| /* |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/licenses/publicdomain |
| */ |
| |
| package java.util.concurrent.locks; |
| |
| import sun.misc.Unsafe; |
| |
| |
| /** |
| * Basic thread blocking primitives for creating locks and other |
| * synchronization classes. |
| * |
| * <p>This class associates, with each thread that uses it, a permit |
| * (in the sense of the {@link java.util.concurrent.Semaphore |
| * Semaphore} class). A call to {@code park} will return immediately |
| * if the permit is available, consuming it in the process; otherwise |
| * it <em>may</em> block. A call to {@code unpark} makes the permit |
| * available, if it was not already available. (Unlike with Semaphores |
| * though, permits do not accumulate. There is at most one.) |
| * |
| * <p>Methods {@code park} and {@code unpark} provide efficient |
| * means of blocking and unblocking threads that do not encounter the |
| * problems that cause the deprecated methods {@code Thread.suspend} |
| * and {@code Thread.resume} to be unusable for such purposes: Races |
| * between one thread invoking {@code park} and another thread trying |
| * to {@code unpark} it will preserve liveness, due to the |
| * permit. Additionally, {@code park} will return if the caller's |
| * thread was interrupted, and timeout versions are supported. The |
| * {@code park} method may also return at any other time, for "no |
| * reason", so in general must be invoked within a loop that rechecks |
| * conditions upon return. In this sense {@code park} serves as an |
| * optimization of a "busy wait" that does not waste as much time |
| * spinning, but must be paired with an {@code unpark} to be |
| * effective. |
| * |
| * <p>The three forms of {@code park} each also support a |
| * {@code blocker} object parameter. This object is recorded while |
| * the thread is blocked to permit monitoring and diagnostic tools to |
| * identify the reasons that threads are blocked. (Such tools may |
| * access blockers using method {@link #getBlocker}.) The use of these |
| * forms rather than the original forms without this parameter is |
| * strongly encouraged. The normal argument to supply as a |
| * {@code blocker} within a lock implementation is {@code this}. |
| * |
| * <p>These methods are designed to be used as tools for creating |
| * higher-level synchronization utilities, and are not in themselves |
| * useful for most concurrency control applications. The {@code park} |
| * method is designed for use only in constructions of the form: |
| * <pre>while (!canProceed()) { ... LockSupport.park(this); }</pre> |
| * where neither {@code canProceed} nor any other actions prior to the |
| * call to {@code park} entail locking or blocking. Because only one |
| * permit is associated with each thread, any intermediary uses of |
| * {@code park} could interfere with its intended effects. |
| * |
| * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out |
| * non-reentrant lock class: |
| * <pre>{@code |
| * class FIFOMutex { |
| * private final AtomicBoolean locked = new AtomicBoolean(false); |
| * private final Queue<Thread> waiters |
| * = new ConcurrentLinkedQueue<Thread>(); |
| * |
| * public void lock() { |
| * boolean wasInterrupted = false; |
| * Thread current = Thread.currentThread(); |
| * waiters.add(current); |
| * |
| * // Block while not first in queue or cannot acquire lock |
| * while (waiters.peek() != current || |
| * !locked.compareAndSet(false, true)) { |
| * LockSupport.park(this); |
| * if (Thread.interrupted()) // ignore interrupts while waiting |
| * wasInterrupted = true; |
| * } |
| * |
| * waiters.remove(); |
| * if (wasInterrupted) // reassert interrupt status on exit |
| * current.interrupt(); |
| * } |
| * |
| * public void unlock() { |
| * locked.set(false); |
| * LockSupport.unpark(waiters.peek()); |
| * } |
| * }}</pre> |
| */ |
| |
| public class LockSupport { |
| private LockSupport() {} // Cannot be instantiated. |
| |
| // Hotspot implementation via intrinsics API |
| private static final Unsafe unsafe = UnsafeAccess.THE_ONE; // android-changed |
| private static final long parkBlockerOffset; |
| |
| static { |
| try { |
| parkBlockerOffset = unsafe.objectFieldOffset |
| (java.lang.Thread.class.getDeclaredField("parkBlocker")); |
| } catch (Exception ex) { throw new Error(ex); } |
| } |
| |
| private static void setBlocker(Thread t, Object arg) { |
| // Even though volatile, hotspot doesn't need a write barrier here. |
| unsafe.putObject(t, parkBlockerOffset, arg); |
| } |
| |
| /** |
| * Makes available the permit for the given thread, if it |
| * was not already available. If the thread was blocked on |
| * {@code park} then it will unblock. Otherwise, its next call |
| * to {@code park} is guaranteed not to block. This operation |
| * is not guaranteed to have any effect at all if the given |
| * thread has not been started. |
| * |
| * @param thread the thread to unpark, or {@code null}, in which case |
| * this operation has no effect |
| */ |
| public static void unpark(Thread thread) { |
| if (thread != null) |
| unsafe.unpark(thread); |
| } |
| |
| /** |
| * Disables the current thread for thread scheduling purposes unless the |
| * permit is available. |
| * |
| * <p>If the permit is available then it is consumed and the call returns |
| * immediately; otherwise |
| * the current thread becomes disabled for thread scheduling |
| * purposes and lies dormant until one of three things happens: |
| * |
| * <ul> |
| * <li>Some other thread invokes {@link #unpark unpark} with the |
| * current thread as the target; or |
| * |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * |
| * <li>The call spuriously (that is, for no reason) returns. |
| * </ul> |
| * |
| * <p>This method does <em>not</em> report which of these caused the |
| * method to return. Callers should re-check the conditions which caused |
| * the thread to park in the first place. Callers may also determine, |
| * for example, the interrupt status of the thread upon return. |
| * |
| * @param blocker the synchronization object responsible for this |
| * thread parking |
| * @since 1.6 |
| */ |
| public static void park(Object blocker) { |
| Thread t = Thread.currentThread(); |
| setBlocker(t, blocker); |
| unsafe.park(false, 0L); |
| setBlocker(t, null); |
| } |
| |
| /** |
| * Disables the current thread for thread scheduling purposes, for up to |
| * the specified waiting time, unless the permit is available. |
| * |
| * <p>If the permit is available then it is consumed and the call |
| * returns immediately; otherwise the current thread becomes disabled |
| * for thread scheduling purposes and lies dormant until one of four |
| * things happens: |
| * |
| * <ul> |
| * <li>Some other thread invokes {@link #unpark unpark} with the |
| * current thread as the target; or |
| * |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} the current |
| * thread; or |
| * |
| * <li>The specified waiting time elapses; or |
| * |
| * <li>The call spuriously (that is, for no reason) returns. |
| * </ul> |
| * |
| * <p>This method does <em>not</em> report which of these caused the |
| * method to return. Callers should re-check the conditions which caused |
| * the thread to park in the first place. Callers may also determine, |
| * for example, the interrupt status of the thread, or the elapsed time |
| * upon return. |
| * |
| * @param blocker the synchronization object responsible for this |
| * thread parking |
| * @param nanos the maximum number of nanoseconds to wait |
| * @since 1.6 |
| */ |
| public static void parkNanos(Object blocker, long nanos) { |
| if (nanos > 0) { |
| Thread t = Thread.currentThread(); |
| setBlocker(t, blocker); |
| unsafe.park(false, nanos); |
| setBlocker(t, null); |
| } |
| } |
| |
| /** |
| * Disables the current thread for thread scheduling purposes, until |
| * the specified deadline, unless the permit is available. |
| * |
| * <p>If the permit is available then it is consumed and the call |
| * returns immediately; otherwise the current thread becomes disabled |
| * for thread scheduling purposes and lies dormant until one of four |
| * things happens: |
| * |
| * <ul> |
| * <li>Some other thread invokes {@link #unpark unpark} with the |
| * current thread as the target; or |
| * |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} the |
| * current thread; or |
| * |
| * <li>The specified deadline passes; or |
| * |
| * <li>The call spuriously (that is, for no reason) returns. |
| * </ul> |
| * |
| * <p>This method does <em>not</em> report which of these caused the |
| * method to return. Callers should re-check the conditions which caused |
| * the thread to park in the first place. Callers may also determine, |
| * for example, the interrupt status of the thread, or the current time |
| * upon return. |
| * |
| * @param blocker the synchronization object responsible for this |
| * thread parking |
| * @param deadline the absolute time, in milliseconds from the Epoch, |
| * to wait until |
| * @since 1.6 |
| */ |
| public static void parkUntil(Object blocker, long deadline) { |
| Thread t = Thread.currentThread(); |
| setBlocker(t, blocker); |
| unsafe.park(true, deadline); |
| setBlocker(t, null); |
| } |
| |
| /** |
| * Returns the blocker object supplied to the most recent |
| * invocation of a park method that has not yet unblocked, or null |
| * if not blocked. The value returned is just a momentary |
| * snapshot -- the thread may have since unblocked or blocked on a |
| * different blocker object. |
| * |
| * @return the blocker |
| * @since 1.6 |
| */ |
| public static Object getBlocker(Thread t) { |
| return unsafe.getObjectVolatile(t, parkBlockerOffset); |
| } |
| |
| /** |
| * Disables the current thread for thread scheduling purposes unless the |
| * permit is available. |
| * |
| * <p>If the permit is available then it is consumed and the call |
| * returns immediately; otherwise the current thread becomes disabled |
| * for thread scheduling purposes and lies dormant until one of three |
| * things happens: |
| * |
| * <ul> |
| * |
| * <li>Some other thread invokes {@link #unpark unpark} with the |
| * current thread as the target; or |
| * |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * |
| * <li>The call spuriously (that is, for no reason) returns. |
| * </ul> |
| * |
| * <p>This method does <em>not</em> report which of these caused the |
| * method to return. Callers should re-check the conditions which caused |
| * the thread to park in the first place. Callers may also determine, |
| * for example, the interrupt status of the thread upon return. |
| */ |
| public static void park() { |
| unsafe.park(false, 0L); |
| } |
| |
| /** |
| * Disables the current thread for thread scheduling purposes, for up to |
| * the specified waiting time, unless the permit is available. |
| * |
| * <p>If the permit is available then it is consumed and the call |
| * returns immediately; otherwise the current thread becomes disabled |
| * for thread scheduling purposes and lies dormant until one of four |
| * things happens: |
| * |
| * <ul> |
| * <li>Some other thread invokes {@link #unpark unpark} with the |
| * current thread as the target; or |
| * |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * |
| * <li>The specified waiting time elapses; or |
| * |
| * <li>The call spuriously (that is, for no reason) returns. |
| * </ul> |
| * |
| * <p>This method does <em>not</em> report which of these caused the |
| * method to return. Callers should re-check the conditions which caused |
| * the thread to park in the first place. Callers may also determine, |
| * for example, the interrupt status of the thread, or the elapsed time |
| * upon return. |
| * |
| * @param nanos the maximum number of nanoseconds to wait |
| */ |
| public static void parkNanos(long nanos) { |
| if (nanos > 0) |
| unsafe.park(false, nanos); |
| } |
| |
| /** |
| * Disables the current thread for thread scheduling purposes, until |
| * the specified deadline, unless the permit is available. |
| * |
| * <p>If the permit is available then it is consumed and the call |
| * returns immediately; otherwise the current thread becomes disabled |
| * for thread scheduling purposes and lies dormant until one of four |
| * things happens: |
| * |
| * <ul> |
| * <li>Some other thread invokes {@link #unpark unpark} with the |
| * current thread as the target; or |
| * |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * |
| * <li>The specified deadline passes; or |
| * |
| * <li>The call spuriously (that is, for no reason) returns. |
| * </ul> |
| * |
| * <p>This method does <em>not</em> report which of these caused the |
| * method to return. Callers should re-check the conditions which caused |
| * the thread to park in the first place. Callers may also determine, |
| * for example, the interrupt status of the thread, or the current time |
| * upon return. |
| * |
| * @param deadline the absolute time, in milliseconds from the Epoch, |
| * to wait until |
| */ |
| public static void parkUntil(long deadline) { |
| unsafe.park(true, deadline); |
| } |
| } |
