import java.util.Timer;
import java.util.TimerTask;

public class Demo22 {
    public static void main(String[] args) {
        Timer timer = new Timer();
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                System.out.println("hello world!!!");
            }
        },3000);
        System.out.println("程序開(kāi)始執(zhí)行嘍!!!");
    }
}

import java.util.Comparator;
import java.util.PriorityQueue;

class MyTimerTask implements Comparable<MyTimerTask> {
    private long time; // 表示任務(wù)什么時(shí)候開(kāi)始執(zhí)行
    private Runnable runnable; // 表示具體任務(wù)是啥

    public MyTimerTask(Runnable runnable,long delay) {
        // delay是一個(gè)相對(duì)的時(shí)間差
        time = System.currentTimeMillis() + delay;// 這里計(jì)算出任務(wù)執(zhí)行的具體時(shí)間
        this.runnable = runnable;
    }

    public long getTime() {
        return time;
    }

    public Runnable getRunnable() {
        return runnable;
    }


    @Override
    public int compareTo(MyTimerTask o) {
        // 時(shí)間最少的元素放在隊(duì)首,即時(shí)間越少優(yōu)先級(jí)越高
        return (int)(this.time - o.time); // time是long類型
    }
}

// 這是定時(shí)器類的本體
class MyTimer {
    // 使用優(yōu)先級(jí)隊(duì)列來(lái)保存上面的N個(gè)任務(wù)
    private PriorityQueue<MyTimerTask> queue = new PriorityQueue<>();
    // locker是用來(lái)加鎖的對(duì)象
    private Object locker = new Object();

    // 定時(shí)器的核心方法,即把要執(zhí)行的任務(wù)添加到隊(duì)列中
    public void schedule(Runnable runnable,long delay) {
        synchronized (locker) {
            MyTimerTask task = new MyTimerTask(runnable,delay);
            queue.offer(task);
            // 每次來(lái)新的任務(wù)之后都會(huì)喚醒一下掃描線程,此時(shí)掃描線程就可以根據(jù)最新的任務(wù)情況來(lái)重新規(guī)劃等待時(shí)間
            locker.notify();
        }
    }
    // MyTimer類中還需要一個(gè)掃描線程,一方面要負(fù)責(zé)檢查隊(duì)首元素是否是此時(shí)應(yīng)該被執(zhí)行的。
    // 另一方面,當(dāng)任務(wù)到點(diǎn)開(kāi)始執(zhí)行之后,需要調(diào)用Runnable中的run方法來(lái)完成任務(wù)
    public MyTimer() {
        // 掃描線程
        Thread t = new Thread(() -> {
            while(true) {
                try {
                    synchronized(locker) {
                        while(queue.isEmpty()) {
                            // 隊(duì)列為空時(shí),此時(shí)不應(yīng)該取這里的元素
                            locker.wait();
                        }
                        MyTimerTask task = queue.peek();
                        long curTime = System.currentTimeMillis();
                        if(curTime > task.getTime()) {
                            // 如果當(dāng)前時(shí)間晚于任務(wù)的執(zhí)行時(shí)間,就意味著我們要執(zhí)行這個(gè)任務(wù)了
                            queue.poll();
                            task.getRunnable().run(); // 至此就可以執(zhí)行該任務(wù)了
                        } else {
                            // 如果當(dāng)前時(shí)間早于任務(wù)的執(zhí)行時(shí)間,誒呀太早了,讓這個(gè)線程(休眠)休息一會(huì)一會(huì)吧!!!
                            // Thread.sleep(task.getTime() - curTime);
                            locker.wait(task.getTime() - curTime);
                        }
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        t.start();
    }

}

public class Demo23 {
    public static void main(String[] args) {
        MyTimer timer = new MyTimer();
        timer.schedule(new Runnable() {
            @Override
            public void run() {
                System.out.println("hello world! 3");
            }
        },3000);
        timer.schedule(new Runnable() {
            @Override
            public void run() {
                System.out.println("hello world! 2");
            }
        },2000);
        timer.schedule(new Runnable() {
            @Override
            public void run() {
                System.out.println("hello world! 1");
            }
        },1000);
        System.out.println("程序開(kāi)始執(zhí)行!!!");
    }
}