为什么java.util.list会添加以前的数据，而在添加新数据时会清除这些数据

Question

I created a spider program .

There is a problem that some data are repeated when inserting data into MySQL database.

For example,when the program have inserted the data of A,it firstly inserts some values of A's field into database before inserting the data of B,then inserting B.

in database>>>>>

please click to check the exmple

B does not have the red data but still inserted . I cleared list before inserting B ,however it does not work.

When I debuging:

inserting A_list [A.size is 45,list.size=45]

list.clear() [list=null]

list=getB()

inserting B_list [list.size=45+49=94][B.size is 49]

why is the size of list at this time 94? It should be 49.list added data of A again .

Here are my codes:

private static void Insert(List<User> rootusers) throws Exception{
    String sql="insert into t_userlist(userid,username,rootuserid,rootusername,visited) values(?,?,?,?,?)";
    String sqlupdate="update t_userlist set visited=1 where userid=?";
    String sqlvisited="SELECT * from t_userlist where visited=1 and userid=";
    Connection conn=null;
    PreparedStatement psmt = null;
    PreparedStatement ps;
    for(int i=0;i<rootusers.size();i++){
        User user=rootusers.get(i);

        List<User> userlist=htmlParser.parseUserlist(user);

        if(userlist!=null){
            conn=DbUtil.getConn();
            conn.setAutoCommit(false);
            try {
                for(int j=0;j<userlist.size();j++){


                        int visitedUser=selectUserID(sqlvisited+"'"+userlist.get(j).getUserid()+"'").size();

                        psmt=conn.prepareStatement(sql);
                        psmt.setString(1, userlist.get(j).getUserid());
                        psmt.setString(2, userlist.get(j).getUsername());
                        psmt.setString(3, user.getUserid());
                        psmt.setString(4, user.getUsername());
                        if(visitedUser!=0){
                            psmt.setInt(5, 1);
                        }
                        else {
                            psmt.setInt(5, 0);
                        }
                        psmt.executeUpdate();
                        psmt.close();
                }


                ps=conn.prepareStatement(sqlupdate);
                ps.setString(1,user.getUserid());
                ps.executeUpdate();
                conn.commit();
                ps.close();
                DbUtil.closeConn(conn);
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }catch (Exception e) {
                try {
                    if(conn!=null)
                    {
                          conn.rollback(); 
                         // conn.setAutoCommit( true);
                    }
                } catch (Exception e1) {

                    e1.printStackTrace();
                }
            }

            **userlist.clear();**
            System.out.println("insert"+user.getUsername()+"successfully");
        }else {
            continue;
        }

    }

}

the htmlParser.parseUserlist(user) codes:

public static ArrayList<User> parseUserlist(User user) throws Exception{
    String userlist=GetUserlist.getUserList(user.getUserid());//just return html 
    ArrayList<User> list=new ArrayList<User>();
    if(userlist!=null){
        Document doc=Jsoup.parse(userlist);
        Elements users=doc.select("a[usercard]");

        for(Element nodeuser:users){
            //System.out.println(nodeuser.toString());
            User u=new User();
            u.setUserid(nodeuser.attr("usercard").substring(3,13));
            u.setUsername(nodeuser.text());
            u.setRootuserid(user.getUserid());
            list.add(u);
            //System.out.println(u.getUserid()+" "+u.getUsername());
        }
    }else {
        System.out.println(user.getUsername()+"没有关注任何人");
    }

    //System.out.println(users.toString());
    //System.out.println(list.get(1).getUsername());
    return list;
}

please help me !

Answer 1

I don't see anything wrong that would be a cause of your problem in the code you have provided. One thing to know is that list.Clear() is not the same as list = null; because an empty list is a list that contains no elements and clear() empties the list. In your insert method you would do better with !userList.isEmpty() (seeing how you are always creating a list and at worst returning an empty list from parseUserList.