前言

日前，看到一个比较奇怪的导出功能。

需要根据不同的页面，以及指定不同的字段列表（任意顺序），然后导出对应的表格。

先假设一个场景：
假如你的系统有多个列表展示页，每页中可以依据筛选条件，调整展示的列的个数，顺序等。然后要求导出的时侯，导出一摸一样的格式。也就是“所见即所得”的表格。

那么基于以上场景，我们就来考虑下如何实现？
本文就是对以上场景功能的一个实现。目前仅支持单sheet，不支持数据聚合等。

正文

本文项目环境：
java 8，springboot2.2.0, easyexcel

一、POM依赖

<dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><version>2.2.0.RELEASE</version></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.2</version></dependency><dependency><groupId>com.alibaba</groupId><artifactId>easyexcel</artifactId><version>2.2.11</version><exclusions><exclusion><groupId>org.slf4j</groupId><artifactId>slf4j-api</artifactId></exclusion></exclusions></dependency></dependencies>

二、核心Java文件

此处粘贴全部的java文件

2.1 自定义表头注解 ExcelColumnTitle

package headbean;import java.lang.annotation.*;/*** 列名标题注解，标注列的标题** @author feng*/
@Documented
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface ExcelColumnTitle {String value();
}

2.2 自定义标题头的映射接口

此接口仅仅是用于规范实体，以及用于辅助实现导出功能。

package headbean;/*** excel头部映射接口，用于规范导出的实体类** @author feng*/
public interface ExcelHeadMapInterface {
}

2.3 自定义有序map存储表内数据

这个是表格导出时，字段数量，顺序的关键。

package headbean;import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;/*** 表格数据专用的map，带顺序，而且初始化的时候，依据指定的表头变量字段名确定导出数据的顺序** @author feng*/
public class ExcelDataLinkedHashMap<V> extends LinkedHashMap<String, V> {private static final long serialVersionUID = -8554095999151235982L;/*** 头部字段名缓存*/private final Set<String> headColumnNamesCache;/*** ExcelDataLinkedHashMap构造器** @param headColumnNames 表头字段变量名，例如：[name,studentNo,age,className]*/public ExcelDataLinkedHashMap(List<String> headColumnNames) {// 字段名去重headColumnNames = headColumnNames.stream().distinct().collect(Collectors.toList());// 构建字段名缓存this.headColumnNamesCache = new HashSet<>(headColumnNames);// 指定列数据排列顺序for (String headColumnName : headColumnNames) {this.put(headColumnName, null);}}@Overridepublic V put(String key, V value) {// 只保存字段名缓存中的key以及valueif (headColumnNamesCache.contains(key)) {return super.put(key, value);}return null;}
}

2.4 表头工厂

负责实现初始化表头字段名，以及后期使用时，从中获取表头信息。
核心功能是解析自定义的表头注解。

package factory;import enums.ExcelHeadBeanFlagEnum;
import headbean.ExcelColumnTitle;
import headbean.ExcelHeadMapInterface;import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;public class ExcelHeadMapFactory {/*** 全局表头名映射，ExcelHeadMapInterface实现类型为key，它内部变量的变量名和中文名映射为value*/private static final Map<Class<? extends ExcelHeadMapInterface>, Map<String, String>> HEAD_NAME_MAP = new HashMap<>();public static void addHeadClass(Class<? extends ExcelHeadMapInterface> headClass) {HEAD_NAME_MAP.put(headClass, mapToPrepareHead(headClass));}public static Map<String, String> getHeadMap(Class<? extends ExcelHeadMapInterface> headClass) {return HEAD_NAME_MAP.get(headClass);}public static Map<String, String> getHeadMapByFlag(String flag) {return getHeadMap(ExcelHeadBeanFlagEnum.getHeadClass(flag));}private static Map<String, String> mapToPrepareHead(Class<?> excelHeadClass) {Map<String, String> namedMap = new HashMap<>();Field[] declaredFields = excelHeadClass.getDeclaredFields();for (Field declaredField : declaredFields) {boolean annotationPresent = declaredField.isAnnotationPresent(ExcelColumnTitle.class);if(annotationPresent) {ExcelColumnTitle excelProperty = declaredField.getAnnotation(ExcelColumnTitle.class);String chineseFieldName = excelProperty.value();// 保存字段名和中文变量名namedMap.put(declaredField.getName(), chineseFieldName);}}return namedMap;}
}

2.5 表flag和表头映射枚举

这个枚举，如果你的系统这类功能很多。可以设计为数据库的方式做映射。然后以查字典表的方式，来处理。当然使用枚举大概率是够用了。

package enums;import headbean.ExcelHeadMapInterface;
import headbean.NameAndFactoryDemo;
import headbean.StudentDemo;
import lombok.AllArgsConstructor;
import lombok.Getter;import java.util.Arrays;/*** 表头flag枚举，映射flag与对应的实体类型；主要是可以根据flag找到对应实体类型。** @author feng*/
@Getter
@AllArgsConstructor
public enum ExcelHeadBeanFlagEnum {NAME_AND_FACTORY_DEMO("NameAndFactoryDemo", NameAndFactoryDemo.class),STUDENT_DEMO("StudentDemo", StudentDemo.class);private final String flag;private final Class<? extends ExcelHeadMapInterface> headClass;public static Class<? extends ExcelHeadMapInterface> getHeadClass(String flag) {return Arrays.stream(values()).filter(bean -> bean.getFlag().equals(flag)).findFirst().orElseThrow(RuntimeException::new).getHeadClass();}
}

2.6 测试用的实体

2.6.1 NameAndFactoryDemo

package headbean;import lombok.Data;@Data
public class NameAndFactoryDemo implements ExcelHeadMapInterface {@ExcelColumnTitle("名字")private String name;@ExcelColumnTitle("工厂")private String factory;
}

2.6.2 StudentDemo

package headbean;import lombok.Data;@Data
public class StudentDemo implements ExcelHeadMapInterface {@ExcelColumnTitle("姓名")private String name;@ExcelColumnTitle("年龄")private Integer age;@ExcelColumnTitle("学号")private String studentNo;@ExcelColumnTitle("班级")private String className;
}

2.7 启动类

主要是项目启动后，注册表头数据到内存。

package org.feng;import factory.ExcelHeadMapFactory;
import headbean.ExcelHeadMapInterface;
import headbean.NameAndFactoryDemo;
import headbean.StudentDemo;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;import java.util.ArrayList;
import java.util.List;@SpringBootApplication
public class ExcelDemoApplication implements CommandLineRunner {public static void main(String[] args) {SpringApplication.run(ExcelDemoApplication.class, args);}@Overridepublic void run(String... args) throws Exception {List<Class<? extends ExcelHeadMapInterface>> needRegisterExcelHeadClassList = new ArrayList<>();needRegisterExcelHeadClassList.add(NameAndFactoryDemo.class);needRegisterExcelHeadClassList.add(StudentDemo.class);needRegisterExcelHeadClassList.forEach(ExcelHeadMapFactory::addHeadClass);}
}

2.8 测试控制器

package org.feng;import com.alibaba.excel.EasyExcel;
import com.alibaba.excel.write.metadata.WriteSheet;
import enums.ExcelHeadBeanFlagEnum;
import factory.ExcelHeadMapFactory;
import headbean.ExcelDataLinkedHashMap;
import headbean.ExcelHeadMapInterface;
import headbean.NameAndFactoryDemo;
import headbean.StudentDemo;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;import javax.annotation.PostConstruct;
import javax.servlet.http.HttpServletResponse;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.*;
import java.util.function.BiFunction;@Controller
@RequestMapping("/excel")
public class ExcelDemoController {@GetMapping("/exportDy")public String exportDy(@RequestParam("flag")String flag,@RequestParam("table")List<String> table, HttpServletResponse response) throws IOException {String fileName = System.currentTimeMillis() + ".xlsx";Class<? extends ExcelHeadMapInterface> headClass = ExcelHeadBeanFlagEnum.getHeadClass(flag);Map<String, String> namedPrepareHeadMap = ExcelHeadMapFactory.getHeadMap(headClass);Map<String, String> head = new LinkedHashMap<>();for (String fieldName : table) {head.put(namedPrepareHeadMap.get(fieldName), fieldName);}List<Map<String, String>> excelDataList = new ArrayList<>();excelDataList.add(head);// 制造假数据for (BiFunction<Class<? extends ExcelHeadMapInterface>, List<Map<String, String>>, Boolean> biFunction : bizList) {Boolean applied = biFunction.apply(headClass, excelDataList);if(applied) {break;}}byte[] bytes = easyOut(excelDataList);response.setHeader("Content-disposition", "attachment;filename=" + fileName);response.setContentType("application/x-msdownload");response.setCharacterEncoding("utf-8");response.getOutputStream().write(bytes);response.getOutputStream().flush();return "success";}static final List<BiFunction<Class<? extends ExcelHeadMapInterface>, List<Map<String, String>>, Boolean>> bizList = new ArrayList<>();@PostConstructprivate void init() {bizList.add(this::genStudentDemoData);bizList.add(this::genNameAndFactoryDemoData);}private boolean genStudentDemoData(Class<? extends ExcelHeadMapInterface> headClass, List<Map<String, String>> excelDataList) {if(headClass == StudentDemo.class) {Map<String, String> headMap = excelDataList.get(0);for (int i = 0; i < 5; i++) {Collection<String> fieldNames = headMap.values();Map<String, String> data = new ExcelDataLinkedHashMap<>(new ArrayList<>(fieldNames));excelDataList.add(data);data.put("name", "张三"+(i+1));data.put("age", "年龄"+(i+1));data.put("studentNo", "学号"+(i+1));data.put("className", "班级"+(i+1));}return true;}return false;}private boolean genNameAndFactoryDemoData(Class<? extends ExcelHeadMapInterface> headClass, List<Map<String, String>> excelDataList) {if(headClass == NameAndFactoryDemo.class) {Map<String, String> headMap = excelDataList.get(0);for (int i = 0; i < 5; i++) {Collection<String> fieldNames = headMap.values();Map<String, String> data = new ExcelDataLinkedHashMap<>(new ArrayList<>(fieldNames));excelDataList.add(data);data.put("name", "张三"+(i+1));data.put("factory", "工厂"+(i+1));}return true;}return false;}/*** 导出数据（单sheet）* @param exportData key 是sheet名称，value是每个sheet里面的数据，支持自定义表头*/public static byte[] easyOut(List<Map<String, String>> exportData) {return easyOut(Collections.singletonMap("Sheet", exportData));}/*** 导出数据（多sheet）* @param exportData key 是sheet名称，value是每个sheet里面的数据，可以自定义*/public static byte[] easyOut(Map<String, List<Map<String, String>>> exportData) {// 导出数据ByteArrayOutputStream out = new ByteArrayOutputStream();com.alibaba.excel.ExcelWriter excelWriter = EasyExcel.write(out).build();int i=0;for (Map.Entry<String, List<Map<String, String>>> entry: exportData.entrySet()) {WriteSheet writeSheet = EasyExcel.writerSheet(i, entry.getKey()).head(head(entry.getValue().get(0))).build();i++;excelWriter.write(data(entry.getValue(), true), writeSheet);}excelWriter.finish();return out.toByteArray();}private static List<List<String>> head(Map<String, String> cellData) {List<List<String>> head = new ArrayList<>();for (String key: cellData.keySet()) {head.add(Collections.singletonList(key));}return head;}private static List<List<String>> data(List<Map<String, String>> sheetData, boolean skipHead) {List<List<String>> data = new ArrayList<>();for (int i = 0; i < sheetData.size(); i++) {if(i == 0 && skipHead) {continue;}data.add(new ArrayList<>(sheetData.get(i).values()));}return data;}
}

三、测试

测试1

http://localhost:8080/excel/exportDy?flag=StudentDemo&table=name,studentNo,age,className

获得的表格内容为：

测试2

http://localhost:8080/excel/exportDy?flag=StudentDemo&table=name,studentNo,age

获得的表格内容为：

测试3

http://localhost:8080/excel/exportDy?flag=NameAndFactoryDemo&table=factory,name
获得的表格内容为：

测试4

http://localhost:8080/excel/exportDy?flag=StudentDemo&table=className,name,studentNo
获得的表格内容为：