java mysql 问题 rs = stmt.executeQuery(sql); 这一句不能执行，怎么改啊

public void showpaper()
{

try{
Statement stmt=null;
ResultSet rs=null;
String sql;
sql="select first from timu";
System.out.print("1");
rs = stmt.executeQuery(sql);

System.out.print("2");

//text_paper.setText("sssss");

while(rs.next())
{
String sno = rs.getString("first");
text_paper.setText("sno");
}

// con.select(sql,tmp_q);
}
catch(Exception e)
{
System.out.print("3");

}

}

//ConnectDB.java
public class ConnectDB
{
private Connection connection;

Statement stat = null;
PreparedStatement pstat = null;
ResultSet rs = null;
public Connection getCon(){
try{
Class.forName("com.mysql.jdbc.Driver").newInstance();
// String url = "jdbc:mysql://localhost:8080/test1?user=root&password=admin&useUnicode=true&characterEncoding=gb2312";
// con = DriverManager.getConnection(url);
String connName = "jdbc:mysql://127.0.0.1/test";
connection = DriverManager.getConnection(connName,"root","admin");

}catch(Exception ex){

ex.printStackTrace();
}
return connection;
}

public ResultSet query(String sql){
try{
connection = getCon();
stat = connection.createStatement();
rs = stat.executeQuery(sql);

}catch(Exception ex){

ex.printStackTrace();
}
return rs;
}

}

你的showpaper()方法中，Statement stmt=null;然后执行 rs = stmt.executeQuery(sql);
stmt都为null，怎么可能执行executeQuery(sql)呢；肯定会报空指针异常呀
给stmt赋值一下，如 stmt = connection.createStatement(); 试试

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