需求： 现在项目中需要对标题的字体进行格式设置，（）的字体进行缩小
实际效果图：

核心代码

//设置同一个单元格多个字体样式
HSSFRichTextString textString = new HSSFRichTextString(fileHead);
textString.applyFont(0,fileHead.indexOf("("), font);
textString.applyFont(fileHead.indexOf("("),fileHead.length(), font3);
cell.setCellValue(textString);

利用applyFont可以分段对文字进行处理，applyFont(样式生效起始index,样式生效结束index, 字体样式)，index是从0开始。
最后一个参数也可以直接传递字体的大小，textString.applyFont(0,fileHead.indexOf("("), （short）15);
注意样式不要重复设置，样式重复会报错。
下面是完整代码

String fileHead="测试数据信息(2022-2-12至2022-03-14)"; 
String[] headStr = {"序号", "测试1", "测试2"};
Integer[] cellWidth = {1800, 4500, 5000};
String fileName = fileHead + ".xls";

Workbook wb = new HSSFWorkbook();

/**创建一个SHEET**/
Sheet sheet = wb.createSheet(fileHead);
Row row = null;
 /**设置大标题**/
CellStyle cellStyle = wb.createCellStyle();
cellStyle.setAlignment(HorizontalAlignment.CENTER);
cellStyle.setVerticalAlignment(VerticalAlignment.CENTER);
//标题字体
Font font = wb.createFont();
font.setFontName("楷体");
font.setFontHeightInPoints((short) 20);
font.setBold(true);
//            日期的字体样式
Font font3 = wb.createFont();
font3.setFontName("楷体");
font3.setFontHeightInPoints((short) 15);
//标题内容行
row = sheet.createRow(0);
row.setHeight((short) 700);
/**合并第一行的列**/
CellRangeAddress titleCRA = new CellRangeAddress(0, 0, 0, headStr.length - 1);
sheet.addMergedRegion(titleCRA);
Cell cell = row.createCell(0);
//设置同一个单元格多个字体样式
HSSFRichTextString textString = new HSSFRichTextString(fileHead);
textString.applyFont(0,fileHead.indexOf("("), font);
textString.applyFont(fileHead.indexOf("("),fileHead.length(), font3);
cell.setCellStyle(cellStyle);
cell.setCellValue(textString);

//... 省略后面的填充数据的过程

package com.hf;


import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.UnsupportedEncodingException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;


import org.apache.poi.hssf.usermodel.HSSFCell;
import org.apache.poi.hssf.usermodel.HSSFCellStyle;
import org.apache.poi.hssf.usermodel.HSSFRow;
import org.apache.poi.hssf.usermodel.HSSFSheet;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.hssf.util.HSSFColor;
import org.apache.poi.poifs.filesystem.POIFSFileSystem;
import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.usermodel.CellStyle;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.util.CellRangeAddress;


public class Main {
private static int index;


/**
* @param args
*/
public static void main(String[] args) {

try {
createXLS();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}


}
public static void createXLS() throws Exception{

File fi=new File("D:/students.xls");
POIFSFileSystem fs = new POIFSFileSystem(new FileInputStream(fi));
HSSFWorkbook wb = new HSSFWorkbook(fs);
HSSFSheet sheet = wb.getSheetAt(0);
TempCell  tempCell = new TempCell();
tempCell.setRow(1);
  //获取模板指定单元格样式,设置到tempCell(写列表数据的时候用)
    Row rowIn = sheet.getRow(tempCell.getRow());
    if (rowIn == null) {
        rowIn = sheet.createRow(tempCell.getRow());
    }
    Cell cellIn = rowIn.getCell(tempCell.getColumn());
    if (cellIn == null) {
        cellIn = rowIn.createCell(tempCell.getColumn());
    }
    tempCell.setCellStyle(cellIn.getCellStyle());

    HSSFRow row = sheet.createRow((int)2);
HSSFCell cell = row.createCell((short)2);  
    List list = Main.getStudent();
    for (int i = 0; i < list.size(); i++) {  
        Student stu = (Student) list.get(i); 
        row = sheet.createRow((int) i + 2);
        cell = row.createCell((short) 0);
        cell.setCellValue((double) stu.getId());
        cell.setCellStyle(tempCell.getCellStyle());
        cell= row.createCell((short) 1);
        cell.setCellValue(stu.getName());
        cell.setCellStyle(tempCell.getCellStyle());
        cell= row.createCell((short) 2);
        cell.setCellValue((double) stu.getAge());
        cell.setCellStyle(tempCell.getCellStyle());
        cell = row.createCell((short) 3); 
        cell.setCellValue(new SimpleDateFormat("yyyy-mm-dd").format(stu  
                .getBirth()));
        cell.setCellStyle(tempCell.getCellStyle());
        index =(int)i+2;
     } 
row = sheet.createRow(index);
cell = row.createCell(0);
cell.setCellValue(index-1);
    cell.setCellStyle(tempCell.getCellStyle());
cell = row.createCell(1);
cell.setCellValue("平均年龄sdfdhgjhklhjlkj;l:");
cell.setCellStyle(tempCell.getCellStyle());
CellRangeAddress cra = new CellRangeAddress(index,index,2,3);
sheet.addMergedRegion(cra);
cell = row.createCell(2);
cell.setCellValue((int)18);
cell.setCellStyle(tempCell.getCellStyle());
cell = row.createCell(3);
cell.setCellStyle(tempCell.getCellStyle());
    
    
FileOutputStream out = new FileOutputStream("D:/students1.xls");
wb.write(out);
out.close();
}

private static List<Student> getStudent() throws Exception{
List list = new ArrayList();
SimpleDateFormat df = new SimpleDateFormat("yyyy-mm-dd");
Student user1 = new Student(1,"张三",12,df.parse("1997-03-12"));
Student user2 = new Student(2,"李四",15,df.parse("1997-03-12"));
Student user3 = new Student(3,"张三张三张三张三",18,df.parse("1997-03-12"));
Student user4 = new Student(4,"张三",21,df.parse("1997-03-12"));
Student user5 = new Student(5,"张三张三张三张三张三张三",17,df.parse("1997-03-12"));
Student user6 = new Student(6,"张三张三张三",19,df.parse("1997-03-12"));
Student user7 = new Student(7,"张三张三张三张三",15,df.parse("1997-03-12"));
Student user8 = new Student(8,"张三张三张三张三张三张三",17,df.parse("1997-03-12"));
Student user9 = new Student(9,"张三张三三张三",12,df.parse("1997-03-12"));
list.add(user1);
list.add(user2);
list.add(user3);
list.add(user4);
list.add(user5);
list.add(user6);
list.add(user7);
list.add(user8);
list.add(user9);
return list;
}

static class  TempCell {
        private int row;
        private int column;
        private CellStyle cellStyle;
        private Object data;
        //用于列表合并,表示几列合并
        private int columnSize = -1;


        public int getColumn() {
            return column;
        }


        public void setColumn(int column) {
            this.column = column;
        }


        public int getRow() {
            return row;
        }


        public void setRow(int row) {
            this.row = row;
        }


        public CellStyle getCellStyle() {
            return cellStyle;
        }


        public void setCellStyle(CellStyle cellStyle) {
            this.cellStyle = cellStyle;
        }


        public Object getData() {
            return data;
        }


        public void setData(Object data) {
            this.data = data;
        }


        public int getColumnSize() {
            return columnSize;
        }


        public void setColumnSize(int columnSize) {
            this.columnSize = columnSize;
        }
    }

}

package com.hf;


import java.util.Date;


public class Student {


private int id;
private String name;
private int age;
private Date birth;

public Student() {

}

public Student(int id,String name,int age,Date birth) {
this.id = id;
this.name = name;
this.age = age;
this.birth = birth;
}


public int getId() {
return id;
}


public void setId(int id) {
this.id = id;
}


public String getName() {
return name;
}


public void setName(String name) {
this.name = name;
}


public int getAge() {
return age;
}


public void setAge(int age) {
this.age = age;
}


public Date getBirth() {
return birth;
}


public void setBirth(Date birth) {
this.birth = birth;
}


}
jar包：http://download.csdn.net/download/u014693839/10195359

效果展示

1. 选中单元格，右键选择“设置单元格格式”
   
2. 数字 — 自定义 — 选择“yyyy/m/d” — 类型 输入8个空格（根据自己需要增减空格）AAAA
   
3. 对齐 — 先勾线“缩小字体填充”，再勾选自动换行（注意：如果"缩小字体填充"呈灰色不能勾选，则需要先取消“自动换行”再进行勾选）