io.questdb.mp.SimpleWaitingLock Maven / Gradle / Ivy
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* Copyright (c) 2014-2019 Appsicle
* Copyright (c) 2019-2023 QuestDB
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
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package io.questdb.mp;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* Simple blocking lock which allows 2 arbitrary threads to compete for ownership. Behaviour is undefined for more than 2 threads.
* The lock is not reentrant.
*/
public final class SimpleWaitingLock {
private final AtomicReference ownerOrWaiter = new AtomicReference<>(null);
public boolean isLocked() {
return ownerOrWaiter.get() != null;
}
/**
* Acquires the lock. The method will block until the lock is acquired.
*/
public void lock() {
tryLock(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
}
/**
* Tries to acquire the lock, allowing for a specified timeout period.
* If the lock is not acquired within the given timeout, the method returns false.
*
* @param timeout the maximum time to wait for acquiring the lock, in the given TimeUnit
* @param unit the time unit of the timeout parameter
* @return true if the lock was acquired within the timeout period, false otherwise
*/
public boolean tryLock(long timeout, TimeUnit unit) {
Thread currentThread = Thread.currentThread();
Thread expectedOwner = null;
long deadline = System.nanoTime() + unit.toNanos(timeout); // this might overflow, but that's OK. we subtract current nanoTime and that makes it positive again
for (long remainingNanos = deadline - System.nanoTime(); remainingNanos > 0; remainingNanos = deadline - System.nanoTime()) {
if (ownerOrWaiter.compareAndSet(expectedOwner, currentThread)) {
if (expectedOwner == null) {
// there was no owner before -> we acquired the lock and we are the new owner. yay!
return true;
}
// CAS succeeded, but there was an owner before -> we are a waiter
LockSupport.parkNanos(remainingNanos);
}
expectedOwner = ownerOrWaiter.get();
}
return false;
}
/**
* Tries to acquire the lock. If the lock is available, it is acquired and the method returns true.
* If the lock is already acquired by another thread, the method returns false.
*
* @return true if the lock was acquired, false otherwise
*/
public boolean tryLock() {
return ownerOrWaiter.compareAndSet(null, Thread.currentThread());
}
/**
* Releases the lock, allowing other threads to acquire it.
*
* If the lock is not currently owned by any thread, an IllegalStateException is thrown.
* If there is another thread waiting to acquire the lock, it is unparked.
* If there is no waiting thread, the method returns without any additional action.
*
* Implementation note: This method does not validate the calling thread is the owner of the lock. The method
* releases the lock regardless of the calling thread. This makes it somewhat similar to a single permit semaphore.
* In such case it may set the unpark flag for the formerly owning thread. The unpark flag should have no negative
* side effect on the formerly owning thread as programs must be designed to handle spurious wakeups anyway.
*
* @throws IllegalStateException if the lock is not owned by any thread
*/
public void unlock() {
Thread currentThread = Thread.currentThread();
Thread waitingOrOwningThread = ownerOrWaiter.getAndSet(null);
if (waitingOrOwningThread == null) {
throw new IllegalStateException("Lock is not owned");
}
if (waitingOrOwningThread == currentThread) {
// no waiters
return;
}
LockSupport.unpark(waitingOrOwningThread);
}
}