Allow parameters with custom annotations in service methods

retrofit/src/main/java/retrofit2/RequestFactory.java

@@ -126,7 +126,10 @@ okhttp3.Request create(Object[] args) throws IOException {
     List<Object> argumentList = new ArrayList<>(argumentCount);
     for (int p = 0; p < argumentCount; p++) {
       argumentList.add(args[p]);
-      handlers[p].apply(requestBuilder, args[p]);
+      ParameterHandler<Object> handler = handlers[p];
+      if (handler != null) {
+        handler.apply(requestBuilder, args[p]);
+      }
     }
 
     return requestBuilder.get().tag(Invocation.class, new Invocation(method, argumentList)).build();
@@ -347,7 +350,9 @@ private Headers parseHeaders(String[] headers) {
           } catch (NoClassDefFoundError ignored) {
           }
         }
-        throw parameterError(method, p, "No Retrofit annotation found.");
+        if (annotations == null || annotations.length == 0) {
+          throw parameterError(method, p, "No annotation found.");
+        }
       }
 
       return result;

retrofit/src/test/java/retrofit2/RequestFactoryTest.java

@@ -15,13 +15,15 @@
  */
 package retrofit2;
 
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
 import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.fail;
 
 import java.io.IOException;
+import java.lang.annotation.Retention;
 import java.lang.reflect.Method;
 import java.math.BigInteger;
 import java.net.URI;
@@ -465,7 +467,7 @@ Call<ResponseBody> method(String a) {
     } catch (IllegalArgumentException e) {
       assertThat(e)
           .hasMessage(
-              "No Retrofit annotation found. (parameter #1)\n    for method Example.method");
+              "No annotation found. (parameter #1)\n    for method Example.method");
     }
   }
 
@@ -3252,6 +3254,24 @@ Call<ResponseBody> method(@Tag List<String> one, @Tag List<Long> two) {
     }
   }
 
+  @Retention(RUNTIME)
+  @interface CustomAnnotation {}
+
+  @Test
+  public void parameterWithCustomAnnotation() {
+    class Example {
+      @GET("/") //
+      Call<ResponseBody> method(@CustomAnnotation String a) {
+        return null;
+      }
+    }
+    Request request = buildRequest(Example.class, "CustomArgument");
+    assertThat(request.url().toString()).isEqualTo("http://example.com/");
+    Invocation invocation = request.tag(Invocation.class);
+    assertThat(invocation.arguments().get(0)).isEqualTo("CustomArgument");
+    assertThat(invocation.method().getParameterAnnotations()[0][0]).isInstanceOf(CustomAnnotation.class);
+  }
+
   private static void assertBody(RequestBody body, String expected) {
     assertThat(body).isNotNull();
     Buffer buffer = new Buffer();