Question

I am working on a web application using java servlet and here is my scenario:

servlet1: Has the application's interface and main logic
servlet2: Suplies values to servlet1 on ajax request from servlet1

Totally there are 250 values that servlet1 can request. But calculating these values in servlet2 takes time(As it involves making GET request to other servers to get values). So calculating these values on request makes the client to wait for a long time.

So is there a way to make servlet2 to start pre-calculating the values when servlet1 is first called(so it can quickly send the values on request)?
Any help on how this can be implemented?

P.S. database or filesystem cannot be used.

Answer 1

If the data that servlet2 needs to precalculate is not dependant on servlet1 input and needs to be calculated only once, either calculate it eagerly in ServletContextListener.contextInitialized() or in GenericServlet.init(). However it the computation takes some time, you’ll better move it to some background thread to avoid long application startup time (these methods block deployment until they are finished).

Here is a simple example:

public class Servlet2 extends HttpServlet {

    private final ExecutorService threadPool = Executors.newSingleThreadExecutor();
    private Future<String> calculationResult;

    @Override
    public void init() throws ServletException {
        calculationResult = threadPool.submit(new PreComputingTask());
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        final String slowResponse = calculationResult.get();
        //...
    }
}

class PreComputingTask implements Callable<String> {

    @Override
    public String call() throws Exception {
        //Call external systems, whatever...
        return "slow response";
    }
}

As you can see, when servlet2 starts, it initiates pre computation task in a separate thread. Then in doGet() you retrieve the result, possibly waiting for it, if it hasn’t finished yet.

If the pre-computation is dependant on input in servlet1 (e.g. request parameters of servlet1 need to be used when calling external system), it’s much more challenging and interesting.

I see at least two options:

• start the Future task in servlet1 and retrieve that future (hopefully already finished) in servlet2. You need to pass it somehow between the servlets, e.g. by placing it in ServletContext

• send a message to jms queue from servlet1. Message listener will process the request, precomputing the results and putting the result in some temporary, unique queue. servlet2 can then later receive message from that queue (you’ll have to agree on some naming scheme somehow), or wait for it a little bit.