public
class
LockSupport
extends Object
| java.lang.Object | |
| ↳ | java.util.concurrent.locks.LockSupport |
Basic thread blocking primitives for creating locks and other synchronization classes.
This class associates, with each thread that uses it, a permit
(in the sense of the Semaphore class). A call to park will return immediately
if the permit is available, consuming it in the process; otherwise
it may block. A call to unpark makes the permit
available, if it was not already available. (Unlike with Semaphores
though, permits do not accumulate. There is at most one.)
Reliable usage requires the use of volatile (or atomic) variables
to control when to park or unpark. Orderings of calls to these
methods are maintained with respect to volatile variable accesses,
but not necessarily non-volatile variable accesses.
Methods park and unpark provide efficient
means of blocking and unblocking threads that do not encounter the
problems that cause the deprecated methods Thread.suspend
and Thread.resume to be unusable for such purposes: Races
between one thread invoking park and another thread trying
to unpark it will preserve liveness, due to the
permit. Additionally, park will return if the caller's
thread was interrupted, and timeout versions are supported. The
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 park serves as an
optimization of a "busy wait" that does not waste as much time
spinning, but must be paired with an unpark to be
effective.
The three forms of park each also support a
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 getBlocker(Thread).)
The use of these forms rather than the original forms without this
parameter is strongly encouraged. The normal argument to supply as
a blocker within a lock implementation is this.
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 park
method is designed for use only in constructions of the form:
Developers
while (!canProceed()) {
// ensure request to unpark is visible to other threads
...
LockSupport.park(this);
}
where no actions by the thread publishing a request to unpark,
prior to the call to park, entail locking or blocking.
Because only one permit is associated with each thread, any
intermediary uses of park, including implicitly via class
loading, could lead to an unresponsive thread (a "lost unpark").
Sample Usage. Here is a sketch of a first-in-first-out non-reentrant lock class:
class FIFOMutex {
private final AtomicBoolean locked = new AtomicBoolean(false);
private final Queue<Thread> waiters
= new ConcurrentLinkedQueue<>();
public void lock() {
boolean wasInterrupted = false;
// publish current thread for unparkers
waiters.add(Thread.currentThread());
// Block while not first in queue or cannot acquire lock
while (waiters.peek() != Thread.currentThread() ||
!locked.compareAndSet(false, true)) {
LockSupport.park(this);
// ignore interrupts while waiting
if (Thread.interrupted())
wasInterrupted = true;
}
waiters.remove();
// ensure correct interrupt status on return
if (wasInterrupted)
Thread.currentThread().interrupt();
}
public void unlock() {
locked.set(false);
LockSupport.unpark(waiters.peek());
}
static {
// Reduce the risk of "lost unpark" due to classloading
Class<?> ensureLoaded = LockSupport.class;
}
}
Public methods | |
|---|---|
static
Object
|
getBlocker(Thread t)
Returns the blocker object supplied to the most recent invocation of a park method that has not yet unblocked, or null if not blocked. |
static
void
|
park()
Disables the current thread for thread scheduling purposes unless the permit is available. |
static
void
|
park(Object blocker)
Disables the current thread for thread scheduling purposes unless the permit is available. |
static
void
|
parkNanos(Object blocker, long nanos)
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available. |
static
void
|
parkNanos(long nanos)
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available. |
static
void
|
parkUntil(Object blocker, long deadline)
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available. |
static
void
|
parkUntil(long deadline)
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available. |
static
void
|
unpark(Thread thread)
Makes available the permit for the given thread, if it was not already available. |
Inherited methods | |
|---|---|
 From
class
java.lang.Object
| |
Object getBlocker (Thread t)
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.
| Parameters | |
|---|---|
t |
Thread:
the thread |
| Returns | |
|---|---|
Object |
the blocker |
| Throws | |
|---|---|
NullPointerException |
if argument is null |
void park ()
Disables the current thread for thread scheduling purposes unless the permit is available.
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:
unpark with the
current thread as the target; or
This method does not 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.
void park (Object blocker)
Disables the current thread for thread scheduling purposes unless the permit is available.
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:
unpark with the
current thread as the target; or
This method does not 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.
| Parameters | |
|---|---|
blocker |
Object:
the synchronization object responsible for this
thread parking |
void parkNanos (Object blocker, long nanos)
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available.
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:
unpark with the
current thread as the target; or
This method does not 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.
| Parameters | |
|---|---|
blocker |
Object:
the synchronization object responsible for this
thread parking |
nanos |
long:
the maximum number of nanoseconds to wait |
void parkNanos (long nanos)
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available.
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:
unpark with the
current thread as the target; or
This method does not 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.
| Parameters | |
|---|---|
nanos |
long:
the maximum number of nanoseconds to wait
|
void parkUntil (Object blocker, long deadline)
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.
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:
unpark with the
current thread as the target; or
This method does not 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.
| Parameters | |
|---|---|
blocker |
Object:
the synchronization object responsible for this
thread parking |
deadline |
long:
the absolute time, in milliseconds from the Epoch,
to wait until |
void parkUntil (long deadline)
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.
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:
unpark with the
current thread as the target; or
This method does not 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.
| Parameters | |
|---|---|
deadline |
long:
the absolute time, in milliseconds from the Epoch,
to wait until
|
void unpark (Thread thread)
Makes available the permit for the given thread, if it
was not already available. If the thread was blocked on
park then it will unblock. Otherwise, its next call
to 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.
| Parameters | |
|---|---|
thread |
Thread:
the thread to unpark, or null, in which case
this operation has no effect
|