In the implementation of the Lock interface, there is a well-known one called ReentrantLock. Its implementation mechanism relies on its internal class Sync, which is a subclass of AbstractQueuedSynchronizer (AQS). The main function of this class is to manage the synchronization state by managing a two-way synchronization queue (FIFO), and realize the security of multi-thread access to the synchronization state. The data structure stored in the synchronization queue is a node Node, which contains references to forward nodes and subsequent nodes similar to a doubly linked list, as well as subsequent waiting node references and constructed threads. The nodes in the waiting queue to be discussed later are the same definitions as this.
For users, it is only necessary to understand that the synchronization queue is managed by the synchronizer through two directional references. The following is the basic structure diagram:
ConditionObject, a subclass of the Condition interface, is also aggregated inside Sync, which provides a monitor method similar to Object to cooperate with Lock to implement the waiting notification mode. Each Condition comes with a waiting queue. After a thread calls the await method, it will be transferred from the synchronization queue to the waiting queue. The waiting queue is actually a singly linked list, similar to a synchronizer, which saves References to the head node and tail node, other methods are similar to synchronous queues. Only when a thread calls the signal method of the condition (wake up the first node), it can be transferred from the waiting queue to the synchronization queue.