diff --git a/build.xml b/build.xml
index 9228d17..cec038a 100644
--- a/build.xml
+++ b/build.xml
@@ -111,6 +111,9 @@
   <property name="maven-plugin-plugin.version" value="3.4"/>
   <property name="maven-compiler-plugin.version" value="3.8.1"/>
   
+  <!-- extra properties (prefixed by "gradle." added to gradle.properties during testing: -->
+  <property name="gradle.org.gradle.warning.mode" value="all"/>
+  
   <!-- with fork=false this somehow takes endless to download the internet, so let's fork - and slow down in an other way (startup) -->
   <property name="maven.fork" value="true"/>
   
diff --git a/src/main/groovy/de/thetaphi/forbiddenapis/gradle/ForbiddenApisPlugin.groovy b/src/main/groovy/de/thetaphi/forbiddenapis/gradle/ForbiddenApisPlugin.groovy
index abc8185..9192406 100644
--- a/src/main/groovy/de/thetaphi/forbiddenapis/gradle/ForbiddenApisPlugin.groovy
+++ b/src/main/groovy/de/thetaphi/forbiddenapis/gradle/ForbiddenApisPlugin.groovy
@@ -46,9 +46,12 @@ public class ForbiddenApisPlugin extends ForbiddenApisPluginBase {
       group = JavaBasePlugin.VERIFICATION_GROUP;
     }
 
+    // retrieve Java Extension, if it is not available fallback to project convention:
+    def javaExtension = project.extensions.findByName('java') ?: project
+
     // Gradle is buggy with it's JavaVersion enum: We use majorVersion property before Java 11 (6,7,8,9,10) and for later we use toString() to be future-proof:
-    Closure targetCompatibilityGetter = { (project.targetCompatibility?.hasProperty('java11Compatible') && project.targetCompatibility?.java11Compatible) ?
-        project.targetCompatibility.toString() : project.targetCompatibility?.majorVersion };
+    Closure targetCompatibilityGetter = { (javaExtension.targetCompatibility?.hasProperty('java11Compatible') && javaExtension.targetCompatibility?.java11Compatible) ?
+        javaExtension.targetCompatibility.toString() : javaExtension.targetCompatibility?.majorVersion };
 
     // Define our tasks (one for each SourceSet):
     project.sourceSets.all{ sourceSet ->
diff --git a/src/test/gradle/build.gradle b/src/test/gradle/build.gradle
index 75e09bc..71c0c0a 100644
--- a/src/test/gradle/build.gradle
+++ b/src/test/gradle/build.gradle
@@ -29,7 +29,16 @@ buildscript {
 apply plugin: 'java'
 apply plugin: 'de.thetaphi.forbiddenapis'
 
-sourceCompatibility = forbiddenSourceCompatibility
+if (project.extensions.findByName('java')) {
+
+  java {
+    sourceCompatibility = JavaVersion.toVersion(forbiddenSourceCompatibility)
+  }
+  
+} else {
+  // TODO: Remove this after Gradle 4.10
+  sourceCompatibility = JavaVersion.toVersion(forbiddenSourceCompatibility)
+}
 
 sourceSets {
   main {