graphql-java
diff --git a/.github/workflows/master.yml b/.github/workflows/master.yml
@@ -17,15 +17,18 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: gradle/actions/wrapper-validation@v4
-      - name: Set up JDK 11
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
-          java-version: '11'
+          java-version: '21'
           distribution: 'corretto'
       - name: build test and publish
         run: ./gradlew assemble && ./gradlew check --info && ./gradlew publishToSonatype closeAndReleaseSonatypeStagingRepository -x check --info --stacktrace
       - name: Publish Test Results
         uses: EnricoMi/publish-unit-test-result-action@v2.20.0
         if: always()
         with:
-          files: '**/build/test-results/test/TEST-*.xml'
+          files: |
+            **/build/test-results/test/TEST-*.xml
+            **/build/test-results/testWithJava11/TEST-*.xml
+            **/build/test-results/testWithJava17/TEST-*.xml
diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
@@ -21,10 +21,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: gradle/actions/wrapper-validation@v4
-      - name: Set up JDK 11
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
-          java-version: '11'
+          java-version: '21'
           distribution: 'corretto'
       - name: build and test
         run: ./gradlew assemble && ./gradlew check --info --stacktrace

diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
@@ -19,10 +19,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: gradle/actions/wrapper-validation@v4
-      - name: Set up JDK 11
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
-          java-version: '11'
+          java-version: '21'
           distribution: 'corretto'
       - name: build test and publish
         run: ./gradlew assemble && ./gradlew check --info && ./gradlew publishToSonatype closeAndReleaseSonatypeStagingRepository -x check --info --stacktrace
diff --git a/build.gradle b/build.gradle
@@ -1,5 +1,8 @@
-import java.text.SimpleDateFormat
+import net.ltgt.gradle.errorprone.CheckSeverity
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 
+import java.text.SimpleDateFormat
 
 plugins {
     id 'java'
@@ -12,11 +15,28 @@ plugins {
     id "io.github.gradle-nexus.publish-plugin" version "2.0.0"
     id "groovy"
     id "me.champeau.jmh" version "0.7.3"
+    id "net.ltgt.errorprone" version '4.2.0'
+    //
+    // Kotlin just for tests - not production code
+    id 'org.jetbrains.kotlin.jvm' version '2.1.21'
 }
 
 java {
     toolchain {
-        languageVersion = JavaLanguageVersion.of(11)
+        languageVersion = JavaLanguageVersion.of(21) // build on 21 - release on 11
+    }
+}
+
+kotlin {
+    compilerOptions {
+        apiVersion = KotlinVersion.KOTLIN_2_0
+        languageVersion = KotlinVersion.KOTLIN_2_0
+        jvmTarget = JvmTarget.JVM_11
+        javaParameters = true
+        freeCompilerArgs = [
+                '-Xemit-jvm-type-annotations',
+                '-Xjspecify-annotations=strict',
+        ]
     }
 }
 
@@ -97,19 +117,15 @@ jar {
         attributes('Automatic-Module-Name': 'com.graphqljava')
     }
 }
-tasks.withType(GroovyCompile) {
-    // Options when compiling Java using the Groovy plugin.
-    // (Groovy itself defaults to UTF-8 for Groovy code)
-    options.encoding = 'UTF-8'
-    groovyOptions.forkOptions.memoryMaximumSize = "4g"
-}
+
 dependencies {
-    implementation 'org.antlr:antlr4-runtime:' + antlrVersion
     api 'com.graphql-java:java-dataloader:5.0.0'
     api 'org.reactivestreams:reactive-streams:' + reactiveStreamsVersion
     api "org.jspecify:jspecify:1.0.0"
-    antlr 'org.antlr:antlr4:' + antlrVersion
+
+    implementation 'org.antlr:antlr4-runtime:' + antlrVersion
     implementation 'com.google.guava:guava:' + guavaVersion
+
     testImplementation group: 'junit', name: 'junit', version: '4.13.2'
     testImplementation 'org.spockframework:spock-core:2.3-groovy-4.0'
     testImplementation 'net.bytebuddy:byte-buddy:1.17.5'
@@ -129,9 +145,17 @@ dependencies {
     testImplementation 'org.testng:testng:7.11.0' // use for reactive streams test inheritance
     testImplementation "com.tngtech.archunit:archunit-junit5:1.4.1"
 
+    antlr 'org.antlr:antlr4:' + antlrVersion
+
     // this is needed for the idea jmh plugin to work correctly
     jmh 'org.openjdk.jmh:jmh-core:1.37'
     jmh 'org.openjdk.jmh:jmh-generator-annprocess:1.37'
+
+    errorprone 'com.uber.nullaway:nullaway:0.12.6'
+    errorprone 'com.google.errorprone:error_prone_core:2.37.0'
+
+    // just tests - no Kotlin otherwise
+    testCompileOnly 'org.jetbrains.kotlin:kotlin-stdlib-jdk8'
 }
 
 shadowJar {
@@ -218,6 +242,36 @@ compileJava {
     source file("build/generated-src"), sourceSets.main.java
 }
 
+tasks.withType(GroovyCompile) {
+    // Options when compiling Java using the Groovy plugin.
+    // (Groovy itself defaults to UTF-8 for Groovy code)
+    options.encoding = 'UTF-8'
+    sourceCompatibility = '11'
+    targetCompatibility = '11'
+    groovyOptions.forkOptions.memoryMaximumSize = "4g"
+}
+
+tasks.withType(JavaCompile) {
+    options.release = 11
+    options.errorprone {
+        disableAllChecks = true
+        check("NullAway", CheckSeverity.ERROR)
+        //
+        // end state has us with this config turned on - eg all classes
+        //
+        //option("NullAway:AnnotatedPackages", "graphql")
+        option("NullAway:CustomContractAnnotations", "graphql.Contract")
+        option("NullAway:OnlyNullMarked", "true")
+        option("NullAway:JSpecifyMode", "true")
+    }
+    // Include to disable NullAway on test code
+    if (name.toLowerCase().contains("test")) {
+        options.errorprone {
+            disable("NullAway")
+        }
+    }
+}
+
 generateGrammarSource {
     includes = ['Graphql.g4']
     maxHeapSize = "64m"
@@ -253,6 +307,7 @@ artifacts {
 List<TestDescriptor> failedTests = []
 
 test {
+    useJUnitPlatform()
     testLogging {
         events "FAILED", "SKIPPED"
         exceptionFormat = "FULL"
@@ -265,6 +320,43 @@ test {
     }
 }
 
+tasks.register('testWithJava17', Test) {
+    javaLauncher = javaToolchains.launcherFor {
+        languageVersion = JavaLanguageVersion.of(17)
+    }
+    useJUnitPlatform()
+    testLogging {
+        events "FAILED", "SKIPPED"
+        exceptionFormat = "FULL"
+    }
+
+    afterTest { TestDescriptor descriptor, TestResult result ->
+        if (result.getFailedTestCount() > 0) {
+            failedTests.add(descriptor)
+        }
+    }
+
+}
+tasks.register('testWithJava11', Test) {
+    javaLauncher = javaToolchains.launcherFor {
+        languageVersion = JavaLanguageVersion.of(11)
+    }
+    useJUnitPlatform()
+    testLogging {
+        events "FAILED", "SKIPPED"
+        exceptionFormat = "FULL"
+    }
+
+    afterTest { TestDescriptor descriptor, TestResult result ->
+        if (result.getFailedTestCount() > 0) {
+            failedTests.add(descriptor)
+        }
+    }
+}
+test.dependsOn testWithJava17
+test.dependsOn testWithJava11
+
+
 /*
  * The gradle.buildFinished callback is deprecated BUT there does not seem to be a decent alternative in gradle 7
  * So progress over perfection here
@@ -374,6 +466,5 @@ tasks.withType(GenerateModuleMetadata) {
     enabled = false
 }
 
-test {
-    useJUnitPlatform()
-}
+
+
diff --git a/src/main/java/graphql/Assert.java b/src/main/java/graphql/Assert.java
@@ -1,6 +1,7 @@
 package graphql;
 
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 import java.util.Collection;
 import java.util.function.Supplier;
@@ -20,71 +21,82 @@ public static <T> T assertNotNullWithNPE(T object, Supplier<String> msg) {
         throw new NullPointerException(msg.get());
     }
 
-    public static <T> T assertNotNull(T object) {
+    @Contract("null -> fail")
+    public static <T> T assertNotNull(@Nullable T object) {
         if (object != null) {
             return object;
         }
         return throwAssert("Object required to be not null");
     }
 
-    public static <T> T assertNotNull(T object, Supplier<String> msg) {
+    @Contract("null,_ -> fail")
+    public static <T> T assertNotNull(@Nullable T object, Supplier<String> msg) {
         if (object != null) {
             return object;
         }
         return throwAssert(msg.get());
     }
 
-    public static <T> T assertNotNull(T object, String constantMsg) {
+    @Contract("null,_ -> fail")
+    public static <T> T assertNotNull(@Nullable T object, String constantMsg) {
         if (object != null) {
             return object;
         }
         return throwAssert(constantMsg);
     }
 
-    public static <T> T assertNotNull(T object, String msgFmt, Object arg1) {
+    @Contract("null,_,_ -> fail")
+    public static <T> T assertNotNull(@Nullable T object, String msgFmt, Object arg1) {
         if (object != null) {
             return object;
         }
         return throwAssert(msgFmt, arg1);
     }
 
-    public static <T> T assertNotNull(T object, String msgFmt, Object arg1, Object arg2) {
+    @Contract("null,_,_,_ -> fail")
+    public static <T> T assertNotNull(@Nullable T object, String msgFmt, Object arg1, Object arg2) {
         if (object != null) {
             return object;
         }
         return throwAssert(msgFmt, arg1, arg2);
     }
 
-    public static <T> T assertNotNull(T object, String msgFmt, Object arg1, Object arg2, Object arg3) {
+    @Contract("null,_,_,_,_ -> fail")
+    public static <T> T assertNotNull(@Nullable T object, String msgFmt, Object arg1, Object arg2, Object arg3) {
         if (object != null) {
             return object;
         }
         return throwAssert(msgFmt, arg1, arg2, arg3);
     }
 
 
-    public static <T> void assertNull(T object, Supplier<String> msg) {
+    @Contract("!null,_ -> fail")
+    public static <T> void assertNull(@Nullable T object, Supplier<String> msg) {
         if (object == null) {
             return;
         }
         throwAssert(msg.get());
     }
 
-    public static <T> void assertNull(T object) {
+    @Contract("!null -> fail")
+    public static <T> void assertNull(@Nullable Object object) {
         if (object == null) {
             return;
         }
         throwAssert("Object required to be null");
     }
 
+    @Contract("-> fail")
     public static <T> T assertNeverCalled() {
         return throwAssert("Should never been called");
     }
 
+    @Contract("_,_-> fail")
     public static <T> T assertShouldNeverHappen(String format, Object... args) {
         return throwAssert("Internal error: should never happen: %s", format(format, args));
     }
 
+    @Contract("-> fail")
     public static <T> T assertShouldNeverHappen() {
         return throwAssert("Internal error: should never happen");
     }
@@ -96,6 +108,7 @@ public static <T> Collection<T> assertNotEmpty(Collection<T> collection) {
         return collection;
     }
 
+    //    @Contract("null,_-> fail")
     public static <T> Collection<T> assertNotEmpty(Collection<T> collection, Supplier<String> msg) {
         if (collection == null || collection.isEmpty()) {
             throwAssert(msg.get());

diff --git a/src/main/java/graphql/Contract.java b/src/main/java/graphql/Contract.java
@@ -0,0 +1,27 @@
+package graphql;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Target;
+
+/**
+ * Custom contract annotation used for jspecify and NullAway checks.
+ *
+ * This is the same as Spring does: we don't want any additional dependencies, therefore we define our own Contract annotation.
+ *
+ * @see <a href="https://raw.githubusercontent.com/spring-projects/spring-framework/refs/heads/main/spring-core/src/main/java/org/springframework/lang/Contract.java">Spring Framework Contract</a>
+ * @see <a href="https://github.com/JetBrains/java-annotations/blob/master/src/jvmMain/java/org/jetbrains/annotations/Contract.java">org.jetbrains.annotations.Contract</a>
+ * @see <a href="https://github.com/uber/NullAway/wiki/Configuration#custom-contract-annotations">
+ * NullAway custom contract annotations</a>
+ */
+@Documented
+@Target(ElementType.METHOD)
+@Internal
+public @interface Contract {
+
+    /**
+     * Describing the contract between call arguments and the returned value.
+     */
+    String value() default "";
+
+}