diff --git a/app/src/main/java/eu/faircode/email/ActivityAnswer.java b/app/src/main/java/eu/faircode/email/ActivityAnswer.java
index bf435c6696..c361ee8577 100644
--- a/app/src/main/java/eu/faircode/email/ActivityAnswer.java
+++ b/app/src/main/java/eu/faircode/email/ActivityAnswer.java
@@ -40,7 +40,7 @@ import java.util.List;
 import java.util.concurrent.ExecutorService;
 
 public class ActivityAnswer extends ActivityBase {
-    private static ExecutorService executor = Helper.getBackgroundExecutor(1, "answer");
+    private static final ExecutorService executor = Helper.getBackgroundExecutor(1, "answer");
 
     @Override
     @RequiresApi(api = Build.VERSION_CODES.M)
diff --git a/app/src/main/java/eu/faircode/email/EditTextMultiAutoComplete.java b/app/src/main/java/eu/faircode/email/EditTextMultiAutoComplete.java
index 2f8c3d1610..7f289a9a3d 100644
--- a/app/src/main/java/eu/faircode/email/EditTextMultiAutoComplete.java
+++ b/app/src/main/java/eu/faircode/email/EditTextMultiAutoComplete.java
@@ -73,7 +73,7 @@ public class EditTextMultiAutoComplete extends AppCompatMultiAutoCompleteTextVie
     private Tokenizer tokenizer;
     private Map<String, Integer> encryption = new ConcurrentHashMap<>();
 
-    private static ExecutorService executor = Helper.getBackgroundExecutor(1, "chips");
+    private static final ExecutorService executor = Helper.getBackgroundExecutor(1, "chips");
 
     private static int[] icons = new int[]{
             R.drawable.twotone_vpn_key_24_p,
diff --git a/app/src/main/java/eu/faircode/email/EntityRule.java b/app/src/main/java/eu/faircode/email/EntityRule.java
index 80547bf42e..9af54708ad 100644
--- a/app/src/main/java/eu/faircode/email/EntityRule.java
+++ b/app/src/main/java/eu/faircode/email/EntityRule.java
@@ -128,7 +128,7 @@ public class EntityRule {
 
     private static final long SEND_DELAY = 5000L; // milliseconds
 
-    private static ExecutorService executor = Helper.getBackgroundExecutor(1, "rule");
+    private static final ExecutorService executor = Helper.getBackgroundExecutor(1, "rule");
 
     static boolean needsHeaders(EntityMessage message, List<EntityRule> rules) {
         return needs(rules, "header");
diff --git a/app/src/main/java/eu/faircode/email/FragmentCompose.java b/app/src/main/java/eu/faircode/email/FragmentCompose.java
index 100a6985c1..cc86f2e670 100644
--- a/app/src/main/java/eu/faircode/email/FragmentCompose.java
+++ b/app/src/main/java/eu/faircode/email/FragmentCompose.java
@@ -338,7 +338,7 @@ public class FragmentCompose extends FragmentBase {
     private static final int REQUEST_SEND = 14;
     private static final int REQUEST_REMOVE_ATTACHMENTS = 15;
 
-    private static ExecutorService executor = Helper.getBackgroundExecutor(1, "compose");
+    private static final ExecutorService executor = Helper.getBackgroundExecutor(1, "compose");
 
     @Override
     public void onCreate(Bundle savedInstanceState) {
diff --git a/app/src/main/java/eu/faircode/email/FragmentMessages.java b/app/src/main/java/eu/faircode/email/FragmentMessages.java
index c9d332cf4f..a388deb4ea 100644
--- a/app/src/main/java/eu/faircode/email/FragmentMessages.java
+++ b/app/src/main/java/eu/faircode/email/FragmentMessages.java
@@ -446,7 +446,7 @@ public class FragmentMessages extends FragmentBase
             "time", "unread", "starred", "priority"
     ));
 
-    private static ExecutorService executor = Helper.getBackgroundExecutor(1, "messages");
+    private static final ExecutorService executor = Helper.getBackgroundExecutor(1, "messages");
 
     @Override
     public void onCreate(Bundle savedInstanceState) {
diff --git a/app/src/main/java/eu/faircode/email/MediaPlayerHelper.java b/app/src/main/java/eu/faircode/email/MediaPlayerHelper.java
index d0ab47687a..3847e962b8 100644
--- a/app/src/main/java/eu/faircode/email/MediaPlayerHelper.java
+++ b/app/src/main/java/eu/faircode/email/MediaPlayerHelper.java
@@ -22,7 +22,7 @@ public class MediaPlayerHelper {
     static final int DEFAULT_SOUND_DURATION = 30; // seconds
     static final int DEFAULT_ALARM_DURATION = 30; // seconds
 
-    private static ExecutorService executor = Helper.getBackgroundExecutor(1, "media");
+    private static final ExecutorService executor = Helper.getBackgroundExecutor(1, "media");
 
     static void queue(Context context, String uri) {
         try {
diff --git a/app/src/main/java/eu/faircode/email/ServiceSend.java b/app/src/main/java/eu/faircode/email/ServiceSend.java
index c995ea46f1..75ea214ad3 100644
--- a/app/src/main/java/eu/faircode/email/ServiceSend.java
+++ b/app/src/main/java/eu/faircode/email/ServiceSend.java
@@ -78,7 +78,7 @@ public class ServiceSend extends ServiceBase implements SharedPreferences.OnShar
     private PowerManager.WakeLock wlOutbox;
     private List<Long> handling = new ArrayList<>();
 
-    private static ExecutorService executor = Helper.getBackgroundExecutor(1, "send");
+    private static final ExecutorService executor = Helper.getBackgroundExecutor(1, "send");
 
     private static final int RETRY_MAX = 3;
     private static final int CONNECTIVITY_DELAY = 5000; // milliseconds