8284161: Implementation of Virtual Threads (Preview)

Co-authored-by: Ron Pressler <rpressler@openjdk.org>
Co-authored-by: Alan Bateman <alanb@openjdk.org>
Co-authored-by: Erik Österlund <eosterlund@openjdk.org>
Co-authored-by: Andrew Haley <aph@openjdk.org>
Co-authored-by: Rickard Bäckman <rbackman@openjdk.org>
Co-authored-by: Markus Grönlund <mgronlun@openjdk.org>
Co-authored-by: Leonid Mesnik <lmesnik@openjdk.org>
Co-authored-by: Serguei Spitsyn <sspitsyn@openjdk.org>
Co-authored-by: Chris Plummer <cjplummer@openjdk.org>
Co-authored-by: Coleen Phillimore <coleenp@openjdk.org>
Co-authored-by: Robbin Ehn <rehn@openjdk.org>
Co-authored-by: Stefan Karlsson <stefank@openjdk.org>
Co-authored-by: Thomas Schatzl <tschatzl@openjdk.org>
Co-authored-by: Sergey Kuksenko <skuksenko@openjdk.org>
Reviewed-by: lancea, eosterlund, rehn, sspitsyn, stefank, tschatzl, dfuchs, lmesnik, dcubed, kevinw, amenkov, dlong, mchung, psandoz, bpb, coleenp, smarks, egahlin, mseledtsov, coffeys, darcy

src/java.base/share/classes/java/lang/ref/Finalizer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2022, 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
@@ -35,7 +35,7 @@ final class Finalizer extends FinalReference<Object> { /* Package-private; must
                                                           same package as the Reference
                                                           class */
 
-    private static ReferenceQueue<Object> queue = new ReferenceQueue<>();
+    private static ReferenceQueue<Object> queue = new NativeReferenceQueue<>();
 
     /** Head of doubly linked list of Finalizers awaiting finalization. */
     private static Finalizer unfinalized = null;
@@ -166,16 +166,6 @@ final class Finalizer extends FinalReference<Object> { /* Package-private; must
             if (running)
                 return;
 
-            // Finalizer thread starts before System.initializeSystemClass
-            // is called.  Wait until JavaLangAccess is available
-            while (VM.initLevel() == 0) {
-                // delay until VM completes initialization
-                try {
-                    VM.awaitInitLevel(1);
-                } catch (InterruptedException x) {
-                    // ignore and continue
-                }
-            }
             final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
             running = true;
             for (;;) {
@@ -189,17 +179,15 @@ final class Finalizer extends FinalReference<Object> { /* Package-private; must
         }
     }
 
-    static {
+    /**
+     * Start the Finalizer thread as a daemon thread.
+     */
+    static void startFinalizerThread(ThreadGroup tg) {
         if (ENABLED) {
-            ThreadGroup tg = Thread.currentThread().getThreadGroup();
-            for (ThreadGroup tgn = tg;
-                 tgn != null;
-                 tg = tgn, tgn = tg.getParent());
             Thread finalizer = new FinalizerThread(tg);
             finalizer.setPriority(Thread.MAX_PRIORITY - 2);
             finalizer.setDaemon(true);
             finalizer.start();
         }
     }
-
 }

src/java.base/share/classes/java/lang/ref/NativeReferenceQueue.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) 2021, 2022, 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.lang.ref;
+
+/**
+ * An implementation of a ReferenceQueue that uses native monitors.
+ * The use of java.util.concurrent.lock locks interacts with various mechanisms,
+ * such as virtual threads and ForkJoinPool, that might not be appropriate for some
+ * low-level mechanisms, in particular MethodType's weak intern set.
+ */
+final class NativeReferenceQueue<T> extends ReferenceQueue<T> {
+    public NativeReferenceQueue() {
+        super(0);
+    }
+
+    private static class Lock { };
+    private final Lock lock = new Lock();
+
+    @Override
+    void signal() {
+        lock.notifyAll();
+    }
+    @Override
+    void await() throws InterruptedException {
+        lock.wait();
+    }
+
+    @Override
+    void await(long timeoutMillis) throws InterruptedException {
+        lock.wait(timeoutMillis);
+    }
+
+    @Override
+    boolean enqueue(Reference<? extends T> r) {
+        synchronized(lock) {
+            return enqueue0(r);
+        }
+    }
+
+    @Override
+    public Reference<? extends T> poll() {
+        if (headIsNull())
+            return null;
+
+        synchronized(lock) {
+            return poll0();
+        }
+    }
+
+    @Override
+    public Reference<? extends T> remove(long timeout)
+            throws IllegalArgumentException, InterruptedException {
+        if (timeout < 0)
+            throw new IllegalArgumentException("Negative timeout value");
+        if (timeout == 0)
+            return remove();
+
+        synchronized(lock) {
+            return remove0(timeout);
+        }
+    }
+
+    @Override
+    public Reference<? extends T> remove() throws InterruptedException {
+        synchronized(lock) {
+            return remove0();
+        }
+    }
+}

src/java.base/share/classes/java/lang/ref/Reference.java

@@ -25,6 +25,7 @@
 
 package java.lang.ref;
 
+import jdk.internal.misc.Unsafe;
 import jdk.internal.vm.annotation.ForceInline;
 import jdk.internal.vm.annotation.IntrinsicCandidate;
 import jdk.internal.access.JavaLangRefAccess;
@@ -192,27 +193,16 @@ public abstract sealed class Reference<T>
     /* High-priority thread to enqueue pending References
      */
     private static class ReferenceHandler extends Thread {
-
-        private static void ensureClassInitialized(Class<?> clazz) {
-            try {
-                Class.forName(clazz.getName(), true, clazz.getClassLoader());
-            } catch (ClassNotFoundException e) {
-                throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
-            }
-        }
-
-        static {
-            // pre-load and initialize Cleaner class so that we don't
-            // get into trouble later in the run loop if there's
-            // memory shortage while loading/initializing it lazily.
-            ensureClassInitialized(Cleaner.class);
-        }
-
         ReferenceHandler(ThreadGroup g, String name) {
             super(g, null, name, 0, false);
         }
 
         public void run() {
+            // pre-load and initialize Cleaner class so that we don't
+            // get into trouble later in the run loop if there's
+            // memory shortage while loading/initializing it lazily.
+            Unsafe.getUnsafe().ensureClassInitialized(Cleaner.class);
+
             while (true) {
                 processPendingReferences();
             }
@@ -302,11 +292,10 @@ public abstract sealed class Reference<T>
         }
     }
 
-    static {
-        ThreadGroup tg = Thread.currentThread().getThreadGroup();
-        for (ThreadGroup tgn = tg;
-             tgn != null;
-             tg = tgn, tgn = tg.getParent());
+    /**
+     * Start the Reference Handler thread as a daemon thread.
+     */
+    static void startReferenceHandlerThread(ThreadGroup tg) {
         Thread handler = new ReferenceHandler(tg, "Reference Handler");
         /* If there were a special system-only priority greater than
          * MAX_PRIORITY, it would be used here
@@ -314,9 +303,21 @@ public abstract sealed class Reference<T>
         handler.setPriority(Thread.MAX_PRIORITY);
         handler.setDaemon(true);
         handler.start();
+    }
 
+    static {
         // provide access in SharedSecrets
         SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
+            @Override
+            public void startThreads() {
+                ThreadGroup tg = Thread.currentThread().getThreadGroup();
+                for (ThreadGroup tgn = tg;
+                     tgn != null;
+                     tg = tgn, tgn = tg.getParent());
+                Reference.startReferenceHandlerThread(tg);
+                Finalizer.startFinalizerThread(tg);
+            }
+
             @Override
             public boolean waitForReferenceProcessing()
                 throws InterruptedException
@@ -328,6 +329,11 @@ public abstract sealed class Reference<T>
             public void runFinalization() {
                 Finalizer.runFinalization();
             }
+
+            @Override
+            public <T> ReferenceQueue<T> newNativeReferenceQueue() {
+                return new NativeReferenceQueue<T>();
+            }
         });
     }
 

src/java.base/share/classes/java/lang/ref/ReferenceQueue.java

@@ -25,6 +25,9 @@
 
 package java.lang.ref;
 
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Consumer;
 import jdk.internal.misc.VM;
 
@@ -38,13 +41,10 @@ import jdk.internal.misc.VM;
  */
 
 public class ReferenceQueue<T> {
-
-    /**
-     * Constructs a new reference-object queue.
-     */
-    public ReferenceQueue() { }
-
     private static class Null extends ReferenceQueue<Object> {
+        public Null() { super(0); }
+
+        @Override
         boolean enqueue(Reference<?> r) {
             return false;
         }
@@ -53,37 +53,65 @@ public class ReferenceQueue<T> {
     static final ReferenceQueue<Object> NULL = new Null();
     static final ReferenceQueue<Object> ENQUEUED = new Null();
 
-    private static class Lock { };
-    private final Lock lock = new Lock();
     private volatile Reference<? extends T> head;
     private long queueLength = 0;
 
-    boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
-        synchronized (lock) {
-            // Check that since getting the lock this reference hasn't already been
-            // enqueued (and even then removed)
-            ReferenceQueue<?> queue = r.queue;
-            if ((queue == NULL) || (queue == ENQUEUED)) {
-                return false;
-            }
-            assert queue == this;
-            // Self-loop end, so if a FinalReference it remains inactive.
-            r.next = (head == null) ? r : head;
-            head = r;
-            queueLength++;
-            // Update r.queue *after* adding to list, to avoid race
-            // with concurrent enqueued checks and fast-path poll().
-            // Volatiles ensure ordering.
-            r.queue = ENQUEUED;
-            if (r instanceof FinalReference) {
-                VM.addFinalRefCount(1);
-            }
-            lock.notifyAll();
-            return true;
-        }
+    private final ReentrantLock lock;
+    private final Condition notEmpty;
+
+    void signal() {
+        notEmpty.signalAll();
     }
 
-    private Reference<? extends T> reallyPoll() {       /* Must hold lock */
+    void await() throws InterruptedException {
+        notEmpty.await();
+    }
+
+    void await(long timeoutMillis) throws InterruptedException {
+        notEmpty.await(timeoutMillis, TimeUnit.MILLISECONDS);
+    }
+
+    /**
+     * Constructs a new reference-object queue.
+     */
+    public ReferenceQueue() {
+        this.lock = new ReentrantLock();
+        this.notEmpty = lock.newCondition();
+    }
+
+    ReferenceQueue(int dummy) {
+        this.lock = null;
+        this.notEmpty = null;
+    }
+
+    final boolean enqueue0(Reference<? extends T> r) { // must hold lock
+        // Check that since getting the lock this reference hasn't already been
+        // enqueued (and even then removed)
+        ReferenceQueue<?> queue = r.queue;
+        if ((queue == NULL) || (queue == ENQUEUED)) {
+            return false;
+        }
+        assert queue == this;
+        // Self-loop end, so if a FinalReference it remains inactive.
+        r.next = (head == null) ? r : head;
+        head = r;
+        queueLength++;
+        // Update r.queue *after* adding to list, to avoid race
+        // with concurrent enqueued checks and fast-path poll().
+        // Volatiles ensure ordering.
+        r.queue = ENQUEUED;
+        if (r instanceof FinalReference) {
+            VM.addFinalRefCount(1);
+        }
+        signal();
+        return true;
+    }
+
+    final boolean headIsNull() {
+        return head == null;
+    }
+
+    final Reference<? extends T> poll0() { // must hold lock
         Reference<? extends T> r = head;
         if (r != null) {
             r.queue = NULL;
@@ -106,6 +134,40 @@ public class ReferenceQueue<T> {
         return null;
     }
 
+    final Reference<? extends T> remove0(long timeout)
+            throws IllegalArgumentException, InterruptedException { // must hold lock
+        Reference<? extends T> r = poll0();
+        if (r != null) return r;
+        long start = System.nanoTime();
+        for (;;) {
+            await(timeout);
+            r = poll0();
+            if (r != null) return r;
+
+            long end = System.nanoTime();
+            timeout -= (end - start) / 1000_000;
+            if (timeout <= 0) return null;
+            start = end;
+        }
+    }
+
+    final Reference<? extends T> remove0() throws InterruptedException { // must hold lock
+        for (;;) {
+            var r = poll0();
+            if (r != null) return r;
+            await();
+        }
+    }
+
+    boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
+        lock.lock();
+        try {
+            return enqueue0(r);
+        } finally {
+            lock.unlock();
+        }
+    }
+
     /**
      * Polls this queue to see if a reference object is available.  If one is
      * available without further delay then it is removed from the queue and
@@ -115,10 +177,13 @@ public class ReferenceQueue<T> {
      *          otherwise {@code null}
      */
     public Reference<? extends T> poll() {
-        if (head == null)
+        if (headIsNull())
             return null;
-        synchronized (lock) {
-            return reallyPoll();
+        lock.lock();
+        try {
+            return poll0();
+        } finally {
+            lock.unlock();
         }
     }
 
@@ -142,27 +207,17 @@ public class ReferenceQueue<T> {
      * @throws  InterruptedException
      *          If the timeout wait is interrupted
      */
-    public Reference<? extends T> remove(long timeout)
-        throws IllegalArgumentException, InterruptedException
-    {
-        if (timeout < 0) {
+    public Reference<? extends T> remove(long timeout) throws InterruptedException {
+        if (timeout < 0)
             throw new IllegalArgumentException("Negative timeout value");
-        }
-        synchronized (lock) {
-            Reference<? extends T> r = reallyPoll();
-            if (r != null) return r;
-            long start = (timeout == 0) ? 0 : System.nanoTime();
-            for (;;) {
-                lock.wait(timeout);
-                r = reallyPoll();
-                if (r != null) return r;
-                if (timeout != 0) {
-                    long end = System.nanoTime();
-                    timeout -= (end - start) / 1000_000;
-                    if (timeout <= 0) return null;
-                    start = end;
-                }
-            }
+        if (timeout == 0)
+            return remove();
+
+        lock.lock();
+        try {
+            return remove0(timeout);
+        } finally {
+            lock.unlock();
         }
     }
 
@@ -174,7 +229,12 @@ public class ReferenceQueue<T> {
      * @throws  InterruptedException  If the wait is interrupted
      */
     public Reference<? extends T> remove() throws InterruptedException {
-        return remove(0);
+        lock.lock();
+        try {
+            return remove0();
+        } finally {
+            lock.unlock();
+        }
     }
 
     /**