Send Data from multiple threads to a single thread

Question

I'm coding a Java socket server that connects to Arduino which in turn send and receive data. As shown by the Java socket documentation I've set up the server to open a new thread for every connection.

My question is, how will I be able to send the data from the socket threads to my main thread? The socket will be constantly open, so the data has to be sent while the thread is running. Any suggestion?

Update: the goal of the server is to send commands to an Arduino (ie. Turn ligh on or off) and receive data from sensors, therefore I need a way to obtain that data from the sensors which are connected to individual threads and to send them into a single one.

Answer 1

Sharing data among threads is always tricky. There is no "correct" answer, it all depends on your use case. I suppose you are not searching for the highest performance, but for easiness of use, right?

For that case, I would recommend looking at synchronized collections, maps, lists or queues perhaps. One class, which seems like a good fit for you, is ConcurrentLinkedQueue .

You can also create synchronized proxies for all usual collections using the factory methods in Collections class:

    Collections.synchronizedList(new ArrayList<String>());

You do not have to synchronize access to them.

Another option, which might be an overkill, is using database. There are some in-memory databases, like H2.

In any case, I suggest you to lower the amount of shared information to the lowest possible level. For example, you can keep the "raw" data separate per thread (eg in ThreadLocal variables) and then just synchronize during aggregation.

Answer 2

You seem to have the right idea - you need a thread to run the connection to the external device and you need a main thread to run your application.

How do you share data between these threads: This isn't in general a problem - different threads can write to the same memory; within the same application threads share memory space.

What you probably want to avoid is the two thread concurrently changing or reading the data - java provides a very useful keyword - synchronized - to handle this sort of situation which is straight forward to use and provides the kind of guarantees you need. This is a bit technical but discusses the concurrency features.

Answer 3

Here is a tutorial you might be able to get some more information on. Please note, a quick google search will bring up lots of answers to your question.

http://tutorials.jenkov.com/java-multithreaded-servers/multithreaded-server.html

In answer to your question, you can send the information from one thread to another by using a number of options - I would recommend if it is a simple setup, just use static variables/methods to pass the information.

Also as reference, for large scale programs, it is not recommended to start a thread for every connection. It works fine on smaller scale (eg a few number of clients), but scales poorly.

Answer 4

If this is a web application and you are just going to show the current readout of any of the sensors, then blocking queue is a huge overkill and will cause more problems than it solves. Just use a volatile static field of the required type. The field itself can be static, or it could reside in a singleton object, or it could be part of a context passed to the worker.

in the SharedState class:

static volatile float temperature;

in the thread:

SharedState.temperature = 13.2f;

In the web interface (assuming jsp):

<%= SharedState.temperature %>

btw: if you want to access last 10 readouts, then it's equally easy: just store an array with last 10 readouts instead of a single value (just don't modifiy what's inside the array, replace the whole array instead - otherwise synchronization issues might occur).