scheduleAtFixedRate
public void scheduleAtFixedRate(TimerTask task,
long delay,
long period)
Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals, separated by the specified period.
In fixed-rate execution, each execution is scheduled relative to the scheduled execution time of the initial execution. If an execution is delayed for any reason (such as garbage collection or other background activity), two or more executions will occur in
rapid succession to "catch up." In the long run, the frequency of execution will be exactly the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).
Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute time, such as ringing a chime every hour on the hour, or running scheduled maintenance every day at a particular time. It is also appropriate for recurring activities
where the total time to perform a fixed number of executions is important, such as a countdown timer that ticks once every second for ten seconds. Finally, fixed-rate execution is appropriate for scheduling multiple repeating timer tasks that must remain synchronized
with respect to one another.
Parameters:
task - task to be scheduled.(要执行的任务)
delay - delay in milliseconds before task is to be executed.（延迟多少毫秒后执行）
period - time in milliseconds between successive task executions.（没过多久执行一次）
Throws:
IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.
IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.