feat: throw IllegalStateException when Signal.get() is called outside a reactive context (#23580)

Signal.get() now throws when called outside a reactive context (effect, computed signal, untracked block, or transaction) to prevent accidental non-reactive reads like `new Span("Name: " + nameSignal.get())` that silently return the value but never update. Users should use peek() for one-time reads.

Key changes:
- UsageTracker: add DELIBERATELY_UNTRACKED sentinel so untracked() sets a sentinel instead of removing the tracker, distinguishing "deliberately untracked" from "no tracking context"
- AbstractLocalSignal/AbstractSignal: guard get() with isGetAllowed() check (shared signals also allow get() inside transactions)
- ComputedSignal: remove peek() override so computed signals can be read outside reactive contexts via inherited AbstractSignal.peek()
- Production code: use peek() where get() was called outside reactive contexts (Html, VaadinSession, SharedMapSignal.toString, Binder)

Fixes #22123

Author: Artur-

flow-data/src/test/java/com/vaadin/flow/data/binder/BinderSignalTest.java

@@ -717,7 +717,7 @@ public void getValidationStatus_signalInitialized() {
                 .getValidationStatus();
         assertNotNull(statusSignal,
                 "getValidationStatus() should setup validation status signal");
-        assertNotNull(statusSignal.get(),
+        assertNotNull(statusSignal.peek(),
                 "validation status signal value should not be null initially");
     }
 
@@ -741,18 +741,18 @@ public void getValidationStatus_statusChangeUpdatesSignal() {
                 .withValidator(value -> !value.isEmpty(), "").bind("lastName");
         binder.setBean(item);
 
-        assertTrue(binder.getValidationStatus().get().isOk());
+        assertTrue(binder.getValidationStatus().peek().isOk());
 
         firstNameField.setValue("");
-        assertFalse(binder.getValidationStatus().get().isOk());
+        assertFalse(binder.getValidationStatus().peek().isOk());
         firstNameField.setValue("foo");
-        assertTrue(binder.getValidationStatus().get().isOk());
+        assertTrue(binder.getValidationStatus().peek().isOk());
         lastNameField.setValue("");
-        assertFalse(binder.getValidationStatus().get().isOk());
+        assertFalse(binder.getValidationStatus().peek().isOk());
         firstNameField.setValue("");
-        assertFalse(binder.getValidationStatus().get().isOk());
+        assertFalse(binder.getValidationStatus().peek().isOk());
         firstNameField.setValue("foo");
-        assertFalse(binder.getValidationStatus().get().isOk());
+        assertFalse(binder.getValidationStatus().peek().isOk());
     }
 
     // verifies that field-specific validation statuses are updated correctly
@@ -769,18 +769,18 @@ public void getValidationStatus_fieldChanged_validationStatusSignalUpdated() {
         binder.setBean(item);
 
         assertTrue(
-                binder.getValidationStatus().get().getFieldValidationStatuses()
+                binder.getValidationStatus().peek().getFieldValidationStatuses()
                         .stream().noneMatch(BindingValidationStatus::isError));
-        assertTrue(binder.getValidationStatus().get().isOk());
+        assertTrue(binder.getValidationStatus().peek().isOk());
 
         lastNameField.setValue(""); // change to invalid state
 
-        assertFalse(binder.getValidationStatus().get().isOk());
-        var firstNameValidationStatuses = binder.getValidationStatus().get()
+        assertFalse(binder.getValidationStatus().peek().isOk());
+        var firstNameValidationStatuses = binder.getValidationStatus().peek()
                 .getFieldValidationStatuses().stream()
                 .filter(status -> status.getBinding() == firstNameBinding)
                 .toList();
-        var otherValidationStatuses = binder.getValidationStatus().get()
+        var otherValidationStatuses = binder.getValidationStatus().peek()
                 .getFieldValidationStatuses().stream()
                 .filter(status -> status.getBinding() != firstNameBinding)
                 .toList();
@@ -794,7 +794,7 @@ public void getValidationStatus_fieldChanged_validationStatusSignalUpdated() {
                 "Expected first name field to have an error");
 
         lastNameField.setValue("Smith");
-        assertTrue(binder.getValidationStatus().get().isOk());
+        assertTrue(binder.getValidationStatus().peek().isOk());
     }
 
     @Test

flow-server/src/main/java/com/vaadin/flow/component/Html.java

@@ -139,7 +139,7 @@ public Html(Signal<String> htmlSignal) {
         if (htmlSignal == null) {
             throw new IllegalArgumentException("HTML signal cannot be null");
         }
-        String outerHtml = htmlSignal.get();
+        String outerHtml = htmlSignal.peek();
         if (outerHtml == null || outerHtml.isEmpty()) {
             throw new IllegalArgumentException("HTML cannot be null or empty");
         }

flow-server/src/main/java/com/vaadin/flow/server/VaadinSession.java

@@ -1167,7 +1167,7 @@ private void writeObject(java.io.ObjectOutputStream stream)
             }
 
             // Sync locale field from signal for serialization
-            locale = localeSignal.get();
+            locale = localeSignal.peek();
             stream.defaultWriteObject();
             if (serializeUIs) {
                 stream.writeObject(uIs);

flow-server/src/main/java/com/vaadin/flow/signals/Signal.java

@@ -69,8 +69,17 @@ public interface Signal<T> extends Serializable {
      * Reading the value inside an {@link #unboundEffect(EffectAction)} or
      * {@link #computed(SignalComputation)} callback sets up that effect or
      * computed signal to depend on the signal.
+     * <p>
+     * This method must only be called within a reactive context such as an
+     * effect, a computed signal, an explicit {@link #untracked(ValueSupplier)}
+     * block, or a {@link #runInTransaction(TransactionTask) transaction}.
+     * Calling it outside such a context throws an
+     * {@link IllegalStateException}. Use {@link #peek()} for one-time reads
+     * that do not need dependency tracking.
      *
      * @return the signal value
+     * @throws IllegalStateException
+     *             if called outside a reactive context
      */
     @Nullable
     T get();
@@ -79,6 +88,10 @@ public interface Signal<T> extends Serializable {
      * Reads the value without setting up any dependencies. This method returns
      * the same value as {@link #get()} but without creating a dependency when
      * used inside a transaction, effect or computed signal.
+     * <p>
+     * Unlike {@link #get()}, this method can be called outside a reactive
+     * context and is the recommended way to read a signal value for one-time
+     * use, such as logging, assertions, or initializing non-reactive UI.
      *
      * @return the signal value
      */

flow-server/src/main/java/com/vaadin/flow/signals/impl/ComputedSignal.java

@@ -273,12 +273,6 @@ public T peekConfirmed() {
                 "Cannot peek a computed signal");
     }
 
-    @Override
-    public T peek() {
-        throw new UnsupportedOperationException(
-                "Cannot peek a computed signal");
-    }
-
     @Override
     public SharedNodeSignal asNode() {
         throw new UnsupportedOperationException(

flow-server/src/main/java/com/vaadin/flow/signals/impl/UsageTracker.java

@@ -165,6 +165,13 @@ public Registration onNextChange(TransientListener listener) {
         }
     };
 
+    /**
+     * Sentinel registrar that represents a deliberately untracked context,
+     * distinguishing it from having no tracking context at all.
+     */
+    private static final UsageRegistrar DELIBERATELY_UNTRACKED = usage -> {
+    };
+
     private static final ThreadLocal<UsageRegistrar> currentTracker = new ThreadLocal<>();
 
     private UsageTracker() {
@@ -255,16 +262,20 @@ public static <T> T track(Supplier<T> task, UsageRegistrar tracker) {
      */
     public static <T> @Nullable T untracked(ValueSupplier<T> task) {
         var previousTracker = currentTracker.get();
-        if (previousTracker == null) {
+        if (previousTracker == DELIBERATELY_UNTRACKED) {
             return task.supply();
         }
 
         try {
-            currentTracker.remove();
+            currentTracker.set(DELIBERATELY_UNTRACKED);
 
             return task.supply();
         } finally {
-            currentTracker.set(previousTracker);
+            if (previousTracker == null) {
+                currentTracker.remove();
+            } else {
+                currentTracker.set(previousTracker);
+            }
         }
     }
 
@@ -282,12 +293,26 @@ public static void registerUsage(Usage usage) {
     }
 
     /**
-     * Checks whether a usage tracker is currently active.
+     * Checks whether calling {@code Signal.get()} is allowed in the current
+     * context. Returns {@code true} when a real usage tracker is active or when
+     * inside a deliberately untracked context (e.g. via
+     * {@code Signal.untracked()} or {@code Signal.peek()}).
+     *
+     * @return {@code true} if calling {@code get()} is allowed
+     */
+    public static boolean isGetAllowed() {
+        return currentTracker.get() != null;
+    }
+
+    /**
+     * Checks whether a usage tracker is currently active. Returns {@code true}
+     * only for real trackers, not the deliberately untracked sentinel.
      *
      * @return <code>true</code> if a usage tracker is active
      */
     public static boolean isActive() {
-        return currentTracker.get() != null;
+        UsageRegistrar tracker = currentTracker.get();
+        return tracker != null && tracker != DELIBERATELY_UNTRACKED;
     }
 
 }

flow-server/src/main/java/com/vaadin/flow/signals/local/AbstractLocalSignal.java

@@ -80,6 +80,14 @@ protected void checkPreconditions() {
 
     @Override
     public @Nullable T get() {
+        if (!UsageTracker.isGetAllowed()) {
+            throw new IllegalStateException(
+                    "Signal.get() was called outside a reactive context. "
+                            + "Use peek() to read the value without setting up "
+                            + "dependency tracking, or use "
+                            + "Signal.untracked(() -> signal.get()) to "
+                            + "explicitly opt out.");
+        }
         lock.lock();
         try {
             checkPreconditions();

flow-server/src/main/java/com/vaadin/flow/signals/shared/AbstractSignal.java

@@ -156,6 +156,14 @@ protected AbstractSignal(SignalTree tree, Id id,
 
     @Override
     public @Nullable T get() {
+        if (!UsageTracker.isGetAllowed() && !Transaction.inTransaction()) {
+            throw new IllegalStateException(
+                    "Signal.get() was called outside a reactive context. "
+                            + "Use peek() to read the value without setting up "
+                            + "dependency tracking, or use "
+                            + "Signal.untracked(() -> signal.get()) to "
+                            + "explicitly opt out.");
+        }
         Transaction transaction = Transaction.getCurrent();
         Data data = data(transaction);
 

flow-server/src/main/java/com/vaadin/flow/signals/shared/SharedMapSignal.java

@@ -369,7 +369,7 @@ public String toString() {
             return "SharedMapSignal[null]";
         }
         return value.entrySet().stream()
-                .map(entry -> entry.getKey() + "=" + entry.getValue().get())
+                .map(entry -> entry.getKey() + "=" + entry.getValue().peek())
                 .collect(Collectors.joining(", ", "SharedMapSignal[", "]"));
     }
 

flow-server/src/main/java/com/vaadin/flow/signals/shared/SharedNumberSignal.java

@@ -121,14 +121,13 @@ public Double peekConfirmed() {
     }
 
     /**
-     * Gets the value of this signal as an integer. This method works in the
-     * same way was {@link #get()} with regards to transactions and dependency
-     * tracking.
+     * Gets the value of this signal as an integer without registering a
+     * dependency. Equivalent to {@code peek().intValue()}.
      *
      * @return the signal value as an integer
      */
     public int getAsInt() {
-        return get().intValue();
+        return peek().intValue();
     }
 
     /**