关于美团的分布式id中的一个小问题
public Result getIdFromSegmentBuffer(final SegmentBuffer buffer) {
while (true) {
buffer.rLock().lock();
try {
final Segment segment = buffer.getCurrent();
if (!buffer.isNextReady() && (segment.getIdle() < 0.9 * segment.getStep()) && buffer.getThreadRunning().compareAndSet(false, true)) {
service.execute(new Runnable() {
@Override
public void run() {
Segment next = buffer.getSegments()[buffer.nextPos()];
boolean updateOk = false;
try {
updateSegmentFromDb(buffer.getKey(), next);
updateOk = true;
logger.info("update segment {} from db {}", buffer.getKey(), next);
} catch (Exception e) {
logger.warn(buffer.getKey() + " updateSegmentFromDb exception", e);
} finally {
if (updateOk) {
buffer.wLock().lock();
buffer.setNextReady(true);
buffer.getThreadRunning().set(false);
buffer.wLock().unlock();
} else {
buffer.getThreadRunning().set(false);
}
}
}
});
}
long value = segment.getValue().getAndIncrement();
if (value < segment.getMax()) {
return new Result(value, Status.SUCCESS);
}
} finally {
buffer.rLock().unlock();
}
waitAndSleep(buffer);
buffer.wLock().lock();
try {
final Segment segment = buffer.getCurrent();
long value = segment.getValue().getAndIncrement();
if (value < segment.getMax()) {
return new Result(value, Status.SUCCESS);
}
if (buffer.isNextReady()) {
buffer.switchPos();
buffer.setNextReady(false);
} else {
logger.error("Both two segments in {} are not ready!", buffer);
return new Result(EXCEPTION_ID_TWO_SEGMENTS_ARE_NULL, Status.EXCEPTION);
}
} finally {
buffer.wLock().unlock();
}
}
}
在第4行中,buffer已经加过读锁了,为什么在没解锁的情况下,第21行代码中又加上了写锁呢,锁不是不能升级的吗?