Java's java.util.concurrent.locks.AbstractQueuedSynchronizer
(AQS) is a powerful tool for building custom locks. In this article, we'll explore how to use AQS to create a simple lock. We'll also look at how to use AQS to create a more complex lock that can be used to implement a fair, reentrant lock.
Let's start by looking at how to create a simple lock using AQS. The first thing we need to do is create a class that extends AbstractQueuedSynchronizer
. This class will be responsible for implementing the functionality of our lock.
public class MyLock extends AbstractQueuedSynchronizer {
}
Next, we need to add the methods that will be used to acquire and release the lock. The acquire
method will be used to acquire the lock, and the release
method will be used to release the lock. We can add these methods by overriding the acquire
and release
methods in our MyLock
class.
@Override
protected boolean tryAcquire(int arg) {
// TODO: implement tryAcquire
}
@Override
protected boolean tryRelease(int arg) {
// TODO: implement tryRelease
}
The tryAcquire
method will be responsible for acquiring the lock. The tryRelease
method will be responsible for releasing the lock.
The tryAcquire
method is responsible for acquiring the lock. In our MyLock
class, we can implement the tryAcquire
method by calling the acquireQueued
method. The acquireQueued
method will attempt to acquire the lock and will return true
if the lock is acquired. Otherwise, it will return false
.
@Override
protected boolean tryAcquire(int arg) {
return acquireQueued(new Node(), arg);
}
The tryRelease
method is responsible for releasing the lock. In our MyLock
class, we can implement the tryRelease
method by calling the release
method. The release
method will release the lock and will return true
if the lock is released. Otherwise, it will return false
.
@Override
protected boolean tryRelease(int arg) {
return release(arg);
}
Now that we've implemented the MyLock
class, let's take a look at how to use it. First, we need to create an instance of the MyLock
class.
MyLock lock = new MyLock();
Next, we can acquire the lock by calling the lock
method.
lock.lock();
Once we have acquired the lock, we can perform the actions that we need to perform while holding the lock. When we are finished, we can release the lock by calling the unlock
method.
lock.unlock();
Now that we've seen how to create a simple lock with AQS, let's take a look at how to create a more complex lock. In this section, we'll implement a fair, reentrant lock using AQS.
A fair, reentrant lock is a lock that is fair and that can be acquired multiple times by the same thread. A lock is fair if it grants access to the resources that it protects in the order in which the threads acquire the lock. A lock is reentrant if a thread can acquire the lock multiple times.
Let's start by looking at how to create a fair, reentrant lock using AQS. The first thing we need to do is create a class that extends AbstractQueuedSynchronizer
. This class will be responsible for implementing the functionality of our lock.
public class MyLock extends AbstractQueuedSynchronizer {
}
Next, we need to add the methods that will be used to acquire and release the lock. The acquire
method will be used to acquire the lock, and the release
method will be used to release the lock. We can add these methods by overriding the acquire
and release
methods in our MyLock
class.
@Override
protected boolean tryAcquire(int arg) {
// TODO: implement tryAcquire
}
@Override
protected boolean tryRelease(int arg) {
// TODO: implement tryRelease
}
The tryAcquire
method will be responsible for acquiring the lock. The tryRelease
method will be responsible for releasing the lock.
The tryAcquire
method is responsible for acquiring the lock. In our MyLock
class, we can implement the tryAcquire
method by calling the acquireQueued
method. The acquireQueued
method will attempt to acquire the lock and will return true
if the lock is acquired. Otherwise, it will return false
.
@Override
protected boolean tryAcquire(int arg) {
return acquireQueued(new Node(), arg);
}
The tryRelease
method is responsible for releasing the lock. In our MyLock
class, we can implement the tryRelease
method by calling the release
method. The release
method will release the lock and will return true
if the lock is released. Otherwise, it will return false
.
@Override
protected boolean tryRelease(int arg) {
return release(arg);
}
Now that we've implemented the MyLock
class, let's take a look at how to use it. First, we need to create an instance of the MyLock
class.
MyLock lock = new MyLock();
Next, we can acquire the lock by calling the lock
method.
lock.lock();
Once we have acquired the lock, we can perform the actions that we need to perform while holding the lock. When we are finished, we can release the lock by calling the unlock
method.
lock.unlock();
In this article, we've explored how to use Java's java.util.concurrent.locks.AbstractQueuedSynchronizer
to create a simple lock and a fair, reentrant lock. We've seen how to use the acquire
and release
methods to acquire and release the lock. We've also seen how to use the lock
and unlock
methods to acquire and release the lock.