i don't understand why my variable state cannot be resolved.
i'm in a java Mysql project.
Here is the Commands class code:
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
public class Commands {
public Commands() throws SQLException{
Connection conn = DbConn.getInstance();
Statement state = conn.createStatement();
}
public String getList(){
System.out.println("Here is a List of our Products:");
// Get list from db
ResultSet result = state.executeQuery("SELECT * FROM products");
ResultSetMetaData resultMeta = result.getMetaData();
// Display the List
System.out.println("List displayed");
return null;
}
}
Here is the DbConn class code:
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class DbConn {
private static String url = "jdbc:mysql://localhost:3306/myDB";
private static String user = "root";
private static String passwd = "";
private static Connection connect;
// create new instance if not exists
public static Connection getInstance(){
if(connect == null){
try {
connect = DriverManager.getConnection(url, user, passwd);
} catch (SQLException e) {
e.printStackTrace();
}
}
return connect;
}
}
My code is not finished yet, but the message come on this line:
ResultSet result = state.executeQuery("SELECT * FROM products");
My Eclipse editor says this message state cannot be resolved
Any idea?
That is a matter of scope. You define the variable here
public Commands() throws SQLException{
Connection conn = DbConn.getInstance();
Statement state = conn.createStatement();
}
And that is the only place the variable is visible - in the constructor. Define it in the class and initialize it in the constructor:
private Connection conn = null;
private Statement state = null;
public Commands() throws SQLException{
conn = DbConn.getInstance();
state = conn.createStatement();
}
Declare "State" outside of that constructor.
Connection conn = null;
Statement state = null;
public Commands() throws SQLException{
conn = DbConn.getInstance();
state = conn.createStatement();
}
Related
My SQL connection keeps saying it's busy even though all previous connections are closed.
The error below results. All others are either closed by the exiting of the JFrame or the .close() method. Does anyone see anything wrong with the class? (All other classes work as intended.)
SEVERE: null
org.sqlite.SQLiteException: [SQLITE_BUSY] The database file is locked (database is locked)
at org.sqlite.core.DB.newSQLException(DB.java:941)
at org.sqlite.core.DB.newSQLException(DB.java:953)
at org.sqlite.core.DB.execute(DB.java:854)
at org.sqlite.core.DB.executeUpdate(DB.java:895)
package teacherreviewproject;
//initialise imports
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JOptionPane;
public class FeedbackForm extends javax.swing.JFrame {
//init. variables
String WWW;
String EBI;
int rating;
String teacher;
String studentUser;
String ratingraw;
String teacherQuery;
public FeedbackForm(String s) {
initComponents();
getTeachersNames();
this.studentUser = s;
}
private void getTeachersNames(){
//get the connection
Connection con = DBConnection.getConnection();
//set up query string
this.teacherQuery = "SELECT * FROM users WHERE type=2";
try {
//prepare statement
PreparedStatement teacherState = con.prepareStatement(teacherQuery);
//execute query
ResultSet teachResult = teacherState.executeQuery();
//clear previous items to avoid duplicates.
jComboBox_teachers.removeAllItems();
//create counter variable to get different teachers in RS
int i = 0;
//while loop
while(teachResult.next()){
//get username then add it to position i at combobox
String tempOption = teachResult.getString("username");
System.out.println(tempOption);
jComboBox_teachers.addItem(tempOption); //thanks mcalpine
//increment i
i++;
}
} catch (SQLException ex) {
Logger.getLogger(FeedbackForm.class.getName()).log(Level.SEVERE, null, ex);
}
Found the bug! I needed to make a close-if feature on my Connection class.
Here's the code, should anyone want it:
public class DBConnection{
public static Connection con = null;
public static Connection getConnection(){
//initialise connection
try{
//creates valid url to access DB with
String url = "jdbc:sqlite:" + System.getProperty("user.dir") + "/TeacherReviewIA.DB";
if(con == null){
con = (Connection) DriverManager.getConnection(url);
}else{
con.close();
con = (Connection) DriverManager.getConnection(url);
}
//as a debug measure and to show connection given
System.out.println(con);
}
catch(SQLException ex){
JOptionPane.showMessageDialog(null,ex,"WARNING",JOptionPane.WARNING_MESSAGE);
}
//allows code that called method to use connection given
return con;
}
}
I have a login app that needs to connect to a server to check the username and password. I am using netbeans and the jbdc is installed and working in the services tab(thanks stack overflow!). By the jbdc is work I mean that i can execute SQL script through it.
I have set this up with MS Server 16 and MySQL so I am convied it is the code:
Connection method:
package dbUtil;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class dbConnection {
private static final String USERNAME = "root";
private static final String PASSWORD = "mess";
private static final String SQCONN = "jdbc:mysql://localhost:1434/MessyLogin?zeroDateTimeBehavior=convertToNull";
public static Connection getConnection()throws SQLException{
try {
Class.forName("com.mysql.jdbc.Driver");
return DriverManager.getConnection(SQCONN, USERNAME, PASSWORD);
}catch (ClassNotFoundException e) {
}
return null;
}
}
loginmodel:
package LogIn;
import dbUtil.dbConnection;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;
import java.util.logging.Logger;
public class LogInModel {
Connection connection;
public LogInModel() {
try{
this.connection = dbConnection.getConnection();
}catch(SQLException e){
}
if(this.connection == null){
System.out.println("here");
// System.exit(1);
}
}
public boolean isDatabaseConnected(){
return this.connection != null;
}
public boolean isLogin(String username, String password) throws Exception{
PreparedStatement pr = null;
ResultSet rs = null;
String sql = "SELECT * FROM MessyLogin where username = ? and Password = ?";
try{
pr = this.connection.prepareStatement(sql);
pr.setString(1, username);
pr.setString(2, password);
rs = pr.executeQuery();
boolean bool1;
if(rs.next()){
return true;
}
return false;
}
catch(SQLException ex){
return false;
}
finally {
{
pr.close();
rs.close();
}
}
}
}
I believe the issue is the return null; from the dbConnection file. The if(this.connection==Null) comes back true and the system is exiting.
Thank you in advance.
Your dbConnection class is a bad idea. Why hard wire those values when you can pass them in?
Your application will only have one Connection if you code it this way. A more practical solution will use a connection pool.
Learn Java coding standards. Your code doesn't follow them; it makes it harder to read and understand.
Here's a couple of recommendations:
package dbUtil;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class dbConnection {
public static final String DRIVER = "com.mysql.jdbc.Driver";
public static final String USERNAME = "root";
public static final String PASSWORD = "mess";
public static final String URL = "jdbc:mysql://localhost:1434/MessyLogin?zeroDateTimeBehavior=convertToNull";
public static Connection getConnection(String driver, String url, String username, String password) throws ClassNotFoundException, SQLException {
Class.forName(driver);
return DriverManager.getConnection(url, username, password);
}
}
I might write that LogInModel this way:
package LogIn;
import dbUtil.dbConnection;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;
import java.util.logging.Logger;
public class LogInModel {
private static final String sql = "SELECT * FROM MessyLogin where username = ? and Password = ?";
private Connection connection;
public LogInModel(Connection connection) {
this.connection = connection;
}
public boolean isLogin(String username, String password) {
boolean isValidUser = false;
PreparedStatement pr = null;
ResultSet rs = null;
try {
pr = this.connection.prepareStatement(sql);
pr.setString(1, username);
pr.setString(2, password);
rs = pr.executeQuery();
while (rs.hasNext()) {
isValidUser = true;
}
} catch (SQLException ex) {
e.printStackTrace();
isValidUser = false;
} finally {
dbUtils.close(rs);
dbUtils.close(pr);
}
return isValidUser;
}
}
Here's my guess as to why your code fails: You don't have the MySQL JDBC driver JAR in your runtime CLASSPATH. There's an exception thrown when it can't find the driver class, but you didn't know it because you swallowed the exception.
I have one method which get connection from database. I run profiler using visualvm. I found out that the very first time I called method which is to get connection from databse takes longer than the rest.
How to avoid getting database connection multiple times?
It's doesn't matter with java servlet or java program or application server, it is just a design idea.
Here is a simple demo, i use MySQL ,hope to explain this design idea:
Class MyConnection:
package mysql;
import java.sql.Connection;
public class MyConnection {
private Connection connection;
private boolean isUsed;
public void setConnection(Connection connection) {
this.connection = connection;
}
public void setUsed(boolean isUsed) {
this.isUsed = isUsed;
}
public Connection getConnection() {
return connection;
}
public boolean isUsed() {
return isUsed;
}
}
Class MyConnectionPool:
package mysql;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
public class MyConnectionPool {
//this pool will hold the connection
static List<MyConnection> connectionPool = new ArrayList<>();
public MyConnectionPool() {
String mysqlConfigString = "jdbc:mysql://IP:3306/dbname?user=username&password=password";
int size = 0;
while (size++ < 10) {
Connection conn;
try {
conn = DriverManager.getConnection(mysqlConfigString);
MyConnection myConnection = new MyConnection();
myConnection.setConnection(conn);
myConnection.setUsed(false);
connectionPool.add(myConnection);
} catch (SQLException e) {
e.printStackTrace();
}
}
}
public MyConnection getConnection(){
for (MyConnection myConnection : connectionPool) {
if(myConnection.isUsed()){
System.out.println("connection is used");
continue;
}
myConnection.setUsed(true);
return myConnection;
}
return null;
}
}
Class test:
package mysql;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
public class MysqlTest {
public static void main(String[] args) throws SQLException {
MyConnectionPool pool = new MyConnectionPool();
// connection one
MyConnection myConnection = pool.getConnection();
Connection conn = myConnection.getConnection();
PreparedStatement ps = conn.prepareStatement("select count(*) as co from asset");
ResultSet rs = ps.executeQuery();
rs.next();
System.out.println(rs.getString("co"));
ps.close();
// connection tow
myConnection = pool.getConnection();
conn = myConnection.getConnection();
ps = conn.prepareStatement("select count(*) as co from asset");
rs = ps.executeQuery();
rs.next();
System.out.println(rs.getString("co"));
// connection three
myConnection = pool.getConnection();
conn = myConnection.getConnection();
ps = conn.prepareStatement("select count(*) as co from asset");
rs = ps.executeQuery();
rs.next();
System.out.println(rs.getString("co"));
//reset connection use status
myConnection.setUsed(false);
// re-use connection three
myConnection = pool.getConnection();
conn = myConnection.getConnection();
ps = conn.prepareStatement("select count(*) as co from asset");
rs = ps.executeQuery();
rs.next();
System.out.println(rs.getString("co"));
}
}
Because i never close the connection in MyConnectionPool, so i can re-use the connection.
It is very simple demo. Real Connection Pool is very Complex.
You can find some Connection Pool jar.
I don't know what ORM framework you choose, the popular ORM framework all support Database Connection Pool.
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
Type mismatch: cannot convert from java.sql.Statement to com.mysql.jdbc.Statement
i am a beginner in java i am trying to use mysql database i have downloaded mysql-connector-java-5.1.23-bin.jar file from mysql.com and i have added this jar file to in my build path of my project but there is an error in the following code
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
Type mismatch: cannot convert from java.sql.Statement to com.mysql.jdbc.Statement
package com.example.demo;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import com.mysql.jdbc.Connection;
import com.mysql.jdbc.Statement;
public class DBConnect
{
private static final String userName = "root";
private static final String userpwd = "sverma";
private static final String CONN_STR = "jdbc:mysql://localhost:3306/phpweb_db";
public static void main(String[] args) throws SQLException
{
Connection conn = null;
Statement st = null;
ResultSet rs = null;
try
{
DriverManager.getConnection(CONN_STR, userName, userpwd);
st=conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
rs = st.executeQuery("select * from user");
rs.last();
System.out.println("No of rows: " + rs.getRow());
// System.out.println("Connected Successfully...");
}
catch (SQLException e)
{
System.err.println(e);
}
finally
{
if (rs != null)
{
rs.close();
}
if (st != null)
{
st.close();
}
if (conn != null)
{
conn.close();
}
}
}
}
Wrong classes
import com.mysql.jdbc.Connection;
import com.mysql.jdbc.Statement;
should be
import java.sql.Connection;
import java.sql.Statement;
In fact, java decouples everything from a specific database engine. One never should need an import of MySQL (or ProgressSQL or ...) classes.
To have those classes available at run-time, the first thing after the try, before getting the connection would be:
Class.forName("com.mysql.jdbc.Driver");
This technique would allow reading all strings from a configuration file, and writing database independent code.
Missing: conn = ...
conn = DriverManager.getConnection(CONN_STR, userName, userpwd);
package com.example.demo;
import java.sql.*;
public class DBConnect
{
private static final String userName = "root";
private static final String userpwd = "sverma";
private static final String CONN_STR = "jdbc:mysql://localhost:3306/phpweb_db";
public static void main(String[] args) throws SQLException
{
Connection conn;
Statement st;
ResultSet rs;
try
{
Class.forName("com.mysql.jdbc.Driver");
DriverManager.getConnection(CONN_STR, userName, userpwd);
st=conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
rs = st.executeQuery("select * from user");
rs.last();
System.out.println("No of rows: " + rs.getRow());
// System.out.println("Connected Successfully...");
}
catch (SQLException e)
{
System.err.println(e);
}
finally
{
if (rs != null)
{
rs.close();
}
if (st != null)
{
st.close();
}
if (conn != null)
{
conn.close();
}
}
}
I am getting the error: java.sql.SQLException: No suitable driver
I have imported the .jar file that is available here http://dev.mysql.com/downloads/mirror.php?id=13598
I am using Eclipse.
I am connecting to the DB with this code:
package dao;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class DBCon {
private static final String host = "jdbc:mysql://localhost";
private static final String port = "3306";
private static final String db = "mydb";
// private static final String user = "root";
// private static final String pwd = "";
private static final String user = "myusername";
private static final String pwd = "mypwd";
public Connection getCon() {
Connection con = null;
try {
String url = host + ":" + port + "/" + db;
con = DriverManager.getConnection(url, user, pwd);
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return con;
}
}
I then query the DB with this code:
package dao;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.sql.Connection;
import model.ClassPojo;
public class ARCSDao {
public ArrayList<ClassPojo> viewClasses() throws SQLException{
ArrayList<ClassPojo> classList = new ArrayList<ClassPojo>();
DBCon db = new DBCon();
Connection con = db.getCon();
Statement stmt;
ResultSet rs;
stmt = con.createStatement();
rs = stmt.executeQuery("SELECT * FROM classes");
while(rs.next()){
ClassPojo aClass = new ClassPojo();
rs.getString("class_name");
rs.getString("class_uri");
classList.add(aClass);
}
return classList;
}
}
Add
Class.forName("com.mysql.jdbc.Driver").newInstance();
before
con = DriverManager.getConnection(url, user, pwd);
if this doesn't work double check that the jdbc driver (jar file) is included in your classpath