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This article mainly explains "what is the use of DelayQueue", the content of the explanation 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 "what is the use of DelayQueue"!

As the name implies, DelayQueue is a delay queue usage:

Assuming that our producer submits a task and the consumer can execute it after 5 seconds, we can define the task in the following format and implement the Delayed interface, where data is the information stored by the task.

/ * * specific task * @ author wangshixiang * / public class Task implements Delayed {/ * data * / private final String data; / * Task execution time * / private final long time; public Task (String data,TimeUnit timeUnit,long time) {this.data=data; this.time=System.currentTimeMillis () + timeUnit.toMillis (time) } @ Override public long getDelay (TimeUnit unit) {long res= time-System.currentTimeMillis (); return unit.convert (res,TimeUnit.MILLISECONDS);} public String getData () {return data;} @ Override public int compareTo (Delayed o) {if (o instanceof Task) {Task task= (Task) o; return (int) (this.time-task.time) } return 0;}}

After defining the task, we need to define a task queue QUEUE_TASK to store messages. The effect is to output Hello... five seconds after the program runs.

Private static final DelayQueue QUEUE_TASK = new DelayQueue (); public static void main (String [] args) throws InterruptedException {QUEUE_TASK .add (new Task ("Hello...", TimeUnit.SECONDS,5)); System.out.println (QUEUE_TASK .take (). GetData ());} usage details

Delayed API definition:

Public interface DeDlayed extends Comparable {long getDelay (TimeUnit unit);}

We find that the Delayed interface inherits the Comparable interface and has a getDelay method that calls this method of the header task while the program is running to return how long the task can be executed. When our task implements this interface, we can store the execution time of the task, and calculate the difference between the execution time and the execution time through the execution time-current time, so we Task defines a variable of the task and sets the execution time of the task when the object is created.

2. DelayQueue delay queue

First, let's take a look at the DelayQueue class inheritance implementation structure diagram

It can be understood that DelayQueue is a blocking queue with delayed execution function.

In-depth understanding

Why does the Delayed interface inherit the Comparable interface?

How does DelayQueue realize that you can't get a task out until the scheduled time?

What happens when you put a task in the queue?

With these questions in mind, let's take a look at the source code of DelayQueeu. First, take a look at the main parameters:

/ / Lock private final transient ReentrantLock lock = new ReentrantLock (); / / the first priority queue with the earliest execution time is private final PriorityQueue q = new PriorityQueue (); / / is there a thread waiting for the task to be executed private Thread leader; / / conditional wake-up private final Condition available = lock.newCondition ()

So let's first look at the add (E e) method, what does the task do when it is queued?

Public boolean add (E e) {return offer (e);} public boolean offer (E e) {final ReentrantLock lock = this.lock; lock.lock (); try {q.offer (e); if (q.peek () = = e) {leader = null; available.signal () } return true;} finally {lock.unlock ();}}

The following steps are taken when you join the queue:

Acquire lock

Put elements (put in priority queue)

If you are in the first place, the originally marked lead thread has been invalidated and directly set to null and wake up the consumer

Release lock

Next, what does the take () method do when you see the queue?

Public E take () throws InterruptedException {final ReentrantLock lock = this.lock; lock.lockInterruptibly (); try {for (;;) {E first = q.peek (); if (first = = null) available.await (); else {long delay = first.getDelay (NANOSECONDS) If (delay

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