What is the principle behind the implementation of a delay queue in Java?

The DelayQueue in Java is a special type of priority queue that allows elements to be retrieved only after a specific delay time. The DelayQueue implementation relies on two key points: using a priority queue to store delayed elements and sorting elements based on their own delay time.

The specific implementation principle is as follows:

1. The DelayQueue internally utilizes a PriorityQueue to store elements.
2. The Delayed element is an interface, and the implementing class must implement the getDelay() method to specify the delay time of the element.
3. When a delayed element is added to the DelayQueue, it will be sorted based on the delay time returned by the getDelay() method of the element, with the element having the shortest delay time being placed in the front of the queue.
4. When using the take() method of DelayQueue to retrieve elements, the method will block until an element expires if there are no elements that have already expired in the queue.
5. The DelayQueue utilizes ReentrantLock and Condition internally to achieve thread safety and blocking wait.
6. The expiration time of an element is calculated when the element is added to the queue. If the delay time of the element is less than or equal to 0, it means that the element has already expired and can be retrieved.