Stabilize unit tests (Local, Static initialization)

- stabilize unit tests that depend on locale (e.g. number formatting in
string output)
- introduce a JUnit extension that handles Var.Provider setting only for
specific tests where required

Author: aschwarte10

core/query/src/test/java/org/eclipse/rdf4j/query/explanation/GenericPlanNodeTest.java

@@ -16,10 +16,28 @@
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.util.Locale;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 class GenericPlanNodeTest {
 
+	private Locale defaultLocale;
+
+	@BeforeEach
+	void setUp() {
+		defaultLocale = Locale.getDefault();
+		// set EN locale explicitly to avoid different number formatting across environment
+		Locale.setDefault(Locale.ENGLISH);
+	}
+
+	@AfterEach
+	void tearDown() {
+		Locale.setDefault(defaultLocale);
+	}
+
 	@Test
 	void toStringIncludesPopulatedTelemetryFields() {
 		GenericPlanNode node = new GenericPlanNode("Join");

core/queryalgebra/evaluation/src/test/java/org/eclipse/rdf4j/query/algebra/evaluation/impl/QueryCostEstimatesTest.java

@@ -12,17 +12,35 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 
+import java.util.Locale;
+
 import org.eclipse.rdf4j.common.exception.RDF4JException;
 import org.eclipse.rdf4j.query.algebra.evaluation.optimizer.QueryJoinOptimizer;
 import org.eclipse.rdf4j.query.parser.ParsedQuery;
 import org.eclipse.rdf4j.query.parser.sparql.SPARQLParser;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 /**
  * Tests that cost estimates are printed as part of the plan
  */
 public class QueryCostEstimatesTest {
 
+	private Locale defaultLocale;
+
+	@BeforeEach
+	void setUp() {
+		defaultLocale = Locale.getDefault();
+		// set EN locale explicitly to avoid different number formatting across environment
+		Locale.setDefault(Locale.ENGLISH);
+	}
+
+	@AfterEach
+	void tearDown() {
+		Locale.setDefault(defaultLocale);
+	}
+
 	@Test
 	public void testBindingSetAssignmentOptimization() throws RDF4JException {
 		String query = "prefix ex: <ex:>" + "select ?s ?p ?o ?x where {" + " ex:s1 ex:pred ?v. "

core/queryalgebra/model/src/main/java/org/eclipse/rdf4j/query/algebra/Var.java

@@ -312,10 +312,31 @@ public boolean isConstant() {
 		return constant;
 	}
 
+	/**
+	 * Installs a custom {@link Provider} to be used by {@link Var#of} and {@link Var#clone}. Intended for test use
+	 * only. Always call {@link #resetProvider()} in a corresponding teardown.
+	 */
+	/* package */ static void setProvider(Provider provider) {
+		Holder.PROVIDER = provider;
+	}
+
+	/**
+	 * Resets the {@link Provider} to the one resolved from the system property and {@link ServiceLoader}. Intended for
+	 * test use only to undo a prior {@link #setProvider} call.
+	 */
+	/* package */ static void resetProvider() {
+		Holder.reset();
+	}
+
 	private static final class Holder {
 		private static final Provider DEFAULT = Var::new;
 
-		static final Provider PROVIDER = initProvider();
+		// Not final so that Var.setProvider() can replace it during tests
+		static Provider PROVIDER = initProvider();
+
+		/* package */ static void reset() {
+			PROVIDER = initProvider();
+		}
 
 		private static Provider initProvider() {
 			// 1) Explicit override via system property (FQCN of Var.Provider)

core/queryalgebra/model/src/test/java/org/eclipse/rdf4j/query/algebra/VarProviderCloneHookTest.java

@@ -23,6 +23,7 @@
 import org.eclipse.rdf4j.model.impl.SimpleValueFactory;
 import org.junit.jupiter.api.Test;
 
+@WithVarProvider(KindAwareVarProvider.class)
 class VarProviderCloneHookTest {
 
 	private final ValueFactory vf = SimpleValueFactory.getInstance();

core/queryalgebra/model/src/test/java/org/eclipse/rdf4j/query/algebra/VarProviderExtension.java

@@ -0,0 +1,52 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Eclipse RDF4J contributors.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Distribution License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *******************************************************************************/
+package org.eclipse.rdf4j.query.algebra;
+
+import java.util.Optional;
+
+import org.junit.jupiter.api.extension.AfterEachCallback;
+import org.junit.jupiter.api.extension.BeforeEachCallback;
+import org.junit.jupiter.api.extension.ExtensionContext;
+
+/**
+ * JUnit 5 extension that installs a custom {@link Var.Provider} for the duration of each test and resets it afterwards.
+ *
+ * <p>
+ * Activated via {@link WithVarProvider} on the test class or method. The method-level annotation takes precedence over
+ * the class-level annotation. The provider is always reset after each test regardless of outcome.
+ *
+ * @see WithVarProvider
+ */
+class VarProviderExtension implements BeforeEachCallback, AfterEachCallback {
+
+	@Override
+	public void beforeEach(ExtensionContext context) throws Exception {
+		Optional<WithVarProvider> annotation = findAnnotation(context);
+		if (annotation.isPresent()) {
+			Var.Provider provider = annotation.get().value().getDeclaredConstructor().newInstance();
+			Var.setProvider(provider);
+		}
+	}
+
+	@Override
+	public void afterEach(ExtensionContext context) {
+		Var.resetProvider();
+	}
+
+	/**
+	 * Returns the {@link WithVarProvider} annotation, preferring the method level over the class level.
+	 */
+	private Optional<WithVarProvider> findAnnotation(ExtensionContext context) {
+		return context.getTestMethod()
+				.map(m -> m.getAnnotation(WithVarProvider.class))
+				.or(() -> context.getTestClass().map(c -> c.getAnnotation(WithVarProvider.class)));
+	}
+}

core/queryalgebra/model/src/test/java/org/eclipse/rdf4j/query/algebra/WithVarProvider.java

@@ -0,0 +1,51 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Eclipse RDF4J contributors.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Distribution License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *******************************************************************************/
+package org.eclipse.rdf4j.query.algebra;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.junit.jupiter.api.extension.ExtendWith;
+
+/**
+ * Installs a custom {@link Var.Provider} for the duration of the annotated test class or test method.
+ *
+ * <p>
+ * The specified provider is instantiated via its no-argument constructor, installed before each test, and removed after
+ * each test via {@link Var#resetProvider()}. When placed on both a class and a method, the method-level annotation
+ * takes precedence.
+ *
+ * <p>
+ * Usage:
+ *
+ * <pre>
+ * &#64;WithVarProvider(MyVarProvider.class)
+ * class MyTest {
+ *
+ * 	&#64;Test
+ * 	void test() {
+ * 		Var v = Var.of("x"); // produced by MyVarProvider
+ * 	}
+ * }
+ * </pre>
+ */
+@Target({ ElementType.TYPE, ElementType.METHOD })
+@Retention(RetentionPolicy.RUNTIME)
+@ExtendWith(VarProviderExtension.class)
+public @interface WithVarProvider {
+
+	/**
+	 * The {@link Var.Provider} implementation to install. Must have a public no-argument constructor.
+	 */
+	Class<? extends Var.Provider> value();
+}

core/queryalgebra/model/src/test/java/org/eclipse/rdf4j/query/algebra/helpers/QueryModelTreeToGenericPlanNodeTest.java

@@ -22,6 +22,7 @@
 import org.eclipse.rdf4j.query.algebra.ExtensionElem;
 import org.eclipse.rdf4j.query.algebra.Filter;
 import org.eclipse.rdf4j.query.algebra.Join;
+import org.eclipse.rdf4j.query.algebra.KindAwareVarProvider;
 import org.eclipse.rdf4j.query.algebra.Projection;
 import org.eclipse.rdf4j.query.algebra.ProjectionElem;
 import org.eclipse.rdf4j.query.algebra.ProjectionElemList;
@@ -30,6 +31,7 @@
 import org.eclipse.rdf4j.query.algebra.StatementPattern;
 import org.eclipse.rdf4j.query.algebra.TupleExpr;
 import org.eclipse.rdf4j.query.algebra.Var;
+import org.eclipse.rdf4j.query.algebra.WithVarProvider;
 import org.eclipse.rdf4j.query.explanation.Explanation;
 import org.eclipse.rdf4j.query.explanation.GenericPlanNode;
 import org.eclipse.rdf4j.query.explanation.TelemetryMetricNames;
@@ -189,6 +191,7 @@ public void doesNotAnnotateConnectedJoinAsCartesianJoin() {
 	}
 
 	@Test
+	@WithVarProvider(KindAwareVarProvider.class)
 	public void skipsCartesianAnnotationForUnsupportedVarSubclass() {
 		Join join = new Join(
 				new StatementPattern(Var.of("s"), Var.of("p"), Var.of("o")),

core/queryalgebra/model/src/test/resources/META-INF/services/org.eclipse.rdf4j.query.algebra.Var$Provider

@@ -15,6 +15,7 @@
 import static org.junit.Assert.assertTrue;
 
 import java.io.File;
+import java.util.Locale;
 
 import org.eclipse.rdf4j.common.iteration.CloseableIteration;
 import org.eclipse.rdf4j.common.iteration.EmptyIteration;
@@ -47,6 +48,8 @@ public class LmdbSailStoreTest {
 
 	protected Repository repo;
 
+	private Locale defaultLocale;
+
 	protected final ValueFactory F = SimpleValueFactory.getInstance();
 
 	protected final IRI CTX_1 = F.createIRI("urn:one");
@@ -62,6 +65,8 @@ public class LmdbSailStoreTest {
 
 	@BeforeEach
 	public void before(@TempDir File dataDir) {
+		defaultLocale = Locale.getDefault();
+		Locale.setDefault(Locale.ENGLISH);
 		repo = new SailRepository(new LmdbStore(dataDir, new LmdbStoreConfig("spoc,posc")));
 		repo.init();
 
@@ -258,5 +263,6 @@ public void testExplainExecutedHidesEstimateStabilityStats() {
 	@AfterEach
 	public void after() {
 		repo.shutDown();
+		Locale.setDefault(defaultLocale);
 	}
 }

core/sail/lmdb/src/test/java/org/eclipse/rdf4j/sail/lmdb/LmdbSailStoreTest.java

@@ -19,6 +19,7 @@
 import java.io.IOException;
 import java.io.StringReader;
 import java.nio.charset.StandardCharsets;
+import java.util.Locale;
 import java.util.function.Predicate;
 
 import org.apache.commons.io.IOUtils;
@@ -43,6 +44,8 @@
 import org.eclipse.rdf4j.repository.sail.SailRepository;
 import org.eclipse.rdf4j.repository.sail.SailRepositoryConnection;
 import org.eclipse.rdf4j.rio.RDFFormat;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
@@ -53,6 +56,19 @@ public class QueryPlanRetrievalTest {
 
 	private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
 
+	private Locale defaultLocale;
+
+	@BeforeEach
+	void setLocale() {
+		defaultLocale = Locale.getDefault();
+		Locale.setDefault(Locale.ENGLISH);
+	}
+
+	@AfterEach
+	void restoreLocale() {
+		Locale.setDefault(defaultLocale);
+	}
+
 	public static final String MAIN_QUERY = String.join("\n", "",
 			"{",
 			"    {",