初识EasyExcel

1、为什么使用easyexcel

java解析excel的框架有很多 poi jxl 存在问题：非常的消耗内存
easyexcel 我们遇到再大的excel都不会出现内存溢出的问题 能够将一个原本3M excel文件，poi来操作将会占用内存100M，
使用easyexcel降低到几KB，使用起来更加简单

poi读 1、创建xsshworkbook/hssfworkbook (inputstream in)
      2、读取sheet
      3、拿到当前sheet所有行row
      4、通过当前行去拿到对应的单元格的值

     
2、easyexcel拟解决的问题

1、excel读写时内存溢出
2、使用简单
3、excel格式解析

3、工作原理

4.springboot集成EasyExcel，使用jpa查询数据

引入依赖：

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>easyexcel</artifactId>
    <version>2.2.6</version>
</dependency>

package com.cxb.excel.entity;

import com.alibaba.excel.annotation.ExcelIgnore;
import com.alibaba.excel.annotation.ExcelProperty;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

/**
 * @author 小石潭记
 * @date 2021/7/5 10:09
 * @Description: ${todo}
 */
@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
@Table(name = "t_area")
public class TArea {

    @Id
    @GeneratedValue
    @ExcelProperty(index = 0, value = "城市ID")
    private int id;

    @ExcelProperty(index = 1, value = "城市名称")
    private String areaName;

    @ExcelProperty(index = 2, value = "父级城市ID")
    private int parentId;

    @ExcelProperty(index = 3, value = "父级城市名称")
    private String parentName;
    @ExcelIgnore
    private String level;
}

package com.cxb.excel.respository;

import com.cxb.excel.entity.TArea;
import org.springframework.data.repository.PagingAndSortingRepository;
import org.springframework.stereotype.Repository;

/**
 * @author 小石潭记
 * @date 2021/7/5 10:18
 * @Description: ${todo}
 */
@Repository
public interface TAreaRepository extends PagingAndSortingRepository<TArea, Integer> {
}

package com.cxb.excel.listener;

import com.alibaba.excel.context.AnalysisContext;
import com.alibaba.excel.event.AnalysisEventListener;
import com.alibaba.excel.metadata.CellData;
import com.cxb.excel.entity.TArea;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * @author 小石潭记
 * @date 2021/7/6 14:18
 * @Description: ${todo} 该listener里面不能依赖注入，可以通过构造器传值，因为该listener不是spring管理的
 */
public class EasyExcelListener extends AnalysisEventListener<TArea> {

    private List<TArea> tAreaList = new ArrayList<>();

    /**
     * 校验excel文件的头部是否被修改
     * @param headMap
     * @param context
     */
    @Override
    public void invokeHead(Map<Integer, CellData> headMap, AnalysisContext context) {
        System.out.println("校验表格头部是否修改");
    }

    /**
     * 每一行数据
     * @param tArea
     * @param analysisContext
     */
    @Override
    public void invoke(TArea tArea, AnalysisContext analysisContext) {
        System.out.println("读取每一行数据");
        tAreaList.add(tArea);
    }

    /**
     * 读取完成执行
     * @param analysisContext
     */
    @Override
    public void doAfterAllAnalysed(AnalysisContext analysisContext) {
        System.out.println("数据读取完成" + tAreaList.size());
    }

    /**
     * 读取异常时的处理
     * @param exception
     * @param context
     * @throws Exception
     */
    @Override
    public void onException(Exception exception, AnalysisContext context) throws Exception {
        System.out.println("数据读取异常");
    }
}

package com.cxb.excel.controller;

import com.alibaba.excel.EasyExcel;
import com.alibaba.excel.ExcelReader;
import com.alibaba.excel.ExcelWriter;
import com.alibaba.excel.read.metadata.ReadSheet;
import com.alibaba.excel.write.metadata.WriteSheet;
import com.cxb.excel.entity.TArea;
import com.cxb.excel.listener.EasyExcelListener;
import com.cxb.excel.respository.TAreaRepository;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.util.ClassUtils;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

/**
 * @author 小石潭记
 * @date 2021/7/5 14:10
 * @Description: ${todo}
 */
@Slf4j
@RestController
@RequestMapping("/area")
public class ExportController {
    @Autowired
    private TAreaRepository tAreaRepository;

    @GetMapping("/export")
    private void export() {
        try {
            String filePath = ClassUtils.getDefaultClassLoader().getResource("").getPath() + "export.xlsx";
            ExcelWriter excelWriter = EasyExcel.write(filePath, TArea.class).build();
            int count = (int) tAreaRepository.count();
            // 总页数
            int pageSize = count % 500 == 0 ? count / 500 : count / 500 + 1;
            for (int i = 0; i < pageSize; i++) {
                PageRequest pageRequest = PageRequest.of(i, 500);
                Page<TArea> allPage = tAreaRepository.findAll(pageRequest);
                WriteSheet sheet = EasyExcel.writerSheet(i).build();
                excelWriter.write(allPage.getContent(), sheet);
            }
            excelWriter.finish();
        } catch (Exception e) {
            log.error("导出异常{}", e);
        }
        System.out.println("导出成功");
    }

    @GetMapping("/import")
    private void importExcel() {
        String filePath = ClassUtils.getDefaultClassLoader().getResource("").getPath() + "export.xlsx";
        ExcelReader build = EasyExcel.read(filePath, TArea.class, new EasyExcelListener()).build();
        ReadSheet readSheet = EasyExcel.readSheet(0).build();
        build.read(readSheet);
        build.finish();
        System.out.println("导入成功");
    }
}