Metric API (#1181)

* Add metrics support to feign

* Introduced capabilities to hystrix

* Addressing PR comments

core/src/main/java/feign/Capability.java

@@ -0,0 +1,115 @@
+/**
+ * Copyright 2012-2020 The Feign Authors
+ *
+ * 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
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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;
+
+import java.lang.reflect.InvocationTargetException;
+import java.util.Arrays;
+import java.util.List;
+import feign.Logger.Level;
+import feign.Request.Options;
+import feign.codec.Decoder;
+import feign.codec.Encoder;
+
+/**
+ * Capabilities expose core feign artifacts to implementations so parts of core can be customized
+ * around the time the client being built.
+ *
+ * For instance, capabilities take the {@link Client}, make changes to it and feed the modified
+ * version back to feign.
+ *
+ * @see Metrics5Capability
+ */
+public interface Capability {
+
+
+  static <E> E enrich(E componentToEnrich, List<Capability> capabilities) {
+    return capabilities.stream()
+        // invoke each individual capability and feed the result to the next one.
+        // This is equivalent to:
+        // Capability cap1 = ...;
+        // Capability cap2 = ...;
+        // Capability cap2 = ...;
+        // Contract contract = ...;
+        // Contract contract1 = cap1.enrich(contract);
+        // Contract contract2 = cap2.enrich(contract1);
+        // Contract contract3 = cap3.enrich(contract2);
+        // or in a more compact version
+        // Contract enrichedContract = cap3.enrich(cap2.enrich(cap1.enrich(contract)));
+        .reduce(
+            componentToEnrich,
+            (component, capability) -> invoke(component, capability),
+            (component, enrichedComponent) -> enrichedComponent);
+  }
+
+  static <E> E invoke(E target, Capability capability) {
+    return Arrays.stream(capability.getClass().getMethods())
+        .filter(method -> method.getName().equals("enrich"))
+        .filter(method -> method.getReturnType().isInstance(target))
+        .findFirst()
+        .map(method -> {
+          try {
+            return (E) method.invoke(capability, target);
+          } catch (IllegalAccessException | IllegalArgumentException
+              | InvocationTargetException e) {
+            throw new RuntimeException("Unable to enrich " + target, e);
+          }
+        })
+        .orElse(target);
+  }
+
+  default Client enrich(Client client) {
+    return client;
+  }
+
+  default Retryer enrich(Retryer retryer) {
+    return retryer;
+  }
+
+  default RequestInterceptor enrich(RequestInterceptor requestInterceptor) {
+    return requestInterceptor;
+  }
+
+  default Logger enrich(Logger logger) {
+    return logger;
+  }
+
+  default Level enrich(Level level) {
+    return level;
+  }
+
+  default Contract enrich(Contract contract) {
+    return contract;
+  }
+
+  default Options enrich(Options options) {
+    return options;
+  }
+
+  default Encoder enrich(Encoder encoder) {
+    return encoder;
+  }
+
+  default Decoder enrich(Decoder decoder) {
+    return decoder;
+  }
+
+  default InvocationHandlerFactory enrich(InvocationHandlerFactory invocationHandlerFactory) {
+    return invocationHandlerFactory;
+  }
+
+  default QueryMapEncoder enrich(QueryMapEncoder queryMapEncoder) {
+    return queryMapEncoder;
+  }
+
+}

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

@@ -18,6 +18,8 @@
 import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.stream.Collectors;
+import feign.Logger.Level;
 import feign.Logger.NoOpLogger;
 import feign.ReflectiveFeign.ParseHandlersByName;
 import feign.Request.Options;
@@ -113,6 +115,7 @@ public static class Builder {
     private boolean closeAfterDecode = true;
     private ExceptionPropagationPolicy propagationPolicy = NONE;
     private boolean forceDecoding = false;
+    private List<Capability> capabilities = new ArrayList<>();
 
     public Builder logLevel(Logger.Level logLevel) {
       this.logLevel = logLevel;
@@ -245,6 +248,11 @@ public Builder exceptionPropagationPolicy(ExceptionPropagationPolicy propagation
       return this;
     }
 
+    public Builder addCapability(Capability capability) {
+      this.capabilities.add(capability);
+      return this;
+    }
+
     /**
      * Internal - used to indicate that the decoder should be immediately called
      */
@@ -262,6 +270,20 @@ public <T> T target(Target<T> target) {
     }
 
     public Feign build() {
+      Client client = Capability.enrich(this.client, capabilities);
+      Retryer retryer = Capability.enrich(this.retryer, capabilities);
+      List<RequestInterceptor> requestInterceptors = this.requestInterceptors.stream()
+          .map(ri -> Capability.enrich(ri, capabilities))
+          .collect(Collectors.toList());
+      Logger logger = Capability.enrich(this.logger, capabilities);
+      Contract contract = Capability.enrich(this.contract, capabilities);
+      Options options = Capability.enrich(this.options, capabilities);
+      Encoder encoder = Capability.enrich(this.encoder, capabilities);
+      Decoder decoder = Capability.enrich(this.decoder, capabilities);
+      InvocationHandlerFactory invocationHandlerFactory =
+          Capability.enrich(this.invocationHandlerFactory, capabilities);
+      QueryMapEncoder queryMapEncoder = Capability.enrich(this.queryMapEncoder, capabilities);
+
       SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
           new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
               logLevel, decode404, closeAfterDecode, propagationPolicy, forceDecoding);

core/src/test/java/feign/CapabilityTest.java

@@ -0,0 +1,69 @@
+/**
+ * Copyright 2012-2020 The Feign Authors
+ *
+ * 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
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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;
+
+import static org.hamcrest.CoreMatchers.nullValue;
+import static org.junit.Assert.assertThat;
+import org.hamcrest.CoreMatchers;
+import org.junit.Test;
+import java.io.IOException;
+import java.util.Arrays;
+import feign.Request.Options;
+
+public class CapabilityTest {
+
+  private class AClient implements Client {
+
+    public AClient(Client client) {}
+
+    @Override
+    public Response execute(Request request, Options options) throws IOException {
+      return null;
+    }
+
+  }
+  private class BClient implements Client {
+
+    public BClient(Client client) {
+      if (!(client instanceof AClient)) {
+        throw new RuntimeException("Test is chaing invokations, expected AClient instace here");
+      }
+    }
+
+    @Override
+    public Response execute(Request request, Options options) throws IOException {
+      return null;
+    }
+
+  }
+
+  @Test
+  public void enrichClient() {
+    Client enriched = Capability.enrich(new Client.Default(null, null), Arrays.asList(
+        new Capability() {
+          @Override
+          public Client enrich(Client client) {
+            return new AClient(client);
+          }
+        }, new Capability() {
+          @Override
+          public Client enrich(Client client) {
+            return new BClient(client);
+          }
+        }));
+
+    assertThat(enriched, CoreMatchers.instanceOf(BClient.class));
+  }
+
+}

hystrix/src/main/java/feign/hystrix/HystrixCapability.java

@@ -0,0 +1,60 @@
+/**
+ * Copyright 2012-2020 The Feign Authors
+ *
+ * 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
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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.hystrix;
+
+import com.netflix.hystrix.HystrixCommand;
+import java.util.HashMap;
+import java.util.Map;
+import feign.Capability;
+import feign.Contract;
+import feign.InvocationHandlerFactory;
+
+/**
+ * Allows Feign interfaces to return HystrixCommand or rx.Observable or rx.Single objects. Also
+ * decorates normal Feign methods with circuit breakers, but calls {@link HystrixCommand#execute()}
+ * directly.
+ */
+public final class HystrixCapability implements Capability {
+
+  private SetterFactory setterFactory = new SetterFactory.Default();
+  private final Map<Class, Object> fallbacks = new HashMap<>();
+
+  /**
+   * Allows you to override hystrix properties such as thread pools and command keys.
+   */
+  public HystrixCapability setterFactory(SetterFactory setterFactory) {
+    this.setterFactory = setterFactory;
+    return this;
+  }
+
+  @Override
+  public Contract enrich(Contract contract) {
+    return new HystrixDelegatingContract(contract);
+  }
+
+  @Override
+  public InvocationHandlerFactory enrich(InvocationHandlerFactory invocationHandlerFactory) {
+    return (target, dispatch) -> new HystrixInvocationHandler(target, dispatch, setterFactory,
+        fallbacks.containsKey(target.type())
+            ? new FallbackFactory.Default<>(fallbacks.get(target.type()))
+            : null);
+  }
+
+  public <E> Capability fallback(Class<E> api, E fallback) {
+    fallbacks.put(api, fallback);
+
+    return this;
+  }
+
+}