为什么不能输出所有数据?

package com.demon.util;
    
    import java.io.File;
    import java.io.IOException;
    import java.lang.reflect.Field;
    import java.util.ArrayList;
    
    import com.demon.bean.Book;
    
    import jxl.Workbook;
    import jxl.write.Label;
    import jxl.write.WritableSheet;
    import jxl.write.WritableWorkbook;
    import jxl.write.WriteException;
    
    public  class ExcelUtil {
    public static void excelOut(ArrayList ar, String str) {
    WritableWorkbook book = null; 
    try {
    book = Workbook.createWorkbook(new File(str));
    WritableSheet sheet = book.createSheet("sheet", 0);
    for (int i = 0; i < ar.size(); i++) {
    Object ob = ar.get(i);
    Class cl = ob.getClass();
    
    Field[] fi = cl.getDeclaredFields();
    for (int j = 0; j < fi.length; j++) {
    fi[j].setAccessible(true);
    Label la = new Label(j, i, String.valueOf(fi[j].get(ob)));
    sheet.addCell(la);
    }
    
    book.write();
    }
    
    } catch (Exception e) {
    // TODO: handle exception
    } finally {
    try {
    book.close();
    } catch (WriteException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
    } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
    }
    }
    }
    public static void main(String[] args) {
    ArrayList<Book> ar=new ArrayList<Book>();
    Book bo=new Book();
    bo.setId(1);
    bo.setName("月子");
    bo.setType("生活");
    
    Book bo2=new Book();
    bo2.setId(2);
    bo2.setName("日子");
    bo2.setType("生活");
          
    ar.add(bo);
    ar.add(bo2); 
    ExcelUtil.excelOut(ar, "/Users/piepie/Desktop/boos.xls");
       
    } 
    
    }