上周發(fā)了一篇關(guān)于Spring Boot中使用@Async來實現(xiàn)異步任務(wù)和線程池控制的文章：《Spring Boot使用@Async實現(xiàn)異步調(diào)用：自定義線程池》。由于最近身邊也發(fā)現(xiàn)了不少異步任務(wù)沒有正確處理而導(dǎo)致的不少問題，所以在本文就接前面內(nèi)容，繼續(xù)說說線程池的優(yōu)雅關(guān)閉，主要針對ThreadPoolTaskScheduler線程池。

問題現(xiàn)象

在上篇文章的例子Chapter4-1-3中，我們定義了一個線程池，然后利用@Async注解寫了3個任務(wù)，并指定了這些任務(wù)執(zhí)行使用的線程池。在上文的單元測試中，我們沒有具體說說shutdown相關(guān)的問題，下面我們就來模擬一個問題現(xiàn)場出來。

第一步：如前文一樣，我們定義一個ThreadPoolTaskScheduler線程池：

@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：改造之前的異步任務(wù)，讓它依賴一個外部資源，比如：Redis

@Slf4j
@Component
public class Task {

    @Autowired
    private StringRedisTemplate stringRedisTemplate;

    @Async("taskExecutor")
    public void doTaskOne() throws Exception {
        log.info("開始做任務(wù)一");
        long start = System.currentTimeMillis();
        log.info(stringRedisTemplate.randomKey());
        long end = System.currentTimeMillis();
        log.info("完成任務(wù)一，耗時：" + (end - start) + "毫秒");
    }

    @Async("taskExecutor")
    public void doTaskTwo() throws Exception {
        log.info("開始做任務(wù)二");
        long start = System.currentTimeMillis();
        log.info(stringRedisTemplate.randomKey());
        long end = System.currentTimeMillis();
        log.info("完成任務(wù)二，耗時：" + (end - start) + "毫秒");
    }

    @Async("taskExecutor")
    public void doTaskThree() throws Exception {
        log.info("開始做任務(wù)三");
        long start = System.currentTimeMillis();
        log.info(stringRedisTemplate.randomKey());
        long end = System.currentTimeMillis();
        log.info("完成任務(wù)三，耗時：" + (end - start) + "毫秒");
    }

}

注意：這里省略了pom.xml中引入依賴和配置redis的步驟

第三步：修改單元測試，模擬高并發(fā)情況下ShutDown的情況：

@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);
            }
        }
    }

}

說明：通過for循環(huán)往上面定義的線程池中提交任務(wù)，由于是異步執(zhí)行，在執(zhí)行過程中，利用System.exit(0)來關(guān)閉程序，此時由于有任務(wù)在執(zhí)行，就可以觀察這些異步任務(wù)的銷毀與Spring容器中其他資源的順序是否安全。

第四步：運行上面的單元測試，我們將碰到下面的異常內(nèi)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

如何解決

原因分析

從異常信息JedisConnectionException: Could not get a resource from the pool來看，我們很容易的可以想到，在應(yīng)用關(guān)閉的時候異步任務(wù)還在執(zhí)行，由于Redis連接池先銷毀了，導(dǎo)致異步任務(wù)中要訪問Redis的操作就報了上面的錯。所以，我們得出結(jié)論，上面的實現(xiàn)方式在應(yīng)用關(guān)閉的時候是不優(yōu)雅的，那么我們要怎么做呢？

解決方法

要解決上面的問題很簡單，Spring的ThreadPoolTaskScheduler為我們提供了相關(guān)的配置，只需要加入如下設(shè)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;
}

說明：setWaitForTasksToCompleteOnShutdown（true）該方法就是這里的關(guān)鍵，用來設(shè)置線程池關(guān)閉的時候等待所有任務(wù)都完成再繼續(xù)銷毀其他的Bean，這樣這些異步任務(wù)的銷毀就會先于Redis線程池的銷毀。同時，這里還設(shè)置了setAwaitTerminationSeconds(60)，該方法用來設(shè)置線程池中任務(wù)的等待時間，如果超過這個時候還沒有銷毀就強制銷毀，以確保應(yīng)用最后能夠被關(guān)閉，而不是阻塞住。

完整示例：

讀者可以根據(jù)喜好選擇下面的兩個倉庫中查看Chapter4-1-4項目：

• Github：https://github.com/dyc87112/SpringBoot-Learning/
• Gitee：https://gitee.com/didispace/SpringBoot-Learning/

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