fix(mock): throw on interaction with null target

docs/release_notes.adoc

@@ -24,6 +24,7 @@ include::include.adoc[]
 
 === Breaking Changes
 
+* Declaring an interaction on a `null` target now throws `InvalidSpecException` instead of being silently ignored (or failing with an unrelated NPE for some mock makers) spockIssue:2339[]
 * Mock/Stub checks on `Comparable<T>` with `T` being something other than `Object` now compare using the java identity hash code instead of always being equal spockIssue:2352[]
 
 == 2.4 (2025-12-11)

spock-core/src/main/java/org/spockframework/mock/constraint/TargetConstraint.java

@@ -31,6 +31,9 @@ public class TargetConstraint implements IInvocationConstraint, IInteractionAwar
   private IMockInteraction interaction;
 
   public TargetConstraint(Object target) {
+    if (target == null) {
+      throw new InvalidSpecException("Interaction on a null object is invalid");
+    }
     this.target = target;
   }
 

spock-specs/src/test/groovy/org/spockframework/mock/runtime/mockito/MockitoMockMakerSpec.groovy

@@ -32,6 +32,7 @@ import org.spockframework.mock.CannotCreateMockException
 import org.spockframework.mock.MockUtil
 import org.spockframework.mock.runtime.ByteBuddyTestClassLoader
 import org.spockframework.runtime.GroovyRuntimeUtil
+import org.spockframework.runtime.InvalidSpecException
 
 import static spock.mock.MockMakers.mockito
 
@@ -516,17 +517,19 @@ Can not mock final classes with the following settings :
     mockUtil.isMock(m)
   }
 
-  @Issue("https://github.com/spockframework/spock/issues/2337")
-  def "NPE when stubbing method on null object"() {
+  @Issue(["https://github.com/spockframework/spock/issues/2337", "https://github.com/spockframework/spock/issues/2339"])
+  def "stubbing method on null object does not NPE and reports InvalidSpecException"() {
     given:
     Object nullObj = null
 
     when:
-    //Issue #2337: MockitoMockMaker passes null to Mockito's getHandler() throws NPE
+    // #2337: MockitoMockMaker must not pass null to Mockito's getHandler()
+    // #2339: null target interactions are rejected with a clear error
     nullObj.toString() >> ""
 
     then:
-    noExceptionThrown()
+    def ex = thrown(InvalidSpecException)
+    ex.message == "Interaction on a null object is invalid"
   }
 }
 

spock-specs/src/test/groovy/org/spockframework/smoke/mock/NullTargetInteraction.groovy

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2026 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 org.spockframework.runtime.InvalidSpecException
+import spock.lang.Issue
+import spock.lang.Specification
+
+@Issue("https://github.com/spockframework/spock/issues/2339")
+class NullTargetInteraction extends Specification {
+  def "interaction on null target throws InvalidSpecException"() {
+    given:
+    Object nullObj = null
+
+    when:
+    nullObj.toString() >> ""
+
+    then:
+    def ex = thrown(InvalidSpecException)
+    ex.message == "Interaction on a null object is invalid"
+  }
+
+  def "required interaction on null target throws InvalidSpecException"() {
+    given:
+    List nullList = null
+
+    when:
+    1 * nullList.size()
+
+    then:
+    def ex = thrown(InvalidSpecException)
+    ex.message == "Interaction on a null object is invalid"
+  }
+
+  def "interaction on real mock is still accepted"() {
+    given:
+    List list = Mock()
+
+    when:
+    list.size() >> 0
+
+    then:
+    notThrown(InvalidSpecException)
+    list.size() == 0
+  }
+}