Как правильно тестировать в JUNIT

    
    public boolean login(String login) throws Exception{
 
        if (checkLogin(login)) {
            userName = login;
            return true;
        }
        return false;
    }
 
    private boolean checkLogin(String login) throws Exception {
        if (connection.isConnected()) {
            try {
                Message message = new Message();
                message.setUser(login);
                message.setCommand(ServerSettings.LOGIN_CHECK);
                connection.sendToServer(message);
 
                message = connection.readFromServer();
                if (ServerSettings.LOGIN_IS_FREE.equals(message.getCommand()))
                    return true;
 
            } catch (Exception e) {
//                System.out.println("Client error:" + e);
                closeConnection();
                throw e;
            }
        }
        return false;
    }

    @Test
    public void loginIsFree() throws Exception {
        ConnectionToServer connection2 = mock(ConnectionToServer.class);
        String user = "user_name";
 
        Message message = new Message();
        message.setUser(user);
        message.setCommand(ServerSettings.LOGIN_CHECK);
        doNothing().when(connection2).sendToServer(message);
 
        Message answer = new Message("", user, ServerSettings.LOGIN_IS_FREE);
        when(connection2.readFromServer()).thenReturn(answer);
 
        Client c = new Client();
        c.connectToServer(connection2);
        assertTrue(c.login(user));
    }

public class Client {
    private String destinationIp;
    private String userName;
 
    private ServerConnection connection;
 
    public Client(String destinationIp) {
        this.destinationIp = destinationIp;
    }
 
    public void connectTo(ServerConnection destinationConnection) {
        connection = destinationConnection;
        connection.establish(destinationIp);
    }
 
    public boolean login(String login) throws Exception {
        if (checkLogin(login)) {
            userName = login;
            return true;
        } else {
            return false;
        }
    }
 
    private boolean checkLogin(String login) throws Exception {
        if (connection.isConnected()) {
            try {
                Message message = new Message("I want to send this");
                message.setUser("evggen0904");
                message.setCommand("This is a command. I'm lazy to make all classes!");
 
                // bla-bla some more code
                connection.send(message);
 
                return true;
            } catch (Exception e) {
                // log and bla-bla
                throw e;
            }
        }
        return false;
    }
}

public class Message {
    private String content;
    private String user;
    private String command;
 
    public Message(String content) {
        this.content = content;
    }
 
    public String getContent() {
        return content;
    }
 
    public void setUser(String user) {
        this.user = user;
    }
 
    public void setCommand(String command) {
        this.command = command;
    }
}

public class ServerConnection {
    protected boolean isConnectionEstablished;
 
    public ServerConnection() {
        isConnectionEstablished = false;
    }
 
    public boolean isConnected() {
        return isConnectionEstablished;
    }
 
    public void establish(String destinationIp) {
        if ((destinationIp != null) && (destinationIp.length() > 0))
            isConnectionEstablished = true;
        else
            isConnectionEstablished = false;
    }
 
    public void send(Message message) throws Exception {
        Thread.sleep(1000);
    }
}

public class MockedConnection extends ServerConnection {
    @Override
    public void establish(String destinationIp) {
        isConnectionEstablished = true;
    }
 
    @Override
    public void send(Message message) throws Exception {
        // no op
        // NOTE: we never throw here so that Client.checkLogin() never fails.
    }
}

public class ClientTest {
    @Test
    public void testLoginIsFree() throws Exception {
        MockedConnection mockedConnection = new MockedConnection();
 
        Client client = new Client("localhost");
        client.connectTo(mockedConnection);
 
        String userNameToCheck = "hell_knight";
 
        assertTrue(client.login(userNameToCheck));
    }
}