Is my usage of nested try-with-resource correct?

Question

I have simple TCP server which prints the message sent by client. I'm trying to modify it to use try-with-resource feature. Is using nested try-with-resource statement in my example correct?

public static void main(String args[]) throws IOException {
    String receivedMessage;

    try (ServerSocket serverSocket = new ServerSocket(777)) {
        while (true) {
            Socket socket = serverSocket.accept();
            System.out.println("Client connected");
            try (BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()))) {
                while ((receivedMessage = in.readLine()) != null) {
                    System.out.println(receivedMessage);
                }
                System.out.println("Client disconnected");
            }
        }
    }
}

}

Answer 1

Yes, your example is correct.

A try-with-resources try block can stand alone because it has an implicit finally block; whereas a traditional try block is required to be followed by a catch block and/or a finally block.

Thus your example code is equivalent to the following (besides the resource variables being visible outside the scope of their respective try blocks):

        final ServerSocket serverSocket = new ServerSocket(777);
        try {
            while (true) {
                Socket socket = serverSocket.accept();
                System.out.println("Client connected");
                final BufferedReader in = new BufferedReader(new InputStreamReader(
                        socket.getInputStream()));
                try {
                    while ((receivedMessage = in.readLine()) != null) {
                        System.out.println(receivedMessage);
                    }
                    System.out.println("Client disconnected");
                } finally {
                    in.close();
                }
            }
        } finally {
            serverSocket.close();
        }

I would have recommended you put both resources inside the same try block, except in your case that doesn't work because it's necessary to call accept() on the serverSocket before asking for its inputStream().

However, there's still an appropriate opportunity in your code example to put multiple resources in your second try-with-resources:

try (Reader stream = new InputStreamReader(socket.getInputStream());
     BufferedReader in = new BufferedReader(stream)) {
  // do stuff, still preferring to use 'in' here rather than 'stream'
}