diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/AsynchronousHook.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/AsynchronousHook.java
index 4e74ece..e89c05e 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/AsynchronousHook.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/AsynchronousHook.java
@@ -14,6 +14,7 @@
 
 package com.googlesource.gerrit.plugins.hooks;
 
+import com.google.gerrit.common.Nullable;
 import java.nio.file.Path;
 
 class AsynchronousHook extends Hook {
@@ -24,12 +25,14 @@
     this.queue = queue;
   }
 
+  @Nullable
   @Override
   HookResult execute(HookArgs args) {
     queue.submit(path, args);
     return null;
   }
 
+  @Nullable
   @Override
   HookResult execute(String projectName, HookArgs args) {
     queue.submit(projectName, path, args);
diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java
index 5b642d3..e83fd4a 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java
@@ -16,6 +16,7 @@
 
 import com.google.common.flogger.FluentLogger;
 import com.google.common.util.concurrent.ThreadFactoryBuilder;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.events.LifecycleListener;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.logging.LoggingContextAwareExecutorService;
@@ -51,6 +52,7 @@
     return submit(null, hook, args);
   }
 
+  @Nullable
   HookResult submit(String projectName, Path hook, HookArgs args) {
     if (!Files.exists(hook)) {
       logger.atFine().log("Hook file not found: %s", hook);
diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookTask.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookTask.java
index 02dfd08..b7a0784 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookTask.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookTask.java
@@ -18,6 +18,7 @@
 
 import com.google.common.flogger.FluentLogger;
 import com.google.common.io.ByteStreams;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.metrics.Timer1;
 import java.io.BufferedReader;
@@ -78,6 +79,7 @@
     return hook.getFileName().toString();
   }
 
+  @Nullable
   public String getOutput() {
     return output != null ? output.toString() : null;
   }