Communications link failure if not connected from localhost

Question

I tried to connect my java application to my database server (MySQL). But it doesn't work. I already had this problem at my JavaPlugins for my Spigot server. But because the Spigot plugins are on the VPS directly they can connect over localhost and this works but if I try to connect trough my domain or IPv4 I have this failure.

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:988)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:341)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2251)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2284)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2083)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:806)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:410)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:328)
    at java.sql.DriverManager.getConnection(DriverManager.java:664)
    at java.sql.DriverManager.getConnection(DriverManager.java:247)
    at kochbuch.MySQL.MySQLConnect(MySQL.java:27)
    at kochbuch.Main.start(Main.java:16)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.net.Socket.connect(Socket.java:589)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:211)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:300)
    ... 25 more`

and this is my MySQL.java Class.

public class MySQL {

public static Connection MySQLConnection;

public void onEnable() {
    System.out.println("");
}

public void onDisable() {
    System.out.println("");
}

public static void MySQLConnect(String host, String port, String database, String username, String password) {
    if (!MySQL.MySQLIsConnected()) {
        try {
            MySQLConnection = DriverManager.getConnection("jdbc:mysql://" + host + ":" + port + "/" + database, username, password);
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

public void createTable(String tablename, String[] values) {
    MySQL.executeCommand("CREATE TABLE IF NOT EXISTS " + tablename + " (" + values + ")");
}

public static void MySQLDisconnect() {
    if (MySQL.MySQLIsConnected()) {
        try {
            MySQLConnection.close();
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

public static void executeCommand(String command) {
    try {
        MySQLConnection.createStatement().executeUpdate(command);
    }
    catch (Exception e) {
        e.printStackTrace();
    }
}

public static boolean MySQLIsConnected() {
    if (MySQLConnection != null) {
        return true;
    }
    return false;
}

public static ResultSet getResult(String qry) {
    if (MySQL.MySQLIsConnected()) {
        try {
            return MySQLConnection.createStatement().executeQuery(qry);
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }
    return null;
    }
}

Thanks for Helping :) Lg Jaxophon