feat: retain local signal values during hotswap by tltv · Pull Request #23854 · vaadin/flow

tltv · 2026-03-12T14:28:25Z

Automatically transfer local signal values from old view fields to matching fields in the new view during hotswap refresh.

Fixes: #23830

This pull request introduces a new mechanism for transferring local signal field values between view instances during a hot-reload (hotswap refresh) in development mode. The main addition is the SignalFieldTransfer utility, which ensures that the state held in ValueSignal and ListSignal fields is preserved when a view is refreshed, improving the developer experience.

The most important changes are:

Signal transfer utility

Added a new class SignalFieldTransfer in flow-server/src/main/java/com/vaadin/flow/internal/SignalFieldTransfer.java that provides static methods to transfer local signal field values (of type ValueSignal and ListSignal) from an old view instance to a new one, matching fields by name and type. This is used to preserve signal state during hot-reload in development.

Integration with navigation

Updated AbstractNavigationStateRenderer#getRouteTarget to call SignalFieldTransfer.transferLocalSignalValues when a view is refreshed (triggered by NavigationTrigger.REFRESH_ROUTE) and the application is not in production mode, ensuring signal state is transferred during development hot-reloads.
Imported the new SignalFieldTransfer class in AbstractNavigationStateRenderer.java to enable the above integration.

Automatically transfer local signal values from old view fields to matching fields in the new view during hotswap refresh. Fixes: #23830

github-actions · 2026-03-12T15:01:37Z

Test Results

1 386 files + 59 1 386 suites +59 1h 27m 56s ⏱️ - 4m 56s
9 922 tests +676 9 851 ✅ +729 71 💤 ±0 0 ❌ - 10
10 395 runs +676 10 315 ✅ +731 80 💤 ±0 0 ❌ - 10

Results for commit ee9747b. ± Comparison against base commit 50aba91.

♻️ This comment has been updated with latest results.

flow-server/src/main/java/com/vaadin/flow/router/internal/AbstractNavigationStateRenderer.java

Legioth

Will also look at the tests but posting findings from reviewing the implementation right away

Legioth · 2026-03-16T07:24:19Z

flow-server/src/main/java/com/vaadin/flow/internal/SignalFieldTransfer.java

+    }
+
+    private static boolean isLocalSignalType(Class<?> type) {
+        return ValueSignal.class.isAssignableFrom(type)


Maybe check for AbstractLocalSignal instead so that any future subclasses are also automatically covered?

Legioth · 2026-03-16T07:28:50Z

flow-server/src/main/java/com/vaadin/flow/internal/SignalFieldTransfer.java

+
+    private static void transferField(HasElement oldInstance,
+            HasElement newInstance, Field newField) throws Exception {
+        Field oldField = findField(oldInstance.getClass(), newField.getName());


Looking through the entire type hierarchy might lead to problems in the edge case case when multiple classes in the hierarchy have a private field with the same name. Would it work to also pass in newClass from the transferLocalSignalValues method and somehow use that either for the lookup or for additional verification?

It's a bit more complicated even, because we can't mix old and new class to read fields since class may have changed. Need to iterate both class hierarchies in parallel instead.

What about matching fields only if the fully qualified class names match?

That's another option. Both probably works, but this might be more efficient even.

Efficiency is relatively irrelevant considering all the other processing that happens in a hotswap scenario. I'm mostly thinking of correctness to avoid accidentally mixing up signal values.

Legioth · 2026-03-16T07:29:52Z

flow-server/src/main/java/com/vaadin/flow/internal/SignalFieldTransfer.java

+        if (oldField == null) {
+            return;
+        }
+        if (!isLocalSignalType(oldField.getType())) {


Could check that it's the same type and not just any local signal type. That would also allow slightly simplifying things further down.

Legioth · 2026-03-16T07:32:51Z

flow-server/src/main/java/com/vaadin/flow/router/internal/AbstractNavigationStateRenderer.java

                ? Optional.empty()
                : findActiveRouteTarget(event, isRouteTargetType);
-        return (T) currentInstance.orElseGet(
+        T result = (T) currentInstance.orElseGet(


Could use a more descriptive variable name, e.g. routeTarget

Legioth · 2026-03-16T07:48:36Z

flow-server/src/test/java/com/vaadin/flow/internal/SignalFieldTransferTest.java

+        var newView = new ViewWithListSignal();
+        SignalFieldTransfer.transferLocalSignalValues(oldView, newView);
+
+        var values = newView.items.peek().stream().map(ValueSignal::peek)


Can be simplified to newView.items.peekValues().toList()

Legioth · 2026-03-16T07:51:46Z

flow-server/src/test/java/com/vaadin/flow/internal/SignalFieldTransferTest.java

+    }
+
+    @Test
+    void exceptionFromSignalSetIsLogged() {


Misleading method name since it doesn't assert anything about logging

vaadin-dev-server/src/test/java/com/vaadin/base/devserver/hotswap/HotswapSignalsTest.java

Legioth

One minimal stylistic remark. Feel free to merge without adjusting or to adjust and then merge right away without waiting for a follow-up review.

Legioth · 2026-03-16T11:01:18Z

flow-server/src/main/java/com/vaadin/flow/internal/SignalFieldTransfer.java

+        Class<?> current = targetClazz;
+        while (current != null && current != Object.class) {
+            try {
+                if (Objects.equals(current.getName(),


I'm confused by the use of Objects.equals here since it implies that either value might be null but I don't see how that could ever happen. (And if still can happen, then I don't think we should match in that case)

removed unnecessary usage of Object.equals.

sonarqubecloud · 2026-03-16T11:30:19Z

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

* feat: retain local signal values during hotswap Automatically transfer local signal values from old view fields to matching fields in the new view during hotswap refresh. Fixes: #23830 * make SignalFieldTransfer Serializable * update javadoc * Add more tests * Add unit test and remove unused constructor * Fix code review findings * remove unnecessary java.util.Objects usage --------- Co-authored-by: Leif Åstrand <leif@vaadin.com>

* feat: retain local signal values during hotswap Automatically transfer local signal values from old view fields to matching fields in the new view during hotswap refresh. Fixes: #23830 * make SignalFieldTransfer Serializable * update javadoc * Add more tests * Add unit test and remove unused constructor * Fix code review findings * remove unnecessary java.util.Objects usage --------- Co-authored-by: Tomi Virtanen <tltv@vaadin.com> Co-authored-by: Leif Åstrand <leif@vaadin.com>

feat: retain local signal values during hotswap

a9ed494

Automatically transfer local signal values from old view fields to matching fields in the new view during hotswap refresh. Fixes: #23830

github-actions bot added the +0.0.1 label Mar 12, 2026

make SignalFieldTransfer Serializable

fcac558

github-actions bot added +0.1.0 and removed +0.0.1 labels Mar 12, 2026

Legioth reviewed Mar 13, 2026

View reviewed changes

flow-server/src/main/java/com/vaadin/flow/router/internal/AbstractNavigationStateRenderer.java Show resolved Hide resolved

tltv added 3 commits March 13, 2026 13:08

update javadoc

d2aef52

Add more tests

9f54e23

Add unit test and remove unused constructor

387c8d1

tltv marked this pull request as ready for review March 13, 2026 11:31

tltv and others added 2 commits March 13, 2026 13:32

Merge branch 'main' into feat/23830-retain-signal-value-on-hotswap

4463d7b

Merge branch 'main' into feat/23830-retain-signal-value-on-hotswap

a67ae8c

Legioth requested changes Mar 16, 2026

View reviewed changes

Fix code review findings

3011512

tltv requested a review from Legioth March 16, 2026 10:45

Legioth previously approved these changes Mar 16, 2026

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mshabarov added target/25.1 live reload signals labels Mar 16, 2026

remove unnecessary java.util.Objects usage

2980ccd

tltv dismissed Legioth’s stale review via 2980ccd March 16, 2026 11:21

Merge branch 'main' into feat/23830-retain-signal-value-on-hotswap

ee9747b

Legioth approved these changes Mar 16, 2026

View reviewed changes

mshabarov merged commit 09d7e0c into main Mar 16, 2026
51 of 52 checks passed

mshabarov deleted the feat/23830-retain-signal-value-on-hotswap branch March 16, 2026 12:04

vaadin-bot added the cherry-picked-25.1 label Mar 16, 2026

vaadin-bot mentioned this pull request Mar 16, 2026

feat: retain local signal values during hotswap (#23854) (CP: 25.1) #23894

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Signal transfer utility

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sonarqubecloud bot commented Mar 16, 2026

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tltv commented Mar 12, 2026 •

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