diff --git a/bundles/org.openhab.core.auth.jaas/src/main/java/org/openhab/core/auth/jaas/internal/JaasAuthenticationProvider.java b/bundles/org.openhab.core.auth.jaas/src/main/java/org/openhab/core/auth/jaas/internal/JaasAuthenticationProvider.java
index ee305aa321..0c71f5a74a 100644
--- a/bundles/org.openhab.core.auth.jaas/src/main/java/org/openhab/core/auth/jaas/internal/JaasAuthenticationProvider.java
+++ b/bundles/org.openhab.core.auth.jaas/src/main/java/org/openhab/core/auth/jaas/internal/JaasAuthenticationProvider.java
@@ -27,6 +27,8 @@ import javax.security.auth.callback.UnsupportedCallbackException;
 import javax.security.auth.login.LoginContext;
 import javax.security.auth.login.LoginException;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.auth.Authentication;
 import org.openhab.core.auth.AuthenticationException;
 import org.openhab.core.auth.AuthenticationProvider;
@@ -48,11 +50,12 @@ import org.osgi.service.component.annotations.Modified;
  * @author Kai Kreuzer - Removed ManagedService and used DS configuration instead
  * @author Yannick Schaus - provides a configuration with the ManagedUserLoginModule as a sufficient login module
  */
+@NonNullByDefault
 @Component(configurationPid = "org.openhab.jaas")
 public class JaasAuthenticationProvider implements AuthenticationProvider {
-    private final String DEFAULT_REALM = "openhab";
+    private static final String DEFAULT_REALM = "openhab";
 
-    private String realmName;
+    private @Nullable String realmName;
 
     @Override
     public Authentication authenticate(final Credentials credentials) throws AuthenticationException {
@@ -70,14 +73,14 @@ public class JaasAuthenticationProvider implements AuthenticationProvider {
 
         final ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
         try {
-
             Principal userPrincipal = new GenericUser(name);
             Subject subject = new Subject(true, Set.of(userPrincipal), Collections.emptySet(), Set.of(userCredentials));
 
             Thread.currentThread().setContextClassLoader(ManagedUserLoginModule.class.getClassLoader());
             LoginContext loginContext = new LoginContext(realmName, subject, new CallbackHandler() {
                 @Override
-                public void handle(Callback[] callbacks) throws IOException, UnsupportedCallbackException {
+                public void handle(@NonNullByDefault({}) Callback[] callbacks)
+                        throws IOException, UnsupportedCallbackException {
                     for (Callback callback : callbacks) {
                         if (callback instanceof PasswordCallback) {
                             ((PasswordCallback) callback).setPassword(password);
diff --git a/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthStoreHandlerImpl.java b/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthStoreHandlerImpl.java
index c0f10f3722..6d37c68602 100644
--- a/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthStoreHandlerImpl.java
+++ b/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/OAuthStoreHandlerImpl.java
@@ -25,7 +25,6 @@ import java.util.concurrent.TimeUnit;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.auth.client.oauth2.AccessTokenResponse;
@@ -124,7 +123,7 @@ public class OAuthStoreHandlerImpl implements OAuthStoreHandler {
     }
 
     @Override
-    public void saveAccessTokenResponse(@NonNull String handle, @Nullable AccessTokenResponse pAccessTokenResponse) {
+    public void saveAccessTokenResponse(String handle, @Nullable AccessTokenResponse pAccessTokenResponse) {
         AccessTokenResponse accessTokenResponse = pAccessTokenResponse;
         if (accessTokenResponse == null) {
             accessTokenResponse = new AccessTokenResponse(); // put empty
diff --git a/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/StorageRecordType.java b/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/StorageRecordType.java
index f33f7b5118..52a7eac6da 100644
--- a/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/StorageRecordType.java
+++ b/bundles/org.openhab.core.auth.oauth2client/src/main/java/org/openhab/core/auth/oauth2client/internal/StorageRecordType.java
@@ -12,13 +12,15 @@
  */
 package org.openhab.core.auth.oauth2client.internal;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 
 /**
  * Enum of types being used in the store
  *
  * @author Gary Tse - Initial contribution
  */
+@NonNullByDefault
 public enum StorageRecordType {
 
     LAST_USED(".LastUsed"),
@@ -31,7 +33,7 @@ public enum StorageRecordType {
         this.suffix = suffix;
     }
 
-    public @NonNull String getKey(String handle) {
+    public String getKey(@Nullable String handle) {
         return (handle == null) ? this.suffix : (handle + this.suffix);
     }
 
diff --git a/bundles/org.openhab.core.automation.module.script/src/main/java/org/openhab/core/automation/module/script/internal/ScriptEngineManagerImpl.java b/bundles/org.openhab.core.automation.module.script/src/main/java/org/openhab/core/automation/module/script/internal/ScriptEngineManagerImpl.java
index 122b28b3a2..184230e3d2 100644
--- a/bundles/org.openhab.core.automation.module.script/src/main/java/org/openhab/core/automation/module/script/internal/ScriptEngineManagerImpl.java
+++ b/bundles/org.openhab.core.automation.module.script/src/main/java/org/openhab/core/automation/module/script/internal/ScriptEngineManagerImpl.java
@@ -21,7 +21,6 @@ import javax.script.Invocable;
 import javax.script.ScriptEngine;
 import javax.script.ScriptException;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.automation.module.script.ScriptEngineContainer;
@@ -112,7 +111,7 @@ public class ScriptEngineManagerImpl implements ScriptEngineManager {
     }
 
     @Override
-    public @Nullable ScriptEngineContainer createScriptEngine(String scriptType, @NonNull String engineIdentifier) {
+    public @Nullable ScriptEngineContainer createScriptEngine(String scriptType, String engineIdentifier) {
         ScriptEngineContainer result = null;
         ScriptEngineFactory engineFactory = findEngineFactory(scriptType);
         if (engineFactory == null) {
diff --git a/bundles/org.openhab.core.config.core/src/main/java/org/openhab/core/config/core/status/ConfigStatusSource.java b/bundles/org.openhab.core.config.core/src/main/java/org/openhab/core/config/core/status/ConfigStatusSource.java
index c1d05f7697..4fa3a4ea7b 100644
--- a/bundles/org.openhab.core.config.core/src/main/java/org/openhab/core/config/core/status/ConfigStatusSource.java
+++ b/bundles/org.openhab.core.config.core/src/main/java/org/openhab/core/config/core/status/ConfigStatusSource.java
@@ -12,7 +12,7 @@
  */
 package org.openhab.core.config.core.status;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 
 /**
  * The {@link ConfigStatusSource} represents a source which would like to propagate its new configuration status. It is
@@ -20,6 +20,7 @@ import org.eclipse.jdt.annotation.NonNull;
  *
  * @author Thomas Höfer - Initial contribution
  */
+@NonNullByDefault
 public abstract class ConfigStatusSource {
 
     /** The id of the entity whose new configuration status is to be propagated. */
@@ -30,7 +31,7 @@ public abstract class ConfigStatusSource {
      *
      * @param entityId the id of the entity whose new configuration status is to be propagated
      */
-    public ConfigStatusSource(@NonNull String entityId) {
+    public ConfigStatusSource(String entityId) {
         super();
         this.entityId = entityId;
     }
diff --git a/bundles/org.openhab.core.config.discovery.mdns/src/main/java/org/openhab/core/config/discovery/mdns/internal/MDNSDiscoveryService.java b/bundles/org.openhab.core.config.discovery.mdns/src/main/java/org/openhab/core/config/discovery/mdns/internal/MDNSDiscoveryService.java
index 5b5d787378..7b9889a89e 100644
--- a/bundles/org.openhab.core.config.discovery.mdns/src/main/java/org/openhab/core/config/discovery/mdns/internal/MDNSDiscoveryService.java
+++ b/bundles/org.openhab.core.config.discovery.mdns/src/main/java/org/openhab/core/config/discovery/mdns/internal/MDNSDiscoveryService.java
@@ -23,7 +23,7 @@ import javax.jmdns.ServiceEvent;
 import javax.jmdns.ServiceInfo;
 import javax.jmdns.ServiceListener;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.config.discovery.AbstractDiscoveryService;
 import org.openhab.core.config.discovery.DiscoveryResult;
@@ -32,7 +32,9 @@ import org.openhab.core.config.discovery.mdns.MDNSDiscoveryParticipant;
 import org.openhab.core.io.transport.mdns.MDNSClient;
 import org.openhab.core.thing.ThingTypeUID;
 import org.openhab.core.thing.ThingUID;
+import org.osgi.service.component.annotations.Activate;
 import org.osgi.service.component.annotations.Component;
+import org.osgi.service.component.annotations.Deactivate;
 import org.osgi.service.component.annotations.Modified;
 import org.osgi.service.component.annotations.Reference;
 import org.osgi.service.component.annotations.ReferenceCardinality;
@@ -48,6 +50,7 @@ import org.slf4j.LoggerFactory;
  * @author Kai Kreuzer - Improved startup behavior and background discovery
  * @author Andre Fuechsel - make {@link #startScan()} asynchronous
  */
+@NonNullByDefault
 @Component(immediate = true, service = DiscoveryService.class, configurationPid = "discovery.mdns")
 public class MDNSDiscoveryService extends AbstractDiscoveryService implements ServiceListener {
 
@@ -59,15 +62,17 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
 
     private final Set<MDNSDiscoveryParticipant> participants = new CopyOnWriteArraySet<>();
 
-    private MDNSClient mdnsClient;
+    private final MDNSClient mdnsClient;
 
-    public MDNSDiscoveryService() {
+    @Activate
+    public MDNSDiscoveryService(final @Nullable Map<String, @Nullable Object> configProperties,
+            final @Reference MDNSClient mdnsClient) {
         super(5);
-    }
 
-    @Reference
-    public void setMDNSClient(MDNSClient mdnsClient) {
         this.mdnsClient = mdnsClient;
+
+        super.activate(configProperties);
+
         if (isBackgroundDiscoveryEnabled()) {
             for (MDNSDiscoveryParticipant participant : participants) {
                 mdnsClient.addServiceListener(participant.getServiceType(), this);
@@ -78,19 +83,22 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
         }
     }
 
-    public void unsetMDNSClient(MDNSClient mdnsClient) {
+    @Deactivate
+    @Override
+    protected void deactivate() {
+        super.deactivate();
+
         for (MDNSDiscoveryParticipant participant : participants) {
             mdnsClient.removeServiceListener(participant.getServiceType(), this);
         }
         for (org.openhab.core.io.transport.mdns.discovery.MDNSDiscoveryParticipant participant : oldParticipants) {
             mdnsClient.removeServiceListener(participant.getServiceType(), this);
         }
-        this.mdnsClient = null;
     }
 
     @Modified
     @Override
-    protected void modified(@Nullable Map<@NonNull String, @Nullable Object> configProperties) {
+    protected void modified(@Nullable Map<String, @Nullable Object> configProperties) {
         super.modified(configProperties);
     }
 
@@ -185,16 +193,14 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
     @Reference(cardinality = ReferenceCardinality.MULTIPLE, policy = ReferencePolicy.DYNAMIC)
     protected void addMDNSDiscoveryParticipant(MDNSDiscoveryParticipant participant) {
         this.participants.add(participant);
-        if (mdnsClient != null && isBackgroundDiscoveryEnabled()) {
+        if (isBackgroundDiscoveryEnabled()) {
             mdnsClient.addServiceListener(participant.getServiceType(), this);
         }
     }
 
     protected void removeMDNSDiscoveryParticipant(MDNSDiscoveryParticipant participant) {
         this.participants.remove(participant);
-        if (mdnsClient != null) {
-            mdnsClient.removeServiceListener(participant.getServiceType(), this);
-        }
+        mdnsClient.removeServiceListener(participant.getServiceType(), this);
     }
 
     @Deprecated
@@ -202,7 +208,7 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
     protected void addMDNSDiscoveryParticipant_old(
             org.openhab.core.io.transport.mdns.discovery.MDNSDiscoveryParticipant participant) {
         this.oldParticipants.add(participant);
-        if (mdnsClient != null && isBackgroundDiscoveryEnabled()) {
+        if (isBackgroundDiscoveryEnabled()) {
             mdnsClient.addServiceListener(participant.getServiceType(), this);
         }
     }
@@ -211,9 +217,7 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
     protected void removeMDNSDiscoveryParticipant_old(
             org.openhab.core.io.transport.mdns.discovery.MDNSDiscoveryParticipant participant) {
         this.oldParticipants.remove(participant);
-        if (mdnsClient != null) {
-            mdnsClient.removeServiceListener(participant.getServiceType(), this);
-        }
+        mdnsClient.removeServiceListener(participant.getServiceType(), this);
     }
 
     @Override
@@ -229,12 +233,12 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
     }
 
     @Override
-    public void serviceAdded(ServiceEvent serviceEvent) {
+    public void serviceAdded(@NonNullByDefault({}) ServiceEvent serviceEvent) {
         considerService(serviceEvent);
     }
 
     @Override
-    public void serviceRemoved(ServiceEvent serviceEvent) {
+    public void serviceRemoved(@NonNullByDefault({}) ServiceEvent serviceEvent) {
         for (MDNSDiscoveryParticipant participant : participants) {
             if (participant.getServiceType().equals(serviceEvent.getType())) {
                 try {
@@ -262,7 +266,7 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
     }
 
     @Override
-    public void serviceResolved(ServiceEvent serviceEvent) {
+    public void serviceResolved(@NonNullByDefault({}) ServiceEvent serviceEvent) {
         considerService(serviceEvent);
     }
 
diff --git a/bundles/org.openhab.core.config.discovery.usbserial/src/main/java/org/openhab/core/config/discovery/usbserial/internal/UsbSerialDiscoveryService.java b/bundles/org.openhab.core.config.discovery.usbserial/src/main/java/org/openhab/core/config/discovery/usbserial/internal/UsbSerialDiscoveryService.java
index 63aaee2e89..76eb23490a 100644
--- a/bundles/org.openhab.core.config.discovery.usbserial/src/main/java/org/openhab/core/config/discovery/usbserial/internal/UsbSerialDiscoveryService.java
+++ b/bundles/org.openhab.core.config.discovery.usbserial/src/main/java/org/openhab/core/config/discovery/usbserial/internal/UsbSerialDiscoveryService.java
@@ -19,7 +19,6 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CopyOnWriteArraySet;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.config.discovery.AbstractDiscoveryService;
@@ -89,7 +88,7 @@ public class UsbSerialDiscoveryService extends AbstractDiscoveryService implemen
 
     @Modified
     @Override
-    protected void modified(@Nullable Map<@NonNull String, @Nullable Object> configProperties) {
+    protected void modified(@Nullable Map<String, @Nullable Object> configProperties) {
         super.modified(configProperties);
     }
 
diff --git a/bundles/org.openhab.core.io.rest.sse/src/main/java/org/openhab/core/io/rest/sse/SseResource.java b/bundles/org.openhab.core.io.rest.sse/src/main/java/org/openhab/core/io/rest/sse/SseResource.java
index dea8c2a77d..f4a175a85f 100644
--- a/bundles/org.openhab.core.io.rest.sse/src/main/java/org/openhab/core/io/rest/sse/SseResource.java
+++ b/bundles/org.openhab.core.io.rest.sse/src/main/java/org/openhab/core/io/rest/sse/SseResource.java
@@ -158,7 +158,6 @@ public class SseResource {
     @ApiOperation(value = "Initiates a new item state tracker connection", response = EventOutput.class)
     @ApiResponses(value = { @ApiResponse(code = 200, message = "OK") })
     public Object getStateEvents() throws IOException, InterruptedException {
-
         final SseStateEventOutput eventOutput = new SseStateEventOutput();
         statesBroadcaster.add(eventOutput);
 
@@ -186,7 +185,6 @@ public class SseResource {
             @ApiResponse(code = 404, message = "Unknown connectionId") })
     public Object updateTrackedItems(@PathParam("connectionId") String connectionId,
             @ApiParam("items") Set<String> itemNames) {
-
         try {
             statesBroadcaster.updateTrackedItems(connectionId, itemNames);
         } catch (IllegalArgumentException e) {
diff --git a/bundles/org.openhab.core.persistence/src/main/java/org/openhab/core/persistence/HistoricItem.java b/bundles/org.openhab.core.persistence/src/main/java/org/openhab/core/persistence/HistoricItem.java
index 99dd1ffeb6..6cbd5d07e5 100644
--- a/bundles/org.openhab.core.persistence/src/main/java/org/openhab/core/persistence/HistoricItem.java
+++ b/bundles/org.openhab.core.persistence/src/main/java/org/openhab/core/persistence/HistoricItem.java
@@ -14,7 +14,7 @@ package org.openhab.core.persistence;
 
 import java.util.Date;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.items.Item;
 import org.openhab.core.types.State;
 
@@ -28,6 +28,7 @@ import org.openhab.core.types.State;
  *
  * @author Kai Kreuzer - Initial contribution
  */
+@NonNullByDefault
 public interface HistoricItem {
 
     /**
@@ -35,7 +36,6 @@ public interface HistoricItem {
      *
      * @return the timestamp of the item
      */
-    @NonNull
     Date getTimestamp();
 
     /**
@@ -43,7 +43,6 @@ public interface HistoricItem {
      *
      * @return the current state
      */
-    @NonNull
     State getState();
 
     /**
@@ -51,7 +50,6 @@ public interface HistoricItem {
      *
      * @return the name of the item
      */
-    @NonNull
     String getName();
 
 }
diff --git a/bundles/org.openhab.core.test.magic/src/main/java/org/openhab/core/magic/binding/handler/MagicThermostatThingHandler.java b/bundles/org.openhab.core.test.magic/src/main/java/org/openhab/core/magic/binding/handler/MagicThermostatThingHandler.java
index 1b8acaf125..f29f013abe 100644
--- a/bundles/org.openhab.core.test.magic/src/main/java/org/openhab/core/magic/binding/handler/MagicThermostatThingHandler.java
+++ b/bundles/org.openhab.core.test.magic/src/main/java/org/openhab/core/magic/binding/handler/MagicThermostatThingHandler.java
@@ -16,7 +16,7 @@ import static org.openhab.core.magic.binding.MagicBindingConstants.*;
 
 import java.util.concurrent.TimeUnit;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.library.types.DecimalType;
 import org.openhab.core.library.types.QuantityType;
 import org.openhab.core.thing.ChannelUID;
@@ -30,6 +30,7 @@ import org.openhab.core.types.Command;
  *
  * @author Henning Treu - Initial contribution
  */
+@NonNullByDefault
 public class MagicThermostatThingHandler extends BaseThingHandler {
 
     public MagicThermostatThingHandler(Thing thing) {
@@ -37,7 +38,7 @@ public class MagicThermostatThingHandler extends BaseThingHandler {
     }
 
     @Override
-    public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+    public void handleCommand(ChannelUID channelUID, Command command) {
         if (channelUID.getId().equals(CHANNEL_SET_TEMPERATURE)) {
             if (command instanceof DecimalType || command instanceof QuantityType) {
                 String state = command.toFullString() + (command instanceof DecimalType ? " °C" : "");
diff --git a/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/internal/CommunicationManager.java b/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/internal/CommunicationManager.java
index 05199078e6..189751dcd7 100644
--- a/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/internal/CommunicationManager.java
+++ b/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/internal/CommunicationManager.java
@@ -27,7 +27,6 @@ import java.util.function.Function;
 
 import javax.measure.Quantity;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.common.AbstractUID;
@@ -650,12 +649,12 @@ public class CommunicationManager implements EventSubscriber, RegistryChangeList
 
     private static class NoOpProfile implements Profile {
         @Override
-        public @NonNull ProfileTypeUID getProfileTypeUID() {
+        public ProfileTypeUID getProfileTypeUID() {
             return new ProfileTypeUID(ProfileTypeUID.SYSTEM_SCOPE, "noop");
         }
 
         @Override
-        public void onStateUpdateFromItem(@NonNull State state) {
+        public void onStateUpdateFromItem(State state) {
             // no-op
         }
     }
diff --git a/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/link/ItemChannelLinkRegistry.java b/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/link/ItemChannelLinkRegistry.java
index 48ae1eae0f..dad557c266 100644
--- a/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/link/ItemChannelLinkRegistry.java
+++ b/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/link/ItemChannelLinkRegistry.java
@@ -17,7 +17,6 @@ import java.util.Set;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.openhab.core.events.EventPublisher;
 import org.openhab.core.items.Item;
@@ -74,7 +73,7 @@ public class ItemChannelLinkRegistry extends AbstractLinkRegistry<ItemChannelLin
     }
 
     public Set<Item> getLinkedItems(final UID uid) {
-        return ((Stream<@NonNull Item>) super.getLinkedItemNames(uid).parallelStream()
+        return ((Stream<Item>) super.getLinkedItemNames(uid).parallelStream()
                 .map(itemName -> itemRegistry.get(itemName)).filter(Objects::nonNull)).collect(Collectors.toSet());
     }
 
@@ -85,7 +84,7 @@ public class ItemChannelLinkRegistry extends AbstractLinkRegistry<ItemChannelLin
      * @return an unmodifiable set of bound things for the given item name
      */
     public Set<Thing> getBoundThings(final String itemName) {
-        return ((Stream<@NonNull Thing>) getBoundChannels(itemName).parallelStream()
+        return ((Stream<Thing>) getBoundChannels(itemName).parallelStream()
                 .map(channelUID -> thingRegistry.get(channelUID.getThingUID())).filter(Objects::nonNull))
                         .collect(Collectors.toSet());
     }
diff --git a/bundles/org.openhab.core.thing/src/test/java/org/openhab/core/thing/type/ThingTypeBuilderTest.java b/bundles/org.openhab.core.thing/src/test/java/org/openhab/core/thing/type/ThingTypeBuilderTest.java
index d8c2ac25f6..8be861e5f0 100644
--- a/bundles/org.openhab.core.thing/src/test/java/org/openhab/core/thing/type/ThingTypeBuilderTest.java
+++ b/bundles/org.openhab.core.thing/src/test/java/org/openhab/core/thing/type/ThingTypeBuilderTest.java
@@ -24,7 +24,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.junit.Before;
 import org.junit.Test;
 import org.openhab.core.thing.ThingTypeUID;
@@ -34,6 +34,7 @@ import org.openhab.core.thing.ThingTypeUID;
  *
  * @author Henning Treu - Initial contribution
  */
+@NonNullByDefault
 public class ThingTypeBuilderTest {
 
     private static final String CONF_URI = "conf:uri";
@@ -44,7 +45,7 @@ public class ThingTypeBuilderTest {
     private static final String THING_TYPE_ID = "thingTypeId";
     private static final String BINDING_ID = "bindingId";
 
-    private ThingTypeBuilder builder;
+    private @NonNullByDefault({}) ThingTypeBuilder builder;
 
     @Before
     public void setup() {
@@ -212,15 +213,15 @@ public class ThingTypeBuilderTest {
         assertThat(bridgeType.getLabel(), is(LABEL));
     }
 
-    private Map<@NonNull String, String> mockProperties() {
-        Map<@NonNull String, String> result = new HashMap<>();
+    private Map<String, String> mockProperties() {
+        Map<String, String> result = new HashMap<>();
         result.put("key1", "value1");
         result.put("key2", "value2");
 
         return result;
     }
 
-    private <T> @NonNull List<T> mockList(Class<T> entityClass, int size) {
+    private <T> List<T> mockList(Class<T> entityClass, int size) {
         List<T> result = new ArrayList<>(size);
         for (int i = 0; i < size; i++) {
             result.add(mock(entityClass));
diff --git a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentProvider.java b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentProvider.java
index 176c71699c..2c68a18388 100644
--- a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentProvider.java
+++ b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentProvider.java
@@ -17,7 +17,6 @@ import java.util.Collection;
 import java.util.List;
 
 import org.apache.commons.lang.StringUtils;
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.common.registry.AbstractProvider;
@@ -62,7 +61,7 @@ public class UIComponentProvider extends AbstractProvider<RootUIComponent>
     }
 
     @Override
-    public void add(@NonNull RootUIComponent element) {
+    public void add(RootUIComponent element) {
         if (StringUtils.isEmpty(element.getUID())) {
             throw new IllegalArgumentException("Invalid UID");
         }
@@ -76,7 +75,7 @@ public class UIComponentProvider extends AbstractProvider<RootUIComponent>
     }
 
     @Override
-    public @Nullable RootUIComponent remove(@NonNull String key) {
+    public @Nullable RootUIComponent remove(String key) {
         RootUIComponent element = storage.remove(key);
         if (element != null) {
             notifyListenersAboutRemovedElement(element);
@@ -87,7 +86,7 @@ public class UIComponentProvider extends AbstractProvider<RootUIComponent>
     }
 
     @Override
-    public @Nullable RootUIComponent update(@NonNull RootUIComponent element) {
+    public @Nullable RootUIComponent update(RootUIComponent element) {
         if (storage.get(element.getUID()) != null) {
             RootUIComponent oldElement = storage.put(element.getUID(), element);
             if (oldElement != null) {
diff --git a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentSitemapProvider.java b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentSitemapProvider.java
index ef0ab7df52..4729e231da 100644
--- a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentSitemapProvider.java
+++ b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/components/UIComponentSitemapProvider.java
@@ -13,13 +13,14 @@
 package org.openhab.core.ui.internal.components;
 
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CopyOnWriteArraySet;
 
 import org.eclipse.emf.common.util.EList;
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.common.registry.RegistryChangeListener;
 import org.openhab.core.config.core.ConfigUtil;
@@ -67,6 +68,7 @@ import org.slf4j.LoggerFactory;
  *
  * @author Yannick Schaus - Initial contribution
  */
+@NonNullByDefault
 @Component(service = SitemapProvider.class)
 public class UIComponentSitemapProvider implements SitemapProvider, RegistryChangeListener<RootUIComponent> {
     private final Logger logger = LoggerFactory.getLogger(UIComponentSitemapProvider.class);
@@ -77,21 +79,26 @@ public class UIComponentSitemapProvider implements SitemapProvider, RegistryChan
     private static final String SITEMAP_SUFFIX = ".sitemap";
 
     private Map<String, Sitemap> sitemaps = new HashMap<>();
-    private UIComponentRegistryFactory componentRegistryFactory;
-    private UIComponentRegistry sitemapComponentRegistry;
+    private @Nullable UIComponentRegistryFactory componentRegistryFactory;
+    private @Nullable UIComponentRegistry sitemapComponentRegistry;
 
     private final Set<ModelRepositoryChangeListener> modelChangeListeners = new CopyOnWriteArraySet<>();
 
     @Override
-    public @Nullable Sitemap getSitemap(@NonNull String sitemapName) {
+    public @Nullable Sitemap getSitemap(String sitemapName) {
         buildSitemap(sitemapName.replaceFirst(SITEMAP_PREFIX, ""));
         return sitemaps.get(sitemapName);
     }
 
     @Override
-    public @NonNull Set<@NonNull String> getSitemapNames() {
+    public Set<String> getSitemapNames() {
+        UIComponentRegistry registry = sitemapComponentRegistry;
+        if (registry == null) {
+            return Collections.emptySet();
+        }
+
         sitemaps.clear();
-        Collection<RootUIComponent> rootComponents = sitemapComponentRegistry.getAll();
+        Collection<RootUIComponent> rootComponents = registry.getAll();
         // try building all sitemaps to leave the invalid ones out
         for (RootUIComponent rootComponent : rootComponents) {
             try {
@@ -105,8 +112,13 @@ public class UIComponentSitemapProvider implements SitemapProvider, RegistryChan
         return sitemaps.keySet();
     }
 
-    protected Sitemap buildSitemap(String sitemapName) {
-        RootUIComponent rootComponent = sitemapComponentRegistry.get(sitemapName);
+    protected @Nullable Sitemap buildSitemap(String sitemapName) {
+        UIComponentRegistry registry = sitemapComponentRegistry;
+        if (registry == null) {
+            return null;
+        }
+
+        RootUIComponent rootComponent = registry.get(sitemapName);
         if (rootComponent != null) {
             try {
                 Sitemap sitemap = buildSitemap(rootComponent);
@@ -141,7 +153,7 @@ public class UIComponentSitemapProvider implements SitemapProvider, RegistryChan
         return sitemap;
     }
 
-    protected Widget buildWidget(UIComponent component) {
+    protected @Nullable Widget buildWidget(UIComponent component) {
         Widget widget = null;
 
         switch (component.getType()) {
@@ -261,8 +273,8 @@ public class UIComponentSitemapProvider implements SitemapProvider, RegistryChan
         return widget;
     }
 
-    private void setWidgetPropertyFromComponentConfig(Widget widget, UIComponent component, String configParamName,
-            int feature) {
+    private void setWidgetPropertyFromComponentConfig(Widget widget, @Nullable UIComponent component,
+            String configParamName, int feature) {
         if (component == null || component.getConfig() == null) {
             return;
         }
@@ -292,12 +304,12 @@ public class UIComponentSitemapProvider implements SitemapProvider, RegistryChan
     }
 
     @Override
-    public void addModelChangeListener(@NonNull ModelRepositoryChangeListener listener) {
+    public void addModelChangeListener(ModelRepositoryChangeListener listener) {
         modelChangeListeners.add(listener);
     }
 
     @Override
-    public void removeModelChangeListener(@NonNull ModelRepositoryChangeListener listener) {
+    public void removeModelChangeListener(ModelRepositoryChangeListener listener) {
         modelChangeListeners.remove(listener);
     }
 
@@ -330,8 +342,12 @@ public class UIComponentSitemapProvider implements SitemapProvider, RegistryChan
     }
 
     protected void unsetComponentRegistryFactory(UIComponentRegistryFactory componentRegistryFactory) {
+        UIComponentRegistry registry = this.sitemapComponentRegistry;
+        if (registry != null) {
+            registry.removeRegistryChangeListener(this);
+        }
+
         this.componentRegistryFactory = null;
-        this.sitemapComponentRegistry.removeRegistryChangeListener(this);
         this.sitemapComponentRegistry = null;
     }
 }
diff --git a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/tiles/TileService.java b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/tiles/TileService.java
index daab0b2ce9..19b5160ae7 100644
--- a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/tiles/TileService.java
+++ b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/internal/tiles/TileService.java
@@ -42,15 +42,15 @@ import org.slf4j.LoggerFactory;
 @Component(immediate = true, name = "org.openhab.core.ui.tiles")
 public class TileService implements TileProvider {
 
-    private static final Logger logger = LoggerFactory.getLogger(TileService.class);
+    private final Logger logger = LoggerFactory.getLogger(TileService.class);
 
     protected ConfigurationAdmin configurationAdmin;
 
     protected Set<Tile> tiles = new CopyOnWriteArraySet<>();
 
-    private final static String LINK_NAME = "link-name";
-    private final static String LINK_URL = "link-url";
-    private final static String LINK_IMAGEURL = "link-imageurl";
+    private static final String LINK_NAME = "link-name";
+    private static final String LINK_URL = "link-url";
+    private static final String LINK_IMAGEURL = "link-imageurl";
 
     @Activate
     protected void activate(ComponentContext componentContext, Map<String, Object> properties) {
diff --git a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/tiles/ExternalServiceTile.java b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/tiles/ExternalServiceTile.java
index bcef666dc9..5e078d12d1 100644
--- a/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/tiles/ExternalServiceTile.java
+++ b/bundles/org.openhab.core.ui/src/main/java/org/openhab/core/ui/tiles/ExternalServiceTile.java
@@ -53,11 +53,11 @@ public class ExternalServiceTile implements Tile {
 
     @Override
     public String toString() {
-        final int MAXLEN = 100;
+        final int maxlen = 100;
 
         String limitedImageUrl = imageUrl;
-        if (limitedImageUrl != null && limitedImageUrl.length() > MAXLEN) {
-            limitedImageUrl = imageUrl.substring(0, MAXLEN) + "...";
+        if (limitedImageUrl != null && limitedImageUrl.length() > maxlen) {
+            limitedImageUrl = imageUrl.substring(0, maxlen) + "...";
         }
 
         return "[name=" + name + ", url=" + url + ", overlay=" + overlay + ", imageUrl=" + limitedImageUrl + "]";
diff --git a/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/GenericUser.java b/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/GenericUser.java
index e302e980e2..f0a0ef89b7 100644
--- a/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/GenericUser.java
+++ b/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/GenericUser.java
@@ -15,7 +15,6 @@ package org.openhab.core.auth;
 import java.util.HashSet;
 import java.util.Set;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 
 /**
@@ -56,7 +55,7 @@ public class GenericUser implements User {
     }
 
     @Override
-    public @NonNull String getUID() {
+    public String getUID() {
         return name;
     }
 
diff --git a/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/UserRegistry.java b/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/UserRegistry.java
index 2973551456..6a49b24303 100644
--- a/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/UserRegistry.java
+++ b/bundles/org.openhab.core/src/main/java/org/openhab/core/auth/UserRegistry.java
@@ -35,7 +35,6 @@ public interface UserRegistry extends Registry<User, String>, AuthenticationProv
      * @param username the username of the new user
      * @param password the user password
      * @param roles the roles attributed to the new user
-     *
      * @return the new registered {@link User} instance
      */
     public User register(String username, String password, Set<String> roles);
diff --git a/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataCommandDescriptionProvider.java b/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataCommandDescriptionProvider.java
index 10ff49a006..26b3e6d5a9 100644
--- a/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataCommandDescriptionProvider.java
+++ b/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataCommandDescriptionProvider.java
@@ -16,7 +16,6 @@ import java.util.Locale;
 import java.util.Map;
 import java.util.stream.Stream;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.internal.types.CommandDescriptionImpl;
@@ -55,7 +54,7 @@ public class MetadataCommandDescriptionProvider implements CommandDescriptionPro
     }
 
     @Override
-    public @Nullable CommandDescription getCommandDescription(@NonNull String itemName, @Nullable Locale locale) {
+    public @Nullable CommandDescription getCommandDescription(String itemName, @Nullable Locale locale) {
         Metadata metadata = metadataRegistry.get(new MetadataKey(COMMANDDESCRIPTION_METADATA_NAMESPACE, itemName));
 
         if (metadata != null) {
diff --git a/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataStateDescriptionFragmentProvider.java b/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataStateDescriptionFragmentProvider.java
index dcb6ff6b21..0b45167588 100644
--- a/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataStateDescriptionFragmentProvider.java
+++ b/bundles/org.openhab.core/src/main/java/org/openhab/core/internal/items/MetadataStateDescriptionFragmentProvider.java
@@ -20,7 +20,6 @@ import java.util.Map;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
-import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.items.Metadata;
@@ -69,8 +68,7 @@ public class MetadataStateDescriptionFragmentProvider implements StateDescriptio
     }
 
     @Override
-    public @Nullable StateDescriptionFragment getStateDescriptionFragment(@NonNull String itemName,
-            @Nullable Locale locale) {
+    public @Nullable StateDescriptionFragment getStateDescriptionFragment(String itemName, @Nullable Locale locale) {
         Metadata metadata = metadataRegistry.get(new MetadataKey(STATEDESCRIPTION_METADATA_NAMESPACE, itemName));
 
         if (metadata != null) {
@@ -145,7 +143,7 @@ public class MetadataStateDescriptionFragmentProvider implements StateDescriptio
     }
 
     @Override
-    public @NonNull Integer getRank() {
+    public Integer getRank() {
         return rank;
     }
 }
diff --git a/bundles/org.openhab.core/src/main/java/org/openhab/core/library/types/StringListType.java b/bundles/org.openhab.core/src/main/java/org/openhab/core/library/types/StringListType.java
index d9249f230f..87c45928fb 100644
--- a/bundles/org.openhab.core/src/main/java/org/openhab/core/library/types/StringListType.java
+++ b/bundles/org.openhab.core/src/main/java/org/openhab/core/library/types/StringListType.java
@@ -33,7 +33,7 @@ import org.openhab.core.types.State;
 @NonNullByDefault
 public class StringListType implements Command, State {
 
-    final protected List<String> typeDetails;
+    protected final List<String> typeDetails;
 
     // constants
     public static final String DELIMITER = ",";
diff --git a/bundles/org.openhab.core/src/test/java/org/openhab/core/items/GenericItemTest.java b/bundles/org.openhab.core/src/test/java/org/openhab/core/items/GenericItemTest.java
index 1398e67325..fc54649594 100644
--- a/bundles/org.openhab.core/src/test/java/org/openhab/core/items/GenericItemTest.java
+++ b/bundles/org.openhab.core/src/test/java/org/openhab/core/items/GenericItemTest.java
@@ -21,7 +21,7 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.Locale;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.junit.Test;
 import org.mockito.ArgumentCaptor;
 import org.openhab.core.events.EventPublisher;
@@ -45,6 +45,7 @@ import org.openhab.core.types.StateOption;
  * @author Christoph Knauf - Initial contribution, event tests
  * @author Simon Kaufmann - migrated from Groovy to Java
  */
+@NonNullByDefault
 @SuppressWarnings("null")
 public class GenericItemTest {
 
@@ -150,7 +151,7 @@ public class GenericItemTest {
         when(commandDescriptionService.getCommandDescription("test", null)).thenReturn(new CommandDescription() {
 
             @Override
-            public @NonNull List<@NonNull CommandOption> getCommandOptions() {
+            public List<CommandOption> getCommandOptions() {
                 return Arrays.asList(new CommandOption("ALERT", "Alert"), new CommandOption("REBOOT", "Reboot"));
             }
         });
@@ -168,7 +169,7 @@ public class GenericItemTest {
                 .thenReturn(new CommandDescription() {
 
                     @Override
-                    public @NonNull List<@NonNull CommandOption> getCommandOptions() {
+                    public List<CommandOption> getCommandOptions() {
                         return Arrays.asList(new CommandOption("C1", "Command 1"), new CommandOption("C2", "Command 2"),
                                 new CommandOption("C3", "Command 3"));
                     }
@@ -183,7 +184,7 @@ public class GenericItemTest {
         TestItem item = new TestItem("test");
 
         StateDescriptionService stateDescriptionService = mock(StateDescriptionService.class);
-        List<@NonNull StateOption> stateOptions = Arrays.asList(new StateOption("STATE1", "State 1"),
+        List<StateOption> stateOptions = Arrays.asList(new StateOption("STATE1", "State 1"),
                 new StateOption("STATE2", "State 2"));
         when(stateDescriptionService.getStateDescription("test", null)).thenReturn(
                 StateDescriptionFragmentBuilder.create().withOptions(stateOptions).build().toStateDescription());
diff --git a/itests/org.openhab.core.automation.tests/src/main/java/org/openhab/core/automation/event/RuleEventTest.java b/itests/org.openhab.core.automation.tests/src/main/java/org/openhab/core/automation/event/RuleEventTest.java
index f4d3607c44..340afc9789 100644
--- a/itests/org.openhab.core.automation.tests/src/main/java/org/openhab/core/automation/event/RuleEventTest.java
+++ b/itests/org.openhab.core.automation.tests/src/main/java/org/openhab/core/automation/event/RuleEventTest.java
@@ -27,7 +27,8 @@ import java.util.Optional;
 import java.util.Set;
 import java.util.stream.Collectors;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.junit.Before;
 import org.junit.Test;
 import org.openhab.core.automation.Action;
@@ -57,7 +58,6 @@ import org.openhab.core.items.events.ItemEventFactory;
 import org.openhab.core.library.items.SwitchItem;
 import org.openhab.core.library.types.OnOffType;
 import org.openhab.core.test.java.JavaOSGiTest;
-import org.openhab.core.test.storage.VolatileStorageService;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -67,13 +67,13 @@ import org.slf4j.LoggerFactory;
  * @author Benedikt Niehues - Initial contribution
  * @author Kai Kreuzer - ported test to Java
  */
+@NonNullByDefault
 public class RuleEventTest extends JavaOSGiTest {
 
-    final Logger logger = LoggerFactory.getLogger(RuleEventTest.class);
-    VolatileStorageService volatileStorageService = new VolatileStorageService();
+    private final Logger logger = LoggerFactory.getLogger(RuleEventTest.class);
 
-    Event itemEvent = null;
-    Event ruleRemovedEvent = null;
+    private @Nullable Event itemEvent = null;
+    private @Nullable Event ruleRemovedEvent = null;
 
     public RuleEventTest() {
     }
@@ -87,7 +87,7 @@ public class RuleEventTest extends JavaOSGiTest {
             }
 
             @Override
-            public @NonNull Collection<@NonNull Item> getAll() {
+            public Collection<Item> getAll() {
                 return Arrays.asList(new Item[] { new SwitchItem("myMotionItem"), new SwitchItem("myPresenceItem"),
                         new SwitchItem("myLampItem"), new SwitchItem("myMotionItem2"),
                         new SwitchItem("myPresenceItem2"), new SwitchItem("myLampItem2") });
@@ -118,7 +118,7 @@ public class RuleEventTest extends JavaOSGiTest {
             }
 
             @Override
-            public EventFilter getEventFilter() {
+            public @Nullable EventFilter getEventFilter() {
                 return null;
             }
 
@@ -186,7 +186,7 @@ public class RuleEventTest extends JavaOSGiTest {
             }
 
             @Override
-            public EventFilter getEventFilter() {
+            public @Nullable EventFilter getEventFilter() {
                 return null;
             }
         };
@@ -215,7 +215,7 @@ public class RuleEventTest extends JavaOSGiTest {
             }
 
             @Override
-            public EventFilter getEventFilter() {
+            public @Nullable EventFilter getEventFilter() {
                 return null;
             }
 
diff --git a/itests/org.openhab.core.config.discovery.tests/src/main/java/org/openhab/core/config/discovery/inbox/DynamicThingUpdateOSGiTest.java b/itests/org.openhab.core.config.discovery.tests/src/main/java/org/openhab/core/config/discovery/inbox/DynamicThingUpdateOSGiTest.java
index 413c558581..0b9d4b3ce6 100644
--- a/itests/org.openhab.core.config.discovery.tests/src/main/java/org/openhab/core/config/discovery/inbox/DynamicThingUpdateOSGiTest.java
+++ b/itests/org.openhab.core.config.discovery.tests/src/main/java/org/openhab/core/config/discovery/inbox/DynamicThingUpdateOSGiTest.java
@@ -18,7 +18,7 @@ import static org.mockito.Mockito.*;
 
 import java.util.Collections;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.junit.Before;
 import org.junit.Test;
@@ -57,6 +57,7 @@ import org.openhab.core.thing.type.ThingTypeBuilder;
  * @author Andre Fuechsel - Added tests for device id
  * @author Simon Kaufmann - ported to Java
  */
+@NonNullByDefault
 public class DynamicThingUpdateOSGiTest extends JavaOSGiTest {
 
     private static final int DEFAULT_TTL = 60;
@@ -73,11 +74,11 @@ public class DynamicThingUpdateOSGiTest extends JavaOSGiTest {
     private static final ThingType THING_TYPE = ThingTypeBuilder.instance(THING_TYPE_UID, "label")
             .withRepresentationProperty(DEVICE_ID_KEY).isListed(true).build();
 
-    private Inbox inbox;
-    private ManagedThingProvider managedThingProvider;
-    private ThingHandler thingHandler;
+    private @NonNullByDefault({}) Inbox inbox;
+    private @NonNullByDefault({}) ManagedThingProvider managedThingProvider;
+    private @NonNullByDefault({}) ThingHandler thingHandler;
 
-    private volatile ThingHandlerCallback callback;
+    private volatile @NonNullByDefault({}) ThingHandlerCallback callback;
 
     @Before
     public void setUp() {
@@ -110,12 +111,12 @@ public class DynamicThingUpdateOSGiTest extends JavaOSGiTest {
         ThingHandlerFactory thingHandlerFactory = new BaseThingHandlerFactory() {
 
             @Override
-            public boolean supportsThingType(@NonNull ThingTypeUID thingTypeUID) {
+            public boolean supportsThingType(ThingTypeUID thingTypeUID) {
                 return THING_TYPE_UID.equals(thingTypeUID);
             }
 
             @Override
-            protected @Nullable ThingHandler createHandler(@NonNull Thing thing) {
+            protected @Nullable ThingHandler createHandler(Thing thing) {
                 thingHandler = createThingHandler(thing);
                 return thingHandler;
             }
diff --git a/itests/org.openhab.core.ephemeris.tests/src/main/java/org/openhab/core/ephemeris/internal/EphemerisManagerImplOSGiTest.java b/itests/org.openhab.core.ephemeris.tests/src/main/java/org/openhab/core/ephemeris/internal/EphemerisManagerImplOSGiTest.java
index 0a9e0d9936..476011adf9 100644
--- a/itests/org.openhab.core.ephemeris.tests/src/main/java/org/openhab/core/ephemeris/internal/EphemerisManagerImplOSGiTest.java
+++ b/itests/org.openhab.core.ephemeris.tests/src/main/java/org/openhab/core/ephemeris/internal/EphemerisManagerImplOSGiTest.java
@@ -295,7 +295,6 @@ public class EphemerisManagerImplOSGiTest extends JavaOSGiTest {
             delay = ephemerisManager.getDaysUntil(today, next);
             assertEquals(4, delay);
         }
-
     }
 
     @Test
diff --git a/itests/org.openhab.core.tests/src/main/java/org/openhab/core/internal/items/ItemUpdaterOSGiTest.java b/itests/org.openhab.core.tests/src/main/java/org/openhab/core/internal/items/ItemUpdaterOSGiTest.java
index 009ee5a8a5..2af7054f98 100644
--- a/itests/org.openhab.core.tests/src/main/java/org/openhab/core/internal/items/ItemUpdaterOSGiTest.java
+++ b/itests/org.openhab.core.tests/src/main/java/org/openhab/core/internal/items/ItemUpdaterOSGiTest.java
@@ -18,7 +18,7 @@ import java.util.Collections;
 import java.util.Set;
 import java.util.concurrent.ConcurrentLinkedQueue;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.junit.Before;
 import org.junit.Test;
@@ -41,10 +41,11 @@ import org.openhab.core.test.java.JavaOSGiTest;
  * @author Andre Fuechsel - extended with tag tests
  * @author Kai Kreuzer - added tests for all items changed cases
  */
+@NonNullByDefault
 public class ItemUpdaterOSGiTest extends JavaOSGiTest {
 
-    private EventPublisher eventPublisher;
-    private ItemRegistry itemRegistry;
+    private @NonNullByDefault({}) EventPublisher eventPublisher;
+    private @NonNullByDefault({}) ItemRegistry itemRegistry;
     private final ConcurrentLinkedQueue<Event> receivedEvents = new ConcurrentLinkedQueue<>();
 
     @Before
@@ -60,12 +61,12 @@ public class ItemUpdaterOSGiTest extends JavaOSGiTest {
 
         EventSubscriber eventSubscriber = new EventSubscriber() {
             @Override
-            public void receive(@NonNull Event event) {
+            public void receive(Event event) {
                 receivedEvents.add(event);
             }
 
             @Override
-            public @NonNull Set<@NonNull String> getSubscribedEventTypes() {
+            public Set<String> getSubscribedEventTypes() {
                 return Collections.singleton(ItemStateChangedEvent.TYPE);
             }
 
diff --git a/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ChannelStateDescriptionProviderOSGiTest.java b/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ChannelStateDescriptionProviderOSGiTest.java
index fb6fde6332..f37663707d 100644
--- a/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ChannelStateDescriptionProviderOSGiTest.java
+++ b/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ChannelStateDescriptionProviderOSGiTest.java
@@ -24,6 +24,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.junit.After;
 import org.junit.Before;
@@ -84,19 +85,20 @@ import org.osgi.service.component.ComponentContext;
  * @author Thomas Höfer - Thing type constructor modified because of thing properties introduction
  * @author Markus Rathgeb - Migrated from Groovy to plain Java
  */
+@NonNullByDefault 
 public class ChannelStateDescriptionProviderOSGiTest extends JavaOSGiTest {
 
     private static final String TEST_BUNDLE_NAME = "thingStatusInfoI18nTest.bundle";
     private static final ChannelTypeUID CHANNEL_TYPE_7_UID = new ChannelTypeUID("hue:num-dynamic");
 
-    private ThingStatusInfoI18nLocalizationService thingStatusInfoI18nLocalizationService;
-    private ItemRegistry itemRegistry;
-    private ItemChannelLinkRegistry linkRegistry;
+    private @NonNullByDefault({}) ThingStatusInfoI18nLocalizationService thingStatusInfoI18nLocalizationService;
+    private @NonNullByDefault({}) ItemRegistry itemRegistry;
+    private @NonNullByDefault({}) ItemChannelLinkRegistry linkRegistry;
 
     @Mock
-    private ComponentContext componentContext;
+    private @NonNullByDefault({}) ComponentContext componentContext;
 
-    private Bundle testBundle;
+    private @NonNullByDefault({}) Bundle testBundle;
 
     @Before
     public void setup() throws Exception {
@@ -145,12 +147,12 @@ public class ChannelStateDescriptionProviderOSGiTest extends JavaOSGiTest {
 
         registerService(new ChannelTypeProvider() {
             @Override
-            public Collection<ChannelType> getChannelTypes(Locale locale) {
+            public Collection<ChannelType> getChannelTypes(@Nullable Locale locale) {
                 return channelTypes;
             }
 
             @Override
-            public ChannelType getChannelType(ChannelTypeUID channelTypeUID, Locale locale) {
+            public @Nullable ChannelType getChannelType(ChannelTypeUID channelTypeUID, @Nullable Locale locale) {
                 for (final ChannelType channelType : channelTypes) {
                     if (channelType.getUID().equals(channelTypeUID)) {
                         return channelType;
@@ -405,9 +407,9 @@ public class ChannelStateDescriptionProviderOSGiTest extends JavaOSGiTest {
      * Helper
      */
     class TestDynamicStateDescriptionProvider extends BaseDynamicStateDescriptionProvider {
-        final StateDescription newState = StateDescriptionFragmentBuilder.create().withMinimum(BigDecimal.valueOf(10))
-                .withMaximum(BigDecimal.valueOf(100)).withStep(BigDecimal.valueOf(5)).withPattern("VALUE %d")
-                .withReadOnly(false)
+        final @Nullable StateDescription newState = StateDescriptionFragmentBuilder.create()
+                .withMinimum(BigDecimal.valueOf(10)).withMaximum(BigDecimal.valueOf(100))
+                .withStep(BigDecimal.valueOf(5)).withPattern("VALUE %d").withReadOnly(false)
                 .withOptions(Arrays.asList(new StateOption("value0", "label0"), new StateOption("value1", "label1")))
                 .build().toStateDescription();
 
@@ -454,7 +456,7 @@ public class ChannelStateDescriptionProviderOSGiTest extends JavaOSGiTest {
         }
 
         @Override
-        protected ThingHandler createHandler(Thing thing) {
+        protected @Nullable ThingHandler createHandler(Thing thing) {
             return new AbstractThingHandler(thing) {
                 @Override
                 public void handleCommand(ChannelUID channelUID, Command command) {
@@ -502,7 +504,7 @@ public class ChannelStateDescriptionProviderOSGiTest extends JavaOSGiTest {
      */
     private class BundleResolverImpl implements BundleResolver {
         @Override
-        public Bundle resolveBundle(Class<?> clazz) {
+        public Bundle resolveBundle(@Nullable Class<?> clazz) {
             if (clazz != null && clazz.equals(AbstractThingHandler.class)) {
                 return testBundle;
             } else {
diff --git a/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingManagerOSGiJavaTest.java b/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingManagerOSGiJavaTest.java
index e0c15d297a..0077bc6f8d 100644
--- a/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingManagerOSGiJavaTest.java
+++ b/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingManagerOSGiJavaTest.java
@@ -26,7 +26,7 @@ import java.util.concurrent.Semaphore;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.junit.After;
 import org.junit.Before;
@@ -93,20 +93,14 @@ import org.osgi.service.component.ComponentContext;
  * @author Simon Kaufmann - Initial contribution
  * @author Christoph Weitkamp - Added preconfigured ChannelGroupBuilder
  */
+@NonNullByDefault
 public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
 
-    private ManagedThingProvider managedThingProvider;
-    private ItemRegistry itemRegistry;
-    private ThingRegistry thingRegistry;
-    private ReadyService readyService;
-    private ItemChannelLinkRegistry itemChannelLinkRegistry;
-    private ThingManager thingManager;
-
     private static final String CONFIG_PARAM_NAME = "test";
 
     private static final String BINDING_ID = "binding";
-
     private static final String CHANNEL_ID = "channel";
+
     private static final ChannelTypeUID CHANNEL_TYPE_UID = new ChannelTypeUID(BINDING_ID, CHANNEL_ID);
 
     private static final String CHANNEL_GROUP_ID = "channel-group";
@@ -123,16 +117,22 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
 
     private static final ChannelGroupUID CHANNEL_GROUP_UID = new ChannelGroupUID(THING_UID, CHANNEL_GROUP_ID);
 
-    private Thing thing;
-    private URI configDescriptionThing;
-    private URI configDescriptionChannel;
+    private @NonNullByDefault({}) ItemChannelLinkRegistry itemChannelLinkRegistry;
+    private @NonNullByDefault({}) ItemRegistry itemRegistry;
+    private @NonNullByDefault({}) ManagedThingProvider managedThingProvider;
+    private @NonNullByDefault({}) ThingRegistry thingRegistry;
+    private @NonNullByDefault({}) ReadyService readyService;
+    private @NonNullByDefault({}) Storage<Boolean> storage;
+    private @NonNullByDefault({}) ThingManager thingManager;
 
-    private Storage<Boolean> storage;
+    private @NonNullByDefault({}) URI configDescriptionChannel;
+    private @NonNullByDefault({}) URI configDescriptionThing;
+    private @NonNullByDefault({}) Thing thing;
 
     @Before
     public void setUp() throws Exception {
-        configDescriptionThing = new URI("test:test");
         configDescriptionChannel = new URI("test:channel");
+        configDescriptionThing = new URI("test:test");
         thing = ThingBuilder.create(THING_TYPE_UID, THING_UID).withChannels(Collections.singletonList( //
                 ChannelBuilder.create(CHANNEL_UID, "Switch").withLabel("Test Label").withDescription("Test Description")
                         .withType(CHANNEL_TYPE_UID).withDefaultTags(Collections.singleton("Test Tag")).build() //
@@ -320,7 +320,7 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
         registerChannelTypeProvider();
         registerThingHandlerFactory(THING_TYPE_UID, thing -> new BaseThingHandler(thing) {
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
         });
 
@@ -354,7 +354,7 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
 
         registerThingHandlerFactory(BRIDGE_TYPE_UID, bridge -> new BaseBridgeHandler((Bridge) bridge) {
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
 
             @Override
@@ -373,7 +373,7 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
         });
         registerThingHandlerFactory(THING_TYPE_UID, thing -> new BaseThingHandler(thing) {
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
 
             @Override
@@ -430,7 +430,7 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
 
         registerThingHandlerFactory(BRIDGE_TYPE_UID, bridge -> new BaseBridgeHandler((Bridge) bridge) {
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
 
             @Override
@@ -449,7 +449,7 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
         });
         registerThingHandlerFactory(THING_TYPE_UID, thing -> new BaseThingHandler(thing) {
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
 
             @Override
@@ -506,7 +506,7 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
 
         registerThingHandlerFactory(BRIDGE_TYPE_UID, bridge -> new BaseBridgeHandler((Bridge) bridge) {
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
 
             @Override
@@ -525,7 +525,7 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
         });
         registerThingHandlerFactory(THING_TYPE_UID, thing -> new BaseThingHandler(thing) {
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
 
             @Override
@@ -1062,12 +1062,12 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
         }
 
         @Override
-        public boolean supportsThingType(@NonNull ThingTypeUID thingTypeUID) {
+        public boolean supportsThingType(ThingTypeUID thingTypeUID) {
             return this.thingTypeUID.equals(thingTypeUID);
         }
 
         @Override
-        protected @Nullable ThingHandler createHandler(@NonNull Thing thing) {
+        protected @Nullable ThingHandler createHandler(Thing thing) {
             return thingHandlerProducer.apply(thing);
         }
     };
@@ -1079,12 +1079,12 @@ public class ThingManagerOSGiJavaTest extends JavaOSGiTest {
         AtomicReference<ThingHandlerCallback> thc = new AtomicReference<>();
         ThingHandlerFactory thingHandlerFactory = new BaseThingHandlerFactory() {
             @Override
-            public boolean supportsThingType(@NonNull ThingTypeUID thingTypeUID) {
+            public boolean supportsThingType(ThingTypeUID thingTypeUID) {
                 return true;
             }
 
             @Override
-            protected @Nullable ThingHandler createHandler(@NonNull Thing thing) {
+            protected @Nullable ThingHandler createHandler(Thing thing) {
                 ThingHandler mockHandler = mock(ThingHandler.class);
                 doAnswer(a -> {
                     thc.set((ThingHandlerCallback) a.getArguments()[0]);
diff --git a/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingRegistryOSGiTest.java b/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingRegistryOSGiTest.java
index 2bc5b8cd93..fa095d7968 100644
--- a/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingRegistryOSGiTest.java
+++ b/itests/org.openhab.core.thing.tests/src/main/java/org/openhab/core/thing/internal/ThingRegistryOSGiTest.java
@@ -23,7 +23,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicReference;
 
-import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
 import org.junit.After;
 import org.junit.Before;
@@ -58,10 +58,11 @@ import org.osgi.framework.ServiceRegistration;
  * @author Stefan Bußweiler - Initial contribution
  * @author Kai Kreuzer - Moved createThing test from managed provider
  */
+@NonNullByDefault
 public class ThingRegistryOSGiTest extends JavaOSGiTest {
 
-    ManagedThingProvider managedThingProvider;
-    ServiceRegistration<?> thingHandlerFactoryServiceReg;
+    private @NonNullByDefault({}) ManagedThingProvider managedThingProvider;
+    private @NonNullByDefault({}) ServiceRegistration<?> thingHandlerFactoryServiceReg;
 
     private static final ThingTypeUID THING_TYPE_UID = new ThingTypeUID("binding:type");
     private static final ThingUID THING_UID = new ThingUID(THING_TYPE_UID, "id");
@@ -69,8 +70,8 @@ public class ThingRegistryOSGiTest extends JavaOSGiTest {
     private static final String THING1_ID = "testThing1";
     private static final String THING2_ID = "testThing2";
 
-    Map<@NonNull String, @NonNull Object> changedParameters = null;
-    Event receivedEvent = null;
+    private @Nullable Map<String, Object> changedParameters = null;
+    private @Nullable Event receivedEvent = null;
 
     @Before
     public void setUp() {
@@ -99,7 +100,7 @@ public class ThingRegistryOSGiTest extends JavaOSGiTest {
             }
 
             @Override
-            public EventFilter getEventFilter() {
+            public @Nullable EventFilter getEventFilter() {
                 return null;
             }
 
@@ -143,12 +144,11 @@ public class ThingRegistryOSGiTest extends JavaOSGiTest {
         ThingHandler thingHandler = new BaseThingHandler(thing) {
 
             @Override
-            public void handleCommand(@NonNull ChannelUID channelUID, @NonNull Command command) {
+            public void handleCommand(ChannelUID channelUID, Command command) {
             }
 
             @Override
-            public void handleConfigurationUpdate(
-                    @NonNull Map<@NonNull String, @NonNull Object> configurationParameters) {
+            public void handleConfigurationUpdate(Map<String, Object> configurationParameters) {
                 changedParameters = configurationParameters;
             }
         };
@@ -158,16 +158,16 @@ public class ThingRegistryOSGiTest extends JavaOSGiTest {
         ThingProvider thingProvider = new ThingProvider() {
 
             @Override
-            public void addProviderChangeListener(ProviderChangeListener<@NonNull Thing> listener) {
+            public void addProviderChangeListener(ProviderChangeListener<Thing> listener) {
             }
 
             @Override
-            public Collection<@NonNull Thing> getAll() {
+            public Collection<Thing> getAll() {
                 return Collections.singleton(thing);
             }
 
             @Override
-            public void removeProviderChangeListener(ProviderChangeListener<@NonNull Thing> listener) {
+            public void removeProviderChangeListener(ProviderChangeListener<Thing> listener) {
             }
 
         };
@@ -208,17 +208,17 @@ public class ThingRegistryOSGiTest extends JavaOSGiTest {
         ThingHandlerFactory thingHandlerFactory = new BaseThingHandlerFactory() {
 
             @Override
-            public boolean supportsThingType(@NonNull ThingTypeUID thingTypeUID) {
+            public boolean supportsThingType(ThingTypeUID thingTypeUID) {
                 return true;
             }
 
             @Override
-            protected @Nullable ThingHandler createHandler(@NonNull Thing thing) {
+            protected @Nullable ThingHandler createHandler(Thing thing) {
                 return null;
             }
 
             @Override
-            public @Nullable Thing createThing(@NonNull ThingTypeUID thingTypeUID, @NonNull Configuration configuration,
+            public @Nullable Thing createThing(ThingTypeUID thingTypeUID, Configuration configuration,
                     @Nullable ThingUID thingUID, @Nullable ThingUID bridgeUID) {
                 assertThat(thingTypeUID, is(expectedThingTypeUID));
                 assertThat(configuration, is(expectedConfiguration));
@@ -237,7 +237,11 @@ public class ThingRegistryOSGiTest extends JavaOSGiTest {
         waitForAssert(() -> {
             assertTrue(thingResultWrapper.get() != null);
         });
-        assertThat(thing, is(thingResultWrapper.get()));
+
+        assertThat(thing, is(notNullValue()));
+        if (thing != null) {
+            assertThat(thing, is(thingResultWrapper.get()));
+        }
     }
 
     private void registerThingHandlerFactory(ThingHandlerFactory thingHandlerFactory) {