diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java b/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
index b205073..53bc9cf 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
@@ -25,6 +25,7 @@
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Lists;
 import com.google.common.io.Files;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.AccountGroup;
 import com.google.gerrit.entities.BranchNameKey;
 import com.google.gerrit.entities.GroupReference;
@@ -453,6 +454,7 @@
     }
   }
 
+  @Nullable
   private PushOne getPendingPush(URIish uri) {
     PushOne e = pending.get(uri);
     if (e != null && !e.wasCanceled()) {
diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/GerritRestApi.java b/src/main/java/com/googlesource/gerrit/plugins/replication/GerritRestApi.java
index fa81dd0..5081e2b 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/GerritRestApi.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/GerritRestApi.java
@@ -18,6 +18,7 @@
 import static com.googlesource.gerrit.plugins.replication.ReplicationQueue.repLog;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.extensions.restapi.Url;
 import com.google.inject.Inject;
@@ -109,6 +110,7 @@
     return ctx;
   }
 
+  @Nullable
   private CredentialsProvider adapt(org.eclipse.jgit.transport.CredentialsProvider cp) {
     CredentialItem.Username user = new CredentialItem.Username();
     CredentialItem.Password pass = new CredentialItem.Password();
diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/PushOne.java b/src/main/java/com/googlesource/gerrit/plugins/replication/PushOne.java
index da3089e..e7b53c6 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/PushOne.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/PushOne.java
@@ -30,6 +30,7 @@
 import com.google.common.collect.ListMultimap;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Sets;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.extensions.events.GitBatchRefUpdateListener;
@@ -755,6 +756,7 @@
     }
   }
 
+  @Nullable
   private RefSpec matchSrc(String ref) {
     for (RefSpec s : config.getPushRefSpecs()) {
       if (s.matchSource(ref)) {
@@ -764,6 +766,7 @@
     return null;
   }
 
+  @Nullable
   private RefSpec matchDst(String ref) {
     for (RefSpec s : config.getPushRefSpecs()) {
       if (s.matchDestination(ref)) {
diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationFileBasedConfig.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationFileBasedConfig.java
index 2424e71..93990f4 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationFileBasedConfig.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationFileBasedConfig.java
@@ -16,6 +16,7 @@
 import static com.googlesource.gerrit.plugins.replication.ReplicationQueue.repLog;
 
 import com.google.common.base.Strings;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.annotations.PluginData;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.inject.Inject;
@@ -59,6 +60,7 @@
     this.pluginDataDir = pluginDataDir;
   }
 
+  @Nullable
   public static String replaceName(String in, String name, boolean keyIsOptional) {
     String key = "${name}";
     int n = in.indexOf(key);
diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java
index 3d04cdd..4dd6060 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationTasksStorage.java
@@ -21,6 +21,7 @@
 import com.google.common.collect.ImmutableSet;
 import com.google.common.flogger.FluentLogger;
 import com.google.common.hash.Hashing;
+import com.google.gerrit.common.Nullable;
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.google.gson.TypeAdapter;
@@ -265,6 +266,7 @@
         out.endObject();
       }
 
+      @Nullable
       @Override
       public ReplicateRefUpdate read(JsonReader in) throws IOException {
         if (in.peek() == JsonToken.NULL) {
@@ -321,6 +323,7 @@
     }
 
     @SuppressWarnings("unchecked")
+    @Nullable
     @Override
     public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       if (type.equals(TypeToken.get(AutoValue_ReplicationTasksStorage_ReplicateRefUpdate.class))
diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/FakeExecutorService.java b/src/test/java/com/googlesource/gerrit/plugins/replication/FakeExecutorService.java
index 2f7059e..ce94187 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/replication/FakeExecutorService.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/FakeExecutorService.java
@@ -14,6 +14,7 @@
 
 package com.googlesource.gerrit.plugins.replication;
 
+import com.google.gerrit.common.Nullable;
 import java.util.Collection;
 import java.util.List;
 import java.util.concurrent.Callable;
@@ -103,6 +104,7 @@
     return null;
   }
 
+  @Nullable
   @Override
   public ScheduledFuture<?> scheduleAtFixedRate(
       Runnable command, long initialDelay, long period, TimeUnit unit) {
@@ -110,6 +112,7 @@
     return null;
   }
 
+  @Nullable
   @Override
   public ScheduledFuture<?> scheduleWithFixedDelay(
       Runnable command, long initialDelay, long delay, TimeUnit unit) {
diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/ForwardingProxy.java b/src/test/java/com/googlesource/gerrit/plugins/replication/ForwardingProxy.java
index 5e6bde8..c8dcae0 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/replication/ForwardingProxy.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/ForwardingProxy.java
@@ -14,6 +14,7 @@
 
 package com.googlesource.gerrit.plugins.replication;
 
+import com.google.gerrit.common.Nullable;
 import java.lang.reflect.InvocationHandler;
 import java.lang.reflect.Method;
 import java.lang.reflect.Proxy;
@@ -55,6 +56,7 @@
       return method.invoke(delegate, args);
     }
 
+    @Nullable
     protected Method getOverriden(Method method) {
       try {
         Method implementedByOverrider =
diff --git a/src/test/java/com/googlesource/gerrit/plugins/replication/ReplicationDaemon.java b/src/test/java/com/googlesource/gerrit/plugins/replication/ReplicationDaemon.java
index b6a3ed1..3bc86c7 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/replication/ReplicationDaemon.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/replication/ReplicationDaemon.java
@@ -22,6 +22,7 @@
 import com.google.gerrit.acceptance.UseLocalDisk;
 import com.google.gerrit.acceptance.WaitUtil;
 import com.google.gerrit.acceptance.testsuite.project.ProjectOperations;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.extensions.api.changes.NotifyHandling;
 import com.google.gerrit.extensions.events.ProjectDeletedListener;
@@ -245,6 +246,7 @@
     return true;
   }
 
+  @Nullable
   protected Ref checkedGetRef(Repository repo, String branchName) {
     try {
       return repo.getRefDatabase().exactRef(branchName);