Introducing the three simplest implementation methods of Java scheduled tasks
Dec 29, 2020 pm 05:45 PM
java基礎(chǔ)教程介紹定時任務(wù)在實際的開發(fā)
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日子匆匆穿過我而行，奔向海洋。
定時任務(wù)在實際的開發(fā)中特別常見，比如電商平臺 30 分鐘后自動取消未支付的訂單，以及凌晨的數(shù)據(jù)匯總和備份等，都需要借助定時任務(wù)來實現(xiàn)，那么我們本文就來看一下定時任務(wù)最簡單的幾種實現(xiàn)方式。
TOP 1：Timer
Timer 是 JDK 自帶的定時任務(wù)執(zhí)行類，無論任何項目都可以直接使用 Timer 來實現(xiàn)定時任務(wù)，所以 Timer 的優(yōu)點就是使用方便，它的實現(xiàn)代碼如下：
public?class?MyTimerTask?{
????public?static?void?main(String[]?args)?{
????????//?定義一個任務(wù)
????????TimerTask?timerTask?=?new?TimerTask()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("Run?timerTask："?+?new?Date());
????????????}
????????};
????????//?計時器
????????Timer?timer?=?new?Timer();
????????//?添加執(zhí)行任務(wù)（延遲?1s?執(zhí)行，每?3s?執(zhí)行一次）
????????timer.schedule(timerTask,?1000,?3000);
????}
}
程序執(zhí)行結(jié)果如下：
Run?timerTask：Mon?Aug?17?21:29:25?CST?2020 Run?timerTask：Mon?Aug?17?21:29:28?CST?2020 Run?timerTask：Mon?Aug?17?21:29:31?CST?2020
Timer 缺點分析
Timer 類實現(xiàn)定時任務(wù)雖然方便，但在使用時需要注意以下問題。
問題 1：任務(wù)執(zhí)行時間長影響其他任務(wù)
當(dāng)一個任務(wù)的執(zhí)行時間過長時，會影響其他任務(wù)的調(diào)度，如下代碼所示：
public?class?MyTimerTask?{
????public?static?void?main(String[]?args)?{
????????//?定義任務(wù)?1
????????TimerTask?timerTask?=?new?TimerTask()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("進(jìn)入?timerTask?1："?+?new?Date());
????????????????try?{
????????????????????//?休眠?5?秒
????????????????????TimeUnit.SECONDS.sleep(5);
????????????????}?catch?(InterruptedException?e)?{
????????????????????e.printStackTrace();
????????????????}
????????????????System.out.println("Run?timerTask?1："?+?new?Date());
????????????}
????????};
????????//?定義任務(wù)?2
????????TimerTask?timerTask2?=?new?TimerTask()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("Run?timerTask?2："?+?new?Date());
????????????}
????????};
????????//?計時器
????????Timer?timer?=?new?Timer();
????????//?添加執(zhí)行任務(wù)（延遲?1s?執(zhí)行，每?3s?執(zhí)行一次）
????????timer.schedule(timerTask,?1000,?3000);
????????timer.schedule(timerTask2,?1000,?3000);
????}
}
程序執(zhí)行結(jié)果如下：
進(jìn)入?timerTask?1：Mon?Aug?17?21:44:08?CST?2020 Run?timerTask?1：Mon?Aug?17?21:44:13?CST?2020 Run?timerTask?2：Mon?Aug?17?21:44:13?CST?2020 進(jìn)入?timerTask?1：Mon?Aug?17?21:44:13?CST?2020 Run?timerTask?1：Mon?Aug?17?21:44:18?CST?2020 進(jìn)入?timerTask?1：Mon?Aug?17?21:44:18?CST?2020 Run?timerTask?1：Mon?Aug?17?21:44:23?CST?2020 Run?timerTask?2：Mon?Aug?17?21:44:23?CST?2020 進(jìn)入?timerTask?1：Mon?Aug?17?21:44:23?CST?2020
從上述結(jié)果中可以看出，當(dāng)任務(wù) 1 運行時間超過設(shè)定的間隔時間時，任務(wù) 2 也會延遲執(zhí)行。 原本任務(wù) 1 和任務(wù) 2 的執(zhí)行時間間隔都是 3s，但因為任務(wù) 1 執(zhí)行了 5s，因此任務(wù) 2 的執(zhí)行時間間隔也變成了 10s（和原定時間不符）。
問題 2：任務(wù)異常影響其他任務(wù)
使用 Timer 類實現(xiàn)定時任務(wù)時，當(dāng)一個任務(wù)拋出異常，其他任務(wù)也會終止運行，如下代碼所示：
public?class?MyTimerTask?{
????public?static?void?main(String[]?args)?{
????????//?定義任務(wù)?1
????????TimerTask?timerTask?=?new?TimerTask()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("進(jìn)入?timerTask?1："?+?new?Date());
????????????????//?模擬異常
????????????????int?num?=?8?/?0;
????????????????System.out.println("Run?timerTask?1："?+?new?Date());
????????????}
????????};
????????//?定義任務(wù)?2
????????TimerTask?timerTask2?=?new?TimerTask()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("Run?timerTask?2："?+?new?Date());
????????????}
????????};
????????//?計時器
????????Timer?timer?=?new?Timer();
????????//?添加執(zhí)行任務(wù)（延遲?1s?執(zhí)行，每?3s?執(zhí)行一次）
????????timer.schedule(timerTask,?1000,?3000);
????????timer.schedule(timerTask2,?1000,?3000);
????}
}
程序執(zhí)行結(jié)果如下：
進(jìn)入?timerTask?1：Mon?Aug?17?22:02:37?CST?2020 Exception?in?thread?"Timer-0"?java.lang.ArithmeticException:?/?by?zero ????at?com.example.MyTimerTask$1.run(MyTimerTask.java:21) ????at?java.util.TimerThread.mainLoop(Timer.java:555) ????at?java.util.TimerThread.run(Timer.java:505) Process?finished?with?exit?code?0
Timer 小結(jié)
Timer 類實現(xiàn)定時任務(wù)的優(yōu)點是方便，因為它是 JDK 自定的定時任務(wù)，但缺點是任務(wù)如果執(zhí)行時間太長或者是任務(wù)執(zhí)行異常，會影響其他任務(wù)調(diào)度，所以在生產(chǎn)環(huán)境下建議謹(jǐn)慎使用。
TOP 2：ScheduledExecutorService
ScheduledExecutorService 也是 JDK 1.5 自帶的 API，我們可以使用它來實現(xiàn)定時任務(wù)的功能，也就是說 ScheduledExecutorService 可以實現(xiàn) Timer 類具備的所有功能，并且它可以解決了 Timer 類存在的所有問題。
ScheduledExecutorService 實現(xiàn)定時任務(wù)的代碼示例如下：
public?class?MyScheduledExecutorService?{
????public?static?void?main(String[]?args)?{
????????//?創(chuàng)建任務(wù)隊列
????????ScheduledExecutorService?scheduledExecutorService?=
????????????????Executors.newScheduledThreadPool(10);?//?10?為線程數(shù)量
????????//?執(zhí)行任務(wù)
????????scheduledExecutorService.scheduleAtFixedRate(()?->?{
????????????System.out.println("Run?Schedule："?+?new?Date());
????????},?1,?3,?TimeUnit.SECONDS);?//?1s?后開始執(zhí)行，每?3s?執(zhí)行一次
????}
}
程序執(zhí)行結(jié)果如下：
Run?Schedule：Mon?Aug?17?21:44:23?CST?2020 Run?Schedule：Mon?Aug?17?21:44:26?CST?2020 Run?Schedule：Mon?Aug?17?21:44:29?CST?2020
ScheduledExecutorService 可靠性測試
① 任務(wù)超時執(zhí)行測試
ScheduledExecutorService 可以解決 Timer 任務(wù)之間相應(yīng)影響的缺點，首先我們來測試一個任務(wù)執(zhí)行時間過長，會不會對其他任務(wù)造成影響，測試代碼如下：
public?class?MyScheduledExecutorService?{
????public?static?void?main(String[]?args)?{
????????//?創(chuàng)建任務(wù)隊列
????????ScheduledExecutorService?scheduledExecutorService?=
????????????????Executors.newScheduledThreadPool(10);
????????//?執(zhí)行任務(wù)?1
????????scheduledExecutorService.scheduleAtFixedRate(()?->?{
????????????System.out.println("進(jìn)入?Schedule："?+?new?Date());
????????????try?{
????????????????//?休眠?5?秒
????????????????TimeUnit.SECONDS.sleep(5);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????????System.out.println("Run?Schedule："?+?new?Date());
????????},?1,?3,?TimeUnit.SECONDS);?//?1s?后開始執(zhí)行，每?3s?執(zhí)行一次
????????//?執(zhí)行任務(wù)?2
????????scheduledExecutorService.scheduleAtFixedRate(()?->?{
????????????System.out.println("Run?Schedule2："?+?new?Date());
????????},?1,?3,?TimeUnit.SECONDS);?//?1s?后開始執(zhí)行，每?3s?執(zhí)行一次
????}
}
程序執(zhí)行結(jié)果如下：
Run?Schedule2：Mon?Aug?17?11:27:55?CST?2020 進(jìn)入?Schedule：Mon?Aug?17?11:27:55?CST?2020 Run?Schedule2：Mon?Aug?17?11:27:58?CST?2020 Run?Schedule：Mon?Aug?17?11:28:00?CST?2020 進(jìn)入?Schedule：Mon?Aug?17?11:28:00?CST?2020 Run?Schedule2：Mon?Aug?17?11:28:01?CST?2020 Run?Schedule2：Mon?Aug?17?11:28:04?CST?2020
從上述結(jié)果可以看出，當(dāng)任務(wù) 1 執(zhí)行時間 5s 超過了執(zhí)行頻率 3s 時，并沒有影響任務(wù) 2 的正常執(zhí)行，因此使用 ScheduledExecutorService 可以避免任務(wù)執(zhí)行時間過長對其他任務(wù)造成的影響。
② 任務(wù)異常測試
接下來我們來測試一下 ScheduledExecutorService 在一個任務(wù)異常時，是否會對其他任務(wù)造成影響，測試代碼如下：
public?class?MyScheduledExecutorService?{
????public?static?void?main(String[]?args)?{
????????//?創(chuàng)建任務(wù)隊列
????????ScheduledExecutorService?scheduledExecutorService?=
????????????????Executors.newScheduledThreadPool(10);
????????//?執(zhí)行任務(wù)?1
????????scheduledExecutorService.scheduleAtFixedRate(()?->?{
????????????System.out.println("進(jìn)入?Schedule："?+?new?Date());
????????????//?模擬異常
????????????int?num?=?8?/?0;
????????????System.out.println("Run?Schedule："?+?new?Date());
????????},?1,?3,?TimeUnit.SECONDS);?//?1s?后開始執(zhí)行，每?3s?執(zhí)行一次
????????//?執(zhí)行任務(wù)?2
????????scheduledExecutorService.scheduleAtFixedRate(()?->?{
????????????System.out.println("Run?Schedule2："?+?new?Date());
????????},?1,?3,?TimeUnit.SECONDS);?//?1s?后開始執(zhí)行，每?3s?執(zhí)行一次
????}
}
程序執(zhí)行結(jié)果如下：
進(jìn)入?Schedule：Mon?Aug?17?22:17:37?CST?2020 Run?Schedule2：Mon?Aug?17?22:17:37?CST?2020 Run?Schedule2：Mon?Aug?17?22:17:40?CST?2020 Run?Schedule2：Mon?Aug?17?22:17:43?CST?2020
從上述結(jié)果可以看出，當(dāng)任務(wù) 1 出現(xiàn)異常時，并不會影響任務(wù) 2 的執(zhí)行。
ScheduledExecutorService 小結(jié)
在單機(jī)生產(chǎn)環(huán)境下建議使用 ScheduledExecutorService 來執(zhí)行定時任務(wù)，它是 JDK 1.5 之后自帶的 API，因此使用起來也比較方便，并且使用 ScheduledExecutorService 來執(zhí)行任務(wù)，不會造成任務(wù)間的相互影響。
TOP 3：Spring Task
如果使用的是 Spring 或 Spring Boot 框架，可以直接使用 Spring Framework 自帶的定時任務(wù)，使用上面兩種定時任務(wù)的實現(xiàn)方式，很難實現(xiàn)設(shè)定了具體時間的定時任務(wù)，比如當(dāng)我們需要每周五來執(zhí)行某項任務(wù)時，但如果使用 Spring Task 就可輕松的實現(xiàn)此需求。
以 Spring Boot 為例，實現(xiàn)定時任務(wù)只需兩步：
- 開啟定時任務(wù)；
- 添加定時任務(wù)。
具體實現(xiàn)步驟如下。
① 開啟定時任務(wù)
開啟定時任務(wù)只需要在 Spring Boot 的啟動類上聲明 @EnableScheduling?即可，實現(xiàn)代碼如下：
@SpringBootApplication
@EnableScheduling?//?開啟定時任務(wù)
public?class?DemoApplication?{
????//?do?someing
}
② 添加定時任務(wù)
定時任務(wù)的添加只需要使用 @Scheduled?注解標(biāo)注即可，如果有多個定時任務(wù)可以創(chuàng)建多個 @Scheduled 注解標(biāo)注的方法，示例代碼如下：
import?org.springframework.scheduling.annotation.Scheduled;
import?org.springframework.stereotype.Component;
@Component?//?把此類托管給?Spring，不能省略
public?class?TaskUtils?{
????//?添加定時任務(wù)
????@Scheduled(cron?=?"59?59?23?0?0?5")?//?cron?表達(dá)式，每周五?23:59:59?執(zhí)行
????public?void?doTask(){
????????System.out.println("我是定時任務(wù)~");
????}
}
注意：定時任務(wù)是自動觸發(fā)的無需手動干預(yù)，也就是說 Spring Boot 啟動后會自動加載并執(zhí)行定時任務(wù)。
Cron 表達(dá)式
Spring Task 的實現(xiàn)需要使用 cron 表達(dá)式來聲明執(zhí)行的頻率和規(guī)則，cron 表達(dá)式是由 6 位或者 7 位組成的（最后一位可以省略），每位之間以空格分隔，每位從左到右代表的含義如下：
其中 * 和 ? 號都表示匹配所有的時間。
cron 表達(dá)式在線生成地址：https://cron.qqe2.com/
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