如何在类之间共享数据库连接？ （JDBC）

Question

I'm trying to figure out how to share an established connection to a database among classes to execute different SQL statements. I read through some of the topics, but as I'm fairly new to programming I have a hard time to adapt the given information to my problem.

Problem: I have two classes with one opening a connection to a database, executing some SQL statments and calling the other class to do the same only using different tables and SQL statments. Now as long as I run the classes separatly with their own main method and connections everything works fine. But as one class calls the other I get different Exceptions, depending on my workarounds I tried so far (either MySQLNonTransientConnectionException: Data source rejected establishment of connection or a StackOverflowException ).

Here is how I'm trying to established a connection that is used to execute some sql operations in two different classes:

public ClassA{

    public static Connection dbConn;

    //Set up a connection to the database
    String dbURL = "jdbc:mysql://<some database>"; //put host, port and database here
    Properties connectionProbs = new Properties();
    connectionProbs.put("user", "root"); //insert USER here
    connectionProbs.put("password", "root"); //insert PASSWORD here

    dbConn = null;
    try{
        dbConn = DriverManager.getConnection(dbURL, connectionProbs);

        PreparedStatement useStmt;
        try{
            useStmt = dbConn.prepareStatement("USE <some database>"); //insert DATABASE here
            useStmt.executeUpdate();
        }
        catch(SQLException e){
            e.printStackTrace();
        }
        //Do some SQL operations
        //Call class B to do some SQL operations using the same connection

    }
    catch(SQLException e){
        System.err.println("There was a problem connecting to the database");
        e.printStackTrace();
    }
    finally{
        if(dbConn != null)
            try{dbConn.close();}catch(SQLException e){e.printStackTrace();}
    }
}

Why can't class B use the connection of ClassA, for instance by doing something like this (This leads to a StackOverflow):

PreparedStatement Stmt = ClassA.dbConn.prepareStatement("INSERT INTO table(ID, name) VALUES (?,?)");

On the other hand, if I'm trying to establish two separate Connections (using the same code as above) to the same database (running at the same time) I get the MySQLNonTransientConnectionException: Data source rejected establishment of connection.

What's the best way to handle this? I stumbled upon ConnectionPooling in the forums, but I couldn't find a beginner friendly source to elaborate how to put this into practice. Is there a straight forward way to ensure different classes can connect and operate on one database?

Thanks for any feedback

Answer 1

I would avoid putting the connection in a static variable. It doesn't make a whole lot of difference in your particular instance but it would be best to not get into the habit of doing it that way.

You should create one controlling class the creates the database connection, then passes it to the other two classes as method parameters. This way the controlling class can handle any exception thrown by the other classes and make sure the connection is closed properly.

Answer 2

You can achieve that by creating a non-static global variable for Connection in Class A and then creating a non-static public method to return this connection, as shown below.

public ClassA{
    // non-static global private Connection object
    private Connection dbConn = null;

    // non-static public method to get dbConn connection object
    public Connection getConnection() {
        // this condition will check if the Connection is not already open then open it.
        if(null == dbConn) {
            //Set up a connection to the database
            String dbURL = "jdbc:mysql://<some database>"; //put host, port and database here
            Properties connectionProbs = new Properties();
            connectionProbs.put("user", "root"); //insert USER here
            connectionProbs.put("password", "root"); //insert PASSWORD here

            try{
                dbConn = DriverManager.getConnection(dbURL, connectionProbs);

                PreparedStatement useStmt;
                try{
                    useStmt = dbConn.prepareStatement("USE <some database>"); //insert DATABASE here
                    useStmt.executeUpdate();
                }
                catch(SQLException e){
                    e.printStackTrace();
                }
                //Do some SQL operations
                //Call class B to do some SQL operations using the same connection

            }
            catch(SQLException e){
                System.err.println("There was a problem connecting to the database");
                e.printStackTrace();
            }
            finally{
                if(dbConn != null)
                    try{dbConn.close();}catch(SQLException e){e.printStackTrace();}
            }
        }
        return dbConn;
    }
}

And then in your class B, you can do something like this.

A a = new A();
PreparedStatement Stmt = a.getConnection().prepareStatement("INSERT INTO table(ID, name) VALUES (?,?)");

Hope this helps you out.

Answer 3

You should create one controlling class the creates the database connection, then passes it to the other two classes as method parameters.

public class ConnectionManager {

    private static String url = "jdbc:mysql://localhost:3306/DbName";
    private static String username = "YourUsername";
    private static String password = "yourDbPass";
    private static Connection con;

    public static Connection getConnection() throws Exception {
      con = DriverManager.getConnection(url, username, password);
        return con;
    }


}

---

public class main {

    public static void main(String[] args) throws Exception {
        Connection con = null;
        con = ConnectionManager.getConnection();


        CrudCity s = new CrudCity();
        s = s.read(con);
        System.out.println(s);

        CrudCountry c = new CrudCountry();
        c = c.read(con);
        System.out.println(c);

        CrudCountryLanguage lang = new CrudCountryLanguage();
        lang = lang.read(con);
        System.out.println(lang);


    }

}

---

  public class CrudCity extends City implements CrudInterface {


    public CrudCity read(Connection con) throws Exception{
        CrudCity p = new CrudCity();

        Statement stmt = con.createStatement();
        ResultSet rs = stmt.executeQuery("SELECT ID,Name,CountryCode FROM world.city");
        while (rs.next()) {
            p.setId(rs.getInt("ID"));
            p.setName(rs.getString("Name"));
            p.setCountryCode(rs.getString("CountryCode"));

        }
        rs.close();
        return p;
    }
}

---

public class CrudCountry extends Country implements CrudInterface{

    public CrudCountry read(Connection con) throws Exception{
        CrudCountry c = new CrudCountry();
        Statement stmt =con.createStatement();
        ResultSet rs = stmt.executeQuery("SELECT Code,Name,Continent FROM world.country");
        while (rs.next()) {
            c.setCountryCode(rs.getString("Code"));
            c.setCountryName(rs.getString("Name"));
            c.setCodeContinent(rs.getString("Continent"));
        }

        rs.close();

        return c;
    }
}