scheduleAtFixedRate是用任务开始时间计算间隔，就是说某任务上次理论启动时间+间隔时间就是下次启动时间。
scheduleWithFixedDelay是用任务结束时间计算间隔，就是说某任务上次结束时间+间隔时间就是下次启动时间。
这段代码模拟了一组10个任务，每个任务都有个name（任务名）和time（任务花费的时间）。第6个任务（任务名为任务5）耗时15s，其他任务耗时1s。这一组任务分别用scheduleAtFixedRate（固定频率）和scheduleWithFixedDelay（固定延迟）处理，截取部分日志解析这两个方法的运行逻辑。
import lombok.extern.slf4j.Slf4j;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.*;
@Slf4j
public class MyConcurrentTest {
public static void main(String[] args) throws ExecutionException, InterruptedException {
ScheduledExecutorService readThreadPool = Executors.newScheduledThreadPool(5);
List<ScheduledFuture> futureList = new ArrayList<>();
//用objects模拟任务列表，任务的time代表耗费的时间
List<Map<String,Object>> objects = new ArrayList<>();
for(int i = 0 ; i < 10 ; i++){
Map<String,Object> object = new HashMap<>();
object.put("name","任务"+i);
object.put("time",1000);
objects.add(object);
}
//第6个任务sleep15s，第6个任务是任务5
objects.get(5).put("time",15000);
System.out.println(objects);
for(Map<String,Object> object : objects){
//任务在0s后开始，任务间隔是5s
ScheduledFuture future = readThreadPool.scheduleAtFixedRate(()->{
log.info(object.toString());
try {
//暂停指定时间
Thread.sleep((int)object.get("time"));
} catch (InterruptedException e) {
e.printStackTrace();
}
},0,5, TimeUnit.SECONDS);
futureList.add(future);
}
for(ScheduledFuture future:futureList){
future.get();
}
}
}
用scheduleAtFixedRate（固定频率）处理的日志。
--10个任务的详情
[{name=任务0, time=1000}, {name=任务1, time=1000}, {name=任务2, time=1000}, {name=任务3, time=1000}, {name=任务4, time=1000}, {name=任务5, time=15000}, {name=任务6, time=1000}, {name=任务7, time=1000}, {name=任务8, time=1000}, {name=任务9, time=1000}]
--线程池有5个核心线程，同时开始了5个任务。
11:06:06.505 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务4, time=1000}
11:06:06.505 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=1000}
11:06:06.505 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务1, time=1000}
11:06:06.505 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务3, time=1000}
11:06:06.505 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务2, time=1000}
--1s之后任务处理完，处理其他的任务。
11:06:07.505 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务5, time=15000}
11:06:07.505 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务9, time=1000}
11:06:07.505 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务6, time=1000}
11:06:07.505 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务7, time=1000}
11:06:07.505 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务8, time=1000}
--4s之后距离任务0/1/2/3/4上次启动已经过去了5s，任务0/1/2/3/4都已经准备好第二次执行，但此时只有四个空闲线程。按照顺序任务4没有执行。
11:06:11.505 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务0, time=1000}
11:06:11.505 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务1, time=1000}
11:06:11.505 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务2, time=1000}
11:06:11.505 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务3, time=1000}
--1s之后除了任务4，任务5/6/7/8/9也都准备好第二次执行，但此时任务5第一次执行还没有结束，所以任务5第二次执行被阻塞。按照顺序，任务9没有执行。
11:06:12.506 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务6, time=1000}
11:06:12.506 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务8, time=1000}
11:06:12.506 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务4, time=1000}
11:06:12.506 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务7, time=1000}
--1s之后任务5/9准备好执行，此时任务5第二次执行依然被阻塞。
11:06:13.521 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务9, time=1000}
--3s之后任务0/1/2/3准备好第三次执行……
11:06:16.506 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=1000}
11:06:16.506 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务2, time=1000}
11:06:16.506 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务1, time=1000}
11:06:16.506 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务3, time=1000}
--1s之后……
11:06:17.506 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务4, time=1000}
11:06:17.506 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务7, time=1000}
11:06:17.506 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务8, time=1000}
11:06:17.506 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务6, time=1000}
--1s之后……
11:06:18.506 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务9, time=1000}
--3s之后……
11:06:21.507 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务1, time=1000}
11:06:21.507 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务3, time=1000}
11:06:21.507 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=1000}
11:06:21.507 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务2, time=1000}
--1s之后，任务5第二次执行，因为任务5第一次执行开始时间到现在时间间隔大于规定的时间间隔，即任务5第二次执行已经“迟到”，应立即执行。
11:06:22.507 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务5, time=15000}
11:06:22.522 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务7, time=1000}
11:06:22.522 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务8, time=1000}
11:06:22.522 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务4, time=1000}
11:06:22.522 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务6, time=1000}
11:06:23.523 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务9, time=1000}
综上所述，scheduleAtFixedRate（固定频率）是在任务上次开始时间+间隔时间开始的，拿任务0来说，第一次启动是在11:06:06，第二次启动是在11:06:11，中间间隔了5s，这正是我们规定的间隔时间，以此来达到频率固定的效果（当然频率也不是绝对固定的，如任务4）。
看java.util.concurrent.ScheduledThreadPoolExecutor.ScheduledFutureTask#run源码得知，只有在任务从队列中取出并执行时才会往任务队列中放入下次任务。下次任务的启动时间是根据本次任务的理论启动时间计算的（任务队列中的每个任务都会有一个理论启动时间，但是可能会因为没有抢到线程而延迟，所以任务的实际启动时间不一定是理论启动时间）。所以，如果任务A执行时间过长，任务A'会一直等待（即使超时），当任务A'执行的时候可能会把任务A''的启动时间设置到任务A'实际启动时间之前。
/**
* Overrides FutureTask version so as to reset/requeue if periodic.
*/
public void run() {
boolean periodic = isPeriodic();
if (!canRunInCurrentRunState(periodic))
cancel(false);
else if (!periodic)
ScheduledFutureTask.super.run();
else if (ScheduledFutureTask.super.runAndReset()) {
/*在任务执行时，修改任务下次启动时间并重新放回队列。
*scheduleAtFixedRate就会以此启动时间+间隔时间计算下次启动时间。
*/
setNextRunTime();
reExecutePeriodic(outerTask);
}
}
注意：如果任务都是第一次处理很耗时间，之后耗时很短的话，会出现连续处理多次某个任务的情况。上面的代码，我们在lambda表达式最后一行添加object.put("time",0);，在处理完第一遍之后把所有任务的处理时间全都设成0s。这样当任务5执行完之后，任务5会立刻执行多次。如下日志：
12:48:14.490 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务4, time=1000}
12:48:14.490 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=1000}
12:48:14.490 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务2, time=1000}
12:48:14.490 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务1, time=1000}
12:48:14.490 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务3, time=1000}
12:48:15.490 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务7, time=1000}
12:48:15.490 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务9, time=1000}
12:48:15.490 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务6, time=1000}
12:48:15.490 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务8, time=1000}
12:48:15.490 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务5, time=15000}
12:48:19.507 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务0, time=0}
12:48:19.507 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务1, time=0}
12:48:19.507 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务2, time=0}
12:48:19.507 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务3, time=0}
12:48:19.507 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务4, time=0}
12:48:19.507 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务6, time=0}
12:48:19.507 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务7, time=0}
12:48:19.507 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务8, time=0}
12:48:19.507 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务9, time=0}
12:48:24.507 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务1, time=0}
12:48:24.507 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=0}
12:48:24.507 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务2, time=0}
12:48:24.507 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务3, time=0}
12:48:24.507 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务4, time=0}
12:48:24.507 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务6, time=0}
12:48:24.507 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务7, time=0}
12:48:24.507 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务8, time=0}
12:48:24.507 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务9, time=0}
12:48:29.509 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务1, time=0}
12:48:29.509 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务2, time=0}
12:48:29.509 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务3, time=0}
12:48:29.509 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务0, time=0}
12:48:29.509 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务4, time=0}
12:48:29.509 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务6, time=0}
12:48:29.509 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务7, time=0}
12:48:29.509 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务8, time=0}
12:48:29.509 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务9, time=0}
--连续执行了三遍任务5
12:48:30.493 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务5, time=0}
12:48:30.493 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务5, time=0}
12:48:30.493 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务5, time=0}
用scheduleWithFixedDelay（固定延迟）处理的日志。
--10个任务的详情
[{name=任务0, time=1000}, {name=任务1, time=1000}, {name=任务2, time=1000}, {name=任务3, time=1000}, {name=任务4, time=1000}, {name=任务5, time=15000}, {name=任务6, time=1000}, {name=任务7, time=1000}, {name=任务8, time=1000}, {name=任务9, time=1000}]
--线程池有5个核心线程，同时开始了5个任务。
11:51:51.450 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务4, time=1000}
11:51:51.450 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务1, time=1000}
11:51:51.450 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=1000}
11:51:51.450 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务2, time=1000}
11:51:51.450 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务3, time=1000}
--1s之后任务处理完，处理其他的任务。
11:51:52.466 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务7, time=1000}
11:51:52.466 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务9, time=1000}
11:51:52.466 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务6, time=1000}
11:51:52.466 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务8, time=1000}
11:51:52.466 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务5, time=15000}
--5s之后，距离任务0/1/2/3/4上次处理结束已经过去了5s，任务0/1/2/3/4都已经准备好第二次执行，但此时只有四个空闲线程。按照顺序任务4没有执行。
11:51:57.467 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务0, time=1000}
11:51:57.467 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务2, time=1000}
11:51:57.467 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务1, time=1000}
11:51:57.467 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务3, time=1000}
--1s之后除了任务4，任务6/7/8/9也都准备好第二次执行，任务5还没有结束第一次执行。
11:51:58.467 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务9, time=1000}
11:51:58.467 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务4, time=1000}
11:51:58.467 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务6, time=1000}
11:51:58.467 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务7, time=1000}
--1s之后执行任务8。
11:51:59.483 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务8, time=1000}
--4s之后任务0/1/2/3准备好第三次执行……
11:52:03.468 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务3, time=1000}
11:52:03.468 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务1, time=1000}
11:52:03.468 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务2, time=1000}
11:52:03.468 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=1000}
--1s之后……
11:52:04.484 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务7, time=1000}
11:52:04.484 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务6, time=1000}
11:52:04.484 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务4, time=1000}
11:52:04.484 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务9, time=1000}
--1s之后……
11:52:05.500 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务8, time=1000}
--4s之后……
11:52:09.469 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务0, time=1000}
11:52:09.469 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务1, time=1000}
11:52:09.469 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务3, time=1000}
11:52:09.469 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务2, time=1000}
--1s之后……
11:52:10.501 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务4, time=1000}
11:52:10.501 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务7, time=1000}
11:52:10.501 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务6, time=1000}
11:52:10.501 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务9, time=1000}
--1s之后……
11:52:11.501 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务8, time=1000}
--1s之后距离任务5处理（11:51:52--11:52:07）结束已经过去了5s，任务5第二次执行。
11:52:12.471 [pool-1-thread-1] INFO MyConcurrentTest - {name=任务5, time=15000}
11:52:15.471 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务1, time=1000}
11:52:15.471 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务0, time=1000}
11:52:15.471 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务3, time=1000}
11:52:15.471 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务2, time=1000}
11:52:16.518 [pool-1-thread-4] INFO MyConcurrentTest - {name=任务4, time=1000}
11:52:16.518 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务7, time=1000}
11:52:16.518 [pool-1-thread-2] INFO MyConcurrentTest - {name=任务9, time=1000}
11:52:16.518 [pool-1-thread-3] INFO MyConcurrentTest - {name=任务6, time=1000}
11:52:17.534 [pool-1-thread-5] INFO MyConcurrentTest - {name=任务8, time=1000}
综上所述，scheduleWithFixedDelay（固定延迟）是在任务上次结束时间+间隔时间开始的，任务如果没有结束，任务队列是没有下次执行计划的。