alien-signals-java

JVM port of stackblitz/alien-signals 3.1.2. Line-for-line, same algorithm, same flags, same DLL semantics.

Why

alien-signals is a small reactive-signals library: signals, computed values, effects, an automatic dependency graph with pull-on-read and push-mark-dirty propagation. It was previously available only on JavaScript runtimes via npm. This artifact ports the same algorithm to the JVM so any Java, Kotlin, Scala, or Clojure consumer can use it with no JS bridge.

Layout

src/main/java/hbt/aliensignals/
  ReactiveFlags.java      Bit-flag constants
  ReactiveNode.java       Base node (deps, depsTail, subs, subsTail, flags)
  Link.java               Doubly-linked-list edge between dep and sub
  ReactiveSystem.java     Algorithm core: link, unlink, propagate,
                          checkDirty, shallowPropagate, startTracking,
                          endTracking (port of system.ts)
  SignalNode.java         value + previousValue
  ComputedNode.java       value + getter
  EffectNode.java         fn
  Signal.java             User-facing get / set
  Computed.java           User-facing get
  Effect.java             User-facing stop
  Signals.java            Static facade: signal, computed, effect, batch,
                          untrack, effectScope, trigger (port of index.ts)

Usage

import static hbt.aliensignals.Signals.*;

Signal<Integer> a = signal(1);
Computed<Integer> b = computed(prev -> a.get() * 2);
Effect e = effect(() -> System.out.println(b.get()));

a.set(5);    // prints 10
batch(() -> {
    a.set(7);
    a.set(9);
});         // prints 18 once
e.stop();

Build

mvn install

Installs hbt:alien-signals:3.1.2 to ~/.m2/repository.

In a deps.edn consumer:

{:deps {hbt/alien-signals {:mvn/version "3.1.2"}}}

Test parity

21 JUnit tests cover the load-bearing cases ported from upstream Vitest:

• signal read/write semantics, value-equality skip
• computed memoisation and chained propagation
• effect lifecycle (eager run, dispose, nested stop)
• batch coalescing
• untrack via Signals.untrack and via direct setActiveSub (matches the upstream pattern in untrack.spec.ts)
• diamond topology — both signal-only and computed-via-checkDirty
• signal-revert: writing then reverting must NOT force recompute
• effectScope dispose

mvn test
[INFO] Tests run: 21, Failures: 0, Errors: 0, Skipped: 0

Threading

Single-threaded, same as upstream. The shared Signals static state (activeSub, activeScope, batchDepth, notify queue) is not synchronised. Calling from multiple threads is unsafe.

Versioning

Tracks upstream npm. When stackblitz publishes a new version, this artifact moves to the same version with the corresponding system.ts and index.ts changes ported.

License

MIT — same as upstream.