I have a problem wth java.sql.sqlexception: the url cannot be null and exception in thread "AWT-eventqueue-0" java.lang.nullpointerexception plzz help me

Question

3.00/5 (2 votes)

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package qlsach;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.logging.Level;
import java.util.logging.Logger;
public class DbConnect {

Connection con;
private String stringCon;

public DbConnect() throws SQLException{
String Stringcon;
Stringcon = "jdbc:sqlserver://MSI:1433;databaseName=QLSach";
try{
con = DriverManager.getConnection(stringCon, "sa", "sa");
}
catch (SQLException ex){
Logger.getLogger(DbConnect.class.getName()).log(Level.SEVERE, null, ex);
}

}
public ResultSet getData(String stringSQL){
ResultSet rs = null;
try{
Statement state = con.createStatement();
rs = state.executeQuery(stringSQL);
} catch (SQLException ex){
Logger.getLogger(DbConnect.class.getName()).log(Level.SEVERE, null, ex);
}
return rs;
}
public int ExcuteSQLInsert(String[] stringSQL){
int rowsInserted = 0;
String sql = "INSERT INTO Sach(MaSach,TenSach,TheLoai,NXB,Sotrang,GiaTien ) VALUE (?,?,?,?,?,?)";
PreparedStatement statement;
try{
statement = con.prepareStatement(sql);
statement.setString(1, stringSQL[0]);
statement.setString(2, stringSQL[1]);
statement.setString(3, stringSQL[2]);
statement.setString(4, stringSQL[3]);
statement.setString(5, stringSQL[4]);
statement.setString(6, stringSQL[5]);
rowsInserted = statement.executeUpdate();
}
catch(SQLException ex){
Logger.getLogger(DbConnect.class.getName()).log(Level.SEVERE, null, ex);
}
return rowsInserted;
}

public int ExcuteSQLUpdate(String[] stringSQL){
int rowsInserted = 0;
String sql = "UPDATE Sach SET TenSach=?,TheLoai=?,NXB=?,SoTrang=?,GiaTien=? WHERE MaSach=?";
PreparedStatement statement;
try{
statement = con.prepareStatement(sql);
statement.setString(1, stringSQL[0]);
statement.setString(2, stringSQL[1]);
statement.setString(3, stringSQL[2]);
statement.setString(4, stringSQL[3]);
statement.setString(5, stringSQL[4]);
statement.setString(6, stringSQL[5]);
rowsInserted = statement.executeUpdate();
}
catch(SQLException ex){
Logger.getLogger(DbConnect.class.getName()).log(Level.SEVERE, null, ex);
}
return rowsInserted;
}
public int ExcuteSQLDelete(String[] stringSQL){
int rowsInserted = 0;
String sql = "DELETE FORM Sach WHERE MaSach=?";
PreparedStatement statement;
try{
statement = con.prepareStatement(sql);
statement.setString(1, stringSQL[0]);
rowsInserted = statement.executeUpdate();
}
catch(SQLException ex){
Logger.getLogger(DbConnect.class.getName()).log(Level.SEVERE, null, ex);
}
return rowsInserted;
}
}

What I have tried:

run:
Sep 25, 2020 2:08:41 AM qlsach.DbConnect <init>
SEVERE: null
java.sql.SQLException: The url cannot be null
	at java.sql.DriverManager.getConnection(DriverManager.java:649)
	at java.sql.DriverManager.getConnection(DriverManager.java:247)
	at qlsach.DbConnect.<init>(DbConnect.java:31)
	at qlsach.frmQuanLySach.<init>(frmQuanLySach.java:28)
	at qlsach.frmQuanLySach.lambda$main$0(frmQuanLySach.java:585)
	at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
	at java.awt.EventQueue.access$500(EventQueue.java:97)
	at java.awt.EventQueue$3.run(EventQueue.java:709)
	at java.awt.EventQueue$3.run(EventQueue.java:703)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at qlsach.frmQuanLySach.showData(frmQuanLySach.java:43)
	at qlsach.frmQuanLySach.<init>(frmQuanLySach.java:29)
	at qlsach.frmQuanLySach.lambda$main$0(frmQuanLySach.java:585)
	at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
	at java.awt.EventQueue.access$500(EventQueue.java:97)
	at java.awt.EventQueue$3.run(EventQueue.java:709)
	at java.awt.EventQueue$3.run(EventQueue.java:703)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
BUILD SUCCESSFUL (total time: 1 second)

Posted 24-Sep-20 9:14am

Member 14947922

Updated 24-Sep-20 16:34pm

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3 solutions

Solution 1

Reference: How to resolve the java.lang.NullPointerException[^]

Quote:
In Java, the java.lang.NullPointerException is thrown when a reference variable is accessed (or de-referenced) and is not pointing to any object. This error can be resolved by using a try-catch block or an if-else condition to check if a reference variable is null before dereferencing it.

Your case, error and line of code is pretty clear:
i.e. while accessing data for method qlsach.frmQuanLySach.showData(frmQuanLySach.java:43), it threw a sqlexception at java.sql.DriverManager.getConnection(DriverManager.java:649)

Debug and see why that is null. Set it up correctly and handle null condition.

If needed, look:
jdb - The Java Debugger[^]
Debugging the Eclipse IDE for Java Developers | The Eclipse Foundation[^]

Posted 24-Sep-20 10:10am

Sandeep Mewara

Solution 3

You have two variables with almost the same names: stringCon and StringCon. You send the uninitialised one to getConnection.

Java

private String stringCon; // NB this reference is null


public DbConnect() throws SQLException{
String Stringcon; // NB this is the one with the URL
Stringcon = "jdbc:sqlserver://MSI:1433;databaseName=QLSach";
try{
// NB and here you pass the null reference
con = DriverManager.getConnection(stringCon, "sa", "sa");
}

Remove the private one from the class as it is never required.

Posted 24-Sep-20 11:07am

Richard MacCutchan

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raddevus · Accepted Answer · 2020-09-24T11:01:00

Check out YOUR code...

Stringcon = "jdbc:sqlserver://MSI:1433;databaseName=QLSach";
try{
con = DriverManager.getConnection(stringCon, "sa", "sa");
}

Compare the case. These are not the same two variables.
Change the 2nd line to :