So basically I'm doing a server and client in java.
In order for it to run, I have to run both a client.java and server.java. I need to close one or both depending on the instance
What I currently need is a way such that when the client passes a parameter x, the client should close but the server should remain open. I also have to implement it in such a way that if client passes parameter y instead, both the client and server should close
I thought of doing:
System.exit(0);
But I'm not sure if this would close both server and client, meaning that it would be useful for the second instance. I also thought of just letting it reach the end of the program, but I'm not really sure what result would be the result of that.
If you want the server to keep listening for more requests for a handshake (even when previous connections terminate), what you need is a multi-client server. A server that can handle multiple clients usually runs a thread that keeps looping and listens for a client that wants to connect. Such a thread would generate a new thread for each connection and store it in an array of threads that all keep listening or talking with each client.
You can learn more about multiple client servers here.
With the array of threads listening to all clients, you could assign a condition that terminates all threads and connections in the server. This could be a String that one of the clients can relay to the server, such as "/exit" and the server can check if information sent by any client-thread is "/exit".
Related
I am completely new to creating a network connection in java so I apologize if this is a stupid question.
I am trying to create a D&D companion in java that will allow a player to create their character and then send it to the DM so that they can view it and make changes and send it back to the player. I want to be able to make it so that any time a field is changed on one computer it will also be changed on the other computer.
After a bunch of research online I have been able to create a socket connection between the DM(server) and the player(client) and pass a message between the two but I am not sure how a socket connection works after this initial connection is made. My research has not been very clear on this. I have found many resources that have said that java closes the socket after a message has been passed and many that say that the socket stays open.
If java closes the socket then my problem is easy enough to solve because then I will just have to open a new socket every time I need to pass data making sure that I pass the IP address of the client to the server the first time I make a connection.
My real questions come in when a socket stays open.
If the socket stays open and multiple clients connect to the server, will the server just shout over the network whenever it transmits a message so that all clients receive the message? (If this is the case then I know I can just attach a username to the front of the message so that the client can determine if the server is talking to it.)
If the server does not shout then how do I specify which client I want the server to talk to?
Will I have to add a loop to my receive methods so that the client/server is constantly listening for a transmission from the server/client or will java automatically do so after I run the method the first time?
I have found many resources that have said that java closes the socket after a message has been passed
You found them where?
and many that say that the socket stays open.
All those are correct. Java never closes connections. The application closes connections.
If java closes the socket then my problem is easy enough to solve because then I will just have to open a new socket every time I need to pass data making sure that I pass the IP address of the client to the server the first time I make a connection.
It doesn't.
My real questions come in when a socket stays open.
If the socket stays open and multiple clients connect to the server, will the server just shout over the network whenever it transmits a message so that all clients receive the message?
No. It will respond via the socket that is connected to the corresponding client.
(If this is the case then I know I can just attach a username to the front of the message so that the client can determine if the server is talking to it.)
Unnecessary.
If the server does not shout then how do I specify which client I want the server to talk to?
The server responds via the same socket it read the request from.
Will I have to add a loop to my receive methods so that the client/server is constantly listening for a transmission from the server/client
No, you will have to add a thread per accepted socket, that loops reading requests until end of stream.
or will java automatically do so after I run the method the first time?
No.
You seem to have been reading some truly appalling drivel. Take a look at the Custom Networking section of the Java Tutorial.
Adding to EJP's wise answer, it might be worth clarifying:
Sounds like you (wisely) use TCP, so your Socket represents a connection between 1 server and 1 client. No "shouting". In examples such as this , when connection is established (namely, client obtains a Socket by calling "new Socket" and server obtains a Socket by calling "accept"), those Sockets are dedicated to those 2 specific endpoints. So if 10 clients connect to 1 server, the server will keep 10 Sockets and won't mix them up. A bit like a poor secretary that has 10 phones on his desk and answers them all - despite the mess, each earpiece is clearly connected to 1 customer.
The connection can hold for a while & serve several messages. It will terminate when either one of the sides calls 'socket.close', or it can be terminated by underlying 3rd parties (operating system, proxies, firewalls).
For your first version, or for simple business requirements, it's probably enough to converse over this 1 simple connection. However, for commercial critical data that requires 'assurance of delivery', you might need to invest some careful thought & possibly tools such as RabbitMQ.
Good luck:)
I'm developing a standard java RMI server with multiple clients. These clients have a menu where they can call the server to do various of things for them.
One method involves a queue, where they can send a job to the queue and wait for it to get handled. The RMI server dispose threads for all the clients automatic, but when it comes to this method and queue, how can I hold this request back, so for example:
client 1 call first, and then client 2 calls just after (here client 1 should receive the message first from the server and client 2 should wait the time it takes for the server to process client 1 request)
Is it to make some kind of singleton only for this task?
What can I do to tackle this problem.
Make the remote method concerned synchronized.
No queue required.
I have classic http client/server application where the server serves the clients data at their will but also performs some kind of call-backs to the list of clients' addresses it has. My two questions are :
1- How would the server know if a client is down (the client did not disconnect but the connection got suddenly interrupted) ?
2- Is there a way to know from the server-side if the process at client-side listening on the call-back port is still up (i.e. client call-back socket is still open) ?
1- How would the server know if a client is down (the client did not disconnect but the connection got suddenly interrupted) ?
Option #1: direct communication
Client tells server "I'm alive" at a periodic interval. You could make your client to ping your server at a configurable interval, and if the server does not receive the signal for a certain time, it'll mark the client as down. Client could even tell server more info(e.g. It's status) in each heartbeat if necessary, this is also the way used in many distributed systems(e.g. Hadoop/Hbase).
Option #2: distributed coordination service
You could treat all clients connected to a server as a group, and use a 3rd party distributed coordination service like Zookeeper to facilitate the membership management. Client registers itself to Zookeeper as a new member of the group right after booting up, and leaves the group if it's down. Zookeeper notifies the server whenever the membership changes.
2- Is there a way to know from the server-side if the process at client-side listening on the call-back port is still up (i.e. client call-back socket is still open) ?
I think this can only be done by the way Option #1 listed above. It could be either the way clients tell server "My callback port is OK" at a fixed interval, or the server asks clients "Are your callback port OK?" and wait its response at a fixed interval
You would have to establish some sort of protocol; and simply spoken: the server keeps track of "messages" that it tried to sent to clients.
If that "send" is acknowledged, fine; if not: then the server might do a limited number of retries; and then regard that client as "gone"; and then drop any other messages for that client.
1- How would the server know if a client is down (the client did not disconnect but the connection got suddenly interrupted) ?
A write to the client will fail.
2- Is there a way to know from the server-side if the process at client-side listening on the call-back port is still up (i.e. client call-back socket is still open
A write to the client will fail.
The write won't necessarily fail immediately, due to TCP buffering, but the write will eventually provoke retries and retry timeouts that will cause a subsequent read or write to fail.
In Java the failure will manifest itself as an IOException: connection reset.
I want to create a simple chat program. But my requirement is some what different I want a single client - server type of thing. Means I don't want a multiple client server.
I am creating a server and waiting for client to connect. After client connects I am waiting for any message from client.
Now my problem is that when I run my client program, I can run multiple instances of it, but the only first client messages are delivered to server as it is single server client program. What I want is some sort of method with which I can limit other clients to connect and say display error message saying that server is busy try after sometime. Can anyone help me do that?
The server should have a list of all client connected so whenever a new client connect; instead of just adding a new client; it check if there is already a user connected and if yes ; it close the connection/return special exception.It is up to the client project you made that decide how it will handle this closing of connection/exception.
I'm making a java program & I want this to be both as server and a client (using sockets). How is this best achieved?
If you mean that you want to both send and receive data, a single regular socket (on each computer) will do just fine. See Socket.getInputStream and Socket.getOutputStream.
The usual "server" / "client" distinction just boils down which host is listening for incoming connections, and which hosts connect to those hosts. Once the connection is setup, you can both send and receive from both ends.
If you want both hosts to listen for incoming connections, then just set up a ServerSocket and call accept on both hosts.
Related links:
Official trail: The Java™ Tutorials, Lesson: All About Sockets
If you want each station to function as a server and a client, like a p2p chat,
you should implement a thread with a ServerSocket, listening for incoming connections, and once it got a connection, open a new thread to handle it so the current one will keep on listening for new connections.
For it to be able to connect to others, simple use SocketAddress and Socket, in a different thread to try to connect to a specified server address (e.g. by a list of the user's friends)
you can find plenty of chat examples by googling.
cheers.
If you want the program to perform the same operations regardless of whether it is a server or a client for a certain connection, I could imagine handing off both the client Socket and the ServerSocket.accept()-produced socket to the same method for processing.
Have a look at jgroups it's a library that allows the creation of groups of processes whose members can send messages to each other. Another option would be to use hazelcast...
You may also look at this question.
The best way to do this is to run the server on a thread:
You run server.accept() therefore while your program is listening for a connection on that thread you can do whatever you want on the main thread, even connect to another server therefore making the program both a server & a client.