Java applet can't access mysql via browser

The reason why is because your applet does not have the security permissions to access local ports on the computer. Java does this to protect peoples computers. Think about if you opened up a webpage and the Applet was allowed to access your local ports. You could have your computer hacked in seconds.

If you want to do it the way you're doing it, you need to package your applet in a jar file, sign it and verify it so that it can receive those permissions. Here is some info for you if you're interested:

How to create a jar file
How to sign and verify a jar file
How to run jar packaged software

All of those are great resources if thats the approach you want to take. What i would do is setup a Servlet (see duffymo's answer) or setup a ProxyServer to relay the information between your computer and the applet. If you want any more information let me know and i can provide it.

Edit:

This is sort of a lot of code but i think you will get the idea, i hope this helps. You of course will have to write the client end that connects to the server and requests information from the database.

DatabaseServer.java:

import java.io.IOException;
import java.net.*;
import java.util.ArrayList;
import java.util.Iterator;

public class DatabaseServer implements Runnable
{
    public DatabaseServer()
    {
        System.out.println("Created new server!");
        clients = new ArrayList<Client>(); // this is a list of all the clients connected to the server
        connected = false;
        running = false;
        connect(); // starts server
    }

    public void run()
    {
        System.out.println("Waiting for clients.../n");

        while(running)
        {
            try 
            {
                Socket socket = server.accept(); // waits for a client to connect
                System.out.println("Client connected! "+socket.getInetAddress());
                Client client = new Client(this, socket); // creates a new client object
                clients.add(client);// adds it to the list


                // cleans the client list everytime a new client connects
                Iterator<Client> it = clients.iterator();
                while(it.hasNext())
                {
                    Client next = it.next();
                    if(!next.connected)
                    {
                        next.disconnect();
                        it.remove();
                    }
                }
            }catch(IOException e){e.printStackTrace();}
        }
    }

    public void connect()
    {
        if(!connected)
        {
            System.out.println("starting server...");
            try
            {
                System.out.println("opening port...");
                server = new ServerSocket(8080); // opens a server socket
                System.out.println("server started on port: "+server.getLocalPort());

                running = true;
                connected = true;

                thread = new Thread(this);// starts the server thread
                thread.start();
            }catch(Exception e){e.printStackTrace(); connected = false; running = false;}
        }
    }

    public void disconnect()
    {
        //stops the server
        try
        {
            server.close();
        }catch(Exception e){e.printStackTrace();}
        server = null;

        if(thread != null)
            thread.interrupt();
        thread = null;

        connected = false;
        running = false;
    }

    public void handleMessage(Client client, String message)
    {
        /* this is where you do your database interactions, based on the message you can pull out specific
         * information from your database and then send it back to the client using client.sendMessage()
        */
    }

    private Thread thread;
    private boolean running;
    private boolean connected;
    private ServerSocket server;
    private ArrayList<Client> clients;

    public static void main(String args[])
    {
        new DatabaseServer(); // makes a new server
    }
}

Client.java:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;

public class Client implements Runnable
{
    public Client(DatabaseServer server, Socket socket)
    {
        this.socket = socket;
        this.server = server;
        try
        {
            connected = true;

            writer = new PrintWriter(socket.getOutputStream()); // opens an output stream
            reader = new BufferedReader(new InputStreamReader(socket.getInputStream())); // opens an input stream

            running = true;
            thread = new Thread(this);
            thread.start();// starts the client thread
        }catch(Exception e){e.printStackTrace(); connected = false;}
    }

    public void run()
    {
        try 
        {
            String message = "";
            while((message = reader.readLine()) != null & running & connected) // waits for a message to be recieved
            {
                server.handleMessage(this, message); // tells server to handle message
            }
        }catch(IOException e){e.printStackTrace(); connected = false;}
    }

    public void disconnect()
    {
        // disconnects client
        try
        {
            socket.close();
        }catch(Exception e){e.printStackTrace();}

        try
        {
            reader.close();
        }catch(Exception e){e.printStackTrace();}

        try
        {
            writer.close();
        }catch(Exception e){e.printStackTrace();}

        socket = null;
        reader = null;
        writer = null;

        if(thread != null)
            thread.interrupt();
        thread = null;

        connected = false;
        running = false;
    }

    public void sendMessage(String message)
    {
        // sends a message back to the client
        writer.println(message);
        writer.flush();
    }

    public boolean connected;

    private boolean running;
    private Thread thread;
    private DatabaseServer server;
    private Socket socket;
    private PrintWriter writer;
    private BufferedReader reader;
}

Your applet will connect to this server, and then you can send messages to the server to request information from the database. All you have to do is add the mysql stuff to the server and then write the client portion of your applet. Good luck!

I'd recommend putting a servlet between the applet and the database. You should not expose the database to the web like that. Better to have the applet send requests to the servlet and let it intercede on its behalf. The servlet can authenticate and authorize, validate and bind inputs, and marshall the response to send back.