Fix regression: if null is passed as parameter to a resolver method, use Java default value

Author: BURJA Lucian

src/main/kotlin/graphql/kickstart/tools/resolver/MethodFieldResolver.kt

@@ -80,7 +80,7 @@ internal class MethodFieldResolver(
                     return@add Optional.empty<Any>()
                 }
 
-                if (value != null && shouldValueBeConverted(value, definition, parameterType, environment)) {
+                if (value == null || shouldValueBeConverted(value, definition, parameterType, environment)) {
                     return@add mapper.convertValue(value, object : TypeReference<Any>() {
                         override fun getType() = parameterType
                     })

src/test/kotlin/graphql/kickstart/tools/ResolverMethodsTest.java

@@ -0,0 +1,52 @@
+package graphql.kickstart.tools;
+
+import graphql.ExecutionInput;
+import graphql.ExecutionResult;
+import graphql.GraphQL;
+import graphql.schema.GraphQLSchema;
+import org.junit.Test;
+
+import java.util.Map;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class ResolverMethodsTest {
+    // Note: don't convert this code to Kotlin or Groovy, since it's quite important that the
+    //       resolver method is defined with an argument of primitive type, like 'boolean', not 'Boolean':
+    //         String testOmittedBoolean(boolean value1, Boolean value2)
+    @Test
+    public void testOmittedBooleanArgument() {
+        // In this schema, the 'value1' argument is optional, but the Java resolver defines it as 'boolean'
+        // Instead of failing with an error, we expect the argument to be set to the Java default (i.e. false for booleans)
+        GraphQLSchema schema = SchemaParser.newParser()
+                .schemaString("" +
+                        "type Query {" +
+                        "   testOmittedBoolean(value1: Boolean, value2: Boolean): String" +
+                        "}")
+                .resolvers(new Resolver())
+                .build()
+                .makeExecutableSchema();
+
+        GraphQL gql = GraphQL.newGraphQL(schema).build();
+
+        ExecutionResult result = gql
+                .execute(ExecutionInput.newExecutionInput()
+                        .query("" +
+                                "query { " +
+                                "  testOmittedBoolean" +
+                                "}")
+                        .context(new Object())
+                        .root(new Object()));
+
+        assertTrue(result.getErrors().isEmpty());
+        assertEquals("false,null", ((Map<?, ?>) result.getData()).get("testOmittedBoolean"));
+    }
+
+    static class Resolver implements GraphQLQueryResolver {
+        @SuppressWarnings("unused")
+        public String testOmittedBoolean(boolean value1, Boolean value2) {
+            return value1 + "," + value2;
+        }
+    }
+}