Grokking responseHandled

[anuraaga]

I'm trying to understand this pattern of code from JettyHandlerAdvice.

    AtomicBoolean responseHandled = new AtomicBoolean(false);

    // In case of async servlets wait for the actual response to be ready
    if (request.isAsyncStarted()) {
      try {
        request
            .getAsyncContext()
            .addListener(new TagSettingAsyncListener(responseHandled, context));
      } catch (IllegalStateException e) {
        // org.eclipse.jetty.server.Request may throw an exception here if request became
        // finished after check above. We just ignore that exception and move on.
      }
    }

    // Check again in case the request finished before adding the listener.
    if (!request.isAsyncStarted() && responseHandled.compareAndSet(false, true)) {
      tracer().end(context, response);
    }

Why do we need to pass in responseHandled from here? This appears equivalent to me so I am confused.

    // In case of async servlets wait for the actual response to be ready
    if (request.isAsyncStarted()) {
      try {
        request
            .getAsyncContext()
            .addListener(new TagSettingAsyncListener(context));
      } catch (IllegalStateException e) {
        // org.eclipse.jetty.server.Request may throw an exception here if request became
        // finished after check above. We just ignore that exception and move on.
      }
    } else {
      // Either synchronous or the async call was already finished so we're already done
      tracer().end(context, response);
    }

[iNikem]

Can it happen that first request.isAsyncStarted() returns true, but when we reach addListener request was already finished? And thus listener will never be called.

[anuraaga]

Ah - I think I ignored the // org.eclipse.jetty.server.Request may throw an exception here if request became which I just read, now I see. Unlike many async APIs, servlet doesn't just call a listener with the value if it's already completed it seems.

I think this could be made a bit clearer by setting a boolean in that catch block instead of the current pattern will see what it looks like.