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This article mainly explains "how to understand ReentrantLock unfair lock source code", the content of the article is simple and clear, easy to learn and understand, the following please follow the editor's ideas slowly in depth, together to study and learn "how to understand ReentrantLock unfair lock source code"!

1. Lock

In java, the way of locking

Synchronized, which is implemented at the bottom of java, that is, the C language.

. Lock, which is under the java.util.concurrent package, is implemented in java.

2.ReentrantLock

ReentrantLock is an implementation of Lock, which is a reentrant fair or unfair lock. The default is an unfair lock.

2.1 creation of Lock

First, take a look at the code for creating and using the lock:

/ / create lock Lock lock = new ReentrantLock (); / / lock lock.lock (); / / release lock lock.unlock ()

Then take a look at the constructor that is created for ReentrantLock:

Public ReentrantLock () {sync = new NonfairSync ();}

NonfairSync is an unfair lock. So ReentrantLock defaults to the implementation of unfair locks.

2.2 lock ()

The logic of locking is more complex because of thread competition. So there are two situations, one is the handling of competition to lock, and the other is the handling of no competition to lock.

First of all, let's take a look at the lock () method, because it is ultimately an unfair implementation, so let's just look at the lock method in NonfairSync.

Final void lock () {if (compareAndSetState (0,1)) setExclusiveOwnerThread (Thread.currentThread ()); else acquire (1);}

2.3 did not acquire the logical acquire of the lock ()

Go directly to the code:

Public final void acquire (int arg) {if (! tryAcquire (arg) & & acquireQueued (addWaiter (Node.EXCLUSIVE), arg) selfInterrupt ();}

There are still three ways, said Ah Fan one by one.

TryAcquire (arg), or first look at the code in the analysis.

Final boolean nonfairTryAcquire (int acquires) {final Thread current = Thread.currentThread (); int c = getState (); if (c = = 0) {if (compareAndSetState (0, acquires)) {setExclusiveOwnerThread (current); return true;}} else if (current = = getExclusiveOwnerThread ()) {int nextc = c + acquires If (nextc < 0) / / overflow throw new Error ("Maximum lock count exceeded"); setState (nextc); return true;} return false;}

a. If the state is equal to 0, it means that the thread that acquired the lock has been released, then the thread will compete for the lock again, which is the embodiment of the unfair lock. If it is a fair lock, there is no such judgment.

b. If the previous thread that acquired the lock did not release the lock, it is determined whether it is the same thread, and if so, the state is incremented by 1. This is the embodiment of reentering the lock.

c. If both are not satisfied, return false.

AcquireQueued (addWaiter (Node.EXCLUSIVE), arg)), if the lock is not successfully acquired again, and the lock is not reentrant, then it is stored in a blocking queue. There is still a little bit of logic in it, so you can take a look at it for yourself if you are interested.

SelfInterrupt (); this is the interrupt flag of the current thread, the function is whether the thread is blocking or not, the client calls the method interrupt () that interrupts the thread, then the thread will respond when it is awakened. It depends on the code logic in the thread's run method.

2.4 unlock ()

Protected final boolean tryRelease (int releases) {int c = getState ()-releases; if (Thread.currentThread ()! = getExclusiveOwnerThread ()) throw new IllegalMonitorStateException (); boolean free = false; if (c = = 0) {free = true; setExclusiveOwnerThread (null);} setState (c); return free;}

State-1. If it is greater than 0, the reentrant lock is released. You only need to modify the state.

If it is equal to 0, it means that to release the lock, you first need to set the exclusive thread to null and then set state to 0.

Thank you for your reading, the above is the content of "how to understand ReentrantLock unfair lock source code". After the study of this article, I believe you have a deeper understanding of how to understand ReentrantLock unfair lock source code, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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