improve memory footprint of the timer

implementation/core/src/main/java/io/smallrye/faulttolerance/core/timeout/Timeout.java

@@ -34,7 +34,7 @@ public V apply(InvocationContext<V> ctx) throws Exception {
     }
 
     private V doApply(InvocationContext<V> ctx) throws Exception {
-        // must extract `AsyncTimeoutNotification` synchronously, because if retries are present,
+        // must extract `AsyncTimeoutNotification` early, because if retries are present,
         // a different `AsyncTimeoutNotification` may be present in the `InvocationContext`
         // by the time the timeout callback is invoked
         AsyncTimeoutNotification notification = ctx.get(AsyncTimeoutNotification.class);

implementation/core/src/main/java/io/smallrye/faulttolerance/core/timeout/TimeoutExecution.java

@@ -1,13 +1,23 @@
 package io.smallrye.faulttolerance.core.timeout;
 
-import java.util.concurrent.atomic.AtomicInteger;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
 
 final class TimeoutExecution {
     private static final int STATE_RUNNING = 0;
     private static final int STATE_FINISHED = 1;
     private static final int STATE_TIMED_OUT = 2;
 
-    private final AtomicInteger state;
+    private static final VarHandle STATE;
+    static {
+        try {
+            STATE = MethodHandles.lookup().findVarHandle(TimeoutExecution.class, "state", int.class);
+        } catch (ReflectiveOperationException e) {
+            throw new ExceptionInInitializerError(e);
+        }
+    }
+
+    private volatile int state;
 
     // can be null, if no thread shall be interrupted upon timeout
     private final Thread executingThread;
@@ -21,7 +31,7 @@ final class TimeoutExecution {
     }
 
     TimeoutExecution(Thread executingThread, long timeoutInMillis, Runnable timeoutAction) {
-        this.state = new AtomicInteger(STATE_RUNNING);
+        this.state = STATE_RUNNING;
         this.executingThread = executingThread;
         this.timeoutInMillis = timeoutInMillis;
         this.timeoutAction = timeoutAction;
@@ -32,25 +42,25 @@ long timeoutInMillis() {
     }
 
     boolean isRunning() {
-        return state.get() == STATE_RUNNING;
+        return state == STATE_RUNNING;
     }
 
     boolean hasFinished() {
-        return state.get() == STATE_FINISHED;
+        return state == STATE_FINISHED;
     }
 
     boolean hasTimedOut() {
-        return state.get() == STATE_TIMED_OUT;
+        return state == STATE_TIMED_OUT;
     }
 
     void finish(Runnable ifFinished) {
-        if (state.compareAndSet(STATE_RUNNING, STATE_FINISHED)) {
+        if (STATE.compareAndSet(this, STATE_RUNNING, STATE_FINISHED)) {
             ifFinished.run();
         }
     }
 
     void timeoutAndInterrupt() {
-        if (state.compareAndSet(STATE_RUNNING, STATE_TIMED_OUT)) {
+        if (STATE.compareAndSet(this, STATE_RUNNING, STATE_TIMED_OUT)) {
             if (executingThread != null) {
                 executingThread.interrupt();
             }

implementation/core/src/main/java/io/smallrye/faulttolerance/core/timer/ThreadTimer.java

@@ -10,51 +10,66 @@
 import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
-import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.LockSupport;
-import java.util.function.Consumer;
 
 import io.smallrye.faulttolerance.core.util.RunnableWrapper;
 
 /**
  * Starts one thread that processes submitted tasks in a loop and when it's time for a task to run,
  * it gets submitted to the executor. The default executor is provided by a caller, so the caller
  * must shut down this timer <em>before</em> shutting down the executor.
+ * <p>
+ * At most one timer may exist.
  */
-// TODO implement a hashed wheel?
 public final class ThreadTimer implements Timer {
-    private static final AtomicInteger COUNTER = new AtomicInteger(0);
-
     private static final Comparator<Task> TASK_COMPARATOR = (o1, o2) -> {
+        // two different instances are never equal
         if (o1 == o2) {
-            // two different instances are never equal
             return 0;
         }
 
-        // must _not_ return 0 if start times are equal, because that isn't consistent
-        // with `equals` (see also above)
-        return o1.startTime <= o2.startTime ? -1 : 1;
+        // must _not_ return 0 if start times are equal, because that isn't consistent with `equals` (see also above)
+        // must _not_ compare `startTime` using `<` because of how `System.nanoTime()` works (see also below)
+        long delta = o1.startTime - o2.startTime;
+        if (delta < 0) {
+            return -1;
+        } else if (delta > 0) {
+            return 1;
+        }
+
+        return System.identityHashCode(o1) < System.identityHashCode(o2) ? -1 : 1;
     };
 
-    private final String name;
+    private static volatile ThreadTimer INSTANCE;
+
+    private final SortedSet<Task> tasks = new ConcurrentSkipListSet<>(TASK_COMPARATOR);
 
-    private final SortedSet<Task> tasks;
+    private final Executor defaultExecutor;
 
     private final Thread thread;
 
     private final AtomicBoolean running = new AtomicBoolean(true);
 
     /**
+     * Creates a timer with given {@code defaultExecutor}, unless a timer already exists,
+     * in which case an exception is thrown.
+     *
      * @param defaultExecutor default {@link Executor} used for running scheduled tasks, unless an executor
      *        is provided when {@linkplain #schedule(long, Runnable, Executor) scheduling} a task
      */
-    public ThreadTimer(Executor defaultExecutor) {
-        checkNotNull(defaultExecutor, "Executor must be set");
+    public static synchronized ThreadTimer create(Executor defaultExecutor) {
+        ThreadTimer instance = INSTANCE;
+        if (instance == null) {
+            instance = new ThreadTimer(defaultExecutor);
+            INSTANCE = instance;
+            return instance;
+        }
+        throw new IllegalStateException("Timer already exists");
+    }
 
-        this.name = "SmallRye Fault Tolerance Timer " + COUNTER.incrementAndGet();
-        LOG.createdTimer(name);
+    private ThreadTimer(Executor defaultExecutor) {
+        this.defaultExecutor = checkNotNull(defaultExecutor, "Executor must be set");
 
-        this.tasks = new ConcurrentSkipListSet<>(TASK_COMPARATOR);
         this.thread = new Thread(() -> {
             while (running.get()) {
                 try {
@@ -77,35 +92,30 @@ public ThreadTimer(Executor defaultExecutor) {
                         // in such case, `taskStartTime` can be positive, `currentTime` can be negative,
                         //  and yet `taskStartTime` is _before_ `currentTime`
                         if (taskStartTime - currentTime <= 0) {
-                            tasks.remove(task);
-                            if (task.state.compareAndSet(Task.STATE_NEW, Task.STATE_RUNNING)) {
-                                Executor executorForTask = task.executorOverride;
+                            boolean removed = tasks.remove(task);
+                            if (removed) {
+                                Executor executorForTask = task.executor();
                                 if (executorForTask == null) {
                                     executorForTask = defaultExecutor;
                                 }
 
-                                executorForTask.execute(() -> {
-                                    LOG.runningTimerTask(task);
-                                    try {
-                                        task.runnable.run();
-                                    } finally {
-                                        task.state.set(Task.STATE_FINISHED);
-                                    }
-                                });
+                                executorForTask.execute(task);
                             }
                         } else {
                             // this is OK even if another timer is scheduled during the sleep (even if that timer should
-                            // fire sooner than `taskStartTime`), because `schedule` always calls` LockSupport.unpark`
+                            // fire sooner than `taskStartTime`), because `schedule` always calls `LockSupport.unpark`
                             LockSupport.parkNanos(taskStartTime - currentTime);
                         }
                     }
-                } catch (Exception e) {
+                } catch (Throwable e) {
                     // can happen e.g. when the executor is shut down sooner than the timer
                     LOG.unexpectedExceptionInTimerLoop(e);
                 }
             }
-        }, name);
+        }, "SmallRye Fault Tolerance Timer");
         thread.start();
+
+        LOG.createdTimer();
     }
 
     @Override
@@ -116,7 +126,10 @@ public TimerTask schedule(long delayInMillis, Runnable task) {
     @Override
     public TimerTask schedule(long delayInMillis, Runnable task, Executor executor) {
         long startTime = System.nanoTime() + TimeUnit.MILLISECONDS.toNanos(delayInMillis);
-        Task timerTask = new Task(startTime, RunnableWrapper.INSTANCE.wrap(task), tasks::remove, executor);
+        task = RunnableWrapper.INSTANCE.wrap(task);
+        Task timerTask = executor == null || executor == defaultExecutor
+                ? new Task(startTime, task)
+                : new TaskWithExecutor(startTime, task, executor);
         tasks.add(timerTask);
         LockSupport.unpark(thread);
         LOG.scheduledTimerTask(timerTask, delayInMillis);
@@ -131,47 +144,84 @@ public int countScheduledTasks() {
     @Override
     public void shutdown() throws InterruptedException {
         if (running.compareAndSet(true, false)) {
-            LOG.shutdownTimer(name);
-            thread.interrupt();
-            thread.join();
+            try {
+                LOG.shutdownTimer();
+                thread.interrupt();
+                thread.join();
+            } finally {
+                INSTANCE = null;
+            }
         }
     }
 
-    private static final class Task implements TimerTask {
-        static final int STATE_NEW = 0; // was scheduled, but isn't running yet
-        static final int STATE_RUNNING = 1; // running on the executor
-        static final int STATE_FINISHED = 2; // finished running
-        static final int STATE_CANCELLED = 3; // cancelled before it could be executed
+    private static class Task implements TimerTask, Runnable {
+        // scheduled: present in the `tasks` queue
+        // running: not present in the `tasks` queue && `runnable != null`
+        // finished or cancelled: not present in the `tasks` queue && `runnable == null`
 
         final long startTime; // in nanos, to be compared with System.nanoTime()
-        final Runnable runnable;
-        final Executor executorOverride; // may be null, which means that the timer's executor shall be used
-        final AtomicInteger state = new AtomicInteger(STATE_NEW);
-
-        private final Consumer<TimerTask> onCancel;
+        volatile Runnable runnable;
 
-        Task(long startTime, Runnable runnable, Consumer<TimerTask> onCancel, Executor executorOverride) {
+        Task(long startTime, Runnable runnable) {
             this.startTime = startTime;
             this.runnable = checkNotNull(runnable, "Runnable task must be set");
-            this.onCancel = checkNotNull(onCancel, "Cancellation callback must be set");
-            this.executorOverride = executorOverride;
         }
 
         @Override
         public boolean isDone() {
-            int state = this.state.get();
-            return state == STATE_FINISHED || state == STATE_CANCELLED;
+            ThreadTimer timer = INSTANCE;
+            if (timer != null) {
+                boolean queued = timer.tasks.contains(this);
+                if (queued) {
+                    return false;
+                } else {
+                    return runnable == null;
+                }
+            }
+            return true; // ?
         }
 
         @Override
         public boolean cancel() {
-            // can't cancel if it's already running
-            if (state.compareAndSet(STATE_NEW, STATE_CANCELLED)) {
-                LOG.cancelledTimerTask(this);
-                onCancel.accept(this);
-                return true;
+            ThreadTimer timer = INSTANCE;
+            if (timer != null) {
+                // can't cancel if it's already running
+                boolean removed = timer.tasks.remove(this);
+                if (removed) {
+                    runnable = null;
+                    LOG.cancelledTimerTask(this);
+                    return true;
+                }
             }
             return false;
         }
+
+        public Executor executor() {
+            return null; // default executor of the timer should be used
+        }
+
+        @Override
+        public void run() {
+            LOG.runningTimerTask(this);
+            try {
+                runnable.run();
+            } finally {
+                runnable = null;
+            }
+        }
+    }
+
+    private static final class TaskWithExecutor extends Task {
+        private final Executor executor;
+
+        TaskWithExecutor(long startTime, Runnable runnable, Executor executor) {
+            super(startTime, runnable);
+            this.executor = checkNotNull(executor, "Executor must be set");
+        }
+
+        @Override
+        public Executor executor() {
+            return executor;
+        }
     }
 }

implementation/core/src/main/java/io/smallrye/faulttolerance/core/timer/TimerLogger.java

@@ -14,13 +14,13 @@
 interface TimerLogger extends BasicLogger {
     TimerLogger LOG = Logger.getMessageLogger(TimerLogger.class, TimerLogger.class.getPackage().getName());
 
-    @Message(id = NONE, value = "Timer %s created")
+    @Message(id = NONE, value = "Timer created")
     @LogMessage(level = Logger.Level.TRACE)
-    void createdTimer(String name);
+    void createdTimer();
 
-    @Message(id = NONE, value = "Timer %s shut down")
+    @Message(id = NONE, value = "Timer shut down")
     @LogMessage(level = Logger.Level.TRACE)
-    void shutdownTimer(String name);
+    void shutdownTimer();
 
     @Message(id = NONE, value = "Scheduled timer task %s to run in %s millis")
     @LogMessage(level = Logger.Level.TRACE)
@@ -36,5 +36,5 @@ interface TimerLogger extends BasicLogger {
 
     @Message(id = 11000, value = "Unexpected exception in timer loop, ignoring")
     @LogMessage(level = Logger.Level.WARN)
-    void unexpectedExceptionInTimerLoop(@Cause Exception e);
+    void unexpectedExceptionInTimerLoop(@Cause Throwable e);
 }

implementation/core/src/test/java/io/smallrye/faulttolerance/core/timeout/RealWorldCompletionStageTimeoutTest.java

@@ -55,7 +55,7 @@ public void setUp() {
         executor = Executors.newSingleThreadExecutor();
 
         timerExecutor = Executors.newSingleThreadExecutor();
-        timer = new ThreadTimer(timerExecutor);
+        timer = ThreadTimer.create(timerExecutor);
         timerWatcher = new TimerTimeoutWatcher(timer);
     }
 

implementation/core/src/test/java/io/smallrye/faulttolerance/core/timeout/TimerTimeoutWatcherTest.java

@@ -27,7 +27,7 @@ public class TimerTimeoutWatcherTest {
     @BeforeEach
     public void setUp() {
         executor = Executors.newSingleThreadExecutor();
-        timer = new ThreadTimer(executor);
+        timer = ThreadTimer.create(executor);
         watcher = new TimerTimeoutWatcher(timer);
     }