Turn on Var and Immutable checks, and fix many reported errors

build.gradle

@@ -576,11 +576,6 @@ subprojects { subproj ->
 			// adds a lot of churn for not a lot of gain
 			'-Xep:InconsistentOverloads:OFF',
 
-			// TODO: turning this check on and applying @com.google.errorprone.annotations.Immutable
-			// to immutable classes causes JDK 9 to fail to compile; investigate and consider
-			// turning it on when error-prone supports JDK 9
-			//'-Xep:Immutable:OFF',
-
 			// adds a lot of churn for not a lot of gain
 			'-Xep:MethodCanBeStatic:OFF',
 
@@ -602,11 +597,6 @@ subprojects { subproj ->
 
 			// we do not follow the Google Java Style Guide
 			'-Xep:UngroupedOverloads:OFF',
-
-			// TODO: turning this check on and applying @com.google.errorprone.annotations.Var to
-			// non-final variables causes JDK 9 to fail to compile; investigate and consider turning
-			// it on when error-prone supports JDK 9
-			//'-Xep:Var:OFF'
 		]
 
 		def errorproneTestFlags = errorproneBaseFlags + [
@@ -620,6 +610,7 @@ subprojects { subproj ->
 		]
 
 		task compileJavaWithErrorProne(type: JavaCompile) {
+			dependsOn compileJava, compileKotlin
 			group = 'verification'
 			toolChain = net.ltgt.gradle.errorprone.ErrorProneToolChain.create(project)
 			classpath = compileJava.classpath + compileKotlin.classpath
@@ -630,10 +621,11 @@ subprojects { subproj ->
 		check.dependsOn(compileJavaWithErrorProne)
 
 		task compileTestJavaWithErrorProne(type: JavaCompile) {
+			dependsOn compileJava, compileKotlin, compileTestJava, compileTestKotlin
 			group = 'verification'
 			toolChain = net.ltgt.gradle.errorprone.ErrorProneToolChain.create(project)
-			classpath = compileTestJava.classpath + compileTestKotlin.classpath
-			source = compileTestJava.source + compileTestKotlin.source
+			classpath = compileJava.classpath + compileKotlin.classpath + compileTestJava.classpath + compileTestKotlin.classpath
+			source = compileJava.source + compileKotlin.source + compileTestJava.source + compileTestKotlin.source
 			options.compilerArgs += compileTestJava.options.compilerArgs + errorproneTestFlags
 			destinationDir = file("$buildDir/classes/error-prone-java/test")
 		}

junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertIterableEquals.java

@@ -23,6 +23,8 @@
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Supplier;
 
+import com.google.errorprone.annotations.Var;
+
 /**
  * {@code AssertIterable} is a collection of utility methods that support asserting
  * Iterable equality in tests.
@@ -60,6 +62,7 @@ private static void assertIterableEquals(Iterable<?> expected, Iterable<?> actua
 		Iterator<?> expectedIterator = expected.iterator();
 		Iterator<?> actualIterator = actual.iterator();
 
+		@Var
 		int processed = 0;
 		while (expectedIterator.hasNext() && actualIterator.hasNext()) {
 			processed++;

junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertLinesMatch.java

@@ -22,6 +22,8 @@
 import java.util.regex.PatternSyntaxException;
 import java.util.stream.IntStream;
 
+import com.google.errorprone.annotations.Var;
+
 /**
  * {@code AssertLinesMatch} is a collection of utility methods that support asserting
  * lines of {@link String} equality or {@link java.util.regex.Pattern}-match in tests.
@@ -70,6 +72,7 @@ private static void assertLinesMatchWithFastForward(List<String> expectedLines,
 		Deque<String> actualDeque = new ArrayDeque<>(actualLines);
 
 		main: while (!expectedDeque.isEmpty()) {
+			@Var
 			String expectedLine = expectedDeque.pop();
 			String actualLine = actualDeque.peek();
 
@@ -147,7 +150,7 @@ private static void fail(List<String> expectedLines, List<String> actualLines, S
 		assertEquals(expected, actual, format(format, args));
 	}
 
-	static boolean isFastForwardLine(String line) {
+	static boolean isFastForwardLine(@Var String line) {
 		line = line.trim();
 		return line.length() >= 4 && line.startsWith(">>") && line.endsWith(">>");
 	}

junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertTimeout.java

@@ -23,6 +23,8 @@
 import java.util.concurrent.TimeoutException;
 import java.util.function.Supplier;
 
+import com.google.errorprone.annotations.Var;
+
 import org.junit.jupiter.api.function.Executable;
 import org.junit.jupiter.api.function.ThrowingSupplier;
 import org.junit.platform.commons.util.ExceptionUtils;
@@ -75,6 +77,7 @@ static <T> T assertTimeout(Duration timeout, ThrowingSupplier<T> supplier, Suppl
 	private static <T> T assertTimeout(Duration timeout, ThrowingSupplier<T> supplier, Object messageOrSupplier) {
 		long timeoutInMillis = timeout.toMillis();
 		long start = System.currentTimeMillis();
+		@Var
 		T result = null;
 		try {
 			result = supplier.get();

junit-jupiter-api/src/main/java/org/junit/jupiter/api/condition/OS.java

@@ -14,6 +14,8 @@
 
 import java.util.Locale;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.platform.commons.logging.Logger;
 import org.junit.platform.commons.logging.LoggerFactory;
@@ -65,6 +67,7 @@ public enum OS {
 	private static final OS CURRENT_OS = determineCurrentOs();
 
 	private static OS determineCurrentOs() {
+		@Var
 		String osName = System.getProperty("os.name");
 
 		if (StringUtils.isBlank(osName)) {

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/JupiterTestEngine.java

@@ -14,6 +14,8 @@
 
 import java.util.Optional;
 
+import com.google.errorprone.annotations.Immutable;
+
 import org.apiguardian.api.API;
 import org.junit.jupiter.engine.descriptor.JupiterEngineDescriptor;
 import org.junit.jupiter.engine.discovery.DiscoverySelectorResolver;
@@ -30,6 +32,7 @@
  * @since 5.0
  */
 @API(status = INTERNAL, since = "5.0")
+@Immutable
 public final class JupiterTestEngine extends HierarchicalTestEngine<JupiterEngineExecutionContext> {
 
 	public static final String ENGINE_ID = "junit-jupiter";

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/AbstractExtensionContext.java

@@ -19,6 +19,8 @@
 import java.util.Optional;
 import java.util.Set;
 
+import com.google.errorprone.annotations.Var;
+
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.engine.execution.ExtensionValuesStore;
 import org.junit.jupiter.engine.execution.NamespaceAwareStore;
@@ -61,6 +63,7 @@ abstract class AbstractExtensionContext<T extends TestDescriptor> implements Ext
 	}
 
 	private ExtensionValuesStore createStore(ExtensionContext parent) {
+		@Var
 		ExtensionValuesStore parentStore = null;
 		if (parent != null) {
 			parentStore = ((AbstractExtensionContext<?>) parent).valuesStore;

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/ClassTestDescriptor.java

@@ -29,6 +29,8 @@
 import java.util.Set;
 import java.util.function.Function;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.TestInstance.Lifecycle;
 import org.junit.jupiter.api.extension.AfterAllCallback;
@@ -295,6 +297,7 @@ private AfterEachMethodAdapter synthesizeAfterEachMethodAdapter(Method method) {
 	}
 
 	private void invokeMethodInExtensionContext(Method method, ExtensionContext context, ExtensionRegistry registry) {
+		@Var
 		Object testInstance = context.getRequiredTestInstance();
 		testInstance = ReflectionUtils.getOutermostInstance(testInstance, method.getDeclaringClass()).orElseThrow(
 			() -> new JUnitException("Failed to find instance for method: " + method.toGenericString()));

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestFactoryTestDescriptor.java

@@ -18,6 +18,8 @@
 import java.util.function.Supplier;
 import java.util.stream.Stream;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.DynamicContainer;
 import org.junit.jupiter.api.DynamicNode;
@@ -83,6 +85,7 @@ protected void invokeTestMethod(JupiterEngineExecutionContext context, DynamicTe
 			TestSource source = getSource().orElseThrow(
 				() -> new JUnitException("Illegal state: TestSource must be present"));
 			try (Stream<DynamicNode> dynamicNodeStream = toDynamicNodeStream(testFactoryMethodResult)) {
+				@Var
 				int index = 1;
 				Iterator<DynamicNode> iterator = dynamicNodeStream.iterator();
 				while (iterator.hasNext()) {

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestInstanceLifecycleUtils.java

@@ -14,6 +14,8 @@
 
 import java.util.Optional;
 
+import com.google.errorprone.annotations.Var;
+
 import org.junit.jupiter.api.TestInstance;
 import org.junit.jupiter.api.TestInstance.Lifecycle;
 import org.junit.platform.commons.logging.Logger;
@@ -55,6 +57,7 @@ static TestInstance.Lifecycle getDefaultTestInstanceLifecycle(ConfigurationParam
 		String propertyName = DEFAULT_TEST_INSTANCE_LIFECYCLE_PROPERTY_NAME;
 
 		Optional<String> optional = configParams.get(propertyName);
+		@Var
 		String constantName = null;
 		if (optional.isPresent()) {
 			try {

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/JavaElementsResolver.java

@@ -28,6 +28,8 @@
 import java.util.Optional;
 import java.util.Set;
 
+import com.google.errorprone.annotations.Var;
+
 import org.junit.jupiter.engine.JupiterTestEngine;
 import org.junit.jupiter.engine.descriptor.ClassTestDescriptor;
 import org.junit.jupiter.engine.descriptor.Filterable;
@@ -107,6 +109,7 @@ private Deque<TestDescriptor> resolveAllSegments(UniqueId uniqueId) {
 			TestDescriptor parent = resolvedDescriptors.getLast();
 			UniqueId partialUniqueId = parent.getUniqueId().append(segment);
 
+			@Var
 			Optional<TestDescriptor> resolvedDescriptor = findTestDescriptorByUniqueId(partialUniqueId);
 			if (!resolvedDescriptor.isPresent()) {
 				// @formatter:off
@@ -137,6 +140,7 @@ else if (numSegmentsResolved != numSegmentsToResolve) {
 			}
 			else {
 				logger.warn(() -> {
+					@Var
 					List<Segment> unresolved = segments.subList(1, segments.size()); // Remove engine ID
 					unresolved = unresolved.subList(numSegmentsResolved, unresolved.size()); // Remove resolved segments
 					return format("Unique ID '%s' could only be partially resolved. "

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/execution/ConditionEvaluator.java

@@ -19,6 +19,8 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.extension.ConditionEvaluationResult;
 import org.junit.jupiter.api.extension.ExecutionCondition;
@@ -116,7 +118,7 @@ private String getDeactivatePatternString(ConfigurationParameters configurationP
 	 * See {@link Constants#DEACTIVATE_CONDITIONS_PATTERN_PROPERTY_NAME} for
 	 * details on the pattern matching syntax.
 	 */
-	private String convertToRegEx(String pattern) {
+	private String convertToRegEx(@Var String pattern) {
 		pattern = Matcher.quoteReplacement(pattern);
 
 		// Match "." against "." and "$" since users may declare a "." instead of a

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/execution/ExecutableInvoker.java

@@ -22,6 +22,8 @@
 import java.util.List;
 import java.util.Optional;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.api.extension.ParameterContext;
@@ -159,6 +161,7 @@ private Object[] resolveParameters(Executable executable, Optional<Object> targe
 
 		Parameter[] parameters = executable.getParameters();
 		Object[] values = new Object[parameters.length];
+		@Var
 		int start = 0;
 
 		// Ensure that the outer instance is resolved as the first parameter if

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/execution/ExtensionValuesStore.java

@@ -22,6 +22,8 @@
 import java.util.function.Function;
 import java.util.function.Supplier;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.api.extension.ExtensionContext.Namespace;
@@ -73,6 +75,7 @@ <T> T get(Namespace namespace, Object key, Class<T> requiredType) {
 
 	<K, V> Object getOrComputeIfAbsent(Namespace namespace, K key, Function<K, V> defaultCreator) {
 		CompositeKey compositeKey = new CompositeKey(namespace, key);
+		@Var
 		Supplier<Object> storedValue = getStoredValue(compositeKey);
 		if (storedValue == null) {
 			storedValue = new MemoizingSupplier(() -> defaultCreator.apply(key));

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/script/Script.java

@@ -15,6 +15,8 @@
 import java.lang.annotation.Annotation;
 import java.util.Objects;
 
+import com.google.errorprone.annotations.Immutable;
+
 import org.apiguardian.api.API;
 import org.junit.platform.commons.util.Preconditions;
 
@@ -27,6 +29,7 @@
  * @see ScriptExecutionManager
  */
 @API(status = INTERNAL, since = "5.1")
+@Immutable
 public final class Script {
 
 	/**

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/script/ScriptExecutionManager.java

@@ -23,6 +23,8 @@
 import javax.script.ScriptEngineManager;
 import javax.script.ScriptException;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.platform.commons.util.Preconditions;
 
@@ -54,6 +56,7 @@ public class ScriptExecutionManager {
 	 */
 	public Object evaluate(Script script, Bindings bindings) throws ScriptException {
 		// Always look for a compiled script in our cache.
+		@Var
 		CompiledScript compiledScript = compiledScripts.get(script);
 
 		// No compiled script found?
@@ -73,6 +76,7 @@ public Object evaluate(Script script, Bindings bindings) throws ScriptException
 	}
 
 	ScriptEngine createScriptEngine(String engine) {
+		@Var
 		ScriptEngine scriptEngine = scriptEngineManager.getEngineByName(engine);
 		if (scriptEngine == null) {
 			scriptEngine = scriptEngineManager.getEngineByExtension(engine);

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ParameterizedTestNameFormatter.java

@@ -16,6 +16,8 @@
 import java.util.Arrays;
 import java.util.stream.IntStream;
 
+import com.google.errorprone.annotations.Var;
+
 import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.util.StringUtils;
 
@@ -37,6 +39,7 @@ String format(int invocationIndex, Object... arguments) {
 	}
 
 	private String prepareMessageFormatPattern(int invocationIndex, Object[] arguments) {
+		@Var
 		String result = namePattern.replace("{index}", String.valueOf(invocationIndex));
 		if (result.contains("{arguments}")) {
 			// @formatter:off

junit-platform-commons-java-9/src/main/java/org/junit/platform/commons/util/ModuleUtils.java

@@ -31,6 +31,8 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.logging.Logger;
@@ -129,6 +131,7 @@ public static List<Class<?>> findAllClassesInModule(String moduleName, ClassFilt
 	 */
 	private static Stream<ResolvedModule> streamResolvedModules(Predicate<String> moduleNamePredicate) {
 		Module module = ModuleUtils.class.getModule();
+		@Var
 		ModuleLayer layer = module.getLayer();
 		if (layer == null) {
 			logger.config(() -> ModuleUtils.class + " is a member of " + module
@@ -203,7 +206,7 @@ List<Class<?>> scan(ModuleReference reference) {
 		/**
 		 * Convert resource name to binary class name.
 		 */
-		private String className(String resourceName) {
+		private String className(@Var String resourceName) {
 			resourceName = resourceName.substring(0, resourceName.length() - 6); // 6 = ".class".length()
 			resourceName = resourceName.replace('/', '.');
 			return resourceName;

junit-platform-commons/src/main/java/org/junit/platform/commons/logging/LoggerFactory.java

@@ -18,6 +18,8 @@
 import java.util.logging.Level;
 import java.util.logging.LogRecord;
 
+import com.google.errorprone.annotations.Var;
+
 import org.apiguardian.api.API;
 import org.junit.platform.commons.JUnitException;
 
@@ -155,8 +157,11 @@ private void log(Level level, Throwable throwable, Supplier<String> messageSuppl
 
 		private LogRecord createLogRecord(Level level, Throwable throwable, Supplier<String> messageSupplier) {
 			StackTraceElement[] stack = new Throwable().getStackTrace();
+			@Var
 			String sourceClassName = null;
+			@Var
 			String sourceMethodName = null;
+			@Var
 			boolean found = false;
 			for (StackTraceElement element : stack) {
 				String className = element.getClassName();