TimerTask 出现IllegalStateException：Task already scheduled or cancelled.

抛出该错误的场景

需求：写一个定时器，定时打开和关闭继电器。可以随时暂停，更改关闭和打开时间 出现问题： 定时器的TimerTask只能被shceduler一次，所以当再次调用时，就会抛出该IllegalStateException：Task already scheduled or cancelled.

查找问题

debug查看代码后发现，task有个标识符state

/** * The state of this task, chosen from the constants below. */ int state = VIRGIN; /** * This task has not yet been scheduled. */ static final int VIRGIN = 0; /** * This task is scheduled for execution. If it is a non-repeating task, * it has not yet been executed. */ static final int SCHEDULED = 1; /** * This non-repeating task has already executed (or is currently * executing) and has not been cancelled. */ static final int EXECUTED = 2; /** * This task has been cancelled (with a call to TimerTask.cancel). */ static final int CANCELLED = 3;

当实例被调度，之后状态就会改变，当状态不等于VIRGIN 时，就会抛出错误，看源码

private void sched(TimerTask task, long time, long period) { if (time < 0) throw new IllegalArgumentException("Illegal execution time."); // Constrain value of period sufficiently to prevent numeric // overflow while still being effectively infinitely large. if (Math.abs(period) > (Long.MAX_VALUE >> 1)) period >>= 1; synchronized(queue) { if (!thread.newTasksMayBeScheduled) throw new IllegalStateException("Timer already cancelled."); synchronized(task.lock) { if (task.state != TimerTask.VIRGIN) //当state不等于0时，就会跑 throw new IllegalStateException( "Task already scheduled or cancelled"); task.nextExecutionTime = time; task.period = period; task.state = TimerTask.SCHEDULED; } queue.add(task); if (queue.getMin() == task) queue.notify(); } }

解决问题

1、通过改变标识符state来达到目的，但state是默认的，访问权限只有包和同个类，所以我们通过反射来设置 timer = new Timer(); task = RelayOpenTask.getIntance(); //task.setMap(uuidMap, this); Field field; try { field = TimerTask.class.getDeclaredField("state"); field.setAccessible(true); field.set(task, 0); } catch (NoSuchFieldException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); }