Concurrency Problem in Java

I am designing a client-server chat application in Java. This is a secure application where the messages are exchanged using cryptographic algorithms. I have one server and it can support many clients. My problem is that when one client logs on the server it works fine, but when another user logs into the system, the server starts giving me bad padding exceptions for the encrypted text.

I am not able to figure out the problem, according to my logic, when new connection request to server is made, the server creates a thread for listening to the client. Is it possible that once the instance of thread class is created, it does all the processing correctly for the first client, but not for the second client because the variables in server listener thread class already have some previous value, and thus the encrypted text is not decrypted properly?

Please advise how I can make this process more robust so that the number of clients does not affect how well the server functions.

Hi, The code is like this :

When Server Starts:

Socket in= serverSocket.accept();

                Receive rlt = new Receive(in);
                Thread receiveReq = new Thread(rlt);
                receiveLoginReq.start();

now the Receive Thread waits for the incoming message and do the process according to message type. When mor开发者_如何学Pythone than one client is invoked, Server works fine, problem starts when one client terminates and then again tries to reconnect. Server always gives the Error in following pattern:

1. First time the HAsh not matched error for second client
2. Second time javax.crypto.BadPaddingException: Given final block not properly padded error

When this happens, I need to restart server and restart both clients, only then both clients works. but again if one client terminates connection and again tries to reconnects, the same 2 errors occurs in the same manner. and then again restart Server.

Any Advise will be highly appreciated. Thanks

Don't share mutable data with threads. Use functional style - no object states. If you really need to share some data with the threads then use message passing.

Check that you close connections in a proper way.

You could use a real server like Jetty that is very easy to install.

I can only guess the reasons without seeing the full source code. I assume you are using CipherInput/OutputStream for your encryption. You should use separate instances of Cipher for each thread (each I/OStream). Every time you create a new connection or re-connect, run the init method of Cipher on both the client and server side and create new CipherInput/OutputStream instances.

The cryptographic objects are stateful, therefore they cannot be shared between threads. Each thread and each connection should have its separate sets of stateful objects.

Check out java.lang.ThreadLocal class.