嗨，有一个方法可以创建compMac对象的LinkedList。 CompMac对象包含来自mysql表行的元素。每行包含一个昵称和位置，因此我使用循环为每行创建一个“ compMac”并将其添加到链表中。我不知道为什么，但是创建了第一个'compMac'并将其添加到链表中，但是之后我得到了
java.sql.SQLException: Column 'nickname' not found.
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1078)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:989)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:975)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:920)
at com.mysql.jdbc.ResultSetImpl.findColumn(ResultSetImpl.java:1163)
at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5729)
at server.DbConnect.getArray(DbConnect.java:224)
at server.doComms.sendArray(doComms.java:127)
at server.doComms.whileChatting(doComms.java:107)
at server.doComms.run(doComms.java:32)
at java.lang.Thread.run(Thread.java:695)
这是我的方法：
public LinkedList<compMac> getArray()
{
LinkedList<compMac> locArray = new LinkedList<compMac>();
try{
String query = "SELECT * FROM online";
rs = st.executeQuery(query);
while(rs.next()){
String nn = rs.getString("nickname");
String loc = rs.getString("location");//mac addresses
String cX=getCoorX(loc);
String cY=getCoorY(loc);
String building=getBuilding(loc);
String floor=getFloor(loc);
locArray.add(new compMac(nn,loc,cX,cY,building,floor));
}
}
catch(Exception e){e.printStackTrace();}
return locArray;
}
但是我确实有一个昵称列，就像我说的那样，创建了第一个compMac，当我弹出链表时，我得到了第一行的信息。
感谢任何人知道我的代码有什么问题。
编辑：这里的类代码：
包服务器；
导入java.sql。*;
导入java.util.LinkedList;
import com.example.client.Mac;
import com.example.client.compMac;
public class DbConnect {
private Connection con;
private Statement st;
private Statement stClean;
private ResultSet rs;
public DbConnect(){
try{
Class.forName("com.mysql.jdbc.Driver").newInstance();
con = DriverManager.getConnection("jdbc:mysql://melucio.no-ip.biz:3306/TOC","XXXXX","XXXXXXX");
st = con.createStatement();
stClean = con.createStatement();
}catch(Exception ex){
System.out.println("Error: "+ex);
}
}
/*This method returns the x coor. of mac address*/
public String getCoorX(String mac){
String cX = "";
try{
String query = "SELECT * FROM coordinate";
rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("coorX");
}
}
}
catch(Exception e){}
return cX;
}
/*This method returns the y coor. of mac address*/
public String getCoorY(String mac){
String cY = "";
try{
String query = "SELECT * FROM coordinate";
rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("coorY");
}
}
}
catch(Exception e){}
return cY;
} public String getBuilding(String mac){
String buil = "";
try{
String query = "SELECT * FROM coordinate";
rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("building");
}
}
}
catch(Exception e){}
return buil;
}
public String getFloor(String mac){
String fl = "";
try{
String query = "SELECT * FROM coordinate";
rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("floor");
}
}
}
catch(Exception e){}
return fl;
}
/*Update the location table*/
public void updateLocation(String userNN,String location){//,long time){
long time = System.currentTimeMillis();
if(!isOnline(userNN))
{System.out.println("not online");
setOnline(userNN,location);}
else{
try{
//String query ="UPDATE online SET location = '" +location +"' WHERE nickname='" +userNN +"'";
String query ="UPDATE online SET location = '" +location +"', lastSeen = "+time +" WHERE nickname='" +userNN +"'";
st.executeUpdate(query);
}
catch(Exception e){System.out.println("ayayaya");}
}
}
public LinkedList<compMac> getArray()
{
LinkedList<compMac> locArray = new LinkedList<compMac>();
try{
String query = "SELECT * FROM online";
rs = st.executeQuery(query);
while(rs.next()){
String nn = rs.getString("nickname");
String loc = rs.getString("location");//mac addresses
String cX=getCoorX(loc);
String cY=getCoorY(loc);
String building=getBuilding(loc);
String floor=getFloor(loc);
locArray.add(new compMac(nn,loc,cX,cY,building,floor));
}
}
catch(Exception e){e.printStackTrace();}
return locArray;
}
public void clean()
{
long currentTime = System.currentTimeMillis();
long acceptableTime = currentTime-120000;
try{
String query = "SELECT * FROM online";
rs = st.executeQuery(query);
while(rs.next()){
String nn = rs.getString("nickname");
Long ls = rs.getLong("lastSeen");
if(ls<acceptableTime)//{}
{stClean.executeUpdate("DELETE FROM online WHERE nickname='"+nn+"'");}
}
}
catch(Exception e){e.printStackTrace();}
}
}
最佳答案
根据您的代码，您将以不同的方法重用同一Statement对象。因此，在第一次迭代之后，最后一个方法getFloor更改了st对象以在坐标表上进行查询。我建议您在每个方法中初始化单独的Statement对象，并在finally块中将其关闭。
import com.example.client.Mac;
import com.example.client.compMac;
public class DbConnect {
private Connection con;
public DbConnect(){
try{
Class.forName("com.mysql.jdbc.Driver").newInstance();
con = DriverManager.getConnection("jdbc:mysql://melucio.no-ip.biz:3306/TOC","XXXXX","XXXXXXX");
}catch(Exception ex){
System.out.println("Error: "+ex);
}
}
/*This method returns the x coor. of mac address*/
public String getCoorX(String mac){
String cX = "";
try{
String query = "SELECT * FROM coordinate";
Statement st = con.createStatement();
ResultSet rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("coorX");
}
}
rs.close();
st.close();
}
catch(Exception e){}
return cX;
}
/*This method returns the y coor. of mac address*/
public String getCoorY(String mac){
String cY = "";
try{
String query = "SELECT * FROM coordinate";
Statement st = con.createStatement();
ResultSet rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("coorY");
}
}
rs.close();
st.close();
}
catch(Exception e){}
return cY;
} public String getBuilding(String mac){
String buil = "";
try{
String query = "SELECT * FROM coordinate";
Statement st = con.createStatement();
ResultSet rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("building");
}
}
rs.close();
st.close();
}
catch(Exception e){}
return buil;
}
public String getFloor(String mac){
String fl = "";
try{
String query = "SELECT * FROM coordinate";
Statement st = con.createStatement();
ResultSet rs = st.executeQuery(query);
while(rs.next()){
if(rs.getString("mac").equals(mac)){
return rs.getString("floor");
}
}
rs.close();
st.close();
}
catch(Exception e){}
return fl;
}
/*Update the location table*/
public void updateLocation(String userNN,String location){//,long time){
long time = System.currentTimeMillis();
if(!isOnline(userNN))
{System.out.println("not online");
setOnline(userNN,location);}
else{
try{
//String query ="UPDATE online SET location = '" +location +"' WHERE nickname='" +userNN +"'";
String query ="UPDATE online SET location = '" +location +"', lastSeen = "+time +" WHERE nickname='" +userNN +"'";
st.executeUpdate(query);
}
catch(Exception e){System.out.println("ayayaya");}
}
}
public LinkedList<compMac> getArray()
{
LinkedList<compMac> locArray = new LinkedList<compMac>();
try{
String query = "SELECT * FROM online";
Statement st = con.createStatement();
ResultSet rs = st.executeQuery(query);
while(rs.next()){
String nn = rs.getString("nickname");
String loc = rs.getString("location");//mac addresses
String cX=getCoorX(loc);
String cY=getCoorY(loc);
String building=getBuilding(loc);
String floor=getFloor(loc);
locArray.add(new compMac(nn,loc,cX,cY,building,floor));
}
rs.close();
st.close();
}
catch(Exception e){e.printStackTrace();}
return locArray;
}
public void clean()
{
long currentTime = System.currentTimeMillis();
long acceptableTime = currentTime-120000;
try{
String query = "SELECT * FROM online";
Statement st = con.createStatement();
ResultSet rs = st.executeQuery(query);
while(rs.next()){
String nn = rs.getString("nickname");
Long ls = rs.getLong("lastSeen");
if(ls<acceptableTime)//{}
{stClean.executeUpdate("DELETE FROM online WHERE nickname='"+nn+"'");}
}
}
catch(Exception e){e.printStackTrace();}
}