update:避免大量内存占用
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---
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id="2018-09-22-15-57-00"
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id="2018-09-22-15-57"
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title="java导出EXCEL文件"
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headWord="最近在java上做了一个EXCEL的导出功能，写了一个通用类，在这里分享分享，该类支持多sheet，且无需手动进行复杂的类型转换."
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tags=["reflex", "java","excel","hssfworksheet"]
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tags=["reflex", "java","excel","SXSSFWorksheet"]
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category="java"
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serie="java工具集"
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---
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**本篇原创发布于：**[FleyX 的个人博客](tapme.top/blog/detail/2018-09-22-15-57)
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**本篇所用到代码**：[github](https://github.com/FleyX/demo-project/blob/master/%E6%9D%82%E9%A1%B9/excel%E5%AF%BC%E5%87%BA.java)
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**更新说明：**之前用的`HSSFWorkbook`有爆内存的风险，当数据量有几百万时该对象会占用大量内存。更换为`SXSSFWorkbook`可解决内存占用。
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## 一、背景
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  最近在 java 上做了一个 EXCEL 的导出功能，写了一个通用类，在这里分享分享，该类支持多 sheet，且无需手动进行复杂的类型转换，只需提供三个参数即可：
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@ -83,7 +89,7 @@ headers.put("ASheet",sheetA);
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headers.put("BSheet",sheetB);
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ExcelUtil excelUtil = new ExcelUtil(fileName,data,headers);
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//获取表格对象
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HSSFWorkbook workbook = excelUtil.createExcel();
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SXSSFWorkbook workbook = excelUtil.createExcel();
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//这里内置了一个写到response的方法（判断浏览器类型设置合适的参数），如果想写到文件也是类似的
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workbook.writeToResponse(workbook,request,response);
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```
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@ -97,9 +103,10 @@ workbook.writeToResponse(workbook,request,response);
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#### 1、遍历 headers 创建 sheet
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```java
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public HSSFWorkbook createExcel() throws Exception {
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public SXSSFWorkbook createExcel() throws Exception {
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try {
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HSSFWorkbook workbook = new HSSFWorkbook();
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//只在内存中保留五百条记录，五百条之前的会写到磁盘上，后面无法再操作
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SXSSFWorkbook workbook = new SXSSFWorkbook(500);
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//遍历headers创建表格
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for (String key : headers.keySet()) {
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this.createSheet(workbook, key, headers.get(key), this.data.get(key));
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@ -119,24 +126,44 @@ workbook.writeToResponse(workbook,request,response);
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  表头也就是一个表格的第一行，通常用来对列进行说明
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```java
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HSSFSheet sheet = workbook.createSheet(sheetName);
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// 列数
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int cellNum = header.length;
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private void createSheet(SXSSFWorkbook workbook, String sheetName, String[][] header, List<?> data) throws Exception {
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Sheet sheet = workbook.createSheet(sheetName);
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// 单元行，单元格
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HSSFRow row;
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HSSFCell cell;
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// 表头单元格样式
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HSSFCellStyle columnTopStyle = this.getColumnTopStyle(workbook);
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// 设置表头
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Row row;
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Cell cell;
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//列数
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int cellNum = header.length;
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//设置表头
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row = sheet.createRow(0);
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for (int i = 0; i < cellNum; i++) {
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cell = row.createCell(i);
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cell.setCellStyle(columnTopStyle);
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String str = header[i][1];
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cell.setCellValue(str);
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// 设置列宽为表头的文字宽度+6个半角符号宽度
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//设置列宽为表头的宽度+4
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sheet.setColumnWidth(i, (str.getBytes("utf-8").length + 6) * 256);
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}
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int rowNum = data.size();
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if (rowNum == 0) {
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return;
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}
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//获取Object 属性名与field属性的映射，后面通过反射获取值来设置到cell
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Field[] fields = data.get(0).getClass().getDeclaredFields();
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Map<String, Field> fieldMap = new HashMap<>(fields.length);
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for (Field field : fields) {
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field.setAccessible(true);
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fieldMap.put(field.getName(), field);
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}
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Object object;
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for (int i = 0; i < rowNum; i++) {
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row = sheet.createRow(i + 1);
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object = data.get(i);
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for (int j = 0; j < cellNum; j++) {
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cell = row.createCell(j);
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this.setCell(cell, object, fieldMap, header[j][0]);
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}
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}
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}
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```
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#### 3、插入行数据
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@ -144,44 +171,41 @@ workbook.writeToResponse(workbook,request,response);
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  这里是最重要的部分，首先通过数据的类对象获取它的反射属性 Field 类，然后将属性名和 Field 做一个 hash 映射，避免循环查找，提高插入速度，接着通过一个 switch 语句，根据属性类别设值，主要代码如下：
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```java
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/**
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* 设置单元格,根据fieldName获取对应的Field类，使用反射得到值
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*
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* @param cell 单元格实例
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* @param obj 存有属性的对象实例
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* @param fieldMap 属性名与Field的映射
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* @param fieldName 属性名
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*/
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private void setCell(HSSFCell cell, Object obj, Map<String, Field> fieldMap, String fieldName) throws Exception {
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//获取该属性的Field对象
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Field field = fieldMap.get(fieldName);
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//通过反射获取属性的值，由于不能确定该值的类型，用下面的判断语句进行合适的转型
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Object value = field.get(obj);
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if (value == null) {
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cell.setCellValue("");
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} else {
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switch (field.getGenericType().getTypeName()) {
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case "java.lang.String":
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cell.setCellValue((String) value);
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break;
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case "java.lang.Integer":
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case "int":
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cell.setCellValue((int) value);
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break;
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case "java.lang.Double":
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case "double":
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cell.setCellValue((double) value);
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break;
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case "java.util.Date":
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cell.setCellValue(this.dateFormat.format((Date) value));
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break;
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default:
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cell.setCellValue(obj.toString());
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private void setCell(Cell cell, Object obj, Map<String, Field> fieldMap, String fieldName) throws Exception {
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Field field = fieldMap.get(fieldName);
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if(field == null){
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throw new Exception("找不到 "+fieldName+" 数据项");
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}
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Object value = field.get(obj);
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if (value == null) {
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cell.setCellValue("");
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} else {
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switch (field.getGenericType().getTypeName()) {
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case "java.lang.String":
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cell.setCellValue((String) value);
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break;
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case "java.lang.Integer":
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case "int":
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cell.setCellValue((int) value);
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break;
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case "java.lang.Double":
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case "double":
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cell.setCellValue((double) value);
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break;
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case "java.util.Date":
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cell.setCellValue(this.dateFormat.format((Date) value));
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break;
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default:
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cell.setCellValue(obj.toString());
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}
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}
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}
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}
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```
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完整代码可以到 github 上查看下载，这里就不列出来了。
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github 地址：[点击跳转]()
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github 地址：
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**本篇所用到代码**：[github](https://github.com/FleyX/demo-project/blob/master/%E6%9D%82%E9%A1%B9/excel%E5%AF%BC%E5%87%BA.java)
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**本篇原创发布于：**[FleyX 的个人博客](tapme.top/blog/detail/2018-09-22-15-57)
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