more optimized promise implementation for completed promises

futur-api/src/main/java/dev/tommyjs/futur/promise/AbstractPromise.java

@@ -9,47 +9,29 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.slf4j.Logger;
 
-import java.lang.invoke.MethodHandles;
-import java.lang.invoke.VarHandle;
-import java.util.Collection;
-import java.util.Collections;
-import java.util.Iterator;
-import java.util.Objects;
 import java.util.concurrent.*;
 import java.util.concurrent.atomic.AtomicReference;
-import java.util.concurrent.locks.AbstractQueuedSynchronizer;
 import java.util.function.Consumer;
 
-@SuppressWarnings({"FieldMayBeFinal", "unchecked"})
-public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T> {
-
-    private static final VarHandle COMPLETION_HANDLE;
-    private static final VarHandle LISTENERS_HANDLE;
-
-    static {
-        try {
-            MethodHandles.Lookup lookup = MethodHandles.lookup();
-            COMPLETION_HANDLE = lookup.findVarHandle(AbstractPromise.class, "completion", PromiseCompletion.class);
-            LISTENERS_HANDLE = lookup.findVarHandle(AbstractPromise.class, "listeners", Collection.class);
-        } catch (ReflectiveOperationException e) {
-            throw new ExceptionInInitializerError(e);
-        }
-    }
-
-    private final Sync sync;
-
-    private volatile Collection<PromiseListener<T>> listeners;
-    private volatile PromiseCompletion<T> completion;
-
-    public AbstractPromise() {
-        this.sync = new Sync();
-        this.listeners = Collections.EMPTY_LIST;
-        this.completion = null;
-    }
+public abstract class AbstractPromise<T, FS, FA> implements Promise<T> {
 
     public abstract @NotNull AbstractPromiseFactory<FS, FA> getFactory();
 
-    private void runCompleter(@NotNull CompletablePromise<?> promise, @NotNull ExceptionalRunnable completer) {
+    protected abstract @NotNull Promise<T> addAnyListener(@NotNull PromiseListener<T> listener);
+
+    protected @NotNull Logger getLogger() {
+        return getFactory().getLogger();
+    }
+
+    protected void callListener(@NotNull PromiseListener<T> listener, @NotNull PromiseCompletion<T> cmp) {
+        if (listener instanceof AsyncPromiseListener) {
+            callListenerAsync(listener, cmp);
+        } else {
+            callListenerNow(listener, cmp);
+        }
+    }
+
+    protected void runCompleter(@NotNull CompletablePromise<?> promise, @NotNull ExceptionalRunnable completer) {
         try {
             completer.run();
         } catch (Error e) {
@@ -60,11 +42,8 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
         }
     }
 
-    private <V> @NotNull Runnable createCompleter(
-        T result,
-        @NotNull CompletablePromise<V> promise,
-        @NotNull ExceptionalFunction<T, V> completer
-    ) {
+    protected <V> @NotNull Runnable createCompleter(T result, @NotNull CompletablePromise<V> promise,
+                                                    @NotNull ExceptionalFunction<T, V> completer) {
         return () -> {
             if (!promise.isCompleted()) {
                 runCompleter(promise, () -> promise.complete(completer.apply(result)));
@@ -72,43 +51,37 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
         };
     }
 
-    protected @NotNull Logger getLogger() {
-        return getFactory().getLogger();
+    protected <V> @NotNull CompletablePromise<V> createLinked() {
+        CompletablePromise<V> promise = getFactory().unresolved();
+        PromiseUtil.propagateCancel(promise, this);
+        return promise;
     }
 
-    @Override
-    public T get() throws InterruptedException, ExecutionException {
-        sync.acquireSharedInterruptibly(1);
-        return joinCompletion();
-    }
-
-    @Override
-    public T get(long time, @NotNull TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
-        boolean success = sync.tryAcquireSharedNanos(1, unit.toNanos(time));
-        if (!success) {
-            throw new TimeoutException("Promise stopped waiting after " + time + " " + unit);
-        }
-
-        return joinCompletion();
-    }
-
-    @Override
-    public T await() {
+    protected void callListenerAsync(PromiseListener<T> listener, PromiseCompletion<T> res) {
         try {
-            sync.acquireSharedInterruptibly(1);
-        } catch (InterruptedException e) {
-            throw new RuntimeException(e);
+            getFactory().getAsyncExecutor().run(() -> callListenerNow(listener, res));
+        } catch (RejectedExecutionException ignored) {
+        } catch (Exception e) {
+            getLogger().warn("Exception caught while running promise listener", e);
         }
-
-        PromiseCompletion<T> completion = Objects.requireNonNull(getCompletion());
-        if (completion.isSuccess()) return completion.getResult();
-        throw new CompletionException(completion.getException());
     }
 
-    private T joinCompletion() throws ExecutionException {
-        PromiseCompletion<T> completion = Objects.requireNonNull(getCompletion());
-        if (completion.isSuccess()) return completion.getResult();
-        throw new ExecutionException(completion.getException());
+    protected void callListenerNow(PromiseListener<T> listener, PromiseCompletion<T> res) {
+        try {
+            listener.handle(res);
+        } catch (Error e) {
+            getLogger().error("Error caught in promise listener", e);
+            throw e;
+        } catch (Throwable e) {
+            getLogger().error("Exception caught in promise listener", e);
+        }
+    }
+
+    protected void callListenerAsyncLastResort(PromiseListener<T> listener, PromiseCompletion<T> completion) {
+        try {
+            getFactory().getAsyncExecutor().run(() -> callListenerNow(listener, completion));
+        } catch (Throwable ignored) {
+        }
     }
 
     @Override
@@ -141,33 +114,68 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
 
     @Override
     public <V> @NotNull Promise<V> thenApply(@NotNull ExceptionalFunction<T, V> task) {
-        CompletablePromise<V> promise = getFactory().unresolved();
-        addDirectListener(
-            res -> createCompleter(res, promise, task).run(),
-            promise::completeExceptionally
-        );
+        PromiseCompletion<T> completion = getCompletion();
+        if (completion == null) {
+            CompletablePromise<V> promise = createLinked();
+            addDirectListener(
+                res -> createCompleter(res, promise, task).run(),
+                promise::completeExceptionally
+            );
 
-        PromiseUtil.propagateCancel(promise, this);
-        return promise;
+            return promise;
+        } else if (completion.isSuccess()) {
+            try {
+                V result = task.apply(completion.getResult());
+                return getFactory().resolve(result);
+            } catch (Exception e) {
+                return getFactory().error(e);
+            }
+        } else {
+            Throwable ex = completion.getException();
+            assert ex != null;
+            return getFactory().error(ex);
+        }
     }
 
     @Override
     public <V> @NotNull Promise<V> thenCompose(@NotNull ExceptionalFunction<T, Promise<V>> task) {
-        CompletablePromise<V> promise = getFactory().unresolved();
-        thenApply(task).addDirectListener(
-            nestedPromise -> {
-                if (nestedPromise == null) {
-                    promise.complete(null);
-                } else {
-                    PromiseUtil.propagateCompletion(nestedPromise, promise);
-                    PromiseUtil.propagateCancel(promise, nestedPromise);
-                }
-            },
-            promise::completeExceptionally
-        );
+        PromiseCompletion<T> completion = getCompletion();
+        if (completion == null) {
+            CompletablePromise<V> promise = createLinked();
+            thenApply(task).addDirectListener(
+                result -> {
+                    if (result == null) {
+                        promise.complete(null);
+                    } else {
+                        PromiseUtil.propagateCompletion(result, promise);
+                        PromiseUtil.propagateCancel(promise, result);
+                    }
+                },
+                promise::completeExceptionally
+            );
 
-        PromiseUtil.propagateCancel(promise, this);
-        return promise;
+            return promise;
+        } else if (completion.isSuccess()) {
+            try {
+                Promise<V> result = task.apply(completion.getResult());
+                if (result == null) {
+                    return getFactory().resolve(null);
+                } else if (result.isCompleted()) {
+                    return result;
+                } else {
+                    CompletablePromise<V> promise = createLinked();
+                    PromiseUtil.propagateCompletion(result, promise);
+                    PromiseUtil.propagateCancel(promise, result);
+                    return promise;
+                }
+            } catch (Exception e) {
+                return getFactory().error(e);
+            }
+        } else {
+            Throwable ex = completion.getException();
+            assert ex != null;
+            return getFactory().error(ex);
+        }
     }
 
     @Override
@@ -214,7 +222,7 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
 
     @Override
     public <V> @NotNull Promise<V> thenApplySync(@NotNull ExceptionalFunction<T, V> task) {
-        CompletablePromise<V> promise = getFactory().unresolved();
+        CompletablePromise<V> promise = createLinked();
         addDirectListener(
             res -> runCompleter(promise, () -> {
                 Runnable runnable = createCompleter(res, promise, task);
@@ -224,13 +232,12 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
             promise::completeExceptionally
         );
 
-        PromiseUtil.propagateCancel(promise, this);
         return promise;
     }
 
     @Override
     public <V> @NotNull Promise<V> thenApplyDelayedSync(@NotNull ExceptionalFunction<T, V> task, long delay, @NotNull TimeUnit unit) {
-        CompletablePromise<V> promise = getFactory().unresolved();
+        CompletablePromise<V> promise = createLinked();
         addDirectListener(
             res -> runCompleter(promise, () -> {
                 Runnable runnable = createCompleter(res, promise, task);
@@ -240,13 +247,12 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
             promise::completeExceptionally
         );
 
-        PromiseUtil.propagateCancel(promise, this);
         return promise;
     }
 
     @Override
     public <V> @NotNull Promise<V> thenComposeSync(@NotNull ExceptionalFunction<T, Promise<V>> task) {
-        CompletablePromise<V> promise = getFactory().unresolved();
+        CompletablePromise<V> promise = createLinked();
         thenApplySync(task).addDirectListener(
             nestedPromise -> {
                 if (nestedPromise == null) {
@@ -259,7 +265,6 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
             promise::completeExceptionally
         );
 
-        PromiseUtil.propagateCancel(promise, this);
         return promise;
     }
 
@@ -307,7 +312,7 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
 
     @Override
     public <V> @NotNull Promise<V> thenApplyAsync(@NotNull ExceptionalFunction<T, V> task) {
-        CompletablePromise<V> promise = getFactory().unresolved();
+        CompletablePromise<V> promise = createLinked();
         addDirectListener(
             (res) -> runCompleter(promise, () -> {
                 Runnable runnable = createCompleter(res, promise, task);
@@ -317,13 +322,12 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
             promise::completeExceptionally
         );
 
-        PromiseUtil.propagateCancel(promise, this);
         return promise;
     }
 
     @Override
     public <V> @NotNull Promise<V> thenApplyDelayedAsync(@NotNull ExceptionalFunction<T, V> task, long delay, @NotNull TimeUnit unit) {
-        CompletablePromise<V> promise = getFactory().unresolved();
+        CompletablePromise<V> promise = createLinked();
         addDirectListener(
             res -> runCompleter(promise, () -> {
                 Runnable runnable = createCompleter(res, promise, task);
@@ -333,13 +337,12 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
             promise::completeExceptionally
         );
 
-        PromiseUtil.propagateCancel(promise, this);
         return promise;
     }
 
     @Override
     public <V> @NotNull Promise<V> thenComposeAsync(@NotNull ExceptionalFunction<T, Promise<V>> task) {
-        CompletablePromise<V> promise = getFactory().unresolved();
+        CompletablePromise<V> promise = createLinked();
         thenApplyAsync(task).addDirectListener(
             nestedPromise -> {
                 if (nestedPromise == null) {
@@ -352,7 +355,6 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
             promise::completeExceptionally
         );
 
-        PromiseUtil.propagateCancel(promise, this);
         return promise;
     }
 
@@ -401,51 +403,6 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
         });
     }
 
-    private @NotNull Promise<T> addAnyListener(PromiseListener<T> listener) {
-        Collection<PromiseListener<T>> prev = listeners, next = null;
-        for (boolean haveNext = false; ; ) {
-            if (!haveNext) {
-                next = prev == Collections.EMPTY_LIST ? new ConcurrentLinkedQueue<>() : prev;
-                if (next != null) next.add(listener);
-            }
-
-            if (LISTENERS_HANDLE.weakCompareAndSet(this, prev, next))
-                break;
-
-            haveNext = (prev == (prev = listeners));
-        }
-
-        if (next == null) {
-            if (listener instanceof AsyncPromiseListener) {
-                callListenerAsync(listener, Objects.requireNonNull(getCompletion()));
-            } else {
-                callListenerNow(listener, Objects.requireNonNull(getCompletion()));
-            }
-        }
-
-        return this;
-    }
-
-    private void callListenerAsync(PromiseListener<T> listener, PromiseCompletion<T> res) {
-        try {
-            getFactory().getAsyncExecutor().run(() -> callListenerNow(listener, res));
-        } catch (RejectedExecutionException ignored) {
-        } catch (Exception e) {
-            getLogger().warn("Exception caught while running promise listener", e);
-        }
-    }
-
-    private void callListenerNow(PromiseListener<T> listener, PromiseCompletion<T> res) {
-        try {
-            listener.handle(res);
-        } catch (Error e) {
-            getLogger().error("Error caught in promise listener", e);
-            throw e;
-        } catch (Throwable e) {
-            getLogger().error("Exception caught in promise listener", e);
-        }
-    }
-
     @Override
     public @NotNull Promise<T> onSuccess(@NotNull Consumer<T> listener) {
         return addAsyncListener(listener, null);
@@ -489,92 +446,11 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
 
     @Override
     public @NotNull Promise<T> orDefault(@NotNull ExceptionalFunction<Throwable, T> function) {
-        CompletablePromise<T> promise = getFactory().unresolved();
-        addDirectListener(promise::complete, e -> {
-            try {
-                T result = function.apply(e);
-                promise.complete(result);
-            } catch (Exception ex) {
-                promise.completeExceptionally(ex);
-            }
-        });
-
-        PromiseUtil.propagateCancel(promise, this);
+        CompletablePromise<T> promise = createLinked();
+        addDirectListener(promise::complete, e -> runCompleter(promise, () -> promise.complete(function.apply(e))));
         return promise;
     }
 
-    @Override
-    public @NotNull Promise<T> timeout(long time, @NotNull TimeUnit unit) {
-        Exception e = new CancellationException("Promise timed out after " + time + " " + unit.toString().toLowerCase());
-        return completeExceptionallyDelayed(e, time, unit);
-    }
-
-    @Override
-    public @NotNull Promise<T> maxWaitTime(long time, @NotNull TimeUnit unit) {
-        Exception e = new TimeoutException("Promise stopped waiting after " + time + " " + unit.toString().toLowerCase());
-        return completeExceptionallyDelayed(e, time, unit);
-    }
-
-    private Promise<T> completeExceptionallyDelayed(Throwable e, long delay, TimeUnit unit) {
-        runCompleter(this, () -> {
-            FA future = getFactory().getAsyncExecutor().run(() -> completeExceptionally(e), delay, unit);
-            addDirectListener(_ -> getFactory().getAsyncExecutor().cancel(future));
-        });
-        return this;
-    }
-
-    private void handleCompletion(@NotNull PromiseCompletion<T> cmp) {
-        if (!COMPLETION_HANDLE.compareAndSet(this, null, cmp)) return;
-        sync.releaseShared(1);
-
-
-        Iterator<PromiseListener<T>> iter = ((Iterable<PromiseListener<T>>) LISTENERS_HANDLE.getAndSet(this, null)).iterator();
-        try {
-            while (iter.hasNext()) {
-                PromiseListener<T> listener = iter.next();
-                if (listener instanceof AsyncPromiseListener) {
-                    callListenerAsync(listener, cmp);
-                } else {
-                    callListenerNow(listener, cmp);
-                }
-            }
-        } finally {
-            iter.forEachRemaining(v -> callListenerAsyncLastResort(v, cmp));
-        }
-    }
-
-    private void callListenerAsyncLastResort(PromiseListener<T> listener, PromiseCompletion<T> completion) {
-        try {
-            getFactory().getAsyncExecutor().run(() -> callListenerNow(listener, completion));
-        } catch (Throwable ignored) {
-        }
-    }
-
-    @Override
-    public void cancel(@NotNull CancellationException e) {
-        completeExceptionally(e);
-    }
-
-    @Override
-    public void complete(@Nullable T result) {
-        handleCompletion(new PromiseCompletion<>(result));
-    }
-
-    @Override
-    public void completeExceptionally(@NotNull Throwable result) {
-        handleCompletion(new PromiseCompletion<>(result));
-    }
-
-    @Override
-    public boolean isCompleted() {
-        return completion != null;
-    }
-
-    @Override
-    public @Nullable PromiseCompletion<T> getCompletion() {
-        return completion;
-    }
-
     @Override
     public @NotNull CompletableFuture<T> toFuture() {
         CompletableFuture<T> future = new CompletableFuture<>();
@@ -591,31 +467,4 @@ public abstract class AbstractPromise<T, FS, FA> implements CompletablePromise<T
     private static class DeferredExecutionException extends ExecutionException {
     }
 
-    private static final class Sync extends AbstractQueuedSynchronizer {
-
-        private Sync() {
-            setState(1);
-        }
-
-        @Override
-        protected int tryAcquireShared(int acquires) {
-            return getState() == 0 ? 1 : -1;
-        }
-
-        @Override
-        protected boolean tryReleaseShared(int releases) {
-            int c1, c2;
-            do {
-                c1 = getState();
-                if (c1 == 0) {
-                    return false;
-                }
-
-                c2 = c1 - 1;
-            } while (!compareAndSetState(c1, c2));
-
-            return c2 == 0;
-        }
-    }
-
 }

futur-api/src/main/java/dev/tommyjs/futur/promise/AbstractPromiseFactory.java

@@ -13,7 +13,6 @@ import java.util.*;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionStage;
 import java.util.concurrent.Future;
-import java.util.stream.Stream;
 
 public abstract class AbstractPromiseFactory<FS, FA> implements PromiseFactory {
 
@@ -23,20 +22,6 @@ public abstract class AbstractPromiseFactory<FS, FA> implements PromiseFactory {
 
     public abstract @NotNull PromiseExecutor<FA> getAsyncExecutor();
 
-    @Override
-    public <T> @NotNull Promise<T> resolve(T value) {
-        CompletablePromise<T> promise = unresolved();
-        promise.complete(value);
-        return promise;
-    }
-
-    @Override
-    public <T> @NotNull Promise<T> error(@NotNull Throwable error) {
-        CompletablePromise<T> promise = unresolved();
-        promise.completeExceptionally(error);
-        return promise;
-    }
-
     @Override
     public <T> @NotNull Promise<T> wrap(@NotNull CompletableFuture<T> future) {
         return wrap(future, future);
@@ -112,14 +97,4 @@ public abstract class AbstractPromiseFactory<FS, FA> implements PromiseFactory {
         return promise;
     }
 
-    @Override
-    public <V> @NotNull Promise<V> race(@NotNull Iterable<Promise<V>> promises, boolean cancelLosers) {
-        return race(promises.iterator(), cancelLosers);
-    }
-
-    @Override
-    public <V> @NotNull Promise<V> race(@NotNull Stream<Promise<V>> promises, boolean cancelLosers) {
-        return race(promises.iterator(), cancelLosers);
-    }
-
 }

futur-api/src/main/java/dev/tommyjs/futur/promise/AsyncPromiseListener.java

@@ -5,5 +5,4 @@ package dev.tommyjs.futur.promise;
  * executed asynchronously by the {@link PromiseFactory} that created the completed promise.
  */
 public interface AsyncPromiseListener<T> extends PromiseListener<T> {
-
 }

futur-api/src/main/java/dev/tommyjs/futur/promise/BasePromise.java

@@ -0,0 +1,194 @@
+package dev.tommyjs.futur.promise;
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.Objects;
+import java.util.concurrent.*;
+import java.util.concurrent.locks.AbstractQueuedSynchronizer;
+
+@SuppressWarnings({"FieldMayBeFinal"})
+public abstract class BasePromise<T, FS, FA> extends AbstractPromise<T, FS, FA> implements CompletablePromise<T> {
+
+    private static final VarHandle COMPLETION_HANDLE;
+    private static final VarHandle LISTENERS_HANDLE;
+
+    static {
+        try {
+            MethodHandles.Lookup lookup = MethodHandles.lookup();
+            COMPLETION_HANDLE = lookup.findVarHandle(BasePromise.class, "completion", PromiseCompletion.class);
+            LISTENERS_HANDLE = lookup.findVarHandle(BasePromise.class, "listeners", Collection.class);
+        } catch (ReflectiveOperationException e) {
+            throw new ExceptionInInitializerError(e);
+        }
+    }
+
+    private final Sync sync;
+
+    private volatile PromiseCompletion<T> completion;
+
+    @SuppressWarnings("FieldMayBeFinal")
+    private volatile Collection<PromiseListener<T>> listeners;
+
+    @SuppressWarnings("unchecked")
+    public BasePromise() {
+        this.sync = new Sync();
+        this.completion = null;
+        this.listeners = Collections.EMPTY_LIST;
+    }
+
+    protected T joinCompletion() throws ExecutionException {
+        PromiseCompletion<T> completion = Objects.requireNonNull(getCompletion());
+        if (completion.isSuccess()) return completion.getResult();
+        throw new ExecutionException(completion.getException());
+    }
+
+    protected void handleCompletion(@NotNull PromiseCompletion<T> cmp) {
+        if (!COMPLETION_HANDLE.compareAndSet(this, null, cmp)) return;
+        sync.releaseShared(1);
+        callListeners(cmp);
+    }
+
+    protected Promise<T> completeExceptionallyDelayed(Throwable e, long delay, TimeUnit unit) {
+        runCompleter(this, () -> {
+            FA future = getFactory().getAsyncExecutor().run(() -> completeExceptionally(e), delay, unit);
+            addDirectListener(_ -> getFactory().getAsyncExecutor().cancel(future));
+        });
+
+        return this;
+    }
+
+    @SuppressWarnings("unchecked")
+    protected void callListeners(@NotNull PromiseCompletion<T> cmp) {
+        Iterator<PromiseListener<T>> iter = ((Iterable<PromiseListener<T>>) LISTENERS_HANDLE.getAndSet(this, null)).iterator();
+        try {
+            while (iter.hasNext()) {
+                callListener(iter.next(), cmp);
+            }
+        } finally {
+            iter.forEachRemaining(v -> callListenerAsyncLastResort(v, cmp));
+        }
+    }
+
+    @Override
+    protected @NotNull Promise<T> addAnyListener(@NotNull PromiseListener<T> listener) {
+        Collection<PromiseListener<T>> prev = listeners, next = null;
+        for (boolean haveNext = false; ; ) {
+            if (!haveNext) {
+                next = prev == Collections.EMPTY_LIST ? new ConcurrentLinkedQueue<>() : prev;
+                if (next != null) next.add(listener);
+            }
+
+            if (LISTENERS_HANDLE.weakCompareAndSet(this, prev, next))
+                break;
+
+            haveNext = (prev == (prev = listeners));
+        }
+
+        if (next == null) {
+            callListener(listener, Objects.requireNonNull(getCompletion()));
+        }
+
+        return this;
+    }
+
+    @Override
+    public T get() throws InterruptedException, ExecutionException {
+        sync.acquireSharedInterruptibly(1);
+        return joinCompletion();
+    }
+
+    @Override
+    public T get(long time, @NotNull TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
+        boolean success = sync.tryAcquireSharedNanos(1, unit.toNanos(time));
+        if (!success) {
+            throw new TimeoutException("Promise stopped waiting after " + time + " " + unit);
+        }
+
+        return joinCompletion();
+    }
+
+    @Override
+    public T await() {
+        try {
+            sync.acquireSharedInterruptibly(1);
+        } catch (InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+
+        PromiseCompletion<T> completion = Objects.requireNonNull(getCompletion());
+        if (completion.isSuccess()) return completion.getResult();
+        throw new CompletionException(completion.getException());
+    }
+
+    @Override
+    public @NotNull Promise<T> timeout(long time, @NotNull TimeUnit unit) {
+        Exception e = new CancellationException("Promise timed out after " + time + " " + unit.toString().toLowerCase());
+        return completeExceptionallyDelayed(e, time, unit);
+    }
+
+    @Override
+    public @NotNull Promise<T> maxWaitTime(long time, @NotNull TimeUnit unit) {
+        Exception e = new TimeoutException("Promise stopped waiting after " + time + " " + unit.toString().toLowerCase());
+        return completeExceptionallyDelayed(e, time, unit);
+    }
+
+    @Override
+    public void cancel(@NotNull CancellationException e) {
+        completeExceptionally(e);
+    }
+
+    @Override
+    public void complete(@Nullable T result) {
+        handleCompletion(new PromiseCompletion<>(result));
+    }
+
+    @Override
+    public void completeExceptionally(@NotNull Throwable result) {
+        handleCompletion(new PromiseCompletion<>(result));
+    }
+
+    @Override
+    public boolean isCompleted() {
+        return completion != null;
+    }
+
+    @Override
+    public @Nullable PromiseCompletion<T> getCompletion() {
+        return completion;
+    }
+
+    private static final class Sync extends AbstractQueuedSynchronizer {
+
+        private Sync() {
+            setState(1);
+        }
+
+        @Override
+        protected int tryAcquireShared(int acquires) {
+            return getState() == 0 ? 1 : -1;
+        }
+
+        @Override
+        protected boolean tryReleaseShared(int releases) {
+            int c1, c2;
+            do {
+                c1 = getState();
+                if (c1 == 0) {
+                    return false;
+                }
+
+                c2 = c1 - 1;
+            } while (!compareAndSetState(c1, c2));
+
+            return c2 == 0;
+        }
+
+    }
+
+}

futur-api/src/main/java/dev/tommyjs/futur/promise/CompletedPromise.java

@@ -0,0 +1,68 @@
+package dev.tommyjs.futur.promise;
+
+import org.jetbrains.annotations.NotNull;
+
+import java.util.concurrent.CancellationException;
+import java.util.concurrent.TimeUnit;
+
+public abstract class CompletedPromise<T, FS, FA> extends AbstractPromise<T, FS, FA> {
+
+    private static final PromiseCompletion<?> EMPTY = new PromiseCompletion<>();
+
+    private final @NotNull PromiseCompletion<T> completion;
+
+    public CompletedPromise(@NotNull PromiseCompletion<T> completion) {
+        this.completion = completion;
+    }
+
+    @SuppressWarnings("unchecked")
+    public CompletedPromise() {
+        this((PromiseCompletion<T>) EMPTY);
+    }
+
+    @Override
+    protected @NotNull Promise<T> addAnyListener(@NotNull PromiseListener<T> listener) {
+        callListener(listener, completion);
+        return this;
+    }
+
+    @Override
+    public @NotNull Promise<T> timeout(long time, @NotNull TimeUnit unit) {
+        return this;
+    }
+
+    @Override
+    public @NotNull Promise<T> maxWaitTime(long time, @NotNull TimeUnit unit) {
+        return this;
+    }
+
+    @Override
+    public void cancel(@NotNull CancellationException exception) {
+    }
+
+    @Override
+    public T get() {
+        return null;
+    }
+
+    @Override
+    public T get(long timeout, @NotNull TimeUnit unit) {
+        return null;
+    }
+
+    @Override
+    public T await() {
+        return null;
+    }
+
+    @Override
+    public @NotNull PromiseCompletion<T> getCompletion() {
+        return completion;
+    }
+
+    @Override
+    public boolean isCompleted() {
+        return true;
+    }
+
+}

futur-api/src/main/java/dev/tommyjs/futur/promise/PromiseFactory.java

@@ -70,9 +70,7 @@ public interface PromiseFactory {
      * @return the new promise
      * @apiNote This method is often useful for starting promise chains.
      */
-    default @NotNull Promise<Void> start() {
-        return resolve(null);
-    }
+    @NotNull Promise<Void> start();
 
     /**
      * Creates a new promise, completed exceptionally with the given error.

futur-api/src/main/java/dev/tommyjs/futur/promise/PromiseFactoryImpl.java

@@ -2,6 +2,7 @@ package dev.tommyjs.futur.promise;
 
 import dev.tommyjs.futur.executor.PromiseExecutor;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.slf4j.Logger;
 
 public class PromiseFactoryImpl<FS, FA> extends AbstractPromiseFactory<FS, FA> {
@@ -25,6 +26,21 @@ public class PromiseFactoryImpl<FS, FA> extends AbstractPromiseFactory<FS, FA> {
         return new PromiseImpl<>();
     }
 
+    @Override
+    public @NotNull <T> Promise<T> resolve(T value) {
+        return new CompletedPromiseImpl<>(value);
+    }
+
+    @Override
+    public @NotNull Promise<Void> start() {
+        return new CompletedPromiseImpl<>();
+    }
+
+    @Override
+    public @NotNull <T> Promise<T> error(@NotNull Throwable error) {
+        return new CompletedPromiseImpl<>(error);
+    }
+
     @Override
     public @NotNull Logger getLogger() {
         return logger;
@@ -40,7 +56,28 @@ public class PromiseFactoryImpl<FS, FA> extends AbstractPromiseFactory<FS, FA> {
         return asyncExecutor;
     }
 
-    public class PromiseImpl<T> extends AbstractPromise<T, FS, FA> {
+    private class PromiseImpl<T> extends BasePromise<T, FS, FA> {
+
+        @Override
+        public @NotNull AbstractPromiseFactory<FS, FA> getFactory() {
+            return PromiseFactoryImpl.this;
+        }
+
+    }
+
+    private class CompletedPromiseImpl<T> extends CompletedPromise<T, FS, FA> {
+
+        public CompletedPromiseImpl(@Nullable T result) {
+            super(new PromiseCompletion<>(result));
+        }
+
+        public CompletedPromiseImpl(@NotNull Throwable exception) {
+            super(new PromiseCompletion<>(exception));
+        }
+
+        public CompletedPromiseImpl() {
+            super();
+        }
 
         @Override
         public @NotNull AbstractPromiseFactory<FS, FA> getFactory() {

futur-api/src/test/java/dev/tommyjs/futur/PromiseTests.java

@@ -1,6 +1,7 @@
 package dev.tommyjs.futur;
 
 import dev.tommyjs.futur.promise.CompletablePromise;
+import dev.tommyjs.futur.promise.CompletedPromise;
 import dev.tommyjs.futur.promise.Promise;
 import dev.tommyjs.futur.promise.PromiseFactory;
 import org.junit.jupiter.api.Test;
@@ -17,7 +18,7 @@ import java.util.stream.Stream;
 public final class PromiseTests {
 
     private final Logger logger = LoggerFactory.getLogger(PromiseTests.class);
-    private final ScheduledExecutorService executor = Executors.newScheduledThreadPool(5);
+    private final ScheduledExecutorService executor = Executors.newScheduledThreadPool(6);
     private final PromiseFactory promises = PromiseFactory.of(logger, executor);
 
     @Test
@@ -229,4 +230,17 @@ public final class PromiseTests {
         assert res.get(3) == 6;
     }
 
+    @Test
+    public void testImmediate1() {
+        var promise = promises.start().thenSupply(() -> 10);
+        assert promise.isCompleted() && promise instanceof CompletedPromise<?,?,?>;
+    }
+
+    @Test
+    public void testImmediate2() {
+        var resolved = promises.resolve(10);
+        var promise = promises.start().thenCompose(_ -> resolved);
+        assert promise.isCompleted() && promise instanceof CompletedPromise<?,?,?>;
+    }
+
 }