Add IDefaultValueProviderExtension

to make EmptyOrDummyResponse extensible for type that need special
handling.

build.gradle

@@ -129,6 +129,9 @@ subprojects {
       groovy {
         srcDir "src/${ss.name}-groovy-le-$gv/groovy"
       }
+      resources {
+        srcDir "src/${ss.name}-groovy-le-$gv/resources"
+      }
     }
     for (jv in javaVersions.findAll { javaVersion <= it }) {
       java {
@@ -137,6 +140,9 @@ subprojects {
       groovy {
         srcDir "src/${ss.name}-java-le-$jv/groovy"
       }
+      resources {
+        srcDir "src/${ss.name}-java-le-$jv/resources"
+      }
     }
     for (gv in variants.findAll { variant >= it }) {
       java {
@@ -145,6 +151,9 @@ subprojects {
       groovy {
         srcDir "src/${ss.name}-groovy-ge-$gv/groovy"
       }
+      resources {
+        srcDir "src/${ss.name}-groovy-ge-$gv/resources"
+      }
     }
     for (jv in javaVersions.findAll { javaVersion >= it }) {
       java {
@@ -153,6 +162,9 @@ subprojects {
       groovy {
         srcDir "src/${ss.name}-java-ge-$jv/groovy"
       }
+      resources {
+        srcDir "src/${ss.name}-java-ge-$jv/resources"
+      }
     }
   }
 

docs/extensions.adoc

@@ -1354,3 +1354,20 @@ The extension generates the following output for the example code above.
 ----
 include::{sourcedir}/extension/ExtensionStoreSpec.groovy[tag=example-result]
 ----
+
+
+[[default-value-provider]]
+=== Default Value Provider Extensions
+
+Stubs use the `EmptyOrDummyResponse` to return a default value for a method call.
+If the `EmptyOrDummyResponse` doesn't have specific instructions for a type it will return a new `Stub` for that type.
+This works for most cases, but can lead to unexpected behavior if the type does have a behavioral contract that is not fulfilled by the `Stub`.
+Another problematic case is when the type is `final` or `sealed` as those are not mockable by default.
+
+To address these issues, Spock 2.4 introduces the `org.spockframework.runtime.extension.IDefaultValueProviderExtension` that is loaded via Java's `ServiceLoader` mechanism.
+This extension allows you to provide a default value if the `EmptyOrDummyResponse` doesn't have specific instructions for a type.
+
+It is primarily for framework developers who want to provide a default value for their framework types.
+Or users of a framework that doesn't provide default values for their special types.
+
+If you want to change the default response behavior for `Stub` have a look at <<interaction_based_testing.adoc#ALaCarteMocks,A la Carte Mocks>> and how to use your own `org.spockframework.mock.IDefaultResponse`.

spock-core/src/main/java/org/spockframework/mock/EmptyOrDummyResponse.java

@@ -14,6 +14,7 @@
 
 package org.spockframework.mock;
 
+import org.spockframework.runtime.extension.IDefaultValueProviderExtension;
 import org.spockframework.util.ReflectionUtil;
 import org.spockframework.util.ThreadSafe;
 import spock.lang.Specification;
@@ -35,8 +36,16 @@
 @ThreadSafe
 public class EmptyOrDummyResponse implements IDefaultResponse {
   public static final EmptyOrDummyResponse INSTANCE = new EmptyOrDummyResponse();
+  private final List<IDefaultValueProviderExtension> defaultValueProviders;
 
-  private EmptyOrDummyResponse() {}
+  private EmptyOrDummyResponse() {
+    ServiceLoader<IDefaultValueProviderExtension> serviceLoader = ServiceLoader.load(IDefaultValueProviderExtension.class);
+    List<IDefaultValueProviderExtension> providers = new ArrayList<>();
+    for (IDefaultValueProviderExtension provider : serviceLoader) {
+      providers.add(provider);
+    }
+    defaultValueProviders = Collections.unmodifiableList(providers);
+  }
 
   @Override
   @SuppressWarnings("rawtypes")
@@ -72,6 +81,8 @@ public Object respond(IMockInvocation invocation) {
       if (returnType == IntStream.class) return IntStream.empty();
       if (returnType == DoubleStream.class) return DoubleStream.empty();
       if (returnType == LongStream.class) return LongStream.empty();
+      Object providedValue = askDefaultValueProviders(invocation);
+      if (providedValue != null) return providedValue;
       return createDummy(invocation);
     }
 
@@ -98,6 +109,9 @@ public Object respond(IMockInvocation invocation) {
     Object emptyWrapper = createEmptyWrapper(returnType);
     if (emptyWrapper != null) return emptyWrapper;
 
+    Object providedValue = askDefaultValueProviders(invocation);
+    if (providedValue != null) return providedValue;
+
     Object emptyObject = createEmptyObject(returnType);
     if (emptyObject != null) return emptyObject;
 
@@ -128,6 +142,19 @@ private Object createEmptyObject(Class<?> type) {
     }
   }
 
+  private Object askDefaultValueProviders(IMockInvocation invocation) {
+    if (defaultValueProviders.isEmpty()) {
+      return null;
+    }
+    Class<?> returnType = invocation.getMethod().getReturnType();
+    Type exactReturnType = invocation.getMethod().getExactReturnType();
+    return defaultValueProviders.stream()
+        .map(provider -> provider.provideDefaultValue(returnType, exactReturnType))
+        .filter(Objects::nonNull)
+        .findFirst()
+        .orElse(null);
+  }
+
   private Object createDummy(IMockInvocation invocation) {
     Class<?> type = invocation.getMethod().getReturnType();
     Type genericType = invocation.getMethod().getExactReturnType();

spock-core/src/main/java/org/spockframework/runtime/extension/IDefaultValueProviderExtension.java

@@ -0,0 +1,51 @@
+/*
+ *  Copyright 2024 the original author or 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
+ *      https://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 org.spockframework.runtime.extension;
+
+import java.lang.reflect.Type;
+
+import org.spockframework.mock.IMockMethod;
+import org.spockframework.util.Beta;
+import org.spockframework.util.Nullable;
+import org.spockframework.util.ThreadSafe;
+
+/**
+ * Allows to enhance {@link org.spockframework.mock.EmptyOrDummyResponse} with custom default values.
+ * <p>
+ * Will be instantiated via the {@link java.util.ServiceLoader} mechanism.
+ * <p>
+ * Implementations must be thread-safe and should not have any state.
+ * <p>
+ * This extension is intended for framework authors and users who want to provide default values for their own types.
+ * The extension will only be called if no sensible default value can be provided by the default mechanism.
+ * If you want to change the default behavior for all types, you should implement a custom {@link org.spockframework.mock.IDefaultResponse}.
+ *
+ * @since 2.4
+ * @see org.spockframework.mock.EmptyOrDummyResponse
+ * @see org.spockframework.mock.IDefaultResponse
+ */
+@Beta
+@ThreadSafe
+public interface IDefaultValueProviderExtension {
+  /**
+   * Provides a default value for the given type.
+   *
+   * @param type the type for which a default value should be provided, see {@link IMockMethod#getReturnType()}
+   * @param exactType the exact type for which a default value should be provided, see {@link IMockMethod#getExactReturnType()}
+   * @return the value or null if no default value can be provided
+   */
+  @Nullable
+  Object provideDefaultValue(Class<?> type, Type exactType);
+}

spock-specs/src/test-java-ge-17/java/org/spockframework/smoke/mock/IEither.java

@@ -0,0 +1,31 @@
+/*
+ *  Copyright 2024 the original author or 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
+ *      https://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 org.spockframework.smoke.mock;
+
+public sealed interface IEither<L, R> {
+  record Left<L, R>(L value) implements IEither<L, R> {
+  }
+
+  record Right<L, R>(R value) implements IEither<L, R> {
+  }
+
+  static <L, R> IEither<L, R> left(L value) {
+    return new Left<>(value);
+  }
+
+  static <L, R> IEither<L, R> right(R value) {
+    return new Right<>(value);
+  }
+}

spock-specs/src/test-java-ge-17/java/org/spockframework/smoke/mock/IMaybe.java

@@ -0,0 +1,32 @@
+/*
+ *  Copyright 2024 the original author or 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
+ *      https://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 org.spockframework.smoke.mock;
+
+public sealed interface IMaybe<T> {
+
+  record Some<T>(T value) implements IMaybe<T> {
+  }
+
+  record None<T>() implements IMaybe<T> {
+  }
+
+  static <T> IMaybe<T> some(T value) {
+    return new Some<>(value);
+  }
+
+  static <T> IMaybe<T> none() {
+    return new None<>();
+  }
+}

spock-specs/src/test-java-ge-17/java/org/spockframework/smoke/mock/MaybeDefaultValueProvider.java

@@ -0,0 +1,30 @@
+/*
+ *  Copyright 2024 the original author or 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
+ *      https://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 org.spockframework.smoke.mock;
+
+import java.lang.reflect.Type;
+
+import org.spockframework.runtime.extension.IDefaultValueProviderExtension;
+import org.spockframework.util.Nullable;
+
+public class MaybeDefaultValueProvider implements IDefaultValueProviderExtension {
+  @Override
+  public @Nullable Object provideDefaultValue(Class<?> type, Type exactType) {
+    if (type == IMaybe.class) {
+      return IMaybe.none();
+    }
+    return null;
+  }
+}

spock-specs/src/test-java-ge-17/resources/META-INF/services/org.spockframework.runtime.extension.IDefaultValueProviderExtension

@@ -0,0 +1,2 @@
+org.spockframework.mock.FinalClassDefaultValueProvider
+org.spockframework.smoke.mock.MaybeDefaultValueProvider

spock-specs/src/test-java-le-11/java/org/spockframework/smoke/mock/IEither.java

@@ -0,0 +1,49 @@
+/*
+ *  Copyright 2024 the original author or 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
+ *      https://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 org.spockframework.smoke.mock;
+
+public interface IEither<L, R> {
+  class Left<L, R> implements IEither<L, R> {
+    private final L value;
+
+    public Left(L value) {
+      this.value = value;
+    }
+
+    public L value() {
+      return value;
+    }
+  }
+
+  class Right<L, R> implements IEither<L, R> {
+    private final R value;
+
+    public Right(R value) {
+      this.value = value;
+    }
+
+    public R value() {
+      return value;
+    }
+  }
+
+  static <L, R> IEither<L, R> left(L value) {
+    return new Left<>(value);
+  }
+
+  static <L, R> IEither<L, R> right(R value) {
+    return new Right<>(value);
+  }
+}

spock-specs/src/test-java-le-11/java/org/spockframework/smoke/mock/IMaybe.java

@@ -0,0 +1,41 @@
+/*
+ *  Copyright 2024 the original author or 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
+ *      https://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 org.spockframework.smoke.mock;
+
+public interface IMaybe<T> {
+
+  class Some<T> implements IMaybe<T> {
+    private final T value;
+
+    Some(T value) {
+      this.value = value;
+    }
+
+    public T getValue() {
+      return value;
+    }
+  }
+
+  class None<T> implements IMaybe<T> {
+  }
+
+  static <T> IMaybe<T> some(T value) {
+    return new Some<>(value);
+  }
+
+  static <T> IMaybe<T> none() {
+    return new None<>();
+  }
+}

spock-specs/src/test-java-le-11/java/org/spockframework/smoke/mock/MaybeDefaultValueProvider.java

@@ -0,0 +1,30 @@
+/*
+ *  Copyright 2024 the original author or 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
+ *      https://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 org.spockframework.smoke.mock;
+
+import java.lang.reflect.Type;
+
+import org.spockframework.runtime.extension.IDefaultValueProviderExtension;
+import org.spockframework.util.Nullable;
+
+public class MaybeDefaultValueProvider implements IDefaultValueProviderExtension {
+  @Override
+  public @Nullable Object provideDefaultValue(Class<?> type, Type exactType) {
+    if (type == IMaybe.class) {
+      return IMaybe.none();
+    }
+    return null;
+  }
+}