Android test for creating files in multiple threads

ExampleInstrumentedTest.java

package com.example.jifang.testio;

import android.content.Context;
import android.support.test.InstrumentationRegistry;
import android.support.test.runner.AndroidJUnit4;
import android.util.Log;

import org.junit.Test;
import org.junit.runner.RunWith;

import java.io.File;
import java.io.IOException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;

import static junit.framework.Assert.assertFalse;

/**
 * Instrumented test, which will execute on an Android device.
 *
 * @see <a href="http://d.android.com/tools/testing">Testing documentation</a>
 */
@RunWith(AndroidJUnit4.class)
public class ExampleInstrumentedTest {

    private static final String TAG = "IO_Test";
    private static final int TEST_NUMBER = 100;
    private static final int TEST_THREAD = 2;

    private Object lock = new Object();

    @Test
    public void testCreatingFiles() throws InterruptedException {

        ExecutorService executor = Executors.newFixedThreadPool(TEST_THREAD);
        final CountDownLatch signal = new CountDownLatch(TEST_THREAD);

        Context appContext = InstrumentationRegistry.getTargetContext();
        final File folder = new File(appContext.getExternalFilesDir(null), "test");
        if (!folder.exists() && !folder.mkdirs()) {
            Log.e(TAG, "Failed to create folder");
        }
        for (File file: folder.listFiles()) {
            if (!file.delete()) {
                Log.e(TAG, "Failed to delete:" + file.getAbsolutePath());
            }
        }

        Log.i(TAG, "Testing folder:" + folder.getAbsolutePath());

        final AtomicInteger index = new AtomicInteger(0);
        final AtomicBoolean failed = new AtomicBoolean(false);
        for (int i = 0; i < TEST_THREAD; ++i) {
            executor.submit(new Runnable() {
                @Override
                public void run() {
                    while (index.get() <  TEST_NUMBER && !failed.get()) {
                        int filename = index.getAndIncrement();
                        File testFile = new File(folder, String.valueOf(filename));
                        try {
                            Log.v(TAG, "Creating file: " + filename);
//                            synchronized (lock) { // Add a lock will make test pass
                                testFile.createNewFile();
//                            }
                        } catch (IOException e) {
                            e.printStackTrace();
                            Log.e(TAG, "Failed to create file: " + testFile.getAbsolutePath());
                            failed.set(true);
                            signal.countDown();
                        }
                    }
                    signal.countDown();
                }
            });
        }

        signal.await();
        assertFalse("Test failed", failed.get());
        Log.i(TAG, "Test finished");
    }
}