| /* |
| * 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 java.util.concurrent.*; |
| 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 <tt>park</tt> will return immediately |
| * if the permit is available, consuming it in the process; otherwise |
| * it <em>may</em> block. A call to <tt>unpark</tt> 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 <tt>park</tt> and <tt>unpark</tt> provide efficient |
| * means of blocking and unblocking threads that do not encounter the |
| * problems that cause the deprecated methods <tt>Thread.suspend</tt> |
| * and <tt>Thread.resume</tt> to be unusable for such purposes: Races |
| * between one thread invoking <tt>park</tt> and another thread trying |
| * to <tt>unpark</tt> it will preserve liveness, due to the |
| * permit. Additionally, <tt>park</tt> will return if the caller's |
| * thread was interrupted, and timeout versions are supported. The |
| * <tt>park</tt> 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 <tt>park</tt> serves as an |
| * optimization of a "busy wait" that does not waste as much time |
| * spinning, but must be paired with an <tt>unpark</tt> to be |
| * effective. |
| * |
| * <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. |
| * |
| * <p><b>Sample Usage.</b> Here is a sketch of a First-in-first-out |
| * non-reentrant lock class. |
| * <pre> |
| * private AtomicBoolean locked = new AtomicBoolean(false); |
| * private 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(); |
| * 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> |
| */ |
| @SuppressWarnings("all") |
| public class LockSupport { |
| private LockSupport() {} // Cannot be instantiated. |
| |
| // BEGIN android-changed |
| private static final Unsafe unsafe = UnsafeAccess.THE_ONE; |
| // END android-changed |
| |
| /** |
| * Make available the permit for the given thread, if it |
| * was not already available. If the thread was blocked on |
| * <tt>park</tt> then it will unblock. Otherwise, its next call |
| * to <tt>park</tt> 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 <tt>null</tt>, 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 <tt>unpark</tt> with the current thread |
| * as the target; or |
| * <li>Some other thread {@link 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 <tt>unpark</tt> with the current thread |
| * as the target; or |
| * <li>Some other thread {@link 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 <tt>unpark</tt> with the current thread |
| * as the target; or |
| * <li>Some other thread {@link 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); |
| } |
| |
| } |
| |
| |
