perf: Schedule session end on shared executor (JAVA-653) (#5819)

* perf: Schedule session end on shared executor (JAVA-653)

LifecycleWatcher created a java.util.Timer whose thread was spawned on
the first background transition and lived for the rest of the process.
Schedule the session-end task on the shared timer executor instead,
whose single worker thread is reused and self-terminates when idle. If
scheduling fails (executor already shut down), the session is ended
right away instead of leaking.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* changelog

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: runningcode

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Performance
+
+- Reduce the number of SDK threads: `LifecycleWatcher` now schedules the session-end task on the shared timer executor instead of creating a dedicated `java.util.Timer` thread ([#5819](https://github.com/getsentry/sentry-java/pull/5819))
+
 ## 8.50.0
 
 ### Android 17 support

sentry-android-core/src/main/java/io/sentry/android/core/LifecycleWatcher.java

@@ -8,9 +8,7 @@
 import io.sentry.transport.CurrentDateProvider;
 import io.sentry.transport.ICurrentDateProvider;
 import io.sentry.util.AutoClosableReentrantLock;
-import io.sentry.util.LazyEvaluator;
-import java.util.Timer;
-import java.util.TimerTask;
+import java.util.concurrent.Future;
 import java.util.concurrent.atomic.AtomicLong;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
@@ -22,9 +20,8 @@ final class LifecycleWatcher implements AppState.AppStateListener {
 
   private final long sessionIntervalMillis;
 
-  private @Nullable TimerTask timerTask;
-  private final @NotNull LazyEvaluator<Timer> timer = new LazyEvaluator<>(() -> new Timer(true));
-  private final @NotNull AutoClosableReentrantLock timerLock = new AutoClosableReentrantLock();
+  private @Nullable Future<?> endSessionFuture;
+  private final @NotNull AutoClosableReentrantLock endSessionLock = new AutoClosableReentrantLock();
   private final @NotNull IScopes scopes;
   private final boolean enableSessionTracking;
   private final boolean enableAppLifecycleBreadcrumbs;
@@ -104,29 +101,40 @@ public void onBackground() {
   }
 
   private void scheduleEndSession() {
-    try (final @NotNull ISentryLifecycleToken ignored = timerLock.acquire()) {
+    try (final @NotNull ISentryLifecycleToken ignored = endSessionLock.acquire()) {
       cancelTask();
-      timerTask =
-          new TimerTask() {
-            @Override
-            public void run() {
-              if (enableSessionTracking) {
-                scopes.endSession();
-              }
-              scopes.getOptions().getReplayController().stop();
-              scopes.getOptions().getContinuousProfiler().close(false);
+      final @NotNull Runnable endSession =
+          () -> {
+            if (enableSessionTracking) {
+              scopes.endSession();
             }
+            scopes.getOptions().getReplayController().stop();
+            scopes.getOptions().getContinuousProfiler().close(false);
           };
 
-      timer.getValue().schedule(timerTask, sessionIntervalMillis);
+      try {
+        endSessionFuture =
+            scopes
+                .getOptions()
+                .getTimerExecutorService()
+                .schedule(endSession, sessionIntervalMillis);
+      } catch (Throwable e) {
+        scopes
+            .getOptions()
+            .getLogger()
+            .log(SentryLevel.WARNING, "Failed to schedule end of session. Ending it now.", e);
+        // if we cannot re-check after the session interval, end the session right away instead of
+        // leaving it open forever
+        endSession.run();
+      }
     }
   }
 
   private void cancelTask() {
-    try (final @NotNull ISentryLifecycleToken ignored = timerLock.acquire()) {
-      if (timerTask != null) {
-        timerTask.cancel();
-        timerTask = null;
+    try (final @NotNull ISentryLifecycleToken ignored = endSessionLock.acquire()) {
+      if (endSessionFuture != null) {
+        endSessionFuture.cancel(false);
+        endSessionFuture = null;
       }
     }
   }
@@ -144,13 +152,7 @@ private void addAppBreadcrumb(final @NotNull String state) {
 
   @TestOnly
   @Nullable
-  TimerTask getTimerTask() {
-    return timerTask;
-  }
-
-  @TestOnly
-  @NotNull
-  Timer getTimer() {
-    return timer.getValue();
+  Future<?> getEndSessionFuture() {
+    return endSessionFuture;
   }
 }

sentry-android-core/src/test/java/io/sentry/android/core/LifecycleWatcherTest.kt

@@ -7,6 +7,7 @@ import io.sentry.IScope
 import io.sentry.IScopes
 import io.sentry.ReplayController
 import io.sentry.ScopeCallback
+import io.sentry.SentryExecutorService
 import io.sentry.SentryLevel
 import io.sentry.SentryOptions
 import io.sentry.Session
@@ -32,7 +33,8 @@ class LifecycleWatcherTest {
   private class Fixture {
     val scopes = mock<IScopes>()
     val dateProvider = mock<ICurrentDateProvider>()
-    val options = SentryOptions()
+    // a real executor so scheduled end-session tasks actually run
+    val options = SentryOptions().apply { setTimerExecutorService(SentryExecutorService(this)) }
     val replayController = mock<ReplayController>()
     val continuousProfiler = mock<IContinuousProfiler>()
 
@@ -115,10 +117,10 @@ class LifecycleWatcherTest {
     watcher.onForeground()
 
     watcher.onBackground()
-    assertNotNull(watcher.timerTask)
+    assertNotNull(watcher.endSessionFuture)
 
     watcher.onForeground()
-    assertNull(watcher.timerTask)
+    assertNull(watcher.endSessionFuture)
 
     verify(fixture.scopes, never()).endSession()
     verify(fixture.replayController, never()).stop()
@@ -186,13 +188,6 @@ class LifecycleWatcherTest {
     verify(fixture.scopes, never()).addBreadcrumb(any<Breadcrumb>())
   }
 
-  @Test
-  fun `timer is created if session tracking is enabled`() {
-    val watcher =
-      fixture.getSUT(enableAutoSessionTracking = true, enableAppLifecycleBreadcrumbs = false)
-    assertNotNull(watcher.timer)
-  }
-
   @Test
   fun `if the scopes has already a fresh session running, don't start new one`() {
     val watcher =