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com.google.common.util.concurrent
Class Monitor
java.lang.Object
com.google.common.util.concurrent.Monitor
A synchronization abstraction supporting waiting on arbitrary boolean conditions.
This class is intended as a replacement for ReentrantLock
. Code using Monitor
is less error-prone and more readable than code using ReentrantLock
, without significant
performance loss. Monitor
even has the potential for performance gain by optimizing the
evaluation and signaling of conditions. Signaling is entirely
implicit.
By eliminating explicit signaling, this class can guarantee that only one thread is awakened
when a condition becomes true (no "signaling storms" due to use of Condition.signalAll
) and that no signals are lost
(no "hangs" due to incorrect use of Condition.signal
).
A thread is said to occupy a monitor if it has entered the monitor but not yet left. Only one thread may occupy a given monitor at any moment. A monitor is also reentrant, so a thread may enter a monitor any number of times, and then must leave the same number of times. The enter and leave operations have the same synchronization semantics as the built-in Java language synchronization primitives.
A call to any of the enter methods with void return type should always be followed immediately by a try/finally block to ensure that the current thread leaves the monitor cleanly:
monitor.enter();
try {
// do things while occupying the monitor
} finally {
monitor.leave();
}
A call to any of the enter methods with boolean return type should always appear as
the condition of an if statement containing a try/finally block to ensure that the
current thread leaves the monitor cleanly: if (monitor.tryEnter()) {
try {
// do things while occupying the monitor
} finally {
monitor.leave();
}
} else {
// do other things since the monitor was not available
}
Comparison with synchronized
and ReentrantLock
The following examples show a simple threadsafe holder expressed using synchronized
,
ReentrantLock
, and Monitor
.
synchronized
This version is the fewest lines of code, largely because the synchronization mechanism used
is built into the language and runtime. But the programmer has to remember to avoid a couple of
common bugs: The wait()
must be inside a while
instead of an if
, and
notifyAll()
must be used instead of notify()
because there are two different
logical conditions being awaited.
public class SafeBox<V> {
private V value;
public synchronized V get() throws InterruptedException {
while (value == null) {
wait();
}
V result = value;
value = null;
notifyAll();
return result;
}
public synchronized void set(V newValue) throws InterruptedException {
while (value != null) {
wait();
}
value = newValue;
notifyAll();
}
}
ReentrantLock
This version is much more verbose than the synchronized
version, and still suffers
from the need for the programmer to remember to use while
instead of if
.
However, one advantage is that we can introduce two separate Condition
objects, which
allows us to use signal()
instead of signalAll()
, which may be a performance
benefit.
public class SafeBox<V> {
private final ReentrantLock lock = new ReentrantLock();
private final Condition valuePresent = lock.newCondition();
private final Condition valueAbsent = lock.newCondition();
private V value;
public V get() throws InterruptedException {
lock.lock();
try {
while (value == null) {
valuePresent.await();
}
V result = value;
value = null;
valueAbsent.signal();
return result;
} finally {
lock.unlock();
}
}
public void set(V newValue) throws InterruptedException {
lock.lock();
try {
while (value != null) {
valueAbsent.await();
}
value = newValue;
valuePresent.signal();
} finally {
lock.unlock();
}
}
}
Monitor
This version adds some verbosity around the Guard
objects, but removes that same
verbosity, and more, from the get
and set
methods. Monitor
implements the
same efficient signaling as we had to hand-code in the ReentrantLock
version above.
Finally, the programmer no longer has to hand-code the wait loop, and therefore doesn't have to
remember to use while
instead of if
.
public class SafeBox<V> {
private final Monitor monitor = new Monitor();
private final Monitor.Guard valuePresent = new Monitor.Guard(monitor) {
public boolean isSatisfied() {
return value != null;
}
};
private final Monitor.Guard valueAbsent = new Monitor.Guard(monitor) {
public boolean isSatisfied() {
return value == null;
}
};
private V value;
public V get() throws InterruptedException {
monitor.enterWhen(valuePresent);
try {
V result = value;
value = null;
return result;
} finally {
monitor.leave();
}
}
public void set(V newValue) throws InterruptedException {
monitor.enterWhen(valueAbsent);
try {
value = newValue;
} finally {
monitor.leave();
}
}
}
- Since:
- 10.0
- Author:
- Justin T. Sampson
Nested Class Summary | |
---|---|
static class |
Monitor.Guard
A boolean condition for which a thread may wait. |
Constructor Summary | |
---|---|
Monitor()
Creates a monitor with a non-fair (but fast) ordering policy. |
|
Monitor(boolean fair)
Creates a monitor with the given ordering policy. |
Method Summary | |
---|---|
void |
enter()
Enters this monitor. |
boolean |
enter(long time,
TimeUnit unit)
Enters this monitor. |
boolean |
enterIf(Monitor.Guard guard)
Enters this monitor if the guard is satisfied. |
boolean |
enterIf(Monitor.Guard guard,
long time,
TimeUnit unit)
Enters this monitor if the guard is satisfied. |
boolean |
enterIfInterruptibly(Monitor.Guard guard)
Enters this monitor if the guard is satisfied. |
boolean |
enterIfInterruptibly(Monitor.Guard guard,
long time,
TimeUnit unit)
Enters this monitor if the guard is satisfied. |
void |
enterInterruptibly()
Enters this monitor. |
boolean |
enterInterruptibly(long time,
TimeUnit unit)
Enters this monitor. |
void |
enterWhen(Monitor.Guard guard)
Enters this monitor when the guard is satisfied. |
boolean |
enterWhen(Monitor.Guard guard,
long time,
TimeUnit unit)
Enters this monitor when the guard is satisfied. |
void |
enterWhenUninterruptibly(Monitor.Guard guard)
Enters this monitor when the guard is satisfied. |
boolean |
enterWhenUninterruptibly(Monitor.Guard guard,
long time,
TimeUnit unit)
Enters this monitor when the guard is satisfied. |
int |
getOccupiedDepth()
Returns the number of times the current thread has entered this monitor in excess of the number of times it has left. |
int |
getQueueLength()
Returns an estimate of the number of threads waiting to enter this monitor. |
int |
getWaitQueueLength(Monitor.Guard guard)
Returns an estimate of the number of threads waiting for the given guard to become satisfied. |
boolean |
hasQueuedThread(Thread thread)
Queries whether the given thread is waiting to enter this monitor. |
boolean |
hasQueuedThreads()
Returns whether any threads are waiting to enter this monitor. |
boolean |
hasWaiters(Monitor.Guard guard)
Queries whether any threads are waiting for the given guard to become satisfied. |
boolean |
isFair()
Returns whether this monitor is using a fair ordering policy. |
boolean |
isOccupied()
Returns whether this monitor is occupied by any thread. |
boolean |
isOccupiedByCurrentThread()
Returns whether the current thread is occupying this monitor (has entered more times than it has left). |
void |
leave()
Leaves this monitor. |
boolean |
tryEnter()
Enters this monitor if it is possible to do so immediately. |
boolean |
tryEnterIf(Monitor.Guard guard)
Enters this monitor if it is possible to do so immediately and the guard is satisfied. |
void |
waitFor(Monitor.Guard guard)
Waits for the guard to be satisfied. |
boolean |
waitFor(Monitor.Guard guard,
long time,
TimeUnit unit)
Waits for the guard to be satisfied. |
void |
waitForUninterruptibly(Monitor.Guard guard)
Waits for the guard to be satisfied. |
boolean |
waitForUninterruptibly(Monitor.Guard guard,
long time,
TimeUnit unit)
Waits for the guard to be satisfied. |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
---|
Monitor
public Monitor()
- Creates a monitor with a non-fair (but fast) ordering policy. Equivalent to
Monitor(false)
.
Monitor
public Monitor(boolean fair)
- Creates a monitor with the given ordering policy.
- Parameters:
fair
- whether this monitor should use a fair ordering policy rather than a non-fair (but fast) one
Method Detail |
---|
enter
public void enter()
- Enters this monitor. Blocks indefinitely.
enterInterruptibly
public void enterInterruptibly() throws InterruptedException
- Enters this monitor. Blocks indefinitely, but may be interrupted.
- Throws:
InterruptedException
enter
public boolean enter(long time, TimeUnit unit)
- Enters this monitor. Blocks at most the given time.
- Returns:
- whether the monitor was entered
enterInterruptibly
public boolean enterInterruptibly(long time, TimeUnit unit) throws InterruptedException
- Enters this monitor. Blocks at most the given time, and may be interrupted.
- Returns:
- whether the monitor was entered
- Throws:
InterruptedException
tryEnter
public boolean tryEnter()
- Enters this monitor if it is possible to do so immediately. Does not block.
Note: This method disregards the fairness setting of this monitor.
- Returns:
- whether the monitor was entered
enterWhen
public void enterWhen(Monitor.Guard guard) throws InterruptedException
- Enters this monitor when the guard is satisfied. Blocks indefinitely, but may be interrupted.
- Throws:
InterruptedException
enterWhenUninterruptibly
public void enterWhenUninterruptibly(Monitor.Guard guard)
- Enters this monitor when the guard is satisfied. Blocks indefinitely.
enterWhen
public boolean enterWhen(Monitor.Guard guard, long time, TimeUnit unit) throws InterruptedException
- Enters this monitor when the guard is satisfied. Blocks at most the given time, including both
the time to acquire the lock and the time to wait for the guard to be satisfied, and may be
interrupted.
- Returns:
- whether the monitor was entered
- Throws:
InterruptedException
enterWhenUninterruptibly
public boolean enterWhenUninterruptibly(Monitor.Guard guard, long time, TimeUnit unit)
- Enters this monitor when the guard is satisfied. Blocks at most the given time, including
both the time to acquire the lock and the time to wait for the guard to be satisfied.
- Returns:
- whether the monitor was entered
enterIf
public boolean enterIf(Monitor.Guard guard)
- Enters this monitor if the guard is satisfied. Blocks indefinitely acquiring the lock, but
does not wait for the guard to be satisfied.
- Returns:
- whether the monitor was entered
enterIfInterruptibly
public boolean enterIfInterruptibly(Monitor.Guard guard) throws InterruptedException
- Enters this monitor if the guard is satisfied. Blocks indefinitely acquiring the lock, but does
not wait for the guard to be satisfied, and may be interrupted.
- Returns:
- whether the monitor was entered
- Throws:
InterruptedException
enterIf
public boolean enterIf(Monitor.Guard guard, long time, TimeUnit unit)
- Enters this monitor if the guard is satisfied. Blocks at most the given time acquiring the
lock, but does not wait for the guard to be satisfied.
- Returns:
- whether the monitor was entered
enterIfInterruptibly
public boolean enterIfInterruptibly(Monitor.Guard guard, long time, TimeUnit unit) throws InterruptedException
- Enters this monitor if the guard is satisfied. Blocks at most the given time acquiring the
lock, but does not wait for the guard to be satisfied, and may be interrupted.
- Returns:
- whether the monitor was entered
- Throws:
InterruptedException
tryEnterIf
public boolean tryEnterIf(Monitor.Guard guard)
- Enters this monitor if it is possible to do so immediately and the guard is satisfied. Does not
block acquiring the lock and does not wait for the guard to be satisfied.
Note: This method disregards the fairness setting of this monitor.
- Returns:
- whether the monitor was entered
waitFor
public void waitFor(Monitor.Guard guard) throws InterruptedException
- Waits for the guard to be satisfied. Waits indefinitely, but may be interrupted. May be
called only by a thread currently occupying this monitor.
- Throws:
InterruptedException
waitForUninterruptibly
public void waitForUninterruptibly(Monitor.Guard guard)
- Waits for the guard to be satisfied. Waits indefinitely. May be called only by a thread
currently occupying this monitor.
waitFor
public boolean waitFor(Monitor.Guard guard, long time, TimeUnit unit) throws InterruptedException
- Waits for the guard to be satisfied. Waits at most the given time, and may be interrupted.
May be called only by a thread currently occupying this monitor.
- Returns:
- whether the guard is now satisfied
- Throws:
InterruptedException
waitForUninterruptibly
public boolean waitForUninterruptibly(Monitor.Guard guard, long time, TimeUnit unit)
- Waits for the guard to be satisfied. Waits at most the given time. May be called only by a
thread currently occupying this monitor.
- Returns:
- whether the guard is now satisfied
leave
public void leave()
- Leaves this monitor. May be called only by a thread currently occupying this monitor.
isFair
public boolean isFair()
- Returns whether this monitor is using a fair ordering policy.
isOccupied
public boolean isOccupied()
- Returns whether this monitor is occupied by any thread. This method is designed for use in
monitoring of the system state, not for synchronization control.
isOccupiedByCurrentThread
public boolean isOccupiedByCurrentThread()
- Returns whether the current thread is occupying this monitor (has entered more times than it
has left).
getOccupiedDepth
public int getOccupiedDepth()
- Returns the number of times the current thread has entered this monitor in excess of the number
of times it has left. Returns 0 if the current thread is not occupying this monitor.
getQueueLength
public int getQueueLength()
- Returns an estimate of the number of threads waiting to enter this monitor. The value is only
an estimate because the number of threads may change dynamically while this method traverses
internal data structures. This method is designed for use in monitoring of the system state,
not for synchronization control.
hasQueuedThreads
public boolean hasQueuedThreads()
- Returns whether any threads are waiting to enter this monitor. Note that because cancellations
may occur at any time, a
true
return does not guarantee that any other thread will ever enter this monitor. This method is designed primarily for use in monitoring of the system state.
hasQueuedThread
public boolean hasQueuedThread(Thread thread)
- Queries whether the given thread is waiting to enter this monitor. Note that because
cancellations may occur at any time, a
true
return does not guarantee that this thread will ever enter this monitor. This method is designed primarily for use in monitoring of the system state.
hasWaiters
public boolean hasWaiters(Monitor.Guard guard)
- Queries whether any threads are waiting for the given guard to become satisfied. Note that
because timeouts and interrupts may occur at any time, a
true
return does not guarantee that the guard becoming satisfied in the future will awaken any threads. This method is designed primarily for use in monitoring of the system state.
getWaitQueueLength
public int getWaitQueueLength(Monitor.Guard guard)
- Returns an estimate of the number of threads waiting for the given guard to become satisfied.
Note that because timeouts and interrupts may occur at any time, the estimate serves only as an
upper bound on the actual number of waiters. This method is designed for use in monitoring of
the system state, not for synchronization control.
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