edu.emory.mathcs.backport.java.util.concurrent
Class Exchanger
java.lang.Object
edu.emory.mathcs.backport.java.util.concurrent.Exchanger
public class Exchanger
extends java.lang.Object
A synchronization point at which threads can pair and swap elements
within pairs. Each thread presents some object on entry to the
exchange
method, matches with a partner thread,
and receives its partner's object on return.
Sample Usage:
Here are the highlights of a class that uses an
Exchanger
to swap buffers between threads so that the thread filling the
buffer gets a freshly emptied one when it needs it, handing off the
filled one to the thread emptying the buffer.
class FillAndEmpty {
Exchanger exchanger = new Exchanger();
DataBuffer initialEmptyBuffer = ... a made-up type
DataBuffer initialFullBuffer = ...
class FillingLoop implements Runnable {
public void run() {
DataBuffer currentBuffer = initialEmptyBuffer;
try {
while (currentBuffer != null) {
addToBuffer(currentBuffer);
if (currentBuffer.isFull())
currentBuffer = exchanger.exchange(currentBuffer);
}
} catch (InterruptedException ex) { ... handle ...
}
}
class EmptyingLoop implements Runnable {
public void run() {
DataBuffer currentBuffer = initialFullBuffer;
try {
while (currentBuffer != null) {
takeFromBuffer(currentBuffer);
if (currentBuffer.isEmpty())
currentBuffer = exchanger.exchange(currentBuffer);
}
} catch (InterruptedException ex) { ... handle ...}
}
}
void start() {
new Thread(new FillingLoop()).start();
new Thread(new EmptyingLoop()).start();
}
}
}
Memory consistency effects: For each pair of threads that
successfully exchange objects via an
Exchanger
, actions
prior to the
exchange()
in each thread
happen-before
those subsequent to a return from the corresponding
exchange()
in the other thread.
- Doug Lea and Bill Scherer and Michael Scott
Object | exchange(Object x) - Waits for another thread to arrive at this exchange point (unless
it is
interrupted ),
and then transfers the given object to it, receiving its object
in return.
|
Object | exchange(Object x, long timeout, TimeUnit unit) - Waits for another thread to arrive at this exchange point (unless
the current thread is
interrupted or
the specified waiting time elapses), and then transfers the given
object to it, receiving its object in return.
|
Exchanger
public Exchanger()
Creates a new Exchanger.
exchange
public Object exchange(Object x)
throws InterruptedException
Waits for another thread to arrive at this exchange point (unless
it is
interrupted
),
and then transfers the given object to it, receiving its object
in return.
If another thread is already waiting at the exchange point then
it is resumed for thread scheduling purposes and receives the object
passed in by the current thread. The current thread returns immediately,
receiving the object passed to the exchange by that other thread.
If no other thread is already waiting at the exchange then the
current thread is disabled for thread scheduling purposes and lies
dormant until one of two things happens:
- Some other thread enters the exchange; or
- Some other thread
interrupts
the current
thread.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
interrupted
while waiting
for the exchange,
then
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
x
- the object to exchange
- the object provided by the other thread
exchange
public Object exchange(Object x,
long timeout,
TimeUnit unit)
throws InterruptedException,
TimeoutException
Waits for another thread to arrive at this exchange point (unless
the current thread is
interrupted
or
the specified waiting time elapses), and then transfers the given
object to it, receiving its object in return.
If another thread is already waiting at the exchange point then
it is resumed for thread scheduling purposes and receives the object
passed in by the current thread. The current thread returns immediately,
receiving the object passed to the exchange by that other thread.
If no other thread is already waiting at the exchange then the
current thread is disabled for thread scheduling purposes and lies
dormant until one of three things happens:
- Some other thread enters the exchange; or
- Some other thread
interrupts
the current
thread; or
- The specified waiting time elapses.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
interrupted
while waiting
for the exchange,
then
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
If the specified waiting time elapses then
TimeoutException
is thrown.
If the time is
less than or equal to zero, the method will not wait at all.
x
- the object to exchangetimeout
- the maximum time to waitunit
- the time unit of the timeout argument
- the object provided by the other thread
TimeoutException
- if the specified waiting time elapses
before another thread enters the exchange