黑马点评：分布式锁与异步订单可靠性

AtomicLong localEpoch = new AtomicLong();
long next = localEpoch.incrementAndGet(); // 仅在这个JVM实例内递增，重启后状态会丢失。
boolean updated = acceptedEpoch.compareAndSet(expectedEpoch, next); // 局部CAS不等于远程资源事务。
if (!updated) retryFromCurrentState(); // 应用策略重新读取，不能无条件重复外部副作用。
// 真正fencing需要可靠代次来源和最终资源共同执行检查。

import java.util.*;
public class Main {
    record Applied(long token,long delta,long result){}
    static class Resource {
        long highest=0,balance;Map<String,Applied> operations=new HashMap<>();
        Resource(long value){if(value<0)throw new IllegalArgumentException("balance");balance=value;}
        long apply(long token,String id,long delta){
            if(token<=0||id==null||id.isBlank())throw new IllegalArgumentException("request");Applied old=operations.get(id);
            if(old!=null){if(old.token()!=token||old.delta()!=delta)throw new IllegalArgumentException("conflict");return old.result();} // 精确重放只返回已存结果。
            if(token<highest)throw new IllegalStateException("stale"); // 检查发生在最终资源，而非只有客户端。
            long next=Math.addExact(balance,delta);if(next<0)throw new IllegalStateException("insufficient");
            balance=next;highest=token;operations.put(id,new Applied(token,delta,next));return next; // 单线程模型一次提交三个状态。
        }
    }
    static void eq(long e,long a){if(e!=a)throw new AssertionError(a);}
    public static void main(String[] args){
        Resource r=new Resource(10);eq(5,r.apply(10,"a",-5));eq(5,r.apply(10,"a",-5));eq(5,r.balance);
        eq(8,r.apply(11,"b",3));
        try{r.apply(10,"late",-2);throw new AssertionError();}catch(IllegalStateException expected){}eq(8,r.balance);
        eq(5,r.apply(10,"a",-5));eq(8,r.balance);
        eq(7,r.apply(11,"c",-1));
        try{r.apply(11,"c",-2);throw new AssertionError();}catch(IllegalArgumentException expected){}
        try{r.apply(12,"bad",-100);throw new AssertionError();}catch(IllegalStateException expected){}eq(11,r.highest);
        try{new Resource(Long.MAX_VALUE).apply(1,"x",1);throw new AssertionError();}catch(ArithmeticException expected){}
        System.out.println("PASS 8 scenarios");
    }
}