黑马点评演练：缓存失效时，数据库会承受多少请求

CompletableFuture<Value> mine = new CompletableFuture<>();
CompletableFuture<Value> previous = inFlight.putIfAbsent(key, mine); // 原子决定本进程内谁负责该键重建。
CompletableFuture<Value> shared = previous == null ? mine : previous;
if (previous == null) startBoundedLoad(key, mine); // 自定义加载器负责超时、异常完成和结果回填。
return shared.thenApply(value -> value); // 返回依赖阶段，避免调用者直接取消共享Future。
// 加载器所有退出路径需用inFlight.remove(key,mine)条件清理，避免删除新一代重建。

import java.util.*;
public class Main {
    record Result(int loads,int peak,List<Long> waits){}
    static Result simulate(long[] arrivals,long delay,long ttl,boolean merge){
        if(delay<=0||ttl<=0)throw new IllegalArgumentException("duration");
        PriorityQueue<Long> pending=new PriorityQueue<>();long expiry=-1,last=-1;int loads=0,peak=0;List<Long> waits=new ArrayList<>();
        for(long time:arrivals){
            if(time<0||time<last)throw new IllegalArgumentException("arrival");last=time;
            while(!pending.isEmpty()&&pending.peek()<=time)expiry=Math.addExact(pending.remove(),ttl); // 先完成回源再处理同刻请求。
            if(time<expiry){waits.add(0L);continue;}
            long end;
            if(merge&&!pending.isEmpty())end=pending.peek(); // 同键等待已有工作，不再创建回源。
            else{end=Math.addExact(time,delay);pending.add(end);loads++;peak=Math.max(peak,pending.size());} // 只统计真正新建的回源。
            waits.add(end-time);
        }
        return new Result(loads,peak,List.copyOf(waits));
    }
    static void eq(Result e,Result a){if(!e.equals(a))throw new AssertionError(a);}
    public static void main(String[] args){
        eq(new Result(1,1,List.of(10L,10L,10L)),simulate(new long[]{0,0,0},10,100,true));
        eq(new Result(3,3,List.of(10L,10L,10L)),simulate(new long[]{0,0,0},10,100,false));
        eq(new Result(2,1,List.of(10L,0L,0L,10L)),simulate(new long[]{0,10,109,110},10,100,true));
        eq(new Result(1,1,List.of(10L,5L)),simulate(new long[]{0,5},10,100,true));
        eq(new Result(0,0,List.of()),simulate(new long[]{},1,1,true));
        try{simulate(new long[]{2,1},1,1,true);throw new AssertionError();}catch(IllegalArgumentException expected){}
        try{simulate(new long[]{Long.MAX_VALUE},1,1,true);throw new AssertionError();}catch(ArithmeticException expected){}
        System.out.println("PASS 7 tests");
    }
}