Allows @param value in default contract to be taken from param names.

JEP 118 introduced javac option `-parameters` that adds to bytecode
method parameter names, so if code is compiled with that option
and @param value is empty we can get that template parameter name
from method parameter name.

Fixes #1297.

Author: krzyk

README.md

@@ -107,7 +107,7 @@ should work.  Feign's default contract defines the following annotations:
 | Annotation     | Interface Target | Usage |
 |----------------|------------------|-------|
 | `@RequestLine` | Method           | Defines the `HttpMethod` and `UriTemplate` for request.  `Expressions`, values wrapped in curly-braces `{expression}` are resolved using their corresponding `@Param` annotated parameters. |
-| `@Param`       | Parameter        | Defines a template variable, whose value will be used to resolve the corresponding template `Expression`, by name. |
+| `@Param`       | Parameter        | Defines a template variable, whose value will be used to resolve the corresponding template `Expression`, by name provided as annotation value. If value is missing it will try to get the name from bytecode method parameter name (if the code was compiled with `-parameters` flag). |
 | `@Headers`     | Method, Type     | Defines a `HeaderTemplate`; a variation on a `UriTemplate`.  that uses `@Param` annotated values to resolve the corresponding `Expressions`.  When used on a `Type`, the template will be applied to every request.  When used on a `Method`, the template will apply only to the annotated method. |
 | `@QueryMap`    | Parameter        | Defines a `Map` of name-value pairs, or POJO, to expand into a query string. |
 | `@HeaderMap`   | Parameter        | Defines a `Map` of name-value pairs, to expand into `Http Headers` |

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

@@ -271,7 +271,14 @@ public Default() {
         data.template().headers(toMap(headersOnMethod));
       });
       super.registerParameterAnnotation(Param.class, (paramAnnotation, data, paramIndex) -> {
-        final String name = paramAnnotation.value();
+        final String annotationName = paramAnnotation.value();
+        final Parameter parameter = data.method().getParameters()[paramIndex];
+        final String name;
+        if (emptyToNull(annotationName) == null && parameter.isNamePresent()) {
+          name = parameter.getName();
+        } else {
+          name = annotationName;
+        }
         checkState(emptyToNull(name) != null, "Param annotation was empty on param %s.",
             paramIndex);
         nameParam(data, name, paramIndex);

core/src/main/java/feign/Param.java

@@ -28,7 +28,7 @@
   /**
    * The name of the template parameter.
    */
-  String value();
+  String value() default "";
 
   /**
    * How to expand the value of this parameter, if {@link ToStringExpander} isn't adequate.

core/src/test/java/feign/DefaultContractTest.java

@@ -200,6 +200,21 @@ public void pathAndQueryParams() throws Exception {
         entry(2, asList("type")));
   }
 
+  @Test
+  public void autoDiscoverParamNames() throws Exception {
+    final MethodMetadata md = parseAndValidateMetadata(AutoDiscoverParamNames.class,
+        "recordsByNameAndType", int.class, String.class,
+        String.class);
+
+    assertThat(md.template())
+        .hasQueries(entry("name", asList("{name}")), entry("type", asList("{type}")));
+
+    assertThat(md.indexToName()).containsExactly(
+        entry(0, asList("domainId")),
+        entry(1, asList("name")),
+        entry(2, asList("type")));
+  }
+
   @Test
   public void bodyWithTemplate() throws Exception {
     final MethodMetadata md = parseAndValidateMetadata(FormParams.class,
@@ -517,6 +532,12 @@ Response recordsByNameAndType(@Param("domainId") int id,
                                   @Param("type") String typeFilter);
   }
 
+  interface AutoDiscoverParamNames {
+
+    @RequestLine("GET /domains/{domainId}/records?name={name}&type={type}")
+    Response recordsByNameAndType(@Param int domainId, @Param String name, @Param() String type);
+  }
+
   interface FormParams {
 
     @RequestLine("POST /")

pom.xml

@@ -417,6 +417,21 @@
               <target>${main.java.version}</target>
             </configuration>
           </execution>
+          <execution>
+            <id>default-test-compile</id>
+            <phase>test-compile</phase>
+            <goals>
+              <goal>testCompile</goal>
+            </goals>
+            <configuration>
+              <fork>true</fork>
+              <compilerArgs>
+                <arg>-parameters</arg>
+              </compilerArgs>
+              <source>${main.java.version}</source>
+              <target>${main.java.version}</target>
+            </configuration>
+          </execution>
         </executions>
       </plugin>