首先，我检查了有关此主题的先前问题，但是没有一个适合我的具体问题。
我得到了以下代码，该代码说明了带有时间戳的传感器和存储在双数组中的数据，另外还有我实现的FairRWLock的实例。
class LockedSensors implements Sensors {
long time = 0;
double data[];
FairRWLock lock = new FairRWLock();
LockedSensors() {
time = 0;
}
// store data and timestamp
// if and only if data stored previously is older (lower timestamp)
public void update(long timestamp, double[] data) {
lock.writeAcquire();
if (timestamp > time) {
if (this.data == null)
this.data = new double[data.length];
time = timestamp;
for (int i = 0; i < data.length; ++i)
this.data[i] = data[i];
}
lock.writeRelease();
}
// pre: val != null
// pre: val.length matches length of data written via update
// if no data has been written previously, return 0
// otherwise return current timestamp and fill current data to array passed
// as val
public long get(double val[]) {
try{
lock.readAcquire();
if (time == 0) return 0;
for (int i = 0; i < data.length; ++i)
val[i] = data[i];
return time;
} finally{lock.readRelease();}
}
}
它支持更新，它取决于接收新数据的时间，而get，它提取存储在特定传感器中的数据。
这是我对FairRWLock的实现：
class FairRWLock{
private int readers = 0, writers = 0, readersWaiting = 0, writersWaiting = 0, writersWait = 0;
private static final int ReaderPriority = 30;
private Lock lock = new ReentrantLock();
private Condition readerPass = lock.newCondition();
private Condition writerPass = lock.newCondition();
/*
* readers denotes the number of current readers, writers equivalent, readersWaiting denotes the number of readers
* awaiting their signal, writersWaiting equivalent. writersWait denotes the number of readers the writers have to
* let pass before they can proceed, this amount is controlled by the ReaderPriority (reset occurs when writer releases)
*/
/*
* increment the number of waiting readers, check if there are any currently working writers OR writers are waiting
* whilst they don't have to let any readers pass. When signaled, decrement readersWaiting, decrement the number of
* readers the writers have to let pass and increment the number of current readers.
*/
public void readAcquire(){
lock.lock();
readersWaiting++;
while(writers > 0 || (writersWaiting > 0 && writersWait <= 0)){
try {
readerPass.await();
} catch (InterruptedException e) {}
}
readersWaiting--;
writersWait--;
readers++;
lock.unlock();
}
/*
* simply decrement number of readers and signal the threads that have to be signaled
*/
public void readRelease(){
lock.lock();
readers--;
signaling();
lock.unlock();
}
/*
* increment number of waiting writers, check if there are currently working writers OR readers OR readers currently
* have priority over the writers. When signaled decrement writersWaiting, increment number of writers
*/
public void writeAcquire(){
lock.lock();
writersWaiting++;
while(writers > 0 || readers > 0 || (readersWaiting > 0 && writersWait > 0)){
try{
writerPass.await();
} catch(InterruptedException e) {}
}
writersWaiting--;
writers++;
lock.unlock();
}
/*
* simply decrement number of current writers, reset the number of readers the writers have to let pass before
* another writer may pass. signal the ones that should be
*/
public void writeRelease(){
lock.lock();
writers--;
writersWait = ReaderPriority;
signaling();
lock.unlock();
}
/*
* check first if readers currently got priority over the writers. if so (readersWaiting??) ? signal them : signalAll,
* if not (writersWaiting??) ? signal them : signalAll
*/
private void signaling(){
if(writersWait > 0){
if(readersWaiting > 0) readerPass.signalAll();
else writerPass.signal();
} else{
if(writersWaiting > 0) writerPass.signal();
else readerPass.signalAll();
}
}
}
我对条件锁定并不十分熟悉，而且我的代码似乎遭受了饥饿甚至死锁。但是我无法找到问题（很可能是在FairRWLock实现中的某个地方）。
答案 0 :(得分：1)
试图在不公平的锁定上建立一个公平的锁定是没有意义的。当线程进入readAcquire()或writeAcquire()时，它们正在调用lock.lock()，如果不立即成功，它们可能会进入等待状态并被任意数量的线程才可以继续。
此时，无论你做什么，都已经不可能重新建立公平。但值得注意的是，你也错过了await()的含义。此操作将暂时释放锁定，因为只有这样才能让其他线程有机会满足您正在等待的条件。当线程获得signal()时，它必须重新获取锁，这又是一个公平的操作。任意数量的线程可以发出新的锁请求，在很久以前调用await()的线程将继续进行之前完全改变这种情况。
最后，你不要求公平。 update操作旨在忽略过时的更新，因此如果较新的update请求可以更快地进行，实际上将是一个胜利，因为待处理的旧请求将变为无操作。对于并发get请求，您实际上根本不想要阻塞，所有读取请求应该能够并发运行，但是，当然，您需要一致性（线程安全）并且不需要编写器饥饿。
最好的解决方案是根本不进行锁定并实现整个无锁操作：
class LockedSensors implements Sensors {
private static final class State {
final long time;
final double[] data;
State(long t, double[] in) {
time = t;
data = in.clone();
}
}
final AtomicReference<State> current = new AtomicReference<>();
LockedSensors() {}
// store data and timestamp
// if and only if data stored previously is older (lower timestamp)
public void update(long timestamp, double[] data) {
State newState = null;
for(;;) {
State old = current.get();
if(old != null && old.time > timestamp) return;
if(newState == null) newState = new State(timestamp, data);
if(current.compareAndSet(old, newState)) return;
}
}
// pre: val != null
// pre: val.length matches length of data written via update
// if no data has been written previously, return 0
// otherwise return current timestamp and fill current data to array passed as val
public long get(double[] val) {
State actual = current.get();
if(actual == null) return 0;
if(actual.data.length != val.length)
throw new IllegalArgumentException();
System.arraycopy(actual.data, 0, val, 0, actual.data.length);
return actual.time;
}
}
在这里，读者总是可以继续返回上次完成更新的结果，而不会阻止任何编写者。即使是编写者也不会相互阻止，但如果在其间发生了另一次更新，则可能必须旋转，但由于每个作者都会在遇到更新的时间戳后立即返回，并且总会有至少一个作者取得进展，没问题。