Issue #39: Should use DataFetcher specialized constructor

src/main/java/graphql/annotations/GraphQLAnnotations.java

@@ -60,6 +60,9 @@
 import static graphql.schema.GraphQLInterfaceType.newInterface;
 import static graphql.schema.GraphQLObjectType.newObject;
 import static graphql.schema.GraphQLUnionType.newUnionType;
+import static java.util.Arrays.stream;
+import static java.util.Objects.isNull;
+import static java.util.Objects.nonNull;
 
 /**
  * A utility class for extracting GraphQL data structures from annotated
@@ -313,7 +316,19 @@ protected GraphQLFieldDefinition getField(Field field) throws GraphQLAnnotations
         }
 
         GraphQLDataFetcher dataFetcher = field.getAnnotation(GraphQLDataFetcher.class);
-        DataFetcher actualDataFetcher = dataFetcher != null ? newInstance(dataFetcher.value()) : null;
+        DataFetcher actualDataFetcher = null;
+        if (nonNull(dataFetcher)) {
+            final String[] args = dataFetcher.args();
+            if (args.length == 0) {
+                actualDataFetcher = newInstance(dataFetcher.value());
+            } else {
+                try {
+                    final Constructor<? extends DataFetcher> ctr = dataFetcher.value().getDeclaredConstructor(
+                      stream(args).map(v -> String.class).toArray(Class[]::new));
+                    actualDataFetcher = constructNewInstance(ctr, (Object[]) args);
+                } catch (final NoSuchMethodException e) {}
+            }
+        }
 
         if (actualDataFetcher == null) {
 

src/main/java/graphql/annotations/GraphQLDataFetcher.java

@@ -25,4 +25,5 @@
 @Retention(RetentionPolicy.RUNTIME)
 public @interface GraphQLDataFetcher {
     Class<? extends DataFetcher> value();
+    String[] args() default {};
 }

src/test/java/graphql/annotations/GraphQLDataFetcherTest.java

@@ -0,0 +1,83 @@
+/**
+ * Copyright 2016 Yurii Rashkovskii
+ *
+ * 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
+ */
+package graphql.annotations;
+
+import java.util.HashMap;
+
+import graphql.ExecutionResult;
+import graphql.GraphQL;
+import graphql.schema.DataFetcher;
+import graphql.schema.DataFetchingEnvironment;
+import graphql.schema.GraphQLObjectType;
+import graphql.schema.GraphQLSchema;
+import graphql.schema.PropertyDataFetcher;
+import org.testng.annotations.Test;
+
+import static graphql.schema.GraphQLSchema.newSchema;
+import static org.testng.Assert.assertNotNull;
+import static org.testng.Assert.assertTrue;
+
+public class GraphQLDataFetcherTest {
+
+  @Test
+  public void shouldUsePreferredConstructor() {
+    // Given
+    final GraphQLObjectType object = GraphQLAnnotations.object(GraphQLDataFetcherTest.TestGraphQLQuery.class);
+    final GraphQLSchema schema = newSchema().query(object).build();
+    final GraphQL graphql = new GraphQL(schema);
+
+    // When
+    final ExecutionResult result = graphql.execute("{sample {isGreat}}");
+
+    // Then
+    final HashMap<String, Object> data = (HashMap) result.getData();
+    assertNotNull(data);
+    assertTrue(((HashMap<String,Boolean>)data.get("sample")).get("isGreat"));
+  }
+
+  @GraphQLName("Query")
+  public static class TestGraphQLQuery {
+    @GraphQLField
+    @GraphQLDataFetcher(SampleDataFetcher.class)
+    public TestSample sample() { // Note that GraphQL uses TestSample to build the graph
+      return null;
+    }
+  }
+
+  public static class TestSample {
+    @GraphQLField
+    @GraphQLDataFetcher(value = PropertyDataFetcher.class, args = "isGreat")
+    private Boolean isGreat = false; // Defaults to FieldDataFetcher
+  }
+
+  public static class SampleDataFetcher implements DataFetcher {
+    @Override
+    public Object get(final DataFetchingEnvironment environment) {
+      return new Sample(); // Notice that it return a Sample, not a TestSample
+    }
+  }
+
+  public static class Sample {
+    private Boolean isGreat = true;
+
+    public Boolean getIsGreat() {
+      return isGreat;
+    }
+
+    public void setIsGreat(final Boolean isGreat) {
+      this.isGreat = isGreat;
+    }
+  }
+}