Fix GJF (and detekt) on Java 16+

plugin-maven/src/test/java/com/diffplug/spotless/maven/MavenIntegrationHarness.java

@@ -41,6 +41,7 @@
 
 import com.diffplug.common.base.Unhandled;
 import com.diffplug.common.io.Resources;
+import com.diffplug.spotless.Jvm;
 import com.diffplug.spotless.ResourceHarness;
 
 public class MavenIntegrationHarness extends ResourceHarness {
@@ -81,6 +82,17 @@ public class MavenIntegrationHarness extends ResourceHarness {
 	@BeforeEach
 	void gitAttributes() throws IOException {
 		setFile(".gitattributes").toContent("* text eol=lf");
+		if (Jvm.version() >= 16) {
+			// for GJF https://github.com/diffplug/spotless/issues/834
+			setFile(".mvn/jvm.config").toContent(
+					"--add-exports jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED" +
+							" --add-exports jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED" +
+							" --add-exports jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED" +
+							" --add-exports jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED" +
+							" --add-exports jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED" +
+							// this last line is for Detekt
+							" --add-opens java.base/java.lang=ALL-UNNAMED");
+		}
 		// copy the mvnw resources
 		copy("mvnw").setExecutable(true);
 		copy("mvnw.cmd");

testlib/build.gradle

@@ -21,7 +21,20 @@ dependencies {
 // we'll hold the testlib to a low standard (prize brevity)
 spotbugs { reportLevel = 'high' } // low|medium|high (low = sensitive to even minor mistakes)
 
-test { useJUnitPlatform() }
+test {
+	useJUnitPlatform()
+	if (JavaVersion.current().isCompatibleWith(JavaVersion.VERSION_16)) {
+		// for GJF https://github.com/diffplug/spotless/issues/834
+		def args = [
+			'--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED',
+			'--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED',
+			'--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED',
+			'--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED',
+			'--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED'
+		]
+		jvmArgs args
+	}
+}
 
 apply from: rootProject.file('gradle/special-tests.gradle')
 

testlib/src/test/java/com/diffplug/spotless/java/GoogleJavaFormatStepTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2021 DiffPlug
+ * Copyright 2016-2022 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 
 import static org.junit.jupiter.api.condition.JRE.JAVA_11;
 import static org.junit.jupiter.api.condition.JRE.JAVA_13;
+import static org.junit.jupiter.api.condition.JRE.JAVA_15;
 
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
@@ -41,7 +42,7 @@ void jvm13Features() throws Exception {
 	}
 
 	@Test
-	@EnabledForJreRange(min = JAVA_11) // google-java-format requires JRE 11+
+	@EnabledForJreRange(min = JAVA_11, max = JAVA_15) // google-java-format requires JRE 11+
 	void behavior18() throws Exception {
 		FormatterStep step = GoogleJavaFormatStep.create("1.8", TestProvisioner.mavenCentral());
 		StepHarness.forStep(step)