diff --git a/.circleci/config.yml b/.circleci/config.yml
index 910f476b8..1a97e73c5 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -90,7 +90,14 @@ workflows:
             - run:
                 name: Lint app
                 command: ./gradlew app:lintPlayRelease
+            - run:
+                name: Lint core
+                command: ./gradlew core:lintPlayRelease
             - store_artifacts:
-                name: Uploading lint reports
+                name: Uploading app lint reports
                 path: app/build/reports/lint-results-playRelease.html
-                destination: lint-results.html
+                destination: lint-results-app.html
+            - store_artifacts:
+                name: Uploading core lint reports
+                path: core/build/reports/lint-results-playRelease.html
+                destination: lint-results-core.html
diff --git a/core/build.gradle b/core/build.gradle
index bfaffd14f..b3c614059 100644
--- a/core/build.gradle
+++ b/core/build.gradle
@@ -52,6 +52,16 @@ android {
             dimension "market"
         }
     }
+
+    lintOptions {
+        disable "InvalidPeriodicWorkRequestInterval", "ObsoleteLintCustomCheck", "DefaultLocale", "UnusedAttribute",
+                "GradleDependency", "ParcelClassLoader", "Typos", "ExtraTranslation", "ImpliedQuantity",
+                "PluralsCandidate", "UnusedQuantity", "StringFormatCount", "TrustAllX509TrustManager",
+                "StaticFieldLeak", "TypographyEllipsis", "IconDensities", "IconDuplicates", "CheckResult"
+
+        warningsAsErrors true
+        abortOnError true
+    }
 }
 
 dependencies {
diff --git a/core/src/main/AndroidManifest.xml b/core/src/main/AndroidManifest.xml
index 1f6c36c40..ae5e56e55 100644
--- a/core/src/main/AndroidManifest.xml
+++ b/core/src/main/AndroidManifest.xml
@@ -1,5 +1,5 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    package="de.danoeh.antennapod.core">
+          xmlns:tools="http://schemas.android.com/tools" package="de.danoeh.antennapod.core">
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
@@ -12,15 +12,19 @@
 
     <application
         android:allowBackup="true"
-        android:icon="@mipmap/ic_launcher">
+        android:icon="@mipmap/ic_launcher"
+        android:supportsRtl="true">
 
         <service
             android:name=".service.download.DownloadService"
             android:enabled="true" />
+
         <service android:name=".service.playback.PlaybackService"
             android:label="@string/app_name"
             android:enabled="true"
-            android:exported="true">
+            android:exported="true"
+            tools:ignore="ExportedService">
+
             <intent-filter>
                 <action android:name="android.media.browse.MediaBrowserService"/>
             </intent-filter>
@@ -39,8 +43,8 @@
 
         <receiver android:name=".receiver.FeedUpdateReceiver"
             android:label="@string/feed_update_receiver_name"
-            android:exported="true"> <!-- allow feeds update to be triggered by external apps -->
-        </receiver>
+            android:exported="true"
+            tools:ignore="ExportedReceiver" /> <!-- allow feeds update to be triggered by external apps -->
     </application>
 
 </manifest>
diff --git a/core/src/main/java/de/danoeh/antennapod/core/preferences/UserPreferences.java b/core/src/main/java/de/danoeh/antennapod/core/preferences/UserPreferences.java
index bcbc041a6..5700bb9a0 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/preferences/UserPreferences.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/preferences/UserPreferences.java
@@ -249,7 +249,7 @@ public class UserPreferences {
     public static void setFeedOrder(String selected) {
         prefs.edit()
                 .putString(PREF_DRAWER_FEED_ORDER, selected)
-                .commit();
+                .apply();
     }
 
     public static int getFeedCounterSetting() {
@@ -1054,7 +1054,7 @@ public class UserPreferences {
     public static void setFeedFilter(String value) {
         prefs.edit()
                 .putString(PREF_FILTER_FEED, value)
-                .commit();
+                .apply();
     }
 
 }
diff --git a/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackServiceTaskManager.java b/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackServiceTaskManager.java
index 883ba6023..9ef34cdac 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackServiceTaskManager.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackServiceTaskManager.java
@@ -8,6 +8,7 @@ import androidx.annotation.NonNull;
 import android.util.Log;
 
 import de.danoeh.antennapod.core.preferences.SleepTimerPreferences;
+import io.reactivex.disposables.Disposable;
 import org.greenrobot.eventbus.EventBus;
 import org.greenrobot.eventbus.Subscribe;
 
@@ -57,7 +58,7 @@ public class PlaybackServiceTaskManager {
     private ScheduledFuture<?> widgetUpdaterFuture;
     private ScheduledFuture<?> sleepTimerFuture;
     private volatile Future<List<FeedItem>> queueFuture;
-    private volatile Future<?> chapterLoaderFuture;
+    private volatile Disposable chapterLoaderFuture;
 
     private SleepTimer sleepTimer;
 
@@ -102,7 +103,7 @@ public class PlaybackServiceTaskManager {
 
     private synchronized void loadQueue() {
         if (!isQueueLoaderActive()) {
-            queueFuture = schedExecutor.submit(DBReader::getQueue);
+            queueFuture = schedExecutor.submit(() -> DBReader.getQueue());
         }
     }
 
@@ -289,28 +290,19 @@ public class PlaybackServiceTaskManager {
         }
     }
 
-    private synchronized void cancelChapterLoader() {
-        if (isChapterLoaderActive()) {
-            chapterLoaderFuture.cancel(true);
-        }
-    }
-
-    private synchronized boolean isChapterLoaderActive() {
-        return chapterLoaderFuture != null && !chapterLoaderFuture.isDone();
-    }
-
     /**
      * Starts a new thread that loads the chapter marks from a playable object. If another chapter loader is already active,
      * it will be cancelled first.
      * On completion, the callback's onChapterLoaded method will be called.
      */
     public synchronized void startChapterLoader(@NonNull final Playable media) {
-        if (isChapterLoaderActive()) {
-            cancelChapterLoader();
+        if (chapterLoaderFuture != null) {
+            chapterLoaderFuture.dispose();
+            chapterLoaderFuture = null;
         }
 
         if (media.getChapters() == null) {
-            Completable.create(emitter -> {
+            chapterLoaderFuture = Completable.create(emitter -> {
                 media.loadChapterMarks();
                 emitter.onComplete();
             })
@@ -330,7 +322,11 @@ public class PlaybackServiceTaskManager {
         cancelWidgetUpdater();
         disableSleepTimer();
         cancelQueueLoader();
-        cancelChapterLoader();
+
+        if (chapterLoaderFuture != null) {
+            chapterLoaderFuture.dispose();
+            chapterLoaderFuture = null;
+        }
     }
 
     /**
diff --git a/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java b/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java
index 16e2825b4..4f2417b7d 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java
@@ -257,7 +257,6 @@ public final class DBTasks {
         EventBus.getDefault().post(new MessageEvent(context.getString(R.string.error_file_not_found)));
     }
 
-    @VisibleForTesting(otherwise = VisibleForTesting.PACKAGE_PRIVATE)
     public static List<? extends FeedItem> enqueueFeedItemsToDownload(final Context context,
                        List<? extends FeedItem> items) throws InterruptedException, ExecutionException {
         List<FeedItem> itemsToEnqueue = new ArrayList<>();
diff --git a/core/src/main/res/layout/player_widget.xml b/core/src/main/res/layout/player_widget.xml
index 6e463e9cd..8e38d7f6e 100644
--- a/core/src/main/res/layout/player_widget.xml
+++ b/core/src/main/res/layout/player_widget.xml
@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:padding="@dimen/widget_margin" >
@@ -8,8 +9,8 @@
         android:id="@+id/widgetLayout"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
-        android:background="#262C31" >
-
+        android:background="#262C31"
+        tools:ignore="UselessParent">
 
         <ImageButton
             android:id="@+id/butPlay"
@@ -41,6 +42,7 @@
                 android:layout_width="@android:dimen/app_icon_size"
                 android:layout_height="match_parent"
                 android:src="@mipmap/ic_launcher_round"
+                android:importantForAccessibility="no"
                 android:layout_margin="12dp" />
 
             <LinearLayout