Support kotlin coroutines (#1706)

* Support kotlin coroutines

Resolves: #1565

Inspired by PlaytikaOSS/feign-reactive#486

## TODO

- [ ] Separate Kotlin support module
- [ ] Enhance test case
- [ ] Refactoring
- [ ] Clean up pom.xml

* Apply optional dependency to kotlin support related dependency

* Seperate Kotlin support module

* Remove unused code from ClassUtils.java

* Remove unused code from ClassUtils.java

* Refactor KotlinDetector

* Move ClassUtils location into KotlinDetector

* Move KotlinDetector location

* Format code

* First attempt to move kotlin work to it's own isolated module

* Coroutine Feign using AyncFeign

* Coroutine Feign using AyncFeign

* Refactor suspending function  detect logic

- Remove KotlinDetector.java
- Add Method.isSuspend extension function

* Cleanup CoroutineFeignTest test code format

* Fix suspend function contract parsing error when using http body

* Rename test names to be meaningful

* Add Github Example With Coroutine

- Copy of GithubExample

* Remove unnecessary dependency

https://github.com/OpenFeign/feign/pull/1706/files#r965389041

Co-authored-by: Marvin Froeder <velo.br@gmail.com>
Co-authored-by: Marvin Froeder <velo@users.noreply.github.com>

core/pom.xml

@@ -54,7 +54,6 @@
     <dependency>
       <groupId>com.fasterxml.jackson.core</groupId>
       <artifactId>jackson-databind</artifactId>
-      <version>${jackson.version}</version>
       <scope>test</scope>
     </dependency>
 

core/src/main/java/feign/AsyncInvocation.java

@@ -20,45 +20,45 @@
  * A specific invocation of an APU
  */
 @Experimental
-class AsyncInvocation<C> {
+public class AsyncInvocation<C> {
 
   private final C context;
   private final MethodInfo methodInfo;
   private final long startNanos;
   private CompletableFuture<Response> responseFuture;
 
-  AsyncInvocation(C context, MethodInfo methodInfo) {
+  public AsyncInvocation(C context, MethodInfo methodInfo) {
     super();
     this.context = context;
     this.methodInfo = methodInfo;
     this.startNanos = System.nanoTime();
   }
 
-  C context() {
+  public C context() {
     return context;
   }
 
-  String configKey() {
+  public String configKey() {
     return methodInfo.configKey();
   }
 
-  long startNanos() {
+  public long startNanos() {
     return startNanos;
   }
 
-  Type underlyingType() {
+  public Type underlyingType() {
     return methodInfo.underlyingReturnType();
   }
 
-  boolean isAsyncReturnType() {
+  public boolean isAsyncReturnType() {
     return methodInfo.isAsyncReturnType();
   }
 
-  void setResponseFuture(CompletableFuture<Response> responseFuture) {
+  public void setResponseFuture(CompletableFuture<Response> responseFuture) {
     this.responseFuture = responseFuture;
   }
 
-  CompletableFuture<Response> responseFuture() {
+  public CompletableFuture<Response> responseFuture() {
     return responseFuture;
   }
 }

core/src/main/java/feign/AsyncResponseHandler.java

@@ -27,7 +27,7 @@
  * handling
  */
 @Experimental
-class AsyncResponseHandler {
+public class AsyncResponseHandler {
 
   private static final long MAX_RESPONSE_BUFFER_SIZE = 8192L;
 
@@ -41,8 +41,9 @@ class AsyncResponseHandler {
 
   private final ResponseInterceptor responseInterceptor;
 
-  AsyncResponseHandler(Level logLevel, Logger logger, Decoder decoder, ErrorDecoder errorDecoder,
-      boolean dismiss404, boolean closeAfterDecode, ResponseInterceptor responseInterceptor) {
+  public AsyncResponseHandler(Level logLevel, Logger logger, Decoder decoder,
+      ErrorDecoder errorDecoder, boolean dismiss404, boolean closeAfterDecode,
+      ResponseInterceptor responseInterceptor) {
     super();
     this.logLevel = logLevel;
     this.logger = logger;
@@ -54,14 +55,15 @@ class AsyncResponseHandler {
   }
 
   boolean isVoidType(Type returnType) {
-    return Void.class == returnType || void.class == returnType;
+    return Void.class == returnType || void.class == returnType
+        || returnType.getTypeName().equals("kotlin.Unit");
   }
 
-  void handleResponse(CompletableFuture<Object> resultFuture,
-                      String configKey,
-                      Response response,
-                      Type returnType,
-                      long elapsedTime) {
+  public void handleResponse(CompletableFuture<Object> resultFuture,
+                             String configKey,
+                             Response response,
+                             Type returnType,
+                             long elapsedTime) {
     // copied fairly liberally from SynchronousMethodHandler
     boolean shouldClose = true;
 

core/src/main/java/feign/Contract.java

@@ -130,6 +130,10 @@ protected MethodMetadata parseAndValidateMetadata(Class<?> targetType, Method me
           data.ignoreParamater(i);
         }
 
+        if ("kotlin.coroutines.Continuation".equals(parameterTypes[i].getName())) {
+          data.ignoreParamater(i);
+        }
+
         if (parameterTypes[i] == URI.class) {
           data.urlIndex(i);
         } else if (!isHttpAnnotation

core/src/main/java/feign/Feign.java

@@ -192,7 +192,7 @@ public Builder addCapability(Capability capability) {
     /**
      * Internal - used to indicate that the decoder should be immediately called
      */
-    Builder forceDecoding() {
+    public /* FIXME should not be public */ Builder forceDecoding() {
       this.forceDecoding = true;
       return this;
     }

core/src/main/java/feign/Logger.java

@@ -13,14 +13,16 @@
  */
 package feign;
 
+import static feign.Util.UTF_8;
+import static feign.Util.decodeOrDefault;
+import static feign.Util.valuesOrEmpty;
+import static java.util.Objects.nonNull;
 import java.io.IOException;
 import java.io.PrintWriter;
 import java.io.StringWriter;
 import java.util.logging.FileHandler;
 import java.util.logging.LogRecord;
 import java.util.logging.SimpleFormatter;
-import static feign.Util.*;
-import static java.util.Objects.nonNull;
 
 /**
  * Simple logging abstraction for debug messages. Adapted from {@code retrofit.RestAdapter.Log}.
@@ -137,10 +139,10 @@ protected Response logAndRebufferResponse(String configKey,
     return response;
   }
 
-  protected IOException logIOException(String configKey,
-                                       Level logLevel,
-                                       IOException ioe,
-                                       long elapsedTime) {
+  public IOException logIOException(String configKey,
+                                    Level logLevel,
+                                    IOException ioe,
+                                    long elapsedTime) {
     log(configKey, "<--- ERROR %s: %s (%sms)", ioe.getClass().getSimpleName(), ioe.getMessage(),
         elapsedTime);
     if (logLevel.ordinal() >= Level.FULL.ordinal()) {
@@ -217,7 +219,7 @@ public JavaLogger() {
 
     /**
      * Constructor for JavaLogger class
-     * 
+     *
      * @param loggerName a name for the logger. This should be a dot-separated name and should
      *        normally be based on the package name or class name of the subsystem, such as java.net
      *        or javax.swing

core/src/main/java/feign/MethodInfo.java

@@ -19,12 +19,12 @@
 import java.util.concurrent.CompletableFuture;
 
 @Experimental
-class MethodInfo {
+public class MethodInfo {
   private final String configKey;
   private final Type underlyingReturnType;
   private final boolean asyncReturnType;
 
-  MethodInfo(String configKey, Type underlyingReturnType, boolean asyncReturnType) {
+  protected MethodInfo(String configKey, Type underlyingReturnType, boolean asyncReturnType) {
     this.configKey = configKey;
     this.underlyingReturnType = underlyingReturnType;
     this.asyncReturnType = asyncReturnType;

core/src/main/java/feign/Types.java

@@ -199,7 +199,7 @@ static Type getSupertype(Type context, Class<?> contextRawType, Class<?> superty
         getGenericSupertype(context, contextRawType, supertype));
   }
 
-  static Type resolve(Type context, Class<?> contextRawType, Type toResolve) {
+  public static Type resolve(Type context, Class<?> contextRawType, Type toResolve) {
     // This implementation is made a little more complicated in an attempt to avoid object-creation.
     while (true) {
       if (toResolve instanceof TypeVariable) {
@@ -350,14 +350,17 @@ static final class ParameterizedTypeImpl implements ParameterizedType {
       }
     }
 
+    @Override
     public Type[] getActualTypeArguments() {
       return typeArguments.clone();
     }
 
+    @Override
     public Type getRawType() {
       return rawType;
     }
 
+    @Override
     public Type getOwnerType() {
       return ownerType;
     }
@@ -395,6 +398,7 @@ private static final class GenericArrayTypeImpl implements GenericArrayType {
       this.componentType = componentType;
     }
 
+    @Override
     public Type getGenericComponentType() {
       return componentType;
     }
@@ -454,10 +458,12 @@ static final class WildcardTypeImpl implements WildcardType {
       }
     }
 
+    @Override
     public Type[] getUpperBounds() {
       return new Type[] {upperBound};
     }
 
+    @Override
     public Type[] getLowerBounds() {
       return lowerBound != null ? new Type[] {lowerBound} : EMPTY_TYPE_ARRAY;
     }

example-github-with-coroutine/README.md

@@ -0,0 +1,10 @@
+GitHub Example With Coroutine
+===================
+
+This is an example of a simple json client.
+
+=== Building example with Gradle
+Install and run `gradle` to produce `build/github`
+
+=== Building example with Maven
+Install and run `mvn` to produce `target/github`