One of the systems our team develops is UI for end-users, where users can view and manage their print related data.

The system is designed as a simple web application, where we make AJAX calls to Spring controllers which delegate the calls to two other systems, no database is present.

One of the requirements on the system was to support about 1000 concurrent users. Since Tomcat has by default 200 threads and the calls to the other systems may take long (fortunately it’s not the case at the moment), we have decided to make use of Servlet 3.0 async. This way, each AJAX call from the browser uses up Tomcat thread only for preparation of a call to other system. The calls are handled by our asynchronous library for communication with SafeQ and asynchronous http client for communication with Payment System which both have own threadpools and fill in the response when they get a reply.

Since we depend so much on other systems’ performance, we wanted to monitor the execution time of the requests for better tracking and debugging of production problems.

There are several endpoints for each system and more can come later, so in order to avoid duplication, we have decided to leverage Spring aspect programming support. We have created an aspect for each business service (one for SafeQ, one for Payment System) and it was time to implement the measurement business logic.

In synchronous scenario, things are pretty simple. Just intercept the call, note start and end time and if it took too long, just log it in a logfile.

public Object restTimingAspect(ProceedingJoinPoint joinPoint) throws Throwable {
    long start = System.currentTimeMillis();

    Object result = joinPoint.proceed();

    long end = System.currentTimeMillis();

    long executionInMillis = end - start;
    if (executionInMillis > remoteServiceCallDurationThresholdInMillis) {
        LOGGER.warn("Execution of {} took {} ms.", joinPoint, executionInMillis);

    return result;

This won’t work in our asynchronous case. The call to joinPoint.proceed(), which will make call to other system returns immediately without waiting for a reply. Reply is processed in a callback provided to one of async communication libraries. So we have to do a bit more.

We know the signature of our business methods. One of the arguments is always callback, which will process the reply.

public void getEntitlement(ListenableFutureCallback callback, String userGuid, Long costCenterId)

If we want to add our monitoring logic in a transparent way, we have to create special callback implementation, which will wrap the original callback and track the total time execution.

class TimingListenableFutureCallback implements ListenableFutureCallback {

    private ListenableFutureCallback delegate;
    private StopWatch timer = new StopWatch();
    private String joinPoint;

    public TimingListenableFutureCallback(ListenableFutureCallback delegate, String joinPoint) {
        this.delegate = delegate;
        this.joinPoint = joinPoint;
    public void onSuccess(Object result) {
        logExecution(timer, joinPoint);

    public void onFailure(Throwable ex) {
        logExecution(timer, joinPoint);

And then we have to call the target business method with properly wrapped callback argument.

@Around("remoteServiceMethod() && args(callback,..)")
public Object restTimingAspect(ProceedingJoinPoint joinPoint, ListenableFutureCallback callback) throws Throwable {
    String joinPointName = computeJoinPointName(joinPoint);
    Object[] wrappedArgs = -> {
        return arg instanceof ListenableFutureCallback ? wrapCallback((ListenableFutureCallback) arg, joinPointName) : arg;
    LOGGER.trace("Calling remote service operation {}.", joinPointName);
    return joinPoint.proceed(wrappedArgs);

There is similar implementation for the other async messaging library.

I hope this solution will help you solve similar problems in your applications in an elegant manner 🙂

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