(key).append(CSV_DELIMITER);
        }
        return tableNames.append(CSV_TAIL).toString();
    }

    public static String buildCsvFileBodyMap(List dataLists) {
        List<Map<String, Object>> mapList = new ArrayList<>();
        for (Object o : dataLists) {
            mapList.add(toMap(o));
        }
        StringBuilder lineBuilder = new StringBuilder();
        for (Map<String, Object> rowData : mapList) {
            for (String key : rowData.keySet()) {
                Object value = rowData.get(key);
                if (Objects.nonNull(value)) {
                    lineBuilder.append(value).append(CSV_DELIMITER);
                } else {
                    lineBuilder.append("--").append(CSV_DELIMITER);
                }
            }
            lineBuilder.append(CSV_TAIL);
        }
        return lineBuilder.toString();
    }

    public static <T> Map<String, Object> toMap(T entity) {
        Class<?> bean = entity.getClass();
        Field[] fields = bean.getDeclaredFields();
        Map<String, Object> map = new HashMap<>(fields.length);
        for (Field field : fields) {
            try {
                if (!"serialVersionUID".equals(field.getName())) {
                    String methodName = "get" + field.getName().substring(0, 1).toUpperCase() + field.getName().substring(1);
                    Method method = bean.getDeclaredMethod(methodName);
                    Object fieldValue = method.invoke(entity);
                    map.put(field.getName(), fieldValue);
                }
            } catch (Exception e) {
                log.warn("toMap() Exception={}", e.getMessage());
            }
        }
        return map;
    }
}

3. Usage Example

A Spring MVC controller method demonstrates how to export a list of District objects.

@RequestMapping("/createCsvFileJcTest")
public void createCsvFileJcTest() {
    List<District> districts = districtService.queryByParentCodes(Arrays.asList("110100"));
    String fileName = "D:\\mycsv\\" + MyCsvFileUtil.buildCsvFileFileName(districts);
    String tableNames = MyCsvFileUtil.buildCsvFileTableNames(districts);
    MyCsvFileUtil.writeFile(fileName, tableNames);
    String contentBody = MyCsvFileUtil.buildCsvFileBodyMap(districts);
    MyCsvFileUtil.writeFile(fileName, contentBody);
}

Extension with Custom Annotation

Define @JcExcelName to specify custom column titles, and use resolveExcelTableName to retrieve them.

@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface JcExcelName {
    String name() default "";
}

public static <T> List<String> resolveExcelTableName(T entity) {
    List<String> tableNamesList = new ArrayList<>();
    Class<?> bean = entity.getClass();
    Field[] fields = bean.getDeclaredFields();
    for (Field field : fields) {
        try {
            if (!"serialVersionUID".equals(field.getName())) {
                String tableTitleName = field.getName();
                JcExcelName ann = field.getAnnotation(JcExcelName.class);
                if (ann != null && StringUtils.hasLength(ann.name())) {
                    tableTitleName = ann.name();
                }
                tableNamesList.add(tableTitleName);
            }
        } catch (Exception e) {
            log.warn("toMap() Exception={}", e.getMessage());
        }
    }
    return tableNamesList;
}

Combine the custom titles with the CSV builder to produce a nicely formatted Excel file.