Servlet 生命周期：实例、请求和异步任务何时结束

import java.util.*;
public class Main {
  enum Life { NEW,READY,STOPPING,DESTROYED }
  static class Request { boolean inService=true,async=false,completionRequested=false; }
  static class Server {
    Life life=Life.NEW; final Map<Integer,Request> live=new HashMap<>();
    void need(boolean ok) { if(!ok) throw new IllegalStateException(); }
    Request request(int id) { Request r=live.get(id); need(r!=null); return r; }
    void init() { need(life==Life.NEW); life=Life.READY; }
    void enter(int id) { need(life==Life.READY && !live.containsKey(id)); live.put(id,new Request()); }
    void async(int id) { Request r=request(id); need(r.inService && !r.async); r.async=true; }
    void leave(int id) {
      Request r=request(id); need(r.inService); r.inService=false;
      if(!r.async || r.completionRequested) live.remove(id); // 同步或已请求完成才结束
    }
    void complete(int id) {
      Request r=request(id); need(r.async && !r.completionRequested);
      r.completionRequested=true;
      if(!r.inService) live.remove(id); // 栈内完成先记标志，不能提前释放
    }
    void dispatch(int id) {
      Request r=request(id); need(r.async && !r.inService && !r.completionRequested);
      r.inService=true; r.async=false; // 新的服务调用可再次启动异步周期
    }
    void stop() { need(life==Life.READY); life=Life.STOPPING; }
    void destroy() { need(life==Life.STOPPING && live.isEmpty()); life=Life.DESTROYED; }
  }
  static void check(boolean ok) { if(!ok) throw new AssertionError(); }
  static void fails(Runnable action) {
    try { action.run(); throw new AssertionError("非法状态应被拒绝"); }
    catch(IllegalStateException expected) { /* 期望状态错误 */ }
  }
  static Server ready() { Server s=new Server(); s.init(); return s; }
  public static void main(String[] args) {
    Server sync=ready(); sync.enter(1); sync.leave(1); check(sync.live.isEmpty());
    Server async=ready(); async.enter(1); async.async(1); async.leave(1); check(async.live.size()==1);
    async.complete(1); check(async.live.isEmpty());
    Server early=ready(); early.enter(1); early.async(1); early.complete(1); check(early.live.size()==1);
    early.leave(1); check(early.live.isEmpty());
    Server dispatch=ready(); dispatch.enter(1); dispatch.async(1); dispatch.leave(1);
    dispatch.dispatch(1); dispatch.leave(1); check(dispatch.live.isEmpty());
    Server stop=ready(); stop.enter(1); stop.stop(); fails(stop::destroy); fails(()->stop.enter(2));
    stop.leave(1); stop.destroy(); check(stop.life==Life.DESTROYED);
    fails(()->new Server().enter(1)); fails(sync::init);
    System.out.println("PASS 11 lifecycle assertions");
  }
}