Code taken from Espresso's AsyncTaskPoolMonitor and adapter to be used as an idling resource for any ThreadPoolExecutor

ThreadPoolIdlingResource.java

/*
 * Copyright (C) 2013 Google, 2014 Stefano Dacchille
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.jimdo.test;

import android.util.Log;
import com.google.android.apps.common.testing.ui.espresso.IdlingResource;

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

/**
 * Provides a way to monitor a Thread Pool Executor's work queue to ensure that there is no work pending
 * or executing (and to allow notification of idleness).
 * <p/>
 * Register a new subclass for each Thread Pool Executor.
 */
public abstract class ThreadPoolIdlingResource implements IdlingResource {

    private static final String TAG = "ThreadPoolIdlingResource";

    private final AtomicReference<IdleMonitor> monitor = new AtomicReference<>(null);
    private final AtomicInteger activeBarrierChecks = new AtomicInteger(0);
    private final ThreadPoolExecutor threadPoolExecutor;

    private ResourceCallback callback;
    private AtomicBoolean isMonitorForIdle = new AtomicBoolean(false);

    private final Runnable idleAction = new Runnable() {
        @Override public void run() {
            try {
                isMonitorForIdle.set(false);
                if (callback != null) callback.onTransitionToIdle();
            } finally {
                cancelIdleMonitor();
            }
        }
    };

    protected ThreadPoolIdlingResource(ThreadPoolExecutor executor) {
        this.threadPoolExecutor = checkNotNull(executor,
                String.format("Trying to instantiate a \'%s\' with a null pool", getName()));
    }

    /**
     * Checks if the minPoolThreads is idle at this moment.
     *
     * @return true if the minPoolThreads is idle, false otherwise.
     */
    @Override public boolean isIdleNow() {
        // The minPoolThreads executor hasn't been injected yet, so we're idling
        if (threadPoolExecutor == null) return true;
        boolean idle;
        if (!threadPoolExecutor.getQueue().isEmpty()) {
            idle = false;
        } else {
            int activeCount = threadPoolExecutor.getActiveCount();
            if (0 != activeCount) {
                if (monitor.get() == null) {
                    // if there's no idle monitor scheduled and there are still barrier
                    // checks running, they are about to exit, ignore them.
                    activeCount = activeCount - activeBarrierChecks.get();
                }
            }
            idle = 0 == activeCount;
        }

        if (!idle && !isMonitorForIdle.get()) {
            isMonitorForIdle.set(true);
            notifyWhenIdle(idleAction);
        }

        return idle;
    }

    @Override public void registerIdleTransitionCallback(final ResourceCallback resourceCallback) {
        this.callback = resourceCallback;
    }

    /**
     * Notifies caller once the minPoolThreads is idle.
     * <p/>
     * We check for idle-ness by submitting the max # of tasks the minPoolThreads will take and
     * blocking
     * the tasks until they are all executing. Then we know there are no other tasks _currently_
     * executing in the minPoolThreads, we look back at the work queue to see if its backed up,
     * if it is
     * we reenqueue ourselves and try again.
     * <p/>
     * Obviously this strategy will fail horribly if 2 parties are doing it at the same time,
     * we prevent recursion here the best we can.
     *
     * @param idleCallback called once the minPoolThreads is idle.
     */
    void notifyWhenIdle(final Runnable idleCallback) {
        checkNotNull(idleCallback);
        IdleMonitor myMonitor = new IdleMonitor(idleCallback);
        checkState(monitor.compareAndSet(null, myMonitor), "cannot monitor for idle recursively!");
        myMonitor.monitorForIdle();
    }

    /**
     * Stops the idle monitoring mechanism if it is in place.
     * <p/>
     * Note: the callback may still be invoked after this method is called. The only thing
     * this method guarantees is that we will stop/cancel any blockign tasks we've placed
     * on the thread minPoolThreads.
     */
    void cancelIdleMonitor() {
        IdleMonitor myMonitor = monitor.getAndSet(null);
        if (null != myMonitor) {
            myMonitor.poison();
        }
    }

    private class IdleMonitor {
        private final Runnable onIdle;
        private final AtomicInteger barrierGeneration = new AtomicInteger(0);
        private final CyclicBarrier barrier;
        // written by main, read by all.
        private volatile boolean poisoned;

        private IdleMonitor(final Runnable onIdle) {
            this.onIdle = checkNotNull(onIdle);
            this.barrier = new CyclicBarrier(minPoolThreads(),
                    new Runnable() {
                        @Override
                        public void run() {
                            if (threadPoolExecutor.getQueue().isEmpty()) {
                                // no one is behind us, so the queue is idle!
                                monitor.compareAndSet(IdleMonitor.this, null);
                                onIdle.run();
                            } else {
                                // work is waiting behind us, enqueue another block of tasks and
                                // hopefully when they're all running, the queue will be empty.
                                monitorForIdle();
                            }

                        }
                    }
            );
        }

        private int minPoolThreads() {
            int corePoolSize = threadPoolExecutor.getCorePoolSize();
            return corePoolSize > 0 ? corePoolSize : Runtime.getRuntime().availableProcessors();
        }

        /**
         * Stops this monitor from using the thread minPoolThreads's resources,
         * it may still cause the
         * callback to be executed though.
         */
        private void poison() {
            poisoned = true;
            barrier.reset();
        }

        private void monitorForIdle() {
            if (poisoned) {
                return;
            }

            if (isIdleNow()) {
                monitor.compareAndSet(this, null);
                onIdle.run();
            } else {
                // Submit N tasks that will block until they are all running on the thread
                // minPoolThreads.
                // at this point we can check the minPoolThreads's queue and verify that there
                // are no new
                // tasks behind us and deem the queue idle.

                final BarrierRestarter restarter = new BarrierRestarter(barrier, barrierGeneration);
                for (int i = 0; i < minPoolThreads(); i++) {
                    threadPoolExecutor.execute(new Runnable() {
                        @Override
                        public void run() {
                            while (!poisoned) {
                                activeBarrierChecks.incrementAndGet();
                                int myGeneration = barrierGeneration.get();
                                try {
                                    barrier.await();
                                    return;
                                } catch (InterruptedException ie) {
                                    // sorry - I cant let you interrupt me!
                                    restarter.restart(myGeneration);
                                } catch (BrokenBarrierException bbe) {
                                    restarter.restart(myGeneration);
                                } finally {
                                    activeBarrierChecks.decrementAndGet();
                                }
                            }
                        }
                    });
                }
            }
        }
    }

    private static class BarrierRestarter {
        private final CyclicBarrier barrier;
        private final AtomicInteger barrierGeneration;

        BarrierRestarter(CyclicBarrier barrier, AtomicInteger barrierGeneration) {
            this.barrier = barrier;
            this.barrierGeneration = barrierGeneration;
        }

        /**
         * restarts the barrier.
         * <p/>
         * After the calling this function it is guaranteed that barrier generation has been
         * incremented
         * and the barrier can be awaited on again.
         *
         * @param fromGeneration the generation that encountered the breaking exception.
         */
        synchronized void restart(int fromGeneration) {
            // must be synchronized. T1 could pass the if check, be suspended before calling
            // reset, T2
            // sails thru - and awaits on the barrier again before T1 has awoken and reset it.
            int nextGen = fromGeneration + 1;
            if (barrierGeneration.compareAndSet(fromGeneration, nextGen)) {
                // first time we've seen fromGeneration request a reset. lets reset the barrier.
                barrier.reset();
            } else {
                // some other thread has already reset the barrier - this request is a no op.
            }
        }
    }
}

Sometimes when trying to use this I get this error:

java.lang.IllegalStateException: Resource com.servicemagic.consumer.test.util.TestUtil$2ThreadPoolIdlingResource isIdleNow() is returning true, but a message indicating that the resource has transitioned from busy to idle was never sent.
at android.support.test.espresso.base.IdlingResourceRegistry$Dispatcher.handleRaceCondition(IdlingResourceRegistry.java:375)
at android.support.test.espresso.base.IdlingResourceRegistry$Dispatcher.handleMessage(IdlingResourceRegistry.java:309)
at android.os.Handler.dispatchMessage(Handler.java:98)
at android.support.test.espresso.base.UiControllerImpl.loopUntil(UiControllerImpl.java:467)
at android.support.test.espresso.base.UiControllerImpl.loopMainThreadUntilIdle(UiControllerImpl.java:362)
at android.support.test.espresso.ViewInteraction$2.run(ViewInteraction.java:161)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:422)
at java.util.concurrent.FutureTask.run(FutureTask.java:237)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Any suggestions on how to fix it?