Question

I am writing a multi-thread program in Java, where i am creating a seperate thread to handle each new client connection. I have:

Socket s;


while(true)
{
    s = server.accept();
    ClientHandler ch = new ClientHandler(s);
    Thread servingThread = new Thread(ch);
    servingThread.start();
}

In the ClientHandler thread i have:

public class ClientHandler implements Runnable
{
    private Socket s;

public ClientHandler(Socket _Socket, boolean _accepted, BaseStation _bs)
{
    this.s = _Socket;
}

If in Java i can’t pass an object but only a reference to it, isn’t that going to cause a problem, to the s instance inside ClientHandler whenever server accepts a new connection?
Is it not going to change inside ClientHandler too and corrupt it? If so, what’s the correct way to do it?

Answer 1

Your concern is correct because the Socket object (like any other object in Java) is passed by reference, therefore there is a probability of it being referenced by different modules, or different threads, causing unpredictable behaviour.

However in case of your program there is no problem. The reason is: whenever server accepts a new connection it will create a new Socket object, which you are then passing to a new instance of ClientHandler. Therefore every new connection from the client will be served by an independent ClientHandler instance, so there are no race conditions to the Socket object to worry about. So you are safe for now.

As a counter-example if you decide to create two ClientHandler threads to read from the same Socket object like this …

s = server.accept();
ClientHandler ch = new ClientHandler(s);
ClientHandler ch2 = new ClientHandler(s);
new Thread(ch).start();
new Thread(ch2).start();

… then you might be in trouble because two ClientHandlers will get the same reference to the same Socket object and if both attempt to read from the same socket they will each get only half of the data. E.g. if you are receiving a string "hello", you might end up with ch reading "hel" and ch2 reading "lo", or any other combination.