给定的时间点，给定的时间间隔或者给定的执行次数自动执行的任务。简而言之，Quartz > Timer，Timer是被动地根据既定时间去调度任务的，Quartz则是自己主动定制时间规则去支持更加丰富地调度方法。
本文主要是讲解Timer工具类的，故而下文中不会过多讨论Quartz。
| Timer, Quartz的使用 | Quartz, Spring的整合 |
|---|---|
| Java编程知识 | Spring基础知识 |
Timer类位于java.util包下，有关Timer类的详细描述信息请点击这里访问Oracle Java的官方api文档查阅。
Timer工具类图是Timer工具类及有关类的类图。
快速开始：
/* * Foo.java -- JDK 1.8 */package timer;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-03 Wed PM 19:15:08 */public class Foo { public static void main(String[] args) { Timer timer = new Timer(); // 1. 创建Timer实例，关联线程不能是daemon(守护/后台)线程 FooTimerTask fooTimerTask = new FooTimerTask(); // 2. 创建任务对象 timer.schedule(fooTimerTask, 3000L, 2000L); // 3. 通过Timer定时定频率调用fooTimerTask的业务代码 }}class FooTimerTask extends TimerTask { @Override public void run() { // TODO 业务代码...... System.out.println("Hello Timer!"); }}注意：Timer类在时间granularity(粒度)是毫秒级的，实际上Timer的schedule系列方法获取时间的方式是System.currentTimeMillis()(当前时间与Unix元年之差，类型为long)，最多只能到毫秒级，而一些操作系统的计时精度会达到1/10毫秒。
public void schedule(TimerTask task, Date time) { sched(task, time.getTime(), 0); } public void schedule(TimerTask task, long delay, long period) { if (delay < 0) throw new IllegalArgumentException("Negative delay."); if (period <= 0) throw new IllegalArgumentException("Non-positive period."); sched(task, System.currentTimeMillis() delay, -period); } public void schedule(TimerTask task, long delay) { if (delay < 0) throw new IllegalArgumentException("Negative delay."); sched(task, System.currentTimeMillis() delay, 0); } public void scheduleAtFixedRate(TimerTask task, long delay, long period) { if (delay < 0) throw new IllegalArgumentException("Negative delay."); if (period <= 0) throw new IllegalArgumentException("Non-positive period."); sched(task, System.currentTimeMillis() delay, period); } public void scheduleAtFixedRate(TimerTask task, long delay, long period) { if (delay < 0) throw new IllegalArgumentException("Negative delay."); if (period <= 0) throw new IllegalArgumentException("Non-positive period."); sched(task, System.currentTimeMillis() delay, period); } public void scheduleAtFixedRate(TimerTask task, Date firstTime, long period) { if (period <= 0) throw new IllegalArgumentException("Non-positive period."); sched(task, firstTime.getTime(), period); }笔记:
- schedule()和scheduleAtFixedRate()方法都能实现对任务的一次或多次调度。
- schedule()按是否可重复执行分为单次和多次，按任务初次执行计算方式分为delay(long型延迟毫秒数)和time(Date型时间)。
- schedule()和scheduleAtFixedRate()最终都是调用Timer类下的sched()方法实现的。
演示代码包含DemoTimer.java和TimerUtils.java，代码清单：
/* * DemoTimer.java -- JDK 1.8 */package timer;import java.util.Calendar;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-03 Wed PM 18:37:03 */public class DemoTimer { public static void main(String[] args) { Calendar calendar = TimerUtils.current(); TimerUtils.miscellaneous("Current time is : ", calendar.getTime()); calendar.add(Calendar.SECOND, 3); // 当前时间加3秒 Timer timer = new Timer(); DemoTimerTask demoTimerTask = new DemoTimerTask("No. 1"); demoTimerTask.setName("schedule1"); timer.schedule(demoTimerTask, calendar.getTime()); // 3.1.1.1 // demoTimerTask.setName("schedule2");// timer.schedule(demoTimerTask, calendar.getTime(), 2000); // 3.1.1.2// // demoTimerTask.setName("schedule3");// timer.schedule(demoTimerTask, 3000); // 3.1.1.3// // demoTimerTask.setName("schedule4");// timer.schedule(demoTimerTask, 3000, 2000); // 3.1.1.4// // demoTimerTask.setName("scheduleAtFixedRate1");// timer.scheduleAtFixedRate(demoTimerTask, calendar.getTime(), 2000); // 3.1.2.1// // demoTimerTask.setName("scheduleAtFixedRate2"); // timer.scheduleAtFixedRate(demoTimerTask, 3000, 2000); // 3.1.2.2 }}class DemoTimerTask extends TimerTask { String name; // 任务名 public DemoTimerTask(String name) { this.name = name; } @Override public void run() { TimerUtils.miscellaneous("Current time is : ", TimerUtils.current().getTime()); System.out.println("Current exec name is : " name); // 打印当前name的内容 } public String getName() { return name; } public void setName(String name) { this.name = name; }}/* * TimerUtils.java -- JDK 1.8 */package timer;import java.text.SimpleDateFormat;import java.util.Calendar;import java.util.Date;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-03 Wed PM 18:40:26 */public class TimerUtils { final static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); // 定义日期格式 static Calendar current() { Calendar calendar = Calendar.getInstance(); // 通过静态工厂方法创建Calendar实例 return calendar; } static void schedtime(TimerTask task) { System.out.println("scheduled time is " sdf.format(task.scheduledExecutionTime())); } static void miscellaneous(String str, Date date) { System.out.println(str sdf.format(date)); }} public boolean cancel() { synchronized(lock) { boolean result = (state == SCHEDULED); state = CANCELLED; return result; } } public long scheduledExecutionTime() { synchronized(lock) { return (period < 0 ? nextExecutionTime period : nextExecutionTime - period); } }不能与fixed-delay执行式的重复任务搭配使用，也就是不用于schedule方法，应为schedule方法的（scheduled execution time）计划执行时间会偏移理想的计划时间，对她使用这个方法没有无意义。
演示代码清单：
/* * CancelTest.java -- JDK 1.8 */package timer;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-03 Wed PM 19:43:04 */public class CancelTest { public static void main(String[] args) { Timer timer = new Timer(); MyTimerTask myTimerTask = new MyTimerTask("schedule"); TimerUtils.miscellaneous("Current time is : ", TimerUtils.current().getTime()); timer.schedule(myTimerTask, 3000, 2000); // 3.2.1// timer.schedule(myTimerTask, 3000); // 3.3.2// TimerUtils.schedtime(myTimerTask); }}class MyTimerTask extends TimerTask { private String name; private Integer count = 0; public MyTimerTask(String name) { this.name = name; } @Override public void run() { if (count < 3) { TimerUtils.miscellaneous("Current time is : ", TimerUtils.current().getTime()); System.out.println("Current exec name is : " name); count ; } else { cancel(); System.out.println("Task canceled"); System.exit(0); } } public String getName() { return name; } public void setName(String name) { this.name = name; }}丢弃所有当前已安排的任务。 public void cancel() { synchronized(queue) { thread.newTasksMayBeScheduled = false; queue.clear(); queue.notify(); // 防止队列已为空的处理 } }已取消的任务。 public int purge() { int result = 0; synchronized(queue) { for (int i = queue.size(); i > 0; i--) { if (queue.get(i).state == TimerTask.CANCELLED) { queue.quickRemove(i); result ; } } if (result != 0) queue.heapify(); } return result; }}演示代碼：
/* * CancelTest.java -- JDK 1.8 */package timer;import java.util.Date;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-03 Wed PM 19:43:04 */public class CancelTest { public static void main(String[] args) throws InterruptedException { Timer timer = new Timer(); MyTimerTask task1 = new MyTimerTask("task1"); MyTimerTask task2 = new MyTimerTask("task2"); TimerUtils.miscellaneous("start time is : ", new Date()); // task1首次执行是距离现在时间2秒后，之后每隔2秒执行一次 // task2首次执行是距离现在时间1秒后，之后每隔2秒执行一次 timer.schedule(task1, 1000, 2000); // 奇数次执行 timer.schedule(task2, 2000, 2000); // 偶数次执行// System.out.println("current canceled task number is : " timer.purge()); Thread.sleep(5000); // 当前线程休眠5秒后cancel生效，没有此句立即触发cancel TimerUtils.miscellaneous("cancel time is : ", new Date()); /*3.2.2.1 下面两句执行完后程序只剩下后台线程，JRE判定当前程序结束 因为当前程序只有后台线程，所有前台线程结束，后台的工作无前台线程使用就是没有意义的 */ timer.cancel(); // 当前线程若检测到timer对队列中的任务进行调度则终止timer并从任务队列移除所有任务 System.out.println("Tasks all canceled!"); // 若此句输出后看到还有任务运行则停止所有运行的程序，这可能是之前运行的程序未终止 // 3.2.2.2// task1.cancel(); // 当前线程每次检测到timer对task1进行schedule取消task1// System.out.println("current canceled task number is : " timer.purge()); }}class MyTimerTask extends TimerTask { private String name; private Integer count = 0; public MyTimerTask(String name) { this.name = name; } @Override public void run() { if (count < 3) { TimerUtils.miscellaneous("Current time is : ", TimerUtils.current() .getTime()); System.out.println("Current exec name is : " name); count ; } else { cancel(); System.out.println("Task canceled"); System.exit(0); } } public String getName() { return name; } public void setName(String name) { this.name = name; }}| 方法 | schedule | scheduleAtFixedRate |
|---|---|---|
| 首次计划执行时间早于当前时间 | "fixed-delay"，用代码的形式理解就是scheduleAtfixedDelay()；如果第一次执行时间被delay了，随后的执行时间按照上一次实际执行完成的时间点进行计算。 | "fixed-rate"，义如其名；如果第一次执行时间按照上一次开始的时间点进行计算，并且为了赶上进度会多次执行任务，因此TimerTask中的执行体需要考虑同步。 |
| 任务执行所需的时间超出任务的执行周期间隔 | 下一次执行时间相对于上一次实际执行完成的时间点，因此执行时间会不断延后。 | 下一次执行时间相对于上一次开始的时间点，因此执行时间一般不会延后，因此存在并发性。 |
Calendar calendar = Calendar.getInstance(); calendar.add(Calendar.SECOND, -6); // L1 Timer timer = new Timer(); // 第一次执行为6秒前，之后么个两秒钟执行一次 timer.schedule(new TimerTask() { @Override public void run() { // L2 System.out.println("Scheduled exec time is : " TimerUtils.sdf.format(scheduledExecutionTime())); System.out.println("Task is being executed!"); } }, calendar.getTime(), 2000);如果任务时间为3000毫秒，第一次执行开始时间2020-01-01 00:01:00，第二次为2020-01-01 00:01:03而不是2020-01-01 00:01:02。
这里使用在任务线程休眠三秒来实现，注释掉L1行代码，在L1处添加代码休眠三秒，代码snippet：
Calendar calendar = Calendar.getInstance(); Timer timer = new Timer(); timer.schedule(new TimerTask() { @Override public void run() { try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Scheduled exec time is : " TimerUtils.sdf.format(scheduledExecutionTime())); System.out.println("Task is being executed!"); } }, calendar.getTime(), 2000);Current time is : 2019-04-03 23:02:58Scheduled exec time is : 2019-04-03 23:02:52Task is being executed!Scheduled exec time is : 2019-04-03 23:02:54Task is being executed!Scheduled exec time is : 2019-04-03 23:02:56Task is being executed!Scheduled exec time is : 2019-04-03 23:02:58Task is being executed!代码snippet：
Calendar calendar = Calendar.getInstance(); calendar.add(Calendar.SECOND, -6); // L1 Timer timer = new Timer(); timer.scheduleAtFixedRate(new TimerTask() { @Override public void run() {// L3 System.out.println("Scheduled exec time is : " TimerUtils.sdf.format(scheduledExecutionTime())); System.out.println("Task is being executed!"); } }, calendar.getTime(), 2000); }如果任务时间为3000毫秒，period为2000毫秒，当前时间2019-04-03 23:23:22，第一次执行开始时间2019-04-03 23:23:22，第二次执行时间2019-04-03 23:23:24，两个任务执行时间段有交集。
注释掉L1所在行，在L3处让任务线程休眠三秒模仿执行时间为3秒的任务，代码snippet：
Calendar calendar = Calendar.getInstance(); calendar.add(Calendar.SECOND, -6); // L1 Timer timer = new Timer(); timer.scheduleAtFixedRate(new TimerTask() { @Override public void run() { try { Thread.sleep(3000); // L3 } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Scheduled exec time is : " TimerUtils.sdf.format(scheduledExecutionTime())); System.out.println("Task is being executed!"); } }, calendar.getTime(), 2000);注释掉不需要的代码观察效果，演示代码：
/* * DifferenceTest.java -- JDK 1.8 */package timer;import java.util.Calendar;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-03 Wed PM 21:47:38 */public class DifferenceTest { public static void main(String[] args) { Calendar calendar = TimerUtils.current(); TimerUtils.miscellaneous("Current time is : ", calendar.getTime()); // 设置成6秒前的时间，若当前时间为2016-12-28 00:00:06 // 那么设置之后时间变成2016-12-28 00:00:00 calendar.add(Calendar.SECOND, -6); // L1 Timer timer = new Timer(); // 第一次执行为6秒前，之后么个两秒钟执行一次 timer.schedule(new TimerTask() { @Override public void run() { try { Thread.sleep(3000); // L2 } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Scheduled exec time is : " TimerUtils.sdf.format(scheduledExecutionTime())); System.out.println("Task is being executed!"); } }, calendar.getTime(), 2000); // 此处有个语法糖，编译器生成一个匿名类继承抽象类TimerTask通过new实例化，这并不违反抽象类不能实例化这一原则 timer.scheduleAtFixedRate(new TimerTask() { @Override public void run() { try { Thread.sleep(3000); // L3 } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Scheduled exec time is : " TimerUtils.sdf.format(scheduledExecutionTime())); System.out.println("Task is being executed!"); } }, calendar.getTime(), 2000); }}实现两个机器人
跳舞机器人：每隔两秒打印最近一次计划的时间和执行内容
灌水机器人：模拟往桶里倒水，直到桶里的水满为止
灌水机器人工作流程
灌水，如果水不满，则一直工作；如果水满，则停止工作。
跳舞机器人
跳舞，如果水不满，则一直工作；如果水满，则跳舞两秒后停止工作。
代码清单
DancingRobot.java、WaterRobot.java和Executor.java。
/* * WaterRobot.java -- JDK 1.8 */package timer;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-02 Tue PM 16:44:17 */public class WaterRobot extends TimerTask { private Timer timer; // 最大容量5L private Integer bucketCapacity = 0; private final String unit = "L"; public WaterRobot(Timer timer) { this.timer = timer; } @Override public void run() { // 灌水到桶满为止 if (bucketCapacity < 5) { System.out.println("Add 1L water into the bucket!"); bucketCapacity ; } else { // 水满之后停止执行 System.out.println("The number of canceled task in timer is : " timer.purge()); cancel(); System.out.println("The waterRot has been aborted"); System.out.println("The number of canceled task in timer is : " timer.purge()); System.out.println("Current water is " bucketCapacity unit); // 等待两秒钟，终止timer里面的所有内容 try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } timer.cancel(); } }}/* * DancingRobot.java -- JDK 1.8 */package timer;import java.text.SimpleDateFormat;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-02 Tue PM 16:35:12 */public class DancingRobot extends TimerTask { @Override public void run() { SimpleDateFormat sdf = new SimpleDateFormat(TimeConstants.DATE_FORMAT); System.out.println("Scheduled exec time is : " sdf.format(scheduledExecutionTime())); // 获得最近一次任务执行的计划时间 System.out.println("Dancing happily!"); }}/* * Executor.java -- JDK 1.8 */package timer;import java.text.SimpleDateFormat;import java.util.Calendar;import java.util.Timer;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-02 Tue PM 16:54:04 */public class Executor { public static void main(String[] args) { Timer timer = new Timer(); Calendar calendar = Calendar.getInstance(); SimpleDateFormat sdf = new SimpleDateFormat(TimeConstants.DATE_FORMAT); System.out.println("Current time is : " sdf.format(calendar.getTime())); DancingRobot dr = new DancingRobot(); WaterRobot wr = new WaterRobot(timer); timer.schedule(dr, calendar.getTime(), 2000); timer.scheduleAtFixedRate(wr, calendar.getTime(), 1000); }}执行结果
Current time is : 2019-04-04 01:19:20Scheduled exec time is : 2019-04-04 01:19:21Dancing happily!Add 1L water into the bucket!Add 1L water into the bucket!Add 1L water into the bucket!Scheduled exec time is : 2019-04-04 01:19:23Dancing happily!Add 1L water into the bucket!Add 1L water into the bucket!Scheduled exec time is : 2019-04-04 01:19:25Dancing happily!The number of canceled task in timer is : 0The waterRot has been abortedThe number of canceled task in timer is : 1Current water is 5L/* * ExTimer.java -- JDK 1.8 */package timer;import java.util.Calendar;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-03 Wed PM 23:37:41 */public class ConTimer { public static void main(String[] args) { Timer timer = new Timer(); ConTimerTask exTimerTask1 = new ConTimerTask("No.1", 2000); ConTimerTask exTimerTask2 = new ConTimerTask("No.2", 2000); Calendar calendar = TimerUtils.current(); TimerUtils.miscellaneous("Current time is : ", calendar .getTime()); timer.schedule(exTimerTask1, calendar.getTime()); timer.schedule(exTimerTask2, calendar.getTime());// timer.scheduleAtFixedRate(exTimerTask1, calendar.getTime(), 2000);// timer.scheduleAtFixedRate(exTimerTask2, calendar.getTime(), 2000); }}class ConTimerTask extends TimerTask { private String name; private long costTime; public ConTimerTask(String name, long costTime) { this.setName(name); this.costTime = costTime; } @Override public void run() { System.out.println(name "'s current exec time is : " TimerUtils.sdf.format(Calendar.getInstance() .getTime())); // 输出当前时间 try { Thread.sleep(costTime); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(name "'s finish time is : " TimerUtils.sdf.format(Calendar.getInstance() .getTime())); // 输出costTime之后的时间 } public String getName() { return name; } public void setName(String name) { this.name = name; }}执行结果：
Current time is : 2019-04-04 01:04:24No.1's current exec time is : 2019-04-04 01:04:24No.1's finish time is : 2019-04-04 01:04:26No.2's current exec time is : 2019-04-04 01:04:26No.2's finish time is : 2019-04-04 01:04:28TimerTask抛出RuntimeException，Timer会停止所有任务的运行。/* * ExTimer.java -- JDK 1.8 */package timer;import java.util.Timer;import java.util.TimerTask;/** * Description: * <p> * <p> * @author ascribed * @date 2019-04-04 Thu AM 00:33:14 */public class ExTimer { public static void main(String[] args) { Timer timer = new Timer(); ExTimerTask task1 = new ExTimerTask("task1"); ExTimerTask task2 = new ExTimerTask("task2"); timer.scheduleAtFixedRate(task1, 1000, 1000); timer.scheduleAtFixedRate(task2, 1000, 2000 ); }}class ExTimerTask extends TimerTask { private String name; public ExTimerTask(String name) { this.name = name; } @Override public void run() { System.out.println(name); throw new IllegalStateException(); } public String getName() { return name; } public void setName(String name) { this.name = name; }}执行结果：
task1Exception in thread "Timer-0" java.lang.IllegalStateException at timer.ExTimerTask.run(ExTimer.java:37) at java.util.TimerThread.mainLoop(Timer.java:555) at java.util.TimerThread.run(Timer.java:505)TimerTask是一次性
定时器（Timer）的TimerTask实例只能schedule一次,再次调用会抛出运行时异常IllegalStateException，这是一个运行时异常。
解决方法有二：一是反射修改state字段，二是每次用new一个TimerTask。
对复杂的任务的调度
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