package com.example.netty.async;

import java.util.concurrent.*;
import java.util.function.Function;

/**
 * java8中CompletableFuture异步处理超时的方法
 *
 * Java 8 的 CompletableFuture 并没有 timeout 机制，虽然可以在 get 的时候指定 timeout，是一个同步堵塞的操作。怎样让 timeout 也是异步的呢？Java 8 内有内建的机
 *  制支持，一般的实现方案是启动一个 ScheduledThreadpoolExecutor 线程在 timeout 时间后直接调用 CompletableFuture.completeExceptionally(new TimeoutException())，
 *  然后用acceptEither() 或者 applyToEither 看是先计算完成还是先超时：
 *
 *  在 java 9 引入了 orTimeout 和 completeOnTimeOut 两个方法支持 异步 timeout 机制：
 *
 * public CompletableFuture orTimeout(long timeout, TimeUnit unit) : completes the CompletableFuture with a TimeoutException after the specified timeout has elapsed.
 * public CompletableFuture completeOnTimeout(T value, long timeout, TimeUnit unit) : provides a default value in the case that the CompletableFuture pipeline times out.
 * 内部实现上跟我们上面的实现方案是一模一样的，只是现在不需要自己实现了。
 *
 * 实际上hystrix等熔断的框架，其实现线程Timeout之后就关闭线程，也是基于同样的道理，所以我们可以看到hystrix中会有一个Timer Thread
 *
 *
 * @author luliang
 * @date 2021-02-24 9:48
 */
public class CompletableFutureTimeout {
    /**
     * Singleton delay scheduler, used only for starting and * cancelling tasks.
     */
    static final class Delayer {
        static ScheduledFuture<?> delay(Runnable command, long delay,
                                        TimeUnit unit) {
            return delayer.schedule(command, delay, unit);
        }

        static final class DaemonThreadFactory implements ThreadFactory {
            @Override
            public Thread newThread(Runnable r) {
                Thread t = new Thread(r);
                t.setDaemon(true);
                t.setName("CompletableFutureDelayScheduler");
                return t;
            }
        }

        static final ScheduledThreadPoolExecutor delayer;

        // 注意，这里使用一个线程就可以搞定 因为这个线程并不真的执行请求 而是仅仅抛出一个异常
        static {
            (delayer = new ScheduledThreadPoolExecutor(
                    1, new CompletableFutureTimeout.Delayer.DaemonThreadFactory())).
                    setRemoveOnCancelPolicy(true);
        }
    }

    public static <T> CompletableFuture<T> timeoutAfter(long timeout, TimeUnit unit) {
        CompletableFuture<T> result = new CompletableFuture<T>();
        // timeout 时间后 抛出TimeoutException 类似于sentinel / watcher
        CompletableFutureTimeout.Delayer.delayer.schedule(() -> result.completeExceptionally(new TimeoutException()), timeout, unit);
        return result;
    }

    /**
     * 哪个先完成 就apply哪一个结果 这是一个关键的API,exceptionally出现异常后返回默认值
     *
     * @param t
     * @param future
     * @param timeout
     * @param unit
     * @param <T>
     * @return
     */
    public static <T> CompletableFuture<T> completeOnTimeout(T t, CompletableFuture<T> future, long timeout, TimeUnit unit) {
        final CompletableFuture<T> timeoutFuture = timeoutAfter(timeout, unit);
        return future.applyToEither(timeoutFuture, Function.identity()).exceptionally((throwable) -> t);
    }

    /**
     * 哪个先完成 就apply哪一个结果 这是一个关键的API，不设置默认值，超时后抛出异常
     *
     * @param t
     * @param future
     * @param timeout
     * @param unit
     * @param <T>
     * @return
     */
    public static <T> CompletableFuture<T> orTimeout(T t, CompletableFuture<T> future, long timeout, TimeUnit unit) {
        final CompletableFuture<T> timeoutFuture = timeoutAfter(timeout, unit);
        return future.applyToEither(timeoutFuture, Function.identity()).exceptionally((throwable) -> t);
    }
}

package com.example.netty.async;

import java.util.concurrent.*;

/**
 * @author luliang
 * @date 2021-02-24 10:05
 */
public class CompletableFutureTimeout2 {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return 10;
        });

        CompletableFuture<Integer> within = CompletableFutureTimeout.completeOnTimeout(1, future, 1, TimeUnit.SECONDS);
        System.out.println(within.get());

        CompletableFuture<String> futureStr = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "正常执行";
        });

        CompletableFuture<String> withinStr = CompletableFutureTimeout.completeOnTimeout("异常执行", futureStr, 1, TimeUnit.SECONDS);
        System.out.println(withinStr.get());

    }
}