diff --git a/build.gradle b/build.gradle
index 9bc7436c7e4a..210e24b3f176 100644
--- a/build.gradle
+++ b/build.gradle
@@ -2,7 +2,7 @@ import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins {
     id 'com.google.osdetector' version '1.6.2' apply false
-    // If you want to change `org.jetbrains.kotlin.jvm` version, 
+    // If you want to change `org.jetbrains.kotlin.jvm` version,
     // you also need to change `org.jetbrains.kotlin:kotlin-allopen` version in `dependencies.yml`.
     id 'org.jetbrains.kotlin.jvm' version '1.3.72' apply false
 }
diff --git a/core/src/main/java/com/linecorp/armeria/client/Client.java b/core/src/main/java/com/linecorp/armeria/client/Client.java
index fd679055ab70..144063c6dd96 100644
--- a/core/src/main/java/com/linecorp/armeria/client/Client.java
+++ b/core/src/main/java/com/linecorp/armeria/client/Client.java
@@ -18,6 +18,8 @@
 
 import static java.util.Objects.requireNonNull;
 
+import javax.annotation.Nullable;
+
 import com.linecorp.armeria.common.HttpRequest;
 import com.linecorp.armeria.common.HttpResponse;
 import com.linecorp.armeria.common.Request;
@@ -71,6 +73,7 @@ public interface Client<I extends Request, O extends Response> extends Unwrappab
      * @see ClientFactory#unwrap(Object, Class)
      * @see Unwrappable
      */
+    @Nullable
     @Override
     default <T> T as(Class<T> type) {
         requireNonNull(type, "type");
diff --git a/core/src/main/java/com/linecorp/armeria/client/ClientRequestContextWrapper.java b/core/src/main/java/com/linecorp/armeria/client/ClientRequestContextWrapper.java
index 76cdc005e35b..074fe4210994 100644
--- a/core/src/main/java/com/linecorp/armeria/client/ClientRequestContextWrapper.java
+++ b/core/src/main/java/com/linecorp/armeria/client/ClientRequestContextWrapper.java
@@ -49,16 +49,19 @@ public ClientRequestContext newDerivedContext(RequestId id, @Nullable HttpReques
         return delegate().newDerivedContext(id, req, rpcReq, endpoint);
     }
 
+    @Nullable
     @Override
     public EndpointGroup endpointGroup() {
         return delegate().endpointGroup();
     }
 
+    @Nullable
     @Override
     public Endpoint endpoint() {
         return delegate().endpoint();
     }
 
+    @Nullable
     @Override
     public String fragment() {
         return delegate().fragment();
diff --git a/core/src/main/java/com/linecorp/armeria/client/DecoratingClientFactory.java b/core/src/main/java/com/linecorp/armeria/client/DecoratingClientFactory.java
index f6fce3356323..b8d99d2170eb 100644
--- a/core/src/main/java/com/linecorp/armeria/client/DecoratingClientFactory.java
+++ b/core/src/main/java/com/linecorp/armeria/client/DecoratingClientFactory.java
@@ -118,11 +118,13 @@ public Object newClient(ClientBuilderParams params) {
         return delegate().newClient(params);
     }
 
+    @Nullable
     @Override
     public <T> ClientBuilderParams clientBuilderParams(T client) {
         return delegate().clientBuilderParams(client);
     }
 
+    @Nullable
     @Override
     public <T> T unwrap(Object client, Class<T> type) {
         return delegate().unwrap(client, type);
diff --git a/core/src/main/java/com/linecorp/armeria/client/DefaultClientFactory.java b/core/src/main/java/com/linecorp/armeria/client/DefaultClientFactory.java
index c91a4ab525d5..7ed3cad6a272 100644
--- a/core/src/main/java/com/linecorp/armeria/client/DefaultClientFactory.java
+++ b/core/src/main/java/com/linecorp/armeria/client/DefaultClientFactory.java
@@ -153,6 +153,7 @@ public Object newClient(ClientBuilderParams params) {
         return factory.newClient(params);
     }
 
+    @Nullable
     @Override
     public <T> T unwrap(Object client, Class<T> type) {
         final T params = ClientFactory.super.unwrap(client, type);
diff --git a/core/src/main/java/com/linecorp/armeria/client/DefaultClientRequestContext.java b/core/src/main/java/com/linecorp/armeria/client/DefaultClientRequestContext.java
index 11d2de961c67..ecacc3f956e2 100644
--- a/core/src/main/java/com/linecorp/armeria/client/DefaultClientRequestContext.java
+++ b/core/src/main/java/com/linecorp/armeria/client/DefaultClientRequestContext.java
@@ -393,11 +393,13 @@ public ClientOptions options() {
 
     @Override
     public EndpointGroup endpointGroup() {
+        checkState(endpointGroup != null, "Should call init(endpoint) before invoking this method.");
         return endpointGroup;
     }
 
     @Override
     public Endpoint endpoint() {
+        checkState(endpoint != null, "Should call init(endpoint) before invoking this method.");
         return endpoint;
     }
 
diff --git a/core/src/main/java/com/linecorp/armeria/client/HttpSessionHandler.java b/core/src/main/java/com/linecorp/armeria/client/HttpSessionHandler.java
index 088f17cde29c..008d7e2ac14d 100644
--- a/core/src/main/java/com/linecorp/armeria/client/HttpSessionHandler.java
+++ b/core/src/main/java/com/linecorp/armeria/client/HttpSessionHandler.java
@@ -122,6 +122,7 @@ final class HttpSessionHandler extends ChannelDuplexHandler implements HttpSessi
         this.pingIntervalMillis = pingIntervalMillis;
     }
 
+    @Nullable
     @Override
     public SessionProtocol protocol() {
         return protocol;
diff --git a/core/src/main/java/com/linecorp/armeria/client/RefreshingAddressResolver.java b/core/src/main/java/com/linecorp/armeria/client/RefreshingAddressResolver.java
index 93ec2400e8db..dd12d43fafb4 100644
--- a/core/src/main/java/com/linecorp/armeria/client/RefreshingAddressResolver.java
+++ b/core/src/main/java/com/linecorp/armeria/client/RefreshingAddressResolver.java
@@ -327,7 +327,7 @@ public void run() {
                 servedFromCache = false;
                 numAttemptsSoFar = 1;
 
-                if (entry.address().equals(address) && entry.ttlMillis() == ttlMillis) {
+                if (address.equals(entry.address()) && entry.ttlMillis() == ttlMillis) {
                     scheduleRefresh(ttlMillis);
                 } else {
                     // Replace the old entry with the new one.
diff --git a/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/NonBlockingCircuitBreaker.java b/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/NonBlockingCircuitBreaker.java
index f6fc1373147d..6f5491d91fa8 100644
--- a/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/NonBlockingCircuitBreaker.java
+++ b/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/NonBlockingCircuitBreaker.java
@@ -289,11 +289,13 @@ public EventCount count() {
             return EventCount.ZERO;
         }
 
+        @Nullable
         @Override
         public EventCount onSuccess() {
             return null;
         }
 
+        @Nullable
         @Override
         public EventCount onFailure() {
             return null;
diff --git a/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/SlidingWindowCounter.java b/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/SlidingWindowCounter.java
index 20af85a58b62..165b40ed3f78 100644
--- a/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/SlidingWindowCounter.java
+++ b/core/src/main/java/com/linecorp/armeria/client/circuitbreaker/SlidingWindowCounter.java
@@ -67,11 +67,13 @@ public EventCount count() {
         return snapshot.get();
     }
 
+    @Nullable
     @Override
     public EventCount onSuccess() {
         return onEvent(Event.SUCCESS);
     }
 
+    @Nullable
     @Override
     public EventCount onFailure() {
         return onEvent(Event.FAILURE);
diff --git a/core/src/main/java/com/linecorp/armeria/client/endpoint/DynamicEndpointGroup.java b/core/src/main/java/com/linecorp/armeria/client/endpoint/DynamicEndpointGroup.java
index 5ebbd02f49bf..1bc5ccc2d4d2 100644
--- a/core/src/main/java/com/linecorp/armeria/client/endpoint/DynamicEndpointGroup.java
+++ b/core/src/main/java/com/linecorp/armeria/client/endpoint/DynamicEndpointGroup.java
@@ -26,6 +26,8 @@
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 
+import javax.annotation.Nullable;
+
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Lists;
 
@@ -81,6 +83,7 @@ public final EndpointSelectionStrategy selectionStrategy() {
         return selectionStrategy;
     }
 
+    @Nullable
     @Override
     public final Endpoint select(ClientRequestContext ctx) {
         return maybeCreateSelector().select(ctx);
diff --git a/core/src/main/java/com/linecorp/armeria/client/endpoint/EndpointGroup.java b/core/src/main/java/com/linecorp/armeria/client/endpoint/EndpointGroup.java
index 78de996eeb9c..ed45b37c5c45 100644
--- a/core/src/main/java/com/linecorp/armeria/client/endpoint/EndpointGroup.java
+++ b/core/src/main/java/com/linecorp/armeria/client/endpoint/EndpointGroup.java
@@ -23,6 +23,8 @@
 import java.util.concurrent.CompletableFuture;
 import java.util.function.Consumer;
 
+import javax.annotation.Nullable;
+
 import com.google.common.collect.ImmutableList;
 
 import com.linecorp.armeria.client.ClientRequestContext;
@@ -139,6 +141,7 @@ static EndpointGroup of(EndpointSelectionStrategy selectionStrategy,
      *         which was specified when constructing this {@link EndpointGroup},
      *         or {@code null} if this {@link EndpointGroup} is empty.
      */
+    @Nullable
     @Override
     Endpoint select(ClientRequestContext ctx);
 
diff --git a/core/src/main/java/com/linecorp/armeria/client/endpoint/OrElseEndpointGroup.java b/core/src/main/java/com/linecorp/armeria/client/endpoint/OrElseEndpointGroup.java
index 2ae694bc1629..59135cbe68f4 100644
--- a/core/src/main/java/com/linecorp/armeria/client/endpoint/OrElseEndpointGroup.java
+++ b/core/src/main/java/com/linecorp/armeria/client/endpoint/OrElseEndpointGroup.java
@@ -21,6 +21,8 @@
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
 
+import javax.annotation.Nullable;
+
 import com.linecorp.armeria.client.ClientRequestContext;
 import com.linecorp.armeria.client.Endpoint;
 import com.linecorp.armeria.common.util.AbstractListenable;
@@ -66,6 +68,7 @@ public EndpointSelectionStrategy selectionStrategy() {
         return first.selectionStrategy();
     }
 
+    @Nullable
     @Override
     public Endpoint select(ClientRequestContext ctx) {
         return selector.select(ctx);
diff --git a/core/src/main/java/com/linecorp/armeria/client/endpoint/RoundRobinStrategy.java b/core/src/main/java/com/linecorp/armeria/client/endpoint/RoundRobinStrategy.java
index 0750eadcc08e..6e5536818f91 100644
--- a/core/src/main/java/com/linecorp/armeria/client/endpoint/RoundRobinStrategy.java
+++ b/core/src/main/java/com/linecorp/armeria/client/endpoint/RoundRobinStrategy.java
@@ -21,6 +21,8 @@
 import java.util.List;
 import java.util.concurrent.atomic.AtomicInteger;
 
+import javax.annotation.Nullable;
+
 import com.linecorp.armeria.client.ClientRequestContext;
 import com.linecorp.armeria.client.Endpoint;
 
@@ -49,6 +51,7 @@ static class RoundRobinSelector implements EndpointSelector {
             this.endpointGroup = requireNonNull(endpointGroup, "endpointGroup");
         }
 
+        @Nullable
         @Override
         public Endpoint select(ClientRequestContext ctx) {
 
diff --git a/core/src/main/java/com/linecorp/armeria/client/endpoint/StaticEndpointGroup.java b/core/src/main/java/com/linecorp/armeria/client/endpoint/StaticEndpointGroup.java
index 311d814d496e..04969b97f8a7 100644
--- a/core/src/main/java/com/linecorp/armeria/client/endpoint/StaticEndpointGroup.java
+++ b/core/src/main/java/com/linecorp/armeria/client/endpoint/StaticEndpointGroup.java
@@ -20,6 +20,8 @@
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
 
+import javax.annotation.Nullable;
+
 import com.google.common.collect.ImmutableList;
 
 import com.linecorp.armeria.client.ClientRequestContext;
@@ -57,6 +59,7 @@ public EndpointSelectionStrategy selectionStrategy() {
         return selectionStrategy;
     }
 
+    @Nullable
     @Override
     public Endpoint select(ClientRequestContext ctx) {
         return selector.select(ctx);
diff --git a/core/src/main/java/com/linecorp/armeria/client/endpoint/StickyEndpointSelectionStrategy.java b/core/src/main/java/com/linecorp/armeria/client/endpoint/StickyEndpointSelectionStrategy.java
index 8e50d7059cbb..5fac5e63cebb 100644
--- a/core/src/main/java/com/linecorp/armeria/client/endpoint/StickyEndpointSelectionStrategy.java
+++ b/core/src/main/java/com/linecorp/armeria/client/endpoint/StickyEndpointSelectionStrategy.java
@@ -20,6 +20,8 @@
 import java.util.List;
 import java.util.function.ToLongFunction;
 
+import javax.annotation.Nullable;
+
 import com.google.common.hash.Hashing;
 
 import com.linecorp.armeria.client.ClientRequestContext;
@@ -80,6 +82,7 @@ private static final class StickyEndpointSelector implements EndpointSelector {
             this.endpointGroup = requireNonNull(endpointGroup, "endpointGroup");
         }
 
+        @Nullable
         @Override
         public Endpoint select(ClientRequestContext ctx) {
 
diff --git a/core/src/main/java/com/linecorp/armeria/client/endpoint/WeightedRoundRobinStrategy.java b/core/src/main/java/com/linecorp/armeria/client/endpoint/WeightedRoundRobinStrategy.java
index 82b03820dafb..bac6c829e1c6 100644
--- a/core/src/main/java/com/linecorp/armeria/client/endpoint/WeightedRoundRobinStrategy.java
+++ b/core/src/main/java/com/linecorp/armeria/client/endpoint/WeightedRoundRobinStrategy.java
@@ -61,6 +61,7 @@ private static final class WeightedRoundRobinSelector implements EndpointSelecto
             endpointGroup.addListener(endpoints -> endpointsAndWeights = new EndpointsAndWeights(endpoints));
         }
 
+        @Nullable
         @Override
         public Endpoint select(ClientRequestContext ctx) {
             final int currentSequence = sequence.getAndIncrement();
diff --git a/core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java b/core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java
index 1bc102dd038f..6aef7cce550a 100644
--- a/core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java
+++ b/core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java
@@ -23,6 +23,8 @@
 import java.util.concurrent.ThreadLocalRandom;
 import java.util.function.Supplier;
 
+import javax.annotation.Nullable;
+
 import com.linecorp.armeria.common.util.Unwrappable;
 
 /**
@@ -149,6 +151,7 @@ static Backoff of(String specification) {
      *
      * @see Unwrappable
      */
+    @Nullable
     @Override
     default <T> T as(Class<T> type) {
         return Unwrappable.super.as(type);
diff --git a/core/src/main/java/com/linecorp/armeria/common/DefaultAggregatedHttpRequest.java b/core/src/main/java/com/linecorp/armeria/common/DefaultAggregatedHttpRequest.java
index ded03fc3e6bb..6ef43c56606f 100644
--- a/core/src/main/java/com/linecorp/armeria/common/DefaultAggregatedHttpRequest.java
+++ b/core/src/main/java/com/linecorp/armeria/common/DefaultAggregatedHttpRequest.java
@@ -47,11 +47,13 @@ public String path() {
         return headers.path();
     }
 
+    @Nullable
     @Override
     public String scheme() {
         return headers.scheme();
     }
 
+    @Nullable
     @Override
     public String authority() {
         return headers.authority();
diff --git a/core/src/main/java/com/linecorp/armeria/common/DefaultCookie.java b/core/src/main/java/com/linecorp/armeria/common/DefaultCookie.java
index 0fbec278283a..9d6ab7f396d5 100644
--- a/core/src/main/java/com/linecorp/armeria/common/DefaultCookie.java
+++ b/core/src/main/java/com/linecorp/armeria/common/DefaultCookie.java
@@ -87,11 +87,13 @@ public boolean isValueQuoted() {
         return valueQuoted;
     }
 
+    @Nullable
     @Override
     public String domain() {
         return domain;
     }
 
+    @Nullable
     @Override
     public String path() {
         return path;
@@ -112,6 +114,7 @@ public boolean isHttpOnly() {
         return httpOnly;
     }
 
+    @Nullable
     @Override
     public String sameSite() {
         return sameSite;
diff --git a/core/src/main/java/com/linecorp/armeria/common/MediaType.java b/core/src/main/java/com/linecorp/armeria/common/MediaType.java
index 9875a1ae0f4a..8f74f8143437 100644
--- a/core/src/main/java/com/linecorp/armeria/common/MediaType.java
+++ b/core/src/main/java/com/linecorp/armeria/common/MediaType.java
@@ -1214,6 +1214,7 @@ private static final class KnownTypesByString {
                       .distinct()
                       .collect(toImmutableMap(MediaType::toString, Function.identity(), (a, b) -> a));
 
+        @Nullable
         static MediaType get(String input) {
             return KNOWN_TYPES_BY_STRING.get(input);
         }
diff --git a/core/src/main/java/com/linecorp/armeria/common/NonWrappingRequestContext.java b/core/src/main/java/com/linecorp/armeria/common/NonWrappingRequestContext.java
index 8388f1afad8c..f1cd7c0a8bfd 100644
--- a/core/src/main/java/com/linecorp/armeria/common/NonWrappingRequestContext.java
+++ b/core/src/main/java/com/linecorp/armeria/common/NonWrappingRequestContext.java
@@ -80,11 +80,13 @@ protected NonWrappingRequestContext(
         this.rpcReq = rpcReq;
     }
 
+    @Nullable
     @Override
     public HttpRequest request() {
         return req;
     }
 
+    @Nullable
     @Override
     public RpcRequest rpcRequest() {
         return rpcReq;
@@ -174,6 +176,7 @@ public final String decodedPath() {
         return this.decodedPath = ArmeriaHttpUtil.decodePath(path);
     }
 
+    @Nullable
     @Override
     public final String query() {
         return query;
diff --git a/core/src/main/java/com/linecorp/armeria/common/RequestContextWrapper.java b/core/src/main/java/com/linecorp/armeria/common/RequestContextWrapper.java
index 9ef1a2d1b8f3..8b2c3038e81b 100644
--- a/core/src/main/java/com/linecorp/armeria/common/RequestContextWrapper.java
+++ b/core/src/main/java/com/linecorp/armeria/common/RequestContextWrapper.java
@@ -168,6 +168,7 @@ public String decodedPath() {
         return delegate().decodedPath();
     }
 
+    @Nullable
     @Override
     public String query() {
         return delegate().query();
diff --git a/core/src/main/java/com/linecorp/armeria/common/logging/DefaultRequestLog.java b/core/src/main/java/com/linecorp/armeria/common/logging/DefaultRequestLog.java
index 95292e57248d..6b2dc2790e84 100644
--- a/core/src/main/java/com/linecorp/armeria/common/logging/DefaultRequestLog.java
+++ b/core/src/main/java/com/linecorp/armeria/common/logging/DefaultRequestLog.java
@@ -566,6 +566,7 @@ public long requestStartTimeNanos() {
         return requestStartTimeNanos;
     }
 
+    @Nullable
     @Override
     public Long requestFirstBytesTransferredTimeNanos() {
         ensureAvailable(RequestLogProperty.REQUEST_FIRST_BYTES_TRANSFERRED_TIME);
@@ -584,6 +585,7 @@ public long requestDurationNanos() {
         return requestEndTimeNanos - requestStartTimeNanos;
     }
 
+    @Nullable
     @Override
     public Throwable requestCause() {
         ensureAvailable(RequestLogProperty.REQUEST_CAUSE);
@@ -623,12 +625,14 @@ private void session0(@Nullable Channel channel, SessionProtocol sessionProtocol
         updateFlags(RequestLogProperty.SESSION);
     }
 
+    @Nullable
     @Override
     public Channel channel() {
         ensureAvailable(RequestLogProperty.SESSION);
         return channel;
     }
 
+    @Nullable
     @Override
     public SSLSession sslSession() {
         ensureAvailable(RequestLogProperty.SESSION);
@@ -771,6 +775,7 @@ public void requestHeaders(RequestHeaders requestHeaders) {
         updateFlags(RequestLogProperty.REQUEST_HEADERS);
     }
 
+    @Nullable
     @Override
     public Object requestContent() {
         ensureAvailable(RequestLogProperty.REQUEST_CONTENT);
@@ -792,12 +797,14 @@ public void requestContent(@Nullable Object requestContent, @Nullable Object raw
         }
     }
 
+    @Nullable
     @Override
     public Object rawRequestContent() {
         ensureAvailable(RequestLogProperty.REQUEST_CONTENT);
         return rawRequestContent;
     }
 
+    @Nullable
     @Override
     public String requestContentPreview() {
         ensureAvailable(RequestLogProperty.REQUEST_CONTENT_PREVIEW);
@@ -983,6 +990,7 @@ public long responseStartTimeNanos() {
         return responseStartTimeNanos;
     }
 
+    @Nullable
     @Override
     public Long responseFirstBytesTransferredTimeNanos() {
         ensureAvailable(RequestLogProperty.RESPONSE_FIRST_BYTES_TRANSFERRED_TIME);
@@ -1007,6 +1015,7 @@ public long totalDurationNanos() {
         return responseEndTimeNanos - requestStartTimeNanos;
     }
 
+    @Nullable
     @Override
     public Throwable responseCause() {
         ensureAvailable(RequestLogProperty.RESPONSE_CAUSE);
@@ -1090,6 +1099,7 @@ public void responseHeaders(ResponseHeaders responseHeaders) {
         updateFlags(RequestLogProperty.RESPONSE_HEADERS);
     }
 
+    @Nullable
     @Override
     public Object responseContent() {
         ensureAvailable(RequestLogProperty.RESPONSE_CONTENT);
@@ -1117,6 +1127,7 @@ public void responseContent(@Nullable Object responseContent, @Nullable Object r
         updateFlags(RequestLogProperty.RESPONSE_CONTENT);
     }
 
+    @Nullable
     @Override
     public String responseContentPreview() {
         ensureAvailable(RequestLogProperty.RESPONSE_CONTENT_PREVIEW);
@@ -1138,6 +1149,7 @@ public void responseContentPreview(@Nullable String responseContentPreview) {
         updateFlags(RequestLogProperty.RESPONSE_CONTENT_PREVIEW);
     }
 
+    @Nullable
     @Override
     public Object rawResponseContent() {
         ensureAvailable(RequestLogProperty.RESPONSE_CONTENT);
@@ -1633,6 +1645,7 @@ public long requestStartTimeNanos() {
             return requestStartTimeNanos;
         }
 
+        @Nullable
         @Override
         public Long requestFirstBytesTransferredTimeNanos() {
             return requestFirstBytesTransferredTimeNanosSet ? requestFirstBytesTransferredTimeNanos : null;
@@ -1747,6 +1760,7 @@ public long responseStartTimeNanos() {
             return responseStartTimeNanos;
         }
 
+        @Nullable
         @Override
         public Long responseFirstBytesTransferredTimeNanos() {
             return responseFirstBytesTransferredTimeNanosSet ? responseFirstBytesTransferredTimeNanos : null;
diff --git a/core/src/main/java/com/linecorp/armeria/common/util/AbstractUnwrappable.java b/core/src/main/java/com/linecorp/armeria/common/util/AbstractUnwrappable.java
index 6aeb315a028e..6fb49c16b53a 100644
--- a/core/src/main/java/com/linecorp/armeria/common/util/AbstractUnwrappable.java
+++ b/core/src/main/java/com/linecorp/armeria/common/util/AbstractUnwrappable.java
@@ -17,6 +17,8 @@
 
 import static java.util.Objects.requireNonNull;
 
+import javax.annotation.Nullable;
+
 /**
  * Skeletal {@link Unwrappable} implementation.
  *
@@ -41,6 +43,7 @@ protected final <U extends T> U delegate() {
         return (U) delegate;
     }
 
+    @Nullable
     @Override
     public final <U> U as(Class<U> type) {
         final U result = Unwrappable.super.as(type);
diff --git a/core/src/main/java/com/linecorp/armeria/internal/common/DefaultTimeoutController.java b/core/src/main/java/com/linecorp/armeria/internal/common/DefaultTimeoutController.java
index c93049108ebb..e2403533e67e 100644
--- a/core/src/main/java/com/linecorp/armeria/internal/common/DefaultTimeoutController.java
+++ b/core/src/main/java/com/linecorp/armeria/internal/common/DefaultTimeoutController.java
@@ -253,6 +253,7 @@ private void invokeTimeoutTask() {
         }
     }
 
+    @Nullable
     @Override
     public Long startTimeNanos() {
         return state != State.INIT ? firstExecutionTimeNanos : null;
@@ -269,6 +270,7 @@ ScheduledFuture<?> timeoutFuture() {
         return timeoutFuture;
     }
 
+    @Nullable
     @VisibleForTesting
     TimeoutTask timeoutTask() {
         return timeoutTask;
diff --git a/core/src/main/java/com/linecorp/armeria/internal/common/util/ObjectCollectingUtil.java b/core/src/main/java/com/linecorp/armeria/internal/common/util/ObjectCollectingUtil.java
index 1e75aab0c128..2bf0bd585b7c 100644
--- a/core/src/main/java/com/linecorp/armeria/internal/common/util/ObjectCollectingUtil.java
+++ b/core/src/main/java/com/linecorp/armeria/internal/common/util/ObjectCollectingUtil.java
@@ -42,6 +42,7 @@ public final class ObjectCollectingUtil {
      * The {@link Class} instance of {@code reactor.core.publisher.Mono} of
      * <a href="https://projectreactor.io/">Project Reactor</a>.
      */
+    @Nullable
     private static final Class<?> MONO_CLASS;
 
     static {
diff --git a/core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotationUtil.java b/core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotationUtil.java
index 37e83988ec9f..2eedc8e82f6c 100644
--- a/core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotationUtil.java
+++ b/core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotationUtil.java
@@ -220,14 +220,14 @@ static <T extends Annotation> List<T> find(AnnotatedElement element, Class<T> an
 
     private static <T extends Annotation> void findMetaAnnotations(
             Builder<T> builder, Annotation annotation,
-            Class<T> annotationType, Class<? extends Annotation> containerType) {
+            Class<T> annotationType, @Nullable Class<? extends Annotation> containerType) {
         findMetaAnnotations(builder, annotation, annotationType, containerType,
                             Collections.newSetFromMap(new IdentityHashMap<>()));
     }
 
     private static <T extends Annotation> boolean findMetaAnnotations(
             Builder<T> builder, Annotation annotation,
-            Class<T> annotationType, Class<? extends Annotation> containerType,
+            Class<T> annotationType, @Nullable Class<? extends Annotation> containerType,
             Set<Class<? extends Annotation>> visitedAnnotationTypes) {
 
         final Class<? extends Annotation> actualAnnotationType = annotation.getClass();
diff --git a/core/src/main/java/com/linecorp/armeria/server/DefaultRoutingContext.java b/core/src/main/java/com/linecorp/armeria/server/DefaultRoutingContext.java
index 7beda3b5ff34..1125c6daa0e7 100644
--- a/core/src/main/java/com/linecorp/armeria/server/DefaultRoutingContext.java
+++ b/core/src/main/java/com/linecorp/armeria/server/DefaultRoutingContext.java
@@ -149,6 +149,7 @@ public void deferStatusException(HttpStatusException deferredCause) {
         this.deferredCause = requireNonNull(deferredCause, "deferredCause");
     }
 
+    @Nullable
     @Override
     public HttpStatusException deferredStatusException() {
         return deferredCause;
diff --git a/core/src/main/java/com/linecorp/armeria/server/RoutingContextWrapper.java b/core/src/main/java/com/linecorp/armeria/server/RoutingContextWrapper.java
index c110ca7e29bf..b62df638dead 100644
--- a/core/src/main/java/com/linecorp/armeria/server/RoutingContextWrapper.java
+++ b/core/src/main/java/com/linecorp/armeria/server/RoutingContextWrapper.java
@@ -85,6 +85,7 @@ public void deferStatusException(HttpStatusException cause) {
         delegate.deferStatusException(cause);
     }
 
+    @Nullable
     @Override
     public HttpStatusException deferredStatusException() {
         return delegate.deferredStatusException();
diff --git a/core/src/main/java/com/linecorp/armeria/server/Service.java b/core/src/main/java/com/linecorp/armeria/server/Service.java
index 92309df74471..58ffbddce647 100644
--- a/core/src/main/java/com/linecorp/armeria/server/Service.java
+++ b/core/src/main/java/com/linecorp/armeria/server/Service.java
@@ -70,6 +70,7 @@ default void serviceAdded(ServiceConfig cfg) throws Exception {}
      *
      * @see Unwrappable
      */
+    @Nullable
     @Override
     default <T> T as(Class<T> type) {
         requireNonNull(type, "type");
diff --git a/core/src/main/java/com/linecorp/armeria/server/annotation/DefaultHttpResult.java b/core/src/main/java/com/linecorp/armeria/server/annotation/DefaultHttpResult.java
index 27a422a23889..be90039128a5 100644
--- a/core/src/main/java/com/linecorp/armeria/server/annotation/DefaultHttpResult.java
+++ b/core/src/main/java/com/linecorp/armeria/server/annotation/DefaultHttpResult.java
@@ -50,6 +50,7 @@ public HttpHeaders headers() {
         return headers;
     }
 
+    @Nullable
     @Override
     public T content() {
         return content;
diff --git a/core/src/main/java/com/linecorp/armeria/server/docs/DocService.java b/core/src/main/java/com/linecorp/armeria/server/docs/DocService.java
index aaf4ad089d65..401d7acd033f 100644
--- a/core/src/main/java/com/linecorp/armeria/server/docs/DocService.java
+++ b/core/src/main/java/com/linecorp/armeria/server/docs/DocService.java
@@ -301,6 +301,7 @@ private static FieldInfo addFieldDocString(NamedTypeInfo parent, FieldInfo field
                              docString(parent.name() + '/' + field.name(), field.docString(), docStrings));
     }
 
+    @Nullable
     private static String docString(
             String key, @Nullable String currentDocString, Map<String, String> docStrings) {
         return currentDocString != null ? currentDocString : docStrings.get(key);
diff --git a/core/src/main/java/com/linecorp/armeria/server/docs/EnumInfo.java b/core/src/main/java/com/linecorp/armeria/server/docs/EnumInfo.java
index 843764e32c9e..2631c5d5ccc0 100644
--- a/core/src/main/java/com/linecorp/armeria/server/docs/EnumInfo.java
+++ b/core/src/main/java/com/linecorp/armeria/server/docs/EnumInfo.java
@@ -99,6 +99,7 @@ public List<EnumValueInfo> values() {
         return values;
     }
 
+    @Nullable
     @Override
     public String docString() {
         return docString;
diff --git a/core/src/main/java/com/linecorp/armeria/server/docs/ExceptionInfo.java b/core/src/main/java/com/linecorp/armeria/server/docs/ExceptionInfo.java
index b6b4bfd7d922..a5ba80d74b82 100644
--- a/core/src/main/java/com/linecorp/armeria/server/docs/ExceptionInfo.java
+++ b/core/src/main/java/com/linecorp/armeria/server/docs/ExceptionInfo.java
@@ -73,6 +73,7 @@ public List<FieldInfo> fields() {
         return fields;
     }
 
+    @Nullable
     @Override
     public String docString() {
         return docString;
diff --git a/core/src/main/java/com/linecorp/armeria/server/docs/StructInfo.java b/core/src/main/java/com/linecorp/armeria/server/docs/StructInfo.java
index 0cf8654b58cb..1e166bf79eda 100644
--- a/core/src/main/java/com/linecorp/armeria/server/docs/StructInfo.java
+++ b/core/src/main/java/com/linecorp/armeria/server/docs/StructInfo.java
@@ -73,6 +73,7 @@ public List<FieldInfo> fields() {
         return fields;
     }
 
+    @Nullable
     @Override
     public String docString() {
         return docString;
diff --git a/core/src/main/java/com/linecorp/armeria/server/file/FileSystemHttpFile.java b/core/src/main/java/com/linecorp/armeria/server/file/FileSystemHttpFile.java
index 6b27d17b2f1c..c89755cb4684 100644
--- a/core/src/main/java/com/linecorp/armeria/server/file/FileSystemHttpFile.java
+++ b/core/src/main/java/com/linecorp/armeria/server/file/FileSystemHttpFile.java
@@ -55,6 +55,7 @@ protected String pathOrUri() {
         return path.toString();
     }
 
+    @Nullable
     @Override
     public HttpFileAttributes readAttributes() throws IOException {
         if (!Files.exists(path)) {
@@ -73,6 +74,7 @@ public HttpFileAttributes readAttributes() throws IOException {
         return null;
     }
 
+    @Nullable
     @Override
     protected ByteChannel newStream() throws IOException {
         try {
diff --git a/core/src/main/java/com/linecorp/armeria/server/file/HttpDataFile.java b/core/src/main/java/com/linecorp/armeria/server/file/HttpDataFile.java
index 000693f0e774..3228877b9ef5 100644
--- a/core/src/main/java/com/linecorp/armeria/server/file/HttpDataFile.java
+++ b/core/src/main/java/com/linecorp/armeria/server/file/HttpDataFile.java
@@ -75,6 +75,7 @@ public HttpFileAttributes readAttributes() {
         return attrs;
     }
 
+    @Nullable
     @Override
     public ResponseHeaders readHeaders() {
         try {
diff --git a/core/src/main/java/com/linecorp/armeria/server/file/NonExistentHttpFile.java b/core/src/main/java/com/linecorp/armeria/server/file/NonExistentHttpFile.java
index 55d89c0acf74..c2837dc36ec2 100644
--- a/core/src/main/java/com/linecorp/armeria/server/file/NonExistentHttpFile.java
+++ b/core/src/main/java/com/linecorp/armeria/server/file/NonExistentHttpFile.java
@@ -45,6 +45,7 @@ public ResponseHeaders readHeaders() {
         return null;
     }
 
+    @Nullable
     @Override
     public HttpResponse read(Executor fileReadExecutor, ByteBufAllocator alloc) {
         return null;
diff --git a/core/src/main/java/com/linecorp/armeria/server/file/StreamingHttpFile.java b/core/src/main/java/com/linecorp/armeria/server/file/StreamingHttpFile.java
index 13e8781a81ee..e64836c3eaa9 100644
--- a/core/src/main/java/com/linecorp/armeria/server/file/StreamingHttpFile.java
+++ b/core/src/main/java/com/linecorp/armeria/server/file/StreamingHttpFile.java
@@ -77,6 +77,7 @@ protected StreamingHttpFile(@Nullable MediaType contentType,
         super(contentType, clock, dateEnabled, lastModifiedEnabled, entityTagFunction, headers);
     }
 
+    @Nullable
     @Override
     protected final HttpResponse doRead(ResponseHeaders headers, long length,
                                         Executor fileReadExecutor, ByteBufAllocator alloc) throws IOException {
diff --git a/core/src/main/java/com/linecorp/armeria/server/logging/AccessLogComponent.java b/core/src/main/java/com/linecorp/armeria/server/logging/AccessLogComponent.java
index 02f12f9a3295..07983db3013b 100644
--- a/core/src/main/java/com/linecorp/armeria/server/logging/AccessLogComponent.java
+++ b/core/src/main/java/com/linecorp/armeria/server/logging/AccessLogComponent.java
@@ -352,6 +352,7 @@ AsciiString headerName() {
             return headerName;
         }
 
+        @Nullable
         @Override
         public Object getMessage0(RequestLog log) {
             return httpHeaders.apply(log).get(headerName);