上文关于Spring Boot中描述使用@Async来实现异步任务和线程池控制：《Spring Boot使用@Async实现异步调用：自定义线程池》，本文就接前面内容，继续说说线程池的优雅关闭，主要针对ThreadPoolTaskScheduler线程池。

一、问题现象

在上篇文章的例子Chapter4-1-3中，我们定义了一个线程池，然后利用@Async注解写了3个任务，并指定了这些任务执行使用的线程池。在上文的单元测试中，我们没有具体说说shutdown相关的问题，下面我们就来模拟一个问题现场出来。

第一步：如前文一样，我们定义一个ThreadPoolTaskScheduler线程池：

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @EnableAsync
    @Configuration
    class TaskPoolConfig {

        @Bean("taskExecutor")
        public Executor taskExecutor() {
            ThreadPoolTaskScheduler executor = new ThreadPoolTaskScheduler();
            executor.setPoolSize(20);
            executor.setThreadNamePrefix("taskExecutor-");
            return executor;
        }

    }

}

第二步：改造之前的异步任务，让它依赖一个外部资源，比如：Redis

@Slf4j
@Component
public class Task {

    @Autowired
    private StringRedisTemplate stringRedisTemplate;

    @Async("taskExecutor")
    public void doTaskOne() throws Exception {
        log.info("开始做任务一");
        long start = System.currentTimeMillis();
        log.info(stringRedisTemplate.randomKey());
        long end = System.currentTimeMillis();
        log.info("完成任务一，耗时：" + (end - start) + "毫秒");
    }

    @Async("taskExecutor")
    public void doTaskTwo() throws Exception {
        log.info("开始做任务二");
        long start = System.currentTimeMillis();
        log.info(stringRedisTemplate.randomKey());
        long end = System.currentTimeMillis();
        log.info("完成任务二，耗时：" + (end - start) + "毫秒");
    }

    @Async("taskExecutor")
    public void doTaskThree() throws Exception {
        log.info("开始做任务三");
        long start = System.currentTimeMillis();
        log.info(stringRedisTemplate.randomKey());
        long end = System.currentTimeMillis();
        log.info("完成任务三，耗时：" + (end - start) + "毫秒");
    }

}

注意：这里省略了pom.xml中引入依赖和配置redis的步骤

第三步：修改单元测试，模拟高并发情况下ShutDown的情况：

@RunWith(SpringJUnit4ClassRunner.class)
@SpringBootTest
public class ApplicationTests {

    @Autowired
    private Task task;

    @Test
    @SneakyThrows
    public void test() {

        for (int i = 0; i < 10000; i++) {
            task.doTaskOne();
            task.doTaskTwo();
            task.doTaskThree();

            if (i == 9999) {
                System.exit(0);
            }
        }
    }

}

说明：通过for循环往上面定义的线程池中提交任务，由于是异步执行，在执行过程中，利用System.exit(0)来关闭程序，此时由于有任务在执行，就可以观察这些异步任务的销毁与Spring容器中其他资源的顺序是否安全。

第四步：运行上面的单元测试，我们将碰到下面的异常内容。

org.springframework.data.redis.RedisConnectionFailureException: Cannot get Jedis connection; nested exception is redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool
	at org.springframework.data.redis.connection.jedis.JedisConnectionFactory.fetchJedisConnector(JedisConnectionFactory.java:204) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at org.springframework.data.redis.connection.jedis.JedisConnectionFactory.getConnection(JedisConnectionFactory.java:348) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at org.springframework.data.redis.core.RedisConnectionUtils.doGetConnection(RedisConnectionUtils.java:129) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at org.springframework.data.redis.core.RedisConnectionUtils.getConnection(RedisConnectionUtils.java:92) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at org.springframework.data.redis.core.RedisConnectionUtils.getConnection(RedisConnectionUtils.java:79) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at org.springframework.data.redis.core.RedisTemplate.execute(RedisTemplate.java:194) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at org.springframework.data.redis.core.RedisTemplate.execute(RedisTemplate.java:169) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at org.springframework.data.redis.core.RedisTemplate.randomKey(RedisTemplate.java:781) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	at com.didispace.async.Task.doTaskOne(Task.java:26) ~[classes/:na]
	at com.didispace.async.Task$$FastClassBySpringCGLIB$$ca3ff9d6.invoke(<generated>) ~[classes/:na]
	at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204) ~[spring-core-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:738) ~[spring-aop-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157) ~[spring-aop-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at org.springframework.aop.interceptor.AsyncExecutionInterceptor$1.call(AsyncExecutionInterceptor.java:115) ~[spring-aop-4.3.14.RELEASE.jar:4.3.14.RELEASE]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) [na:1.8.0_151]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) [na:1.8.0_151]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) [na:1.8.0_151]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_151]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_151]
	at java.lang.Thread.run(Thread.java:748) [na:1.8.0_151]
Caused by: redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool
	at redis.clients.util.Pool.getResource(Pool.java:53) ~[jedis-2.9.0.jar:na]
	at redis.clients.jedis.JedisPool.getResource(JedisPool.java:226) ~[jedis-2.9.0.jar:na]
	at redis.clients.jedis.JedisPool.getResource(JedisPool.java:16) ~[jedis-2.9.0.jar:na]
	at org.springframework.data.redis.connection.jedis.JedisConnectionFactory.fetchJedisConnector(JedisConnectionFactory.java:194) ~[spring-data-redis-1.8.10.RELEASE.jar:na]
	... 19 common frames omitted
Caused by: java.lang.InterruptedException: null
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.reportInterruptAfterWait(AbstractQueuedSynchronizer.java:2014) ~[na:1.8.0_151]
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2088) ~[na:1.8.0_151]
	at org.apache.commons.pool2.impl.LinkedBlockingDeque.pollFirst(LinkedBlockingDeque.java:635) ~[commons-pool2-2.4.3.jar:2.4.3]
	at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:442) ~[commons-pool2-2.4.3.jar:2.4.3]
	at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:361) ~[commons-pool2-2.4.3.jar:2.4.3]
	at redis.clients.util.Pool.getResource(Pool.java:49) ~[jedis-2.9.0.jar:na]
	... 22 common frames omitted

二、如何解决

1.原因分析

从异常信息JedisConnectionException: Could not get a resource from the pool来看，我们很容易的可以想到，在应用关闭的时候异步任务还在执行，由于Redis连接池先销毁了，导致异步任务中要访问Redis的操作就报了上面的错。所以，我们得出结论，上面的实现方式在应用关闭的时候是不优雅的，那么我们要怎么做呢？

2.解决方法

要解决上面的问题很简单，Spring的ThreadPoolTaskScheduler为我们提供了相关的配置，只需要加入如下设置即可：

@Bean("taskExecutor")
public Executor taskExecutor() {
    ThreadPoolTaskScheduler executor = new ThreadPoolTaskScheduler();
    executor.setPoolSize(20);
    executor.setThreadNamePrefix("taskExecutor-");
    executor.setWaitForTasksToCompleteOnShutdown(true);
    executor.setAwaitTerminationSeconds(60);
    return executor;
}

说明：setWaitForTasksToCompleteOnShutdown（true） 该方法就是这里的关键，用来设置线程池关闭的时候等待所有任务都完成再继续销毁其他的Bean，这样这些异步任务的销毁就会先于Redis线程池的销毁。同时，这里还设置了setAwaitTerminationSeconds(60)，该方法用来设置线程池中任务的等待时间，如果超过这个时候还没有销毁就强制销毁，以确保应用最后能够被关闭，而不是阻塞住。