Add ThreadLocalAccessor for LocaleContext and RequestAttributes

Add `ThreadLocalAccessor` implementations:
- `LocaleThreadLocalAccessor`
- `RequestAttributesThreadLocalAccessor`

See gh-32243

spring-context/src/test/java/org/springframework/context/i18n/LocaleContextThreadLocalAccessorTests.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2002-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.context.i18n;
+
+import java.util.Locale;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+import io.micrometer.context.ContextRegistry;
+import io.micrometer.context.ContextSnapshot;
+import io.micrometer.context.ContextSnapshotFactory;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import org.springframework.lang.Nullable;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link LocaleContextThreadLocalAccessor}.
+ *
+ * @author Tadaya Tsuyukubo
+ */
+class LocaleContextThreadLocalAccessorTests {
+
+	private final ContextRegistry registry = new ContextRegistry()
+			.registerThreadLocalAccessor(new LocaleContextThreadLocalAccessor());
+
+	@AfterEach
+	void cleanUp() {
+		LocaleContextHolder.resetLocaleContext();
+	}
+
+	@ParameterizedTest
+	@MethodSource
+	void propagation(@Nullable LocaleContext previous, LocaleContext current) throws Exception {
+		LocaleContextHolder.setLocaleContext(current);
+		ContextSnapshot snapshot = ContextSnapshotFactory.builder()
+				.contextRegistry(this.registry)
+				.clearMissing(true)
+				.build()
+				.captureAll();
+
+		AtomicReference<LocaleContext> previousHolder = new AtomicReference<>();
+		AtomicReference<LocaleContext> currentHolder = new AtomicReference<>();
+		CountDownLatch latch = new CountDownLatch(1);
+		new Thread(() -> {
+			LocaleContextHolder.setLocaleContext(previous);
+			try (ContextSnapshot.Scope scope = snapshot.setThreadLocals()) {
+				currentHolder.set(LocaleContextHolder.getLocaleContext());
+			}
+			previousHolder.set(LocaleContextHolder.getLocaleContext());
+			latch.countDown();
+		}).start();
+
+		latch.await(1, TimeUnit.SECONDS);
+		assertThat(previousHolder).hasValueSatisfying(value -> assertThat(value).isSameAs(previous));
+		assertThat(currentHolder).hasValueSatisfying(value -> assertThat(value).isSameAs(current));
+	}
+
+	private static Stream<Arguments> propagation() {
+		LocaleContext previous = new SimpleLocaleContext(Locale.ENGLISH);
+		LocaleContext current = new SimpleLocaleContext(Locale.ENGLISH);
+		return Stream.of(
+				Arguments.of(null, current),
+				Arguments.of(previous, current)
+		);
+	}
+}