黑马点评：商户缓存与一致性边界

import java.util.*;

public class Main {
  enum Kind { MISS, ABSENT, VALUE }
  record Value(boolean absent, String text) {
    Value {
      if (absent && text!=null || !absent && text==null) throw new IllegalArgumentException("value");
    }
    static Value empty() { return new Value(true,null); }
    static Value of(String text) { return new Value(false,text); }
  }
  record Ticket(String key, long generation) {}
  record Cell(long generation, Value value, long expiresAt) {}
  record Answer(Kind kind, String text) {}
  static final class Cache {
    private final Map<String,Cell> cells = new HashMap<>();
    private Cell cell(String key) {
      Objects.requireNonNull(key);
      return cells.computeIfAbsent(key,k -> new Cell(0,null,0));
    }
    synchronized Ticket begin(String key) { return new Ticket(key,cell(key).generation()); }
    synchronized void invalidate(String key) {
      Cell old=cell(key);
      cells.put(key,new Cell(Math.addExact(old.generation(),1),null,0)); // 保留并推进代次
    }
    synchronized boolean publish(Ticket ticket, Value value, long now, long ttl) {
      Objects.requireNonNull(value);
      if(now<0 || ttl<=0) throw new IllegalArgumentException("time");
      long expiry=Math.addExact(now,ttl);
      Cell current=cell(ticket.key());
      if(current.generation()!=ticket.generation()) return false; // 原子拒绝跨失效点的旧加载
      cells.put(ticket.key(),new Cell(current.generation(),value,expiry));
      return true;
    }
    synchronized Answer get(String key, long now) {
      if(now<0) throw new IllegalArgumentException("time");
      Cell current=cell(key);
      if(current.value()==null) return new Answer(Kind.MISS,null);
      if(now>=current.expiresAt()) {
        cells.put(key,new Cell(current.generation(),null,0)); // 过期清值，但不能将代次归零
        return new Answer(Kind.MISS,null);
      }
      Value v=current.value();
      return new Answer(v.absent()?Kind.ABSENT:Kind.VALUE,v.text());
    }
  }
  static void check(boolean ok) { if(!ok) throw new AssertionError(); }
  public static void main(String[] args) {
    Cache c=new Cache();
    check(c.get("shop:1",0).kind()==Kind.MISS);
    Ticket old=c.begin("shop:1");
    c.invalidate("shop:1");
    check(!c.publish(old,Value.of("old"),1,10));
    Ticket fresh=c.begin("shop:1");
    check(c.publish(fresh,Value.of("new"),2,10));
    check(c.get("shop:1",11).equals(new Answer(Kind.VALUE,"new")));
    check(c.get("shop:1",12).kind()==Kind.MISS);
    check(c.begin("shop:1").generation()==1);
    check(!c.publish(old,Value.of("old"),13,10)); // 过期后也不接受代次零
    check(c.publish(c.begin("shop:2"),Value.empty(),10,5));
    check(c.get("shop:2",14).kind()==Kind.ABSENT && c.get("shop:2",15).kind()==Kind.MISS);
    Ticket absent=c.begin("shop:2");
    c.invalidate("shop:2");
    check(!c.publish(absent,Value.empty(),16,5)); // 商户创建也应使旧空值加载失效
    c.invalidate("shop:1");
    check(!c.publish(fresh,Value.of("new"),20,10));
    System.out.println("11 cache generation checks passed");
  }
}