Java 25: Scoped Values vs ThreadLocal (JEP 506)

JavaReleases/src/test/java/pl/mperor/lab/java/Java25.java

@@ -4,6 +4,8 @@
 import org.junit.jupiter.api.Test;
 import pl.mperor.lab.common.TestUtils;
 
+import java.util.NoSuchElementException;
+
 /// Java 25 (September 2025)
 /// [JDK 25](https://openjdk.org/projects/jdk/25)
 ///
@@ -98,4 +100,52 @@ void describe() {
         }
     }
 
+    static final ScopedValue<String> USER = ScopedValue.newInstance();
+
+    @Test
+    public void testScopedValueIsBoundOnlyInsideRunScope() {
+        Assertions.assertFalse(USER.isBound());
+
+        ScopedValue.where(USER, "John Doe").run(() -> {
+            Assertions.assertTrue(USER.isBound());
+            Assertions.assertEquals("John Doe", USER.get());
+        });
+
+        // the binding is torn down as soon as run() returns
+        Assertions.assertFalse(USER.isBound());
+        Assertions.assertThrows(NoSuchElementException.class, USER::get);
+    }
+
+    @Test
+    public void testNestedRebindingRestoresOuterValueAfterward() {
+        ScopedValue.where(USER, "John Doe").run(() -> {
+            Assertions.assertEquals("John Doe", USER.get());
+
+            ScopedValue.where(USER, "Jane Doe").run(() -> Assertions.assertEquals("Jane Doe", USER.get()));
+
+            // the nested run() only rebinds USER for its own body; the outer binding is unaffected
+            Assertions.assertEquals("John Doe", USER.get());
+        });
+    }
+
+    @Test
+    public void testCallReturnsValueComputedUnderTheBinding() throws Exception {
+        String greeting = ScopedValue.where(USER, "John Doe").call(() -> "Hello, " + USER.get());
+        Assertions.assertEquals("Hello, John Doe", greeting);
+    }
+
+    @Test
+    public void testScopedValueHasNoSetUnlikeThreadLocal() {
+        var legacy = new ThreadLocal<String>();
+        legacy.set("John Doe");
+        legacy.set("Jane Doe"); // ThreadLocal is mutable: set() silently replaces the value, anywhere, anytime
+        Assertions.assertEquals("Jane Doe", legacy.get());
+        legacy.remove(); // must be cleared by hand, or it leaks for the thread's lifetime (e.g. in a pool)
+
+        // ScopedValue has no set()/remove(): the only way to associate a value is where(...).run()/.call(...),
+        // and the binding is automatically torn down when that call returns - no leak is possible.
+        ScopedValue.where(USER, "John Doe").run(() -> Assertions.assertEquals("John Doe", USER.get()));
+        Assertions.assertFalse(USER.isBound());
+    }
+
 }