diff --git a/java11/pom.xml b/java11/pom.xml
index 6188ed12f..69c5277ba 100644
--- a/java11/pom.xml
+++ b/java11/pom.xml
@@ -69,6 +69,11 @@
       <type>jar</type>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>com.google.code.gson</groupId>
+      <artifactId>gson</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <build>
diff --git a/java11/src/main/java/feign/http2client/Http2Client.java b/java11/src/main/java/feign/http2client/Http2Client.java
index a68871c79..99664b480 100644
--- a/java11/src/main/java/feign/http2client/Http2Client.java
+++ b/java11/src/main/java/feign/http2client/Http2Client.java
@@ -1,5 +1,5 @@
 /**
- * Copyright 2012-2020 The Feign Authors
+ * Copyright 2012-2021 The Feign Authors
  *
  * <p>Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
  * except in compliance with the License. You may obtain a copy of the License at
@@ -13,6 +13,7 @@
  */
 package feign.http2client;
 
+import feign.AsyncClient;
 import feign.Client;
 import feign.Request;
 import feign.Request.Options;
@@ -32,11 +33,22 @@
 import java.net.http.HttpResponse;
 import java.net.http.HttpResponse.BodyHandlers;
 import java.time.Duration;
-import java.util.*;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.OptionalLong;
+import java.util.Set;
+import java.util.TreeSet;
+import java.util.concurrent.CompletableFuture;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
-public class Http2Client implements Client {
+public class Http2Client implements Client, AsyncClient<Object> {
 
   private final HttpClient client;
 
@@ -54,9 +66,14 @@ public Http2Client(HttpClient client) {
 
   @Override
   public Response execute(Request request, Options options) throws IOException {
-    final HttpRequest httpRequest = newRequestBuilder(request, options).build();
-    HttpClient clientForRequest = getOrCreateClient(options);
+    final HttpRequest httpRequest;
+    try {
+      httpRequest = newRequestBuilder(request, options).version(client.version()).build();
+    } catch (URISyntaxException e) {
+      throw new IOException("Invalid uri " + request.url(), e);
+    }
 
+    HttpClient clientForRequest = getOrCreateClient(options);
     HttpResponse<byte[]> httpResponse;
     try {
       httpResponse = clientForRequest.send(httpRequest, BodyHandlers.ofByteArray());
@@ -65,19 +82,37 @@ public Response execute(Request request, Options options) throws IOException {
       throw new IOException("Invalid uri " + request.url(), e);
     }
 
+    return toFeignResponse(request, httpResponse);
+  }
+
+  @Override
+  public CompletableFuture<Response> execute(
+      Request request, Options options, Optional<Object> requestContext) {
+    HttpRequest httpRequest;
+    try {
+      httpRequest = newRequestBuilder(request, options).build();
+    } catch (URISyntaxException e) {
+      throw new IllegalArgumentException("Invalid uri " + request.url(), e);
+    }
+
+    HttpClient clientForRequest = getOrCreateClient(options);
+    CompletableFuture<HttpResponse<byte[]>> future =
+        clientForRequest.sendAsync(httpRequest, HttpResponse.BodyHandlers.ofByteArray());
+    return future.thenApply(httpResponse -> toFeignResponse(request, httpResponse));
+  }
+
+  protected Response toFeignResponse(Request request, HttpResponse<byte[]> httpResponse) {
     final OptionalLong length = httpResponse.headers().firstValueAsLong("Content-Length");
 
-    final Response response =
-        Response.builder()
-            .body(
-                new ByteArrayInputStream(httpResponse.body()),
-                length.isPresent() ? (int) length.getAsLong() : null)
-            .reason(httpResponse.headers().firstValue("Reason-Phrase").orElse("OK"))
-            .request(request)
-            .status(httpResponse.statusCode())
-            .headers(castMapCollectType(httpResponse.headers().map()))
-            .build();
-    return response;
+    return Response.builder()
+        .body(
+            new ByteArrayInputStream(httpResponse.body()),
+            length.isPresent() ? (int) length.getAsLong() : null)
+        .reason(httpResponse.headers().firstValue("Reason-Phrase").orElse("OK"))
+        .request(request)
+        .status(httpResponse.statusCode())
+        .headers(castMapCollectType(httpResponse.headers().map()))
+        .build();
   }
 
   private HttpClient getOrCreateClient(Options options) {
@@ -114,13 +149,8 @@ private static java.net.http.HttpClient.Builder newClientBuilder(Options options
         .connectTimeout(Duration.ofMillis(options.connectTimeoutMillis()));
   }
 
-  private Builder newRequestBuilder(Request request, Options options) throws IOException {
-    URI uri;
-    try {
-      uri = new URI(request.url());
-    } catch (final URISyntaxException e) {
-      throw new IOException("Invalid uri " + request.url(), e);
-    }
+  private Builder newRequestBuilder(Request request, Options options) throws URISyntaxException {
+    URI uri = new URI(request.url());
 
     final BodyPublisher body;
     final byte[] data = request.body();
@@ -134,7 +164,7 @@ private Builder newRequestBuilder(Request request, Options options) throws IOExc
         HttpRequest.newBuilder()
             .uri(uri)
             .timeout(Duration.ofMillis(options.readTimeoutMillis()))
-            .version(Version.HTTP_2);
+            .version(client.version());
 
     final Map<String, Collection<String>> headers = filterRestrictedHeaders(request.headers());
     if (!headers.isEmpty()) {
diff --git a/java11/src/test/java/feign/http2client/test/CustomPojo.java b/java11/src/test/java/feign/http2client/test/CustomPojo.java
new file mode 100644
index 000000000..d24ed5cc1
--- /dev/null
+++ b/java11/src/test/java/feign/http2client/test/CustomPojo.java
@@ -0,0 +1,25 @@
+/**
+ * Copyright 2012-2020 The Feign Authors
+ *
+ * <p>Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of the License at
+ *
+ * <p>http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * <p>Unless required by applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package feign.http2client.test;
+
+public class CustomPojo {
+
+  private final String name;
+  private final Integer number;
+
+  CustomPojo(String name, Integer number) {
+    this.name = name;
+    this.number = number;
+  }
+}
diff --git a/java11/src/test/java/feign/http2client/test/Http2ClientAsyncTest.java b/java11/src/test/java/feign/http2client/test/Http2ClientAsyncTest.java
new file mode 100644
index 000000000..78db6a2ca
--- /dev/null
+++ b/java11/src/test/java/feign/http2client/test/Http2ClientAsyncTest.java
@@ -0,0 +1,1116 @@
+/**
+ * Copyright 2012-2021 The Feign Authors
+ *
+ * <p>Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of the License at
+ *
+ * <p>http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * <p>Unless required by applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package feign.http2client.test;
+
+import static feign.assertj.MockWebServerAssertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.data.MapEntry.entry;
+import static org.hamcrest.CoreMatchers.isA;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+import com.google.gson.Gson;
+import com.google.gson.reflect.TypeToken;
+import feign.AsyncClient;
+import feign.AsyncFeign;
+import feign.Body;
+import feign.ChildPojo;
+import feign.Feign;
+import feign.Feign.ResponseMappingDecoder;
+import feign.FeignException;
+import feign.HeaderMap;
+import feign.Headers;
+import feign.Param;
+import feign.PropertyPojo;
+import feign.QueryMap;
+import feign.QueryMapEncoder;
+import feign.Request;
+import feign.Request.HttpMethod;
+import feign.RequestInterceptor;
+import feign.RequestLine;
+import feign.RequestTemplate;
+import feign.Response;
+import feign.ResponseMapper;
+import feign.Target;
+import feign.Target.HardCodedTarget;
+import feign.Util;
+import feign.codec.DecodeException;
+import feign.codec.Decoder;
+import feign.codec.EncodeException;
+import feign.codec.Encoder;
+import feign.codec.ErrorDecoder;
+import feign.codec.StringDecoder;
+import feign.http2client.Http2Client;
+import feign.querymap.BeanQueryMapEncoder;
+import feign.querymap.FieldQueryMapEncoder;
+import java.io.IOException;
+import java.lang.reflect.Type;
+import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Date;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+import okhttp3.mockwebserver.MockResponse;
+import okhttp3.mockwebserver.MockWebServer;
+import okio.Buffer;
+import org.assertj.core.api.Assertions;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+public class Http2ClientAsyncTest {
+  @Rule public final ExpectedException thrown = ExpectedException.none();
+  @Rule public final MockWebServer server = new MockWebServer();
+
+  @Test
+  public void iterableQueryParams() throws Exception {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    api.queryParams("user", Arrays.asList("apple", "pear"));
+
+    assertThat(server.takeRequest()).hasPath("/?1=user&2=apple&2=pear");
+  }
+
+  @Test
+  public void postTemplateParamsResolve() throws Exception {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    api.login("netflix", "denominator", "password");
+
+    assertThat(server.takeRequest())
+        .hasBody(
+            "{\"customer_name\": \"netflix\", \"user_name\": \"denominator\", \"password\":"
+                + " \"password\"}");
+  }
+
+  @Test
+  public void responseCoercesToStringBody() throws Throwable {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final Response response = unwrap(api.response());
+    assertTrue(response.body().isRepeatable());
+    assertEquals("foo", Util.toString(response.body().asReader(StandardCharsets.UTF_8)));
+  }
+
+  @Test
+  public void postFormParams() throws Exception {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.form("netflix", "denominator", "password");
+
+    assertThat(server.takeRequest())
+        .hasBody(
+            "{\"customer_name\":\"netflix\",\"user_name\":\"denominator\",\"password\":\"password\"}");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void postBodyParam() throws Exception {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.body(Arrays.asList("netflix", "denominator", "password"));
+
+    assertThat(server.takeRequest())
+        .hasHeaders(entry("Content-Length", Collections.singletonList("32")))
+        .hasBody("[netflix, denominator, password]");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  /**
+   * The type of a parameter value may not be the desired type to encode as. Prefer the interface
+   * type.
+   */
+  @Test
+  public void bodyTypeCorrespondsWithParameterType() throws Exception {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final AtomicReference<Type> encodedType = new AtomicReference<Type>();
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .encoder(
+                new Encoder.Default() {
+                  @Override
+                  public void encode(Object object, Type bodyType, RequestTemplate template) {
+                    encodedType.set(bodyType);
+                  }
+                })
+            .target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.body(Arrays.asList("netflix", "denominator", "password"));
+
+    server.takeRequest();
+
+    Assertions.assertThat(encodedType.get()).isEqualTo(new TypeToken<List<String>>() {}.getType());
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void singleInterceptor() throws Exception {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .requestInterceptor(new ForwardedForInterceptor())
+            .target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.post();
+
+    assertThat(server.takeRequest())
+        .hasHeaders(entry("X-Forwarded-For", Collections.singletonList("origin.host.com")));
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void multipleInterceptor() throws Exception {
+    server.enqueue(new MockResponse().setBody("foo"));
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .requestInterceptor(new ForwardedForInterceptor())
+            .requestInterceptor(new UserAgentInterceptor())
+            .target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.post();
+
+    assertThat(server.takeRequest())
+        .hasHeaders(
+            entry("X-Forwarded-For", Collections.singletonList("origin.host.com")),
+            entry("User-Agent", Collections.singletonList("Feign")));
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void customExpander() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.expand(new Date(1234l));
+
+    assertThat(server.takeRequest()).hasPath("/?date=1234");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void customExpanderListParam() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf =
+        api.expandList(Arrays.asList(new Date(1234l), new Date(12345l)));
+
+    assertThat(server.takeRequest()).hasPath("/?date=1234&date=12345");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void customExpanderNullParam() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.expandList(Arrays.asList(new Date(1234l), null));
+
+    assertThat(server.takeRequest()).hasPath("/?date=1234");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void headerMap() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final Map<String, Object> headerMap = new LinkedHashMap<String, Object>();
+    headerMap.put("Content-Type", "myContent");
+    headerMap.put("Custom-Header", "fooValue");
+    final CompletableFuture<?> cf = api.headerMap(headerMap);
+
+    assertThat(server.takeRequest())
+        .hasHeaders(
+            entry("Content-Type", Arrays.asList("myContent")),
+            entry("Custom-Header", Arrays.asList("fooValue")));
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void headerMapWithHeaderAnnotations() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final Map<String, Object> headerMap = new LinkedHashMap<String, Object>();
+    headerMap.put("Custom-Header", "fooValue");
+    api.headerMapWithHeaderAnnotations(headerMap);
+
+    // header map should be additive for headers provided by annotations
+    assertThat(server.takeRequest())
+        .hasHeaders(
+            entry("Content-Encoding", Arrays.asList("deflate")),
+            entry("Custom-Header", Arrays.asList("fooValue")));
+
+    server.enqueue(new MockResponse());
+    headerMap.put("Content-Encoding", "overrideFromMap");
+
+    final CompletableFuture<?> cf = api.headerMapWithHeaderAnnotations(headerMap);
+
+    /*
+     * @HeaderMap map values no longer override @Header parameters. This caused confusion as it is
+     * valid to have more than one value for a header.
+     */
+    assertThat(server.takeRequest())
+        .hasHeaders(
+            entry("Content-Encoding", Arrays.asList("deflate", "overrideFromMap")),
+            entry("Custom-Header", Arrays.asList("fooValue")));
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void queryMap() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final Map<String, Object> queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("name", "alice");
+    queryMap.put("fooKey", "fooValue");
+    final CompletableFuture<?> cf = api.queryMap(queryMap);
+
+    assertThat(server.takeRequest()).hasPath("/?name=alice&fooKey=fooValue");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void queryMapIterableValuesExpanded() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final Map<String, Object> queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("name", Arrays.asList("Alice", "Bob"));
+    queryMap.put("fooKey", "fooValue");
+    queryMap.put("emptyListKey", new ArrayList<String>());
+    queryMap.put("emptyStringKey", ""); // empty values are ignored.
+    final CompletableFuture<?> cf = api.queryMap(queryMap);
+
+    assertThat(server.takeRequest())
+        .hasPath("/?name=Alice&name=Bob&fooKey=fooValue&emptyStringKey");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void queryMapWithQueryParams() throws Exception {
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    server.enqueue(new MockResponse());
+    Map<String, Object> queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("fooKey", "fooValue");
+    api.queryMapWithQueryParams("alice", queryMap);
+    // query map should be expanded after built-in parameters
+    assertThat(server.takeRequest()).hasPath("/?name=alice&fooKey=fooValue");
+
+    server.enqueue(new MockResponse());
+    queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("name", "bob");
+    api.queryMapWithQueryParams("alice", queryMap);
+    // queries are additive
+    assertThat(server.takeRequest()).hasPath("/?name=alice&name=bob");
+
+    server.enqueue(new MockResponse());
+    queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("name", null);
+    api.queryMapWithQueryParams("alice", queryMap);
+    // null value for a query map key removes query parameter
+    assertThat(server.takeRequest()).hasPath("/?name=alice");
+  }
+
+  @Test
+  public void queryMapValueStartingWithBrace() throws Exception {
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    server.enqueue(new MockResponse());
+    Map<String, Object> queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("name", "{alice");
+    api.queryMap(queryMap);
+    assertThat(server.takeRequest()).hasPath("/?name=%7Balice");
+
+    server.enqueue(new MockResponse());
+    queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("{name", "alice");
+    api.queryMap(queryMap);
+    assertThat(server.takeRequest()).hasPath("/?%7Bname=alice");
+
+    server.enqueue(new MockResponse());
+    queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("name", "%7Balice");
+    api.queryMapEncoded(queryMap);
+    assertThat(server.takeRequest()).hasPath("/?name=%7Balice");
+
+    server.enqueue(new MockResponse());
+    queryMap = new LinkedHashMap<String, Object>();
+    queryMap.put("%7Bname", "%7Balice");
+    api.queryMapEncoded(queryMap);
+    assertThat(server.takeRequest()).hasPath("/?%7Bname=%7Balice");
+  }
+
+  @Test
+  public void queryMapPojoWithFullParams() throws Exception {
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CustomPojo customPojo = new CustomPojo("Name", 3);
+
+    server.enqueue(new MockResponse());
+    final CompletableFuture<?> cf = api.queryMapPojo(customPojo);
+    assertThat(server.takeRequest()).hasQueryParams(Arrays.asList("name=Name", "number=3"));
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void queryMapPojoWithPartialParams() throws Exception {
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CustomPojo customPojo = new CustomPojo("Name", null);
+
+    server.enqueue(new MockResponse());
+    final CompletableFuture<?> cf = api.queryMapPojo(customPojo);
+    assertThat(server.takeRequest()).hasPath("/?name=Name");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void queryMapPojoWithEmptyParams() throws Exception {
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CustomPojo customPojo = new CustomPojo(null, null);
+
+    server.enqueue(new MockResponse());
+    api.queryMapPojo(customPojo);
+    assertThat(server.takeRequest()).hasPath("/");
+  }
+
+  @Test
+  public void configKeyFormatsAsExpected() throws Exception {
+    assertEquals(
+        "TestInterfaceAsync#post()",
+        Feign.configKey(
+            TestInterfaceAsync.class, TestInterfaceAsync.class.getDeclaredMethod("post")));
+    assertEquals(
+        "TestInterfaceAsync#uriParam(String,URI,String)",
+        Feign.configKey(
+            TestInterfaceAsync.class,
+            TestInterfaceAsync.class.getDeclaredMethod(
+                "uriParam", String.class, URI.class, String.class)));
+  }
+
+  @Test
+  public void configKeyUsesChildType() throws Exception {
+    assertEquals(
+        "List#iterator()",
+        Feign.configKey(List.class, Iterable.class.getDeclaredMethod("iterator")));
+  }
+
+  private <T> T unwrap(CompletableFuture<T> cf) throws Throwable {
+    try {
+      return cf.get(1, TimeUnit.SECONDS);
+    } catch (final ExecutionException e) {
+      throw e.getCause();
+    }
+  }
+
+  @Test
+  public void canOverrideErrorDecoder() throws Throwable {
+    server.enqueue(new MockResponse().setResponseCode(400).setBody("foo"));
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("bad zone name");
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .errorDecoder(new IllegalArgumentExceptionOn400())
+            .target("http://localhost:" + server.getPort());
+
+    unwrap(api.post());
+  }
+
+  @Test
+  public void overrideTypeSpecificDecoder() throws Throwable {
+    server.enqueue(new MockResponse().setBody("success!"));
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .decoder((response, type) -> "fail")
+            .target("http://localhost:" + server.getPort());
+
+    assertEquals("fail", unwrap(api.post()));
+  }
+
+  @Test
+  public void doesntRetryAfterResponseIsSent() throws Throwable {
+    server.enqueue(new MockResponse().setBody("success!"));
+    thrown.expect(FeignException.class);
+    thrown.expectMessage("timeout reading POST http://");
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .decoder(
+                (response, type) -> {
+                  throw new IOException("timeout");
+                })
+            .target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.post();
+    server.takeRequest();
+    unwrap(cf);
+  }
+
+  @Test
+  public void throwsFeignExceptionIncludingBody() throws Throwable {
+    server.enqueue(new MockResponse().setBody("success!"));
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .decoder(
+                (response, type) -> {
+                  throw new IOException("timeout");
+                })
+            .target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.body("Request body");
+    server.takeRequest();
+    try {
+      unwrap(cf);
+    } catch (final FeignException e) {
+      Assertions.assertThat(e.getMessage())
+          .isEqualTo("timeout reading POST http://localhost:" + server.getPort() + "/");
+      Assertions.assertThat(e.contentUTF8()).isEqualTo("Request body");
+      return;
+    }
+    fail();
+  }
+
+  @Test
+  public void throwsFeignExceptionWithoutBody() {
+    server.enqueue(new MockResponse().setBody("success!"));
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .decoder(
+                (response, type) -> {
+                  throw new IOException("timeout");
+                })
+            .target("http://localhost:" + server.getPort());
+
+    try {
+      api.noContent();
+    } catch (final FeignException e) {
+      Assertions.assertThat(e.getMessage())
+          .isEqualTo("timeout reading POST http://localhost:" + server.getPort() + "/");
+      Assertions.assertThat(e.contentUTF8()).isEqualTo("");
+    }
+  }
+
+  @SuppressWarnings("deprecation")
+  @Test
+  public void whenReturnTypeIsResponseNoErrorHandling() throws Throwable {
+    final Map<String, Collection<String>> headers = new LinkedHashMap<String, Collection<String>>();
+    headers.put("Location", Arrays.asList("http://bar.com"));
+    final Response response =
+        Response.builder()
+            .status(302)
+            .reason("Found")
+            .headers(headers)
+            .request(Request.create(HttpMethod.GET, "/", Collections.emptyMap(), null, Util.UTF_8))
+            .body(new byte[0])
+            .build();
+
+    final ExecutorService execs = Executors.newSingleThreadExecutor();
+
+    // fake client as Client.Default follows redirects.
+    final TestInterfaceAsync api =
+        AsyncFeign.<Void>asyncBuilder()
+            .client(new AsyncClient.Default<>((request, options) -> response, execs))
+            .target(TestInterfaceAsync.class, "http://localhost:" + server.getPort());
+
+    assertThat(unwrap(api.response()).headers().get("Location")).contains("http://bar.com");
+
+    execs.shutdown();
+  }
+
+  @Test
+  public void okIfDecodeRootCauseHasNoMessage() throws Throwable {
+    server.enqueue(new MockResponse().setBody("success!"));
+    thrown.expect(DecodeException.class);
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .decoder(
+                (response, type) -> {
+                  throw new RuntimeException();
+                })
+            .target("http://localhost:" + server.getPort());
+
+    unwrap(api.post());
+  }
+
+  @Test
+  public void decodingExceptionGetWrappedInDecode404Mode() throws Throwable {
+    server.enqueue(new MockResponse().setResponseCode(404));
+    thrown.expect(DecodeException.class);
+    thrown.expectCause(isA(NoSuchElementException.class));
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .decode404()
+            .decoder(
+                (response, type) -> {
+                  assertEquals(404, response.status());
+                  throw new NoSuchElementException();
+                })
+            .target("http://localhost:" + server.getPort());
+
+    unwrap(api.post());
+  }
+
+  @Test
+  public void decodingDoesNotSwallow404ErrorsInDecode404Mode() throws Throwable {
+    server.enqueue(new MockResponse().setResponseCode(404));
+    thrown.expect(IllegalArgumentException.class);
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .decode404()
+            .errorDecoder(new IllegalArgumentExceptionOn404())
+            .target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<Void> cf = api.queryMap(Collections.<String, Object>emptyMap());
+    server.takeRequest();
+    unwrap(cf);
+  }
+
+  @Test
+  public void okIfEncodeRootCauseHasNoMessage() throws Throwable {
+    server.enqueue(new MockResponse().setBody("success!"));
+    thrown.expect(EncodeException.class);
+
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .encoder(
+                (object, bodyType, template) -> {
+                  throw new RuntimeException();
+                })
+            .target("http://localhost:" + server.getPort());
+
+    unwrap(api.body(Arrays.asList("foo")));
+  }
+
+  @Test
+  public void equalsHashCodeAndToStringWork() {
+    final Target<TestInterfaceAsync> t1 =
+        new HardCodedTarget<TestInterfaceAsync>(TestInterfaceAsync.class, "http://localhost:8080");
+    final Target<TestInterfaceAsync> t2 =
+        new HardCodedTarget<TestInterfaceAsync>(TestInterfaceAsync.class, "http://localhost:8888");
+    final Target<OtherTestInterfaceAsync> t3 =
+        new HardCodedTarget<OtherTestInterfaceAsync>(
+            OtherTestInterfaceAsync.class, "http://localhost:8080");
+    final TestInterfaceAsync i1 = newAsyncBuilder().target(t1);
+    final TestInterfaceAsync i2 = newAsyncBuilder().target(t1);
+    final TestInterfaceAsync i3 = newAsyncBuilder().target(t2);
+    final OtherTestInterfaceAsync i4 = newAsyncBuilder().target(t3);
+
+    Assertions.assertThat(i1).isEqualTo(i2).isNotEqualTo(i3).isNotEqualTo(i4);
+
+    Assertions.assertThat(i1.hashCode())
+        .isEqualTo(i2.hashCode())
+        .isNotEqualTo(i3.hashCode())
+        .isNotEqualTo(i4.hashCode());
+
+    Assertions.assertThat(i1.toString())
+        .isEqualTo(i2.toString())
+        .isNotEqualTo(i3.toString())
+        .isNotEqualTo(i4.toString());
+
+    Assertions.assertThat(t1).isNotEqualTo(i1);
+
+    Assertions.assertThat(t1.hashCode()).isEqualTo(i1.hashCode());
+
+    Assertions.assertThat(t1.toString()).isEqualTo(i1.toString());
+  }
+
+  @SuppressWarnings("resource")
+  @Test
+  public void decodeLogicSupportsByteArray() throws Throwable {
+    final byte[] expectedResponse = {12, 34, 56};
+    server.enqueue(new MockResponse().setBody(new Buffer().write(expectedResponse)));
+
+    final OtherTestInterfaceAsync api =
+        newAsyncBuilder()
+            .target(
+                new HardCodedTarget<>(
+                    OtherTestInterfaceAsync.class, "http://localhost:" + server.getPort()));
+
+    Assertions.assertThat(unwrap(api.binaryResponseBody())).containsExactly(expectedResponse);
+  }
+
+  @Test
+  public void encodeLogicSupportsByteArray() throws Exception {
+    final byte[] expectedRequest = {12, 34, 56};
+    server.enqueue(new MockResponse());
+
+    final OtherTestInterfaceAsync api =
+        newAsyncBuilder()
+            .encoder(new Encoder.Default())
+            .target(
+                new HardCodedTarget<>(
+                    OtherTestInterfaceAsync.class, "http://localhost:" + server.getPort()));
+
+    final CompletableFuture<?> cf = api.binaryRequestBody(expectedRequest);
+
+    assertThat(server.takeRequest()).hasBody(expectedRequest);
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void encodedQueryParam() throws Exception {
+    server.enqueue(new MockResponse());
+
+    final TestInterfaceAsync api = newAsyncBuilder().target("http://localhost:" + server.getPort());
+
+    final CompletableFuture<?> cf = api.encodedQueryParam("5.2FSi+");
+
+    assertThat(server.takeRequest()).hasPath("/?trim=5.2FSi%2B");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  private void checkCFCompletedSoon(CompletableFuture<?> cf) {
+    try {
+      unwrap(cf);
+    } catch (final RuntimeException e) {
+      throw e;
+    } catch (final Throwable t) {
+      throw new RuntimeException(t);
+    }
+  }
+
+  @Test
+  public void responseMapperIsAppliedBeforeDelegate() throws IOException {
+    final ResponseMappingDecoder decoder =
+        new ResponseMappingDecoder(upperCaseResponseMapper(), new StringDecoder());
+    final String output = (String) decoder.decode(responseWithText("response"), String.class);
+
+    Assertions.assertThat(output).isEqualTo("RESPONSE");
+  }
+
+  private static TestInterfaceAsyncBuilder newAsyncBuilder() {
+    return new TestInterfaceAsyncBuilder();
+  }
+
+  private ResponseMapper upperCaseResponseMapper() {
+    return new ResponseMapper() {
+      @SuppressWarnings("deprecation")
+      @Override
+      public Response map(Response response, Type type) {
+        try {
+          return response.toBuilder()
+              .body(Util.toString(response.body().asReader()).toUpperCase().getBytes())
+              .build();
+        } catch (final IOException e) {
+          throw new RuntimeException(e);
+        }
+      }
+    };
+  }
+
+  @SuppressWarnings("deprecation")
+  private Response responseWithText(String text) {
+    return Response.builder()
+        .body(text, Util.UTF_8)
+        .status(200)
+        .request(Request.create(HttpMethod.GET, "/api", Collections.emptyMap(), null, Util.UTF_8))
+        .headers(new HashMap<>())
+        .build();
+  }
+
+  @Test
+  public void mapAndDecodeExecutesMapFunction() throws Throwable {
+    server.enqueue(new MockResponse().setBody("response!"));
+
+    final TestInterfaceAsync api =
+        AsyncFeign.asyncBuilder()
+            .mapAndDecode(upperCaseResponseMapper(), new StringDecoder())
+            .target(TestInterfaceAsync.class, "http://localhost:" + server.getPort());
+
+    assertEquals("RESPONSE!", unwrap(api.post()));
+  }
+
+  @Test
+  public void beanQueryMapEncoderWithPrivateGetterIgnored() throws Exception {
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .queryMapEndcoder(new BeanQueryMapEncoder())
+            .target("http://localhost:" + server.getPort());
+
+    final PropertyPojo.ChildPojoClass propertyPojo = new PropertyPojo.ChildPojoClass();
+    propertyPojo.setPrivateGetterProperty("privateGetterProperty");
+    propertyPojo.setName("Name");
+    propertyPojo.setNumber(1);
+
+    server.enqueue(new MockResponse());
+    final CompletableFuture<?> cf = api.queryMapPropertyPojo(propertyPojo);
+    assertThat(server.takeRequest()).hasQueryParams(Arrays.asList("name=Name", "number=1"));
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void queryMap_with_child_pojo() throws Exception {
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .queryMapEndcoder(new FieldQueryMapEncoder())
+            .target("http://localhost:" + server.getPort());
+
+    final ChildPojo childPojo = new ChildPojo();
+    childPojo.setChildPrivateProperty("first");
+    childPojo.setParentProtectedProperty("second");
+    childPojo.setParentPublicProperty("third");
+
+    server.enqueue(new MockResponse());
+    final CompletableFuture<?> cf = api.queryMapPropertyInheritence(childPojo);
+    assertThat(server.takeRequest())
+        .hasQueryParams(
+            "parentPublicProperty=third",
+            "parentProtectedProperty=second",
+            "childPrivateProperty=first");
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void beanQueryMapEncoderWithNullValueIgnored() throws Exception {
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .queryMapEndcoder(new BeanQueryMapEncoder())
+            .target("http://localhost:" + server.getPort());
+
+    final PropertyPojo.ChildPojoClass propertyPojo = new PropertyPojo.ChildPojoClass();
+    propertyPojo.setName(null);
+    propertyPojo.setNumber(1);
+
+    server.enqueue(new MockResponse());
+    final CompletableFuture<?> cf = api.queryMapPropertyPojo(propertyPojo);
+
+    assertThat(server.takeRequest()).hasQueryParams("number=1");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  @Test
+  public void beanQueryMapEncoderWithEmptyParams() throws Exception {
+    final TestInterfaceAsync api =
+        newAsyncBuilder()
+            .queryMapEndcoder(new BeanQueryMapEncoder())
+            .target("http://localhost:" + server.getPort());
+
+    final PropertyPojo.ChildPojoClass propertyPojo = new PropertyPojo.ChildPojoClass();
+
+    server.enqueue(new MockResponse());
+    final CompletableFuture<?> cf = api.queryMapPropertyPojo(propertyPojo);
+    assertThat(server.takeRequest()).hasQueryParams("/");
+
+    checkCFCompletedSoon(cf);
+  }
+
+  public interface TestInterfaceAsync {
+
+    @RequestLine("POST /")
+    CompletableFuture<Response> response();
+
+    @RequestLine("POST /")
+    CompletableFuture<String> post() throws TestInterfaceException;
+
+    @RequestLine("POST /")
+    @Body(
+        "%7B\"customer_name\": \"{customer_name}\", \"user_name\": \"{user_name}\", \"password\":"
+            + " \"{password}\"%7D")
+    CompletableFuture<Void> login(
+        @Param("customer_name") String customer,
+        @Param("user_name") String user,
+        @Param("password") String password);
+
+    @RequestLine("POST /")
+    CompletableFuture<Void> body(List<String> contents);
+
+    @RequestLine("POST /")
+    CompletableFuture<String> body(String content);
+
+    @RequestLine("POST /")
+    CompletableFuture<String> noContent();
+
+    @RequestLine("POST /")
+    @Headers("Content-Encoding: gzip")
+    CompletableFuture<Void> gzipBody(List<String> contents);
+
+    @RequestLine("POST /")
+    @Headers("Content-Encoding: deflate")
+    CompletableFuture<Void> deflateBody(List<String> contents);
+
+    @RequestLine("POST /")
+    CompletableFuture<Void> form(
+        @Param("customer_name") String customer,
+        @Param("user_name") String user,
+        @Param("password") String password);
+
+    @RequestLine("GET /{1}/{2}")
+    CompletableFuture<Response> uriParam(
+        @Param("1") String one, URI endpoint, @Param("2") String two);
+
+    @RequestLine("GET /?1={1}&2={2}")
+    CompletableFuture<Response> queryParams(
+        @Param("1") String one, @Param("2") Iterable<String> twos);
+
+    @RequestLine("POST /?date={date}")
+    CompletableFuture<Void> expand(@Param(value = "date", expander = DateToMillis.class) Date date);
+
+    @RequestLine("GET /?date={date}")
+    CompletableFuture<Void> expandList(
+        @Param(value = "date", expander = DateToMillis.class) List<Date> dates);
+
+    @RequestLine("GET /?date={date}")
+    CompletableFuture<Void> expandArray(
+        @Param(value = "date", expander = DateToMillis.class) Date[] dates);
+
+    @RequestLine("GET /")
+    CompletableFuture<Void> headerMap(@HeaderMap Map<String, Object> headerMap);
+
+    @RequestLine("GET /")
+    @Headers("Content-Encoding: deflate")
+    CompletableFuture<Void> headerMapWithHeaderAnnotations(
+        @HeaderMap Map<String, Object> headerMap);
+
+    @RequestLine("GET /")
+    CompletableFuture<Void> queryMap(@QueryMap Map<String, Object> queryMap);
+
+    @RequestLine("GET /")
+    CompletableFuture<Void> queryMapEncoded(@QueryMap(encoded = true) Map<String, Object> queryMap);
+
+    @RequestLine("GET /?name={name}")
+    CompletableFuture<Void> queryMapWithQueryParams(
+        @Param("name") String name, @QueryMap Map<String, Object> queryMap);
+
+    @RequestLine("GET /?trim={trim}")
+    CompletableFuture<Void> encodedQueryParam(@Param(value = "trim", encoded = true) String trim);
+
+    @RequestLine("GET /")
+    CompletableFuture<Void> queryMapPojo(@QueryMap CustomPojo object);
+
+    @RequestLine("GET /")
+    CompletableFuture<Void> queryMapPropertyPojo(@QueryMap PropertyPojo object);
+
+    @RequestLine("GET /")
+    CompletableFuture<Void> queryMapPropertyInheritence(@QueryMap ChildPojo object);
+
+    class DateToMillis implements Param.Expander {
+
+      @Override
+      public String expand(Object value) {
+        return String.valueOf(((Date) value).getTime());
+      }
+    }
+  }
+
+  class TestInterfaceException extends Exception {
+    private static final long serialVersionUID = 1L;
+
+    TestInterfaceException(String message) {
+      super(message);
+    }
+  }
+
+  public interface OtherTestInterfaceAsync {
+
+    @RequestLine("POST /")
+    CompletableFuture<String> post();
+
+    @RequestLine("POST /")
+    CompletableFuture<byte[]> binaryResponseBody();
+
+    @RequestLine("POST /")
+    CompletableFuture<Void> binaryRequestBody(byte[] contents);
+  }
+
+  static class ForwardedForInterceptor implements RequestInterceptor {
+
+    @Override
+    public void apply(RequestTemplate template) {
+      template.header("X-Forwarded-For", "origin.host.com");
+    }
+  }
+
+  static class UserAgentInterceptor implements RequestInterceptor {
+
+    @Override
+    public void apply(RequestTemplate template) {
+      template.header("User-Agent", "Feign");
+    }
+  }
+
+  static class IllegalArgumentExceptionOn400 extends ErrorDecoder.Default {
+
+    @Override
+    public Exception decode(String methodKey, Response response) {
+      if (response.status() == 400) {
+        return new IllegalArgumentException("bad zone name");
+      }
+      return super.decode(methodKey, response);
+    }
+  }
+
+  static class IllegalArgumentExceptionOn404 extends ErrorDecoder.Default {
+
+    @Override
+    public Exception decode(String methodKey, Response response) {
+      if (response.status() == 404) {
+        return new IllegalArgumentException("bad zone name");
+      }
+      return super.decode(methodKey, response);
+    }
+  }
+
+  static final class TestInterfaceAsyncBuilder {
+
+    private final AsyncFeign.AsyncBuilder<Object> delegate =
+        AsyncFeign.asyncBuilder()
+            .client(new Http2Client())
+            .decoder(new Decoder.Default())
+            .encoder(
+                new Encoder() {
+
+                  @SuppressWarnings("deprecation")
+                  @Override
+                  public void encode(Object object, Type bodyType, RequestTemplate template) {
+                    if (object instanceof Map) {
+                      template.body(new Gson().toJson(object));
+                    } else {
+                      template.body(object.toString());
+                    }
+                  }
+                });
+
+    TestInterfaceAsyncBuilder requestInterceptor(RequestInterceptor requestInterceptor) {
+      delegate.requestInterceptor(requestInterceptor);
+      return this;
+    }
+
+    TestInterfaceAsyncBuilder encoder(Encoder encoder) {
+      delegate.encoder(encoder);
+      return this;
+    }
+
+    TestInterfaceAsyncBuilder decoder(Decoder decoder) {
+      delegate.decoder(decoder);
+      return this;
+    }
+
+    TestInterfaceAsyncBuilder errorDecoder(ErrorDecoder errorDecoder) {
+      delegate.errorDecoder(errorDecoder);
+      return this;
+    }
+
+    TestInterfaceAsyncBuilder decode404() {
+      delegate.decode404();
+      return this;
+    }
+
+    TestInterfaceAsyncBuilder queryMapEndcoder(QueryMapEncoder queryMapEncoder) {
+      delegate.queryMapEncoder(queryMapEncoder);
+      return this;
+    }
+
+    TestInterfaceAsync target(String url) {
+      return delegate.target(TestInterfaceAsync.class, url);
+    }
+
+    <T> T target(Target<T> target) {
+      return delegate.target(target);
+    }
+  }
+
+  static final class ExtendedCF<T> extends CompletableFuture<T> {}
+
+  abstract static class NonInterface {
+    @RequestLine("GET /")
+    abstract CompletableFuture<Void> x();
+  }
+
+  interface NonCFApi {
+    @RequestLine("GET /")
+    void x();
+  }
+
+  interface ExtendedCFApi {
+    @RequestLine("GET /")
+    ExtendedCF<Void> x();
+  }
+
+  interface LowerWildApi {
+    @RequestLine("GET /")
+    CompletableFuture<? extends Object> x();
+  }
+
+  interface UpperWildApi {
+    @RequestLine("GET /")
+    CompletableFuture<? super Object> x();
+  }
+
+  interface WildApi {
+    @RequestLine("GET /")
+    CompletableFuture<?> x();
+  }
+}