diff --git a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/InboxAutoApprovePredicate.java b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/InboxAutoApprovePredicate.java
index 7c16770ca..a4b59a0e4 100644
--- a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/InboxAutoApprovePredicate.java
+++ b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/InboxAutoApprovePredicate.java
@@ -14,6 +14,7 @@ package org.openhab.core.config.discovery.inbox;
 
 import java.util.function.Predicate;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.config.discovery.DiscoveryResult;
 import org.openhab.core.config.discovery.internal.AutomaticInboxProcessor;
 import org.osgi.service.component.annotations.Component;
@@ -34,6 +35,7 @@ import org.osgi.service.component.annotations.Component;
  *
  * @author Henning Sudbrock - Initial contribution
  */
+@NonNullByDefault
 public interface InboxAutoApprovePredicate extends Predicate<DiscoveryResult> {
 
 }
diff --git a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/AbstractInboxEvent.java b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/AbstractInboxEvent.java
index c68f5ec0f..369df5b28 100644
--- a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/AbstractInboxEvent.java
+++ b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/AbstractInboxEvent.java
@@ -12,6 +12,7 @@
  */
 package org.openhab.core.config.discovery.inbox.events;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
 import org.openhab.core.config.discovery.inbox.Inbox;
 import org.openhab.core.events.AbstractEvent;
@@ -22,6 +23,7 @@ import org.openhab.core.events.AbstractEvent;
  *
  * @author Stefan Bußweiler - Initial contribution
  */
+@NonNullByDefault
 public abstract class AbstractInboxEvent extends AbstractEvent {
 
     private final DiscoveryResultDTO discoveryResult;
@@ -40,7 +42,7 @@ public abstract class AbstractInboxEvent extends AbstractEvent {
 
     /**
      * Gets the discovery result as data transfer object.
-     * 
+     *
      * @return the discoveryResult
      */
     public DiscoveryResultDTO getDiscoveryResult() {
diff --git a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxAddedEvent.java b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxAddedEvent.java
index 0207b5b2d..0b5e8b580 100644
--- a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxAddedEvent.java
+++ b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxAddedEvent.java
@@ -12,6 +12,7 @@
  */
 package org.openhab.core.config.discovery.inbox.events;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
 
 /**
@@ -20,6 +21,7 @@ import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
  *
  * @author Stefan Bußweiler - Initial contribution
  */
+@NonNullByDefault
 public class InboxAddedEvent extends AbstractInboxEvent {
 
     /**
diff --git a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxEventFactory.java b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxEventFactory.java
index a03b15558..5094272a9 100644
--- a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxEventFactory.java
+++ b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxEventFactory.java
@@ -12,9 +12,10 @@
  */
 package org.openhab.core.config.discovery.inbox.events;
 
-import java.util.stream.Collectors;
-import java.util.stream.Stream;
+import java.util.Set;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.config.discovery.DiscoveryResult;
 import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
 import org.openhab.core.config.discovery.dto.DiscoveryResultDTOMapper;
@@ -28,6 +29,7 @@ import org.osgi.service.component.annotations.Component;
  *
  * @author Stefan Bußweiler - Initial contribution
  */
+@NonNullByDefault
 @Component(immediate = true, service = EventFactory.class)
 public class InboxEventFactory extends AbstractEventFactory {
 
@@ -41,12 +43,12 @@ public class InboxEventFactory extends AbstractEventFactory {
      * Constructs a new InboxEventFactory.
      */
     public InboxEventFactory() {
-        super(Stream.of(InboxAddedEvent.TYPE, InboxUpdatedEvent.TYPE, InboxRemovedEvent.TYPE)
-                .collect(Collectors.toSet()));
+        super(Set.of(InboxAddedEvent.TYPE, InboxUpdatedEvent.TYPE, InboxRemovedEvent.TYPE));
     }
 
     @Override
-    protected Event createEventByType(String eventType, String topic, String payload, String source) throws Exception {
+    protected Event createEventByType(String eventType, String topic, String payload, @Nullable String source)
+            throws Exception {
         if (InboxAddedEvent.TYPE.equals(eventType)) {
             return createAddedEvent(topic, payload);
         } else if (InboxRemovedEvent.TYPE.equals(eventType)) {
diff --git a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxRemovedEvent.java b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxRemovedEvent.java
index 5fbd84eea..2d5d57a4d 100644
--- a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxRemovedEvent.java
+++ b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxRemovedEvent.java
@@ -12,6 +12,7 @@
  */
 package org.openhab.core.config.discovery.inbox.events;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
 
 /**
@@ -20,6 +21,7 @@ import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
  *
  * @author Stefan Bußweiler - Initial contribution
  */
+@NonNullByDefault
 public class InboxRemovedEvent extends AbstractInboxEvent {
 
     /**
diff --git a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxUpdatedEvent.java b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxUpdatedEvent.java
index beafc75d8..2253b0a41 100644
--- a/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxUpdatedEvent.java
+++ b/bundles/org.openhab.core.config.discovery/src/main/java/org/openhab/core/config/discovery/inbox/events/InboxUpdatedEvent.java
@@ -12,6 +12,7 @@
  */
 package org.openhab.core.config.discovery.inbox.events;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
 
 /**
@@ -20,6 +21,7 @@ import org.openhab.core.config.discovery.dto.DiscoveryResultDTO;
  *
  * @author Stefan Bußweiler - Initial contribution
  */
+@NonNullByDefault
 public class InboxUpdatedEvent extends AbstractInboxEvent {
 
     /**