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java.lang.Objectedu.emory.mathcs.backport.java.util.concurrent.Semaphore
A counting semaphore. Conceptually, a semaphore maintains a set of
permits. Each acquire()
blocks if necessary until a permit is
available, and then takes it. Each release()
adds a permit,
potentially releasing a blocking acquirer.
However, no actual permit objects are used; the Semaphore just
keeps a count of the number available and acts accordingly.
Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource. For example, here is a class that uses a semaphore to control access to a pool of items:
class Pool { private static final int MAX_AVAILABLE = 100; private final Semaphore available = new Semaphore(MAX_AVAILABLE, true); public Object getItem() throws InterruptedException { available.acquire(); return getNextAvailableItem(); } public void putItem(Object x) { if (markAsUnused(x)) available.release(); } // Not a particularly efficient data structure; just for demo protected Object[] items = ... whatever kinds of items being managed protected boolean[] used = new boolean[MAX_AVAILABLE]; protected synchronized Object getNextAvailableItem() { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (!used[i]) { used[i] = true; return items[i]; } } return null; // not reached } protected synchronized boolean markAsUnused(Object item) { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (item == items[i]) { if (used[i]) { used[i] = false; return true; } else return false; } } return false; } }
Before obtaining an item each thread must acquire a permit from
the semaphore, guaranteeing that an item is available for use. When
the thread has finished with the item it is returned back to the
pool and a permit is returned to the semaphore, allowing another
thread to acquire that item. Note that no synchronization lock is
held when acquire()
is called as that would prevent an item
from being returned to the pool. The semaphore encapsulates the
synchronization needed to restrict access to the pool, separately
from any synchronization needed to maintain the consistency of the
pool itself.
A semaphore initialized to one, and which is used such that it
only has at most one permit available, can serve as a mutual
exclusion lock. This is more commonly known as a binary
semaphore, because it only has two states: one permit
available, or zero permits available. When used in this way, the
binary semaphore has the property (unlike many Lock
implementations), that the "lock" can be released by a
thread other than the owner (as semaphores have no notion of
ownership). This can be useful in some specialized contexts, such
as deadlock recovery.
The constructor for this class optionally accepts a
fairness parameter. When set false, this class makes no
guarantees about the order in which threads acquire permits. In
particular, barging is permitted, that is, a thread
invoking acquire()
can be allocated a permit ahead of a
thread that has been waiting - logically the new thread places itself at
the head of the queue of waiting threads. When fairness is set true, the
semaphore guarantees that threads invoking any of the acquire
methods are selected to obtain permits in the order in
which their invocation of those methods was processed
(first-in-first-out; FIFO). Note that FIFO ordering necessarily
applies to specific internal points of execution within these
methods. So, it is possible for one thread to invoke
acquire before another, but reach the ordering point after
the other, and similarly upon return from the method.
Also note that the untimed tryAcquire
methods do not
honor the fairness setting, but will take any permits that are
available.
Generally, semaphores used to control resource access should be initialized as fair, to ensure that no thread is starved out from accessing a resource. When using semaphores for other kinds of synchronization control, the throughput advantages of non-fair ordering often outweigh fairness considerations.
Memory consistency effects: Actions in a thread prior to calling a "release" method such as happen-before actions following a successful "acquire" method such as in another thread.
Constructor Summary | |
---|---|
Semaphore(int permits)
Creates a Semaphore with the given number of permits and nonfair fairness setting. |
|
Semaphore(int permits,
boolean fair)
Creates a Semaphore with the given number of permits and the given fairness setting. |
Method Summary | |
---|---|
void |
acquire()
Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted . |
void |
acquireUninterruptibly()
Acquires a permit from this semaphore, blocking until one is available. |
int |
availablePermits()
Returns the current number of permits available in this semaphore. |
int |
drainPermits()
Acquires and returns all permits that are immediately available. |
protected java.util.Collection |
getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire. |
int |
getQueueLength()
Returns an estimate of the number of threads waiting to acquire. |
boolean |
hasQueuedThreads()
Queries whether any threads are waiting to acquire. |
boolean |
isFair()
Returns true if this semaphore has fairness set true. |
protected void |
reducePermits(int reduction)
Shrinks the number of available permits by the indicated reduction. |
void |
release()
Releases a permit, returning it to the semaphore. |
void |
release(int permits)
Releases the given number of permits, returning them to the semaphore. |
java.lang.String |
toString()
Returns a string identifying this semaphore, as well as its state. |
boolean |
tryAcquire()
Acquires a permit from this semaphore, only if one is available at the time of invocation. |
boolean |
tryAcquire(long timeout,
TimeUnit unit)
Acquires a permit from this semaphore, if one becomes available within the given waiting time and the current thread has not been interrupted . |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait |
Constructor Detail |
public Semaphore(int permits)
permits
- the initial number of permits available. This
value may be negative, in which case releases must
occur before any acquires will be granted.public Semaphore(int permits, boolean fair)
permits
- the initial number of permits available. This
value may be negative, in which case releases must
occur before any acquires will be granted.fair
- true if this semaphore will guarantee first-in
first-out granting of permits under contention, else false.Method Detail |
public void acquire() throws java.lang.InterruptedException
interrupted
.
Acquires a permit, if one is available and returns immediately, reducing the number of available permits by one.
If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:
release()
method for this
semaphore and the current thread is next to be assigned a permit; or
interrupts
the current
thread.
If the current thread:
interrupted
while waiting
for a permit,
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
java.lang.InterruptedException
- if the current thread is interruptedThread.interrupt()
public void acquireUninterruptibly()
Acquires a permit, if one is available and returns immediately, reducing the number of available permits by one.
If no permit is available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
some other thread invokes the release()
method for this
semaphore and the current thread is next to be assigned a permit.
If the current thread
is interrupted
while waiting
for a permit then it will continue to wait, but the time at which
the thread is assigned a permit may change compared to the time it
would have received the permit had no interruption occurred. When the
thread does return from this method its interrupt status will be set.
public boolean tryAcquire()
Acquires a permit, if one is available and returns immediately, with the value true, reducing the number of available permits by one.
If no permit is available then this method will return immediately with the value false.
Even when this semaphore has been set to use a
fair ordering policy, a call to tryAcquire() will
immediately acquire a permit if one is available, whether or not
other threads are currently waiting.
This "barging" behavior can be useful in certain
circumstances, even though it breaks fairness. If you want to honor
the fairness setting, then use
tryAcquire(0, TimeUnit.SECONDS)
which is almost equivalent (it also detects interruption).
public boolean tryAcquire(long timeout, TimeUnit unit) throws java.lang.InterruptedException
interrupted
.
Acquires a permit, if one is available and returns immediately, with the value true, reducing the number of available permits by one.
If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:
release()
method for this
semaphore and the current thread is next to be assigned a permit; or
interrupts
the current
thread; or
If a permit is acquired then the value true is returned.
If the current thread:
interrupted
while waiting to acquire
a permit,
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all.
timeout
- the maximum time to wait for a permitunit
- the time unit of the timeout argument.
java.lang.InterruptedException
- if the current thread is interruptedThread.interrupt()
public void release()
Releases a permit, increasing the number of available permits by one. If any threads are trying to acquire a permit, then one is selected and given the permit that was just released. That thread is (re)enabled for thread scheduling purposes.
There is no requirement that a thread that releases a permit must
have acquired that permit by calling acquire()
.
Correct usage of a semaphore is established by programming convention
in the application.
public void release(int permits)
Releases the given number of permits, increasing the number of available permits by that amount. If any threads are trying to acquire permits, then one is selected and given the permits that were just released. If the number of available permits satisfies that thread's request then that thread is (re)enabled for thread scheduling purposes; otherwise the thread will wait until sufficient permits are available. If there are still permits available after this thread's request has been satisfied, then those permits are assigned in turn to other threads trying to acquire permits.
There is no requirement that a thread that releases a permit must
have acquired that permit by calling acquire
.
Correct usage of a semaphore is established by programming convention
in the application.
permits
- the number of permits to release
java.lang.IllegalArgumentException
- if permits less than zero.public int availablePermits()
This method is typically used for debugging and testing purposes.
public int drainPermits()
protected void reducePermits(int reduction)
reduction
- the number of permits to remove
java.lang.IllegalArgumentException
- if reduction is negativepublic boolean isFair()
public final boolean hasQueuedThreads()
public final int getQueueLength()
protected java.util.Collection getQueuedThreads()
public java.lang.String toString()
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