Simple UnboundedExecutor implementation

Author: AlanBateman

src/java.base/share/classes/java/util/concurrent/Executors.java

@@ -237,6 +237,30 @@ public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
                                       threadFactory);
     }
 
+    /**
+     * Creates an Executor that runs each task in its own thread. The number of
+     * threads is unbounded.
+     *
+     * @apiNote This is a prototype API. It is intended to be used to run tasks
+     * in virtual threads as in the following example:
+     * <pre> {@code
+     *     ThreadFactory factory = Thread.builder().virtual().factory();
+     *     try (ExecutorService executor = Executors.newUnboundedExecutor(factory)) {
+     *         executor.submit(task1);
+     *         executor.submit(task2);
+     *     }
+     * }</pre>
+     *
+     * @param threadFactory the factory to use when creating new threads
+     * @return a newly created executor
+     * @throws NullPointerException if factory is null
+
+     * @since 99
+     */
+    public static ExecutorService newUnboundedExecutor(ThreadFactory threadFactory) {
+        return new UnboundedExecutor(threadFactory);
+    }
+
     /**
      * Creates a single-threaded executor that can schedule commands
      * to run after a given delay, or to execute periodically.

src/java.base/share/classes/java/util/concurrent/UnboundedExecutor.java

@@ -0,0 +1,185 @@
+/*
+ * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package java.util.concurrent;
+
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
+import java.util.List;
+import java.util.Objects;
+import java.util.Set;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * ExecutorService that executes each task in its own thread. Threads are not
+ * re-used and the number of threads/tasks is unbounded.
+ *
+ * This is a inefficient/simple implementation for now, it will likely be replaced.
+ */
+class UnboundedExecutor extends AbstractExecutorService {
+    private final ThreadFactory factory;
+
+    private static final VarHandle STATE;
+    static {
+        try {
+            MethodHandles.Lookup l = MethodHandles.lookup();
+            STATE = l.findVarHandle(UnboundedExecutor.class, "state", int.class);
+        } catch (Exception e) {
+            throw new InternalError(e);
+        }
+    }
+    private volatile int state;
+
+    // states: RUNNING -> SHUTDOWN -> TERMINATED
+    private static final int RUNNING    = 0;
+    private static final int SHUTDOWN   = 1;
+    private static final int TERMINATED = 2;
+
+    private final Set<Thread> threads = ConcurrentHashMap.newKeySet();
+    private final ReentrantLock terminationLock = new ReentrantLock();
+    private final Condition terminationCondition = terminationLock.newCondition();
+
+    UnboundedExecutor(ThreadFactory factory) {
+        this.factory = Objects.requireNonNull(factory);
+    }
+
+    /**
+     * Sets the state to TERMINATED if there are no remaining threads.
+     */
+    private boolean tryTerminate() {
+        assert state >= SHUTDOWN;
+        if (threads.isEmpty()) {
+            terminationLock.lock();
+            try {
+                STATE.set(this, TERMINATED);
+                terminationCondition.signalAll();
+            } finally {
+                terminationLock.unlock();
+            }
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * Sets the state to SHUTDOWN and attempts to terminate if not already shutdown
+     * @throws SecurityException if denied by security manager
+     */
+    private void tryShutdownAndTerminate() {
+        SecurityManager sm = System.getSecurityManager();
+        if (sm != null) {
+            sm.checkPermission(new RuntimePermission("modifyThread"));
+        }
+        if (STATE.compareAndSet(this, RUNNING, SHUTDOWN)) {
+            tryTerminate();
+        }
+    }
+
+    /**
+     * Removes the thread from the set of threads and attempts to terminate
+     * the executor if shutdown but not terminated.
+     */
+    private void onTerminate(Thread thread) {
+        threads.remove(thread);
+        if (state == SHUTDOWN) {
+            tryTerminate();
+        }
+    }
+
+    @Override
+    public void shutdown() {
+        tryShutdownAndTerminate();
+    }
+
+    @Override
+    public List<Runnable> shutdownNow() {
+        tryShutdownAndTerminate();
+        threads.forEach(Thread::interrupt);
+        return List.of();
+    }
+
+    @Override
+    public boolean isShutdown() {
+        return state >= SHUTDOWN;
+    }
+
+    @Override
+    public boolean isTerminated() {
+        return state >= TERMINATED;
+    }
+
+    @Override
+    public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
+        Objects.requireNonNull(unit);
+        if (isTerminated()) {
+            return true;
+        } else {
+            terminationLock.lock();
+            try {
+                if (!isTerminated()) {
+                    terminationCondition.await(timeout, unit);
+                }
+            } finally {
+                terminationLock.unlock();
+            }
+            return isTerminated();
+        }
+    }
+
+    @Override
+    public void execute(Runnable task) {
+        Objects.requireNonNull(task);
+        if (state >= SHUTDOWN) {
+            // shutdown or terminated
+            reject();
+        }
+        Runnable wrapper = () -> {
+            try {
+                task.run();
+            } finally {
+                onTerminate(Thread.currentThread());
+            }
+        };
+        Thread thread = factory.newThread(wrapper);
+        threads.add(thread);
+        boolean started = false;
+        try {
+            if (state == RUNNING) {
+                thread.start();
+                started = true;
+            }
+        } finally {
+            if (!started) {
+                onTerminate(thread);
+                reject();
+            }
+        }
+    }
+
+    private static void reject() {
+        throw new RejectedExecutionException();
+    }
+}