jsp中Dao的错误. - 黑马程序员技术交流社区

package dao;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.text.SimpleDateFormat;
import java.util.Date;
import com.yourcompany.struts.form.BorrowForm;
import common.DbConnection;
public class BorrowDao
{
public int borrowBook(BorrowForm bf)
{
int a = 0;
String sno = bf.getSno();
String book_id = bf.getBook_id();
String tname = bf.getTname();
String id = bf.getId();
Connection con = DbConnection.getConnection();
long time=System.currentTimeMillis();
long rtime=20*1000*60*60*24; //可借20天
long returntime = time+rtime;
SimpleDateFormat format1 =new SimpleDateFormat("yyyy-MM-dd");
Date btime = new Date();
String ndate = format1.format(btime);
System.out.println(ndate);
java.sql.Date obtime = java.sql.Date.valueOf(ndate);
SimpleDateFormat format2 = new SimpleDateFormat("yyyy-MM-dd");
Date myDate = new Date(returntime);
String returntime1 = format2.format(myDate);
java.sql.Date returntime2= java.sql.Date.valueOf(returntime1);
System.out.println(returntime1);
int nownum=0,borrowed=0; //booksinfo表中的现有数量-1,借阅次数+1;
boolean bool=true;
try {
PreparedStatement prst2;
prst2 = con.prepareStatement("select nownum,borrowed from booksinfo where id=?"); [color=Red] //提示这行有错误!!!空指针异常.哪异常?[/color]
prst2.setString(1, book_id);
ResultSet rs2 = prst2.executeQuery();
if(rs2.next())
{
nownum=rs2.getInt(1);
borrowed=rs2.getInt(2);
if(nownum<=0)
{ //防止图书借没了还能借出
bool=false;
}
else
{
nownum--;
borrowed++;
}
}
} catch (SQLException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
if(bool)
{
try
{
PreparedStatement prst3;
prst3 = con.prepareStatement("update booksinfo set nownum="+nownum+",borrowed="+borrowed+" where id='"+book_id+"'");
PreparedStatement prst4 =
con.prepareStatement
("insert into borrowinfo (id,sno,tname,book_id,borrowedtime,returntime) values('"+id+"','"+sno+"','"+tname+"','"+book_id+"','"+obtime+"','"+returntime2+"'");
a = prst4.executeUpdate();
con.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return a;
}
private SimpleDateFormat SimpleDateFormat(String string) {
// TODO Auto-generated method stub
return null;
}
}

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