Now does not overwrite a types default data fetcher unless its nonnull

src/main/java/graphql/schema/idl/RuntimeWiring.java

@@ -116,10 +116,31 @@ public Map<String, Map<String, DataFetcher>> getDataFetchers() {
         return dataFetchers;
     }
 
+    /**
+     * This is deprecated because the name has the wrong plural case.
+     *
+     * @param typeName the type for fetch a map of per field data fetchers for
+     *
+     * @return a map of field data fetchers for a type
+     *
+     * @deprecated See {@link #getDataFetchersForType(String)}
+     */
+    @Deprecated(since = "2024-04-28")
     public Map<String, DataFetcher> getDataFetcherForType(String typeName) {
         return dataFetchers.computeIfAbsent(typeName, k -> new LinkedHashMap<>());
     }
 
+    /**
+     * This returns a map of the data fetchers per field on that named type.
+     *
+     * @param typeName the type for fetch a map of per field data fetchers for
+     *
+     * @return a map of field data fetchers for a type
+     */
+    public Map<String, DataFetcher> getDataFetchersForType(String typeName) {
+        return dataFetchers.computeIfAbsent(typeName, k -> new LinkedHashMap<>());
+    }
+
     public DataFetcher getDefaultDataFetcherForType(String typeName) {
         return defaultDataFetchers.get(typeName);
     }
@@ -284,7 +305,10 @@ public Builder type(TypeRuntimeWiring typeRuntimeWiring) {
             }
             typeDataFetchers.putAll(typeRuntimeWiring.getFieldDataFetchers());
 
-            defaultDataFetchers.put(typeName, typeRuntimeWiring.getDefaultDataFetcher());
+            DataFetcher<?> defaultDataFetcher = typeRuntimeWiring.getDefaultDataFetcher();
+            if (defaultDataFetcher != null) {
+                defaultDataFetchers.put(typeName, defaultDataFetcher);
+            }
 
             TypeResolver typeResolver = typeRuntimeWiring.getTypeResolver();
             if (typeResolver != null) {

src/main/java/graphql/schema/idl/SchemaGeneratorHelper.java

@@ -839,7 +839,7 @@ private DataFetcherFactory<?> buildDataFetcherFactory(BuildContext buildCtx,
                 dataFetcher = wiringFactory.getDataFetcher(wiringEnvironment);
                 assertNotNull(dataFetcher, () -> "The WiringFactory indicated it provides a data fetcher but then returned null");
             } else {
-                dataFetcher = runtimeWiring.getDataFetcherForType(parentTypeName).get(fieldName);
+                dataFetcher = runtimeWiring.getDataFetchersForType(parentTypeName).get(fieldName);
                 if (dataFetcher == null) {
                     dataFetcher = runtimeWiring.getDefaultDataFetcherForType(parentTypeName);
                     if (dataFetcher == null) {

src/main/java/graphql/schema/idl/TypeRuntimeWiring.java

@@ -184,6 +184,9 @@ private void assertFieldStrictly(String fieldName) {
          */
         public Builder defaultDataFetcher(DataFetcher dataFetcher) {
             assertNotNull(dataFetcher);
+            if (strictMode && defaultDataFetcher != null) {
+                throw new StrictModeWiringException(format("The type %s has already has a default data fetcher defined", typeName));
+            }
             defaultDataFetcher = dataFetcher;
             return this;
         }

src/test/groovy/graphql/schema/idl/RuntimeWiringTest.groovy

@@ -195,6 +195,7 @@ class RuntimeWiringTest extends Specification {
 
     def "strict mode can stop certain redefinitions"() {
         DataFetcher DF1 = env -> "x"
+        DataFetcher DF2 = env -> "x"
         TypeResolver TR1 = env -> null
         EnumValuesProvider EVP1 = name -> null
 
@@ -236,5 +237,45 @@ class RuntimeWiringTest extends Specification {
         def e4 = thrown(StrictModeWiringException)
         e4.message == "The scalar String is already defined"
 
+        when:
+        TypeRuntimeWiring.newTypeWiring("Foo")
+                .strictMode()
+                .defaultDataFetcher(DF1)
+                .defaultDataFetcher(DF2)
+
+        then:
+        def e5 = thrown(StrictModeWiringException)
+        e5.message == "The type Foo has already has a default data fetcher defined"
+    }
+
+    def "dont over default data fetchers if they are null"() {
+
+        DataFetcher DF1 = env -> "x"
+        DataFetcher DF2 = env -> "x"
+        DataFetcher DEFAULT_DF = env -> null
+        DataFetcher DEFAULT_DF2 = env -> null
+
+        when:
+        def runtimeWiring = RuntimeWiring.newRuntimeWiring()
+                .type(TypeRuntimeWiring.newTypeWiring("Foo").defaultDataFetcher(DEFAULT_DF))
+                .type(TypeRuntimeWiring.newTypeWiring("Foo").dataFetcher("foo", DF1))
+                .type(TypeRuntimeWiring.newTypeWiring("Foo").dataFetcher("bar", DF2))
+                .build()
+
+        then:
+        runtimeWiring.getDefaultDataFetcherForType("Foo") == DEFAULT_DF
+
+        when:
+        runtimeWiring = RuntimeWiring.newRuntimeWiring()
+                .type(TypeRuntimeWiring.newTypeWiring("Foo").defaultDataFetcher(DEFAULT_DF))
+                .type(TypeRuntimeWiring.newTypeWiring("Foo").dataFetcher("foo", DF1))
+                .type(TypeRuntimeWiring.newTypeWiring("Foo").dataFetcher("bar", DF2))
+                // we can specifically overwrite it later
+                .type(TypeRuntimeWiring.newTypeWiring("Foo").defaultDataFetcher(DEFAULT_DF2))
+                .build()
+
+        then:
+        runtimeWiring.getDefaultDataFetcherForType("Foo") == DEFAULT_DF2
+
     }
 }