调度服务 ScheduledExecutorService 经常卡顿问题的排查及解决方法

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问题描述

首先，给出调度服务的 Java 代码示例：

@Slf4j
@Component
public class TaskProcessSchedule { 
   

    // 核心线程数
    private static final int THREAD_COUNT = 10;

    // 查询数据步长
    private static final int ROWS_STEP = 30;

    @Resource
    private TaskDao taskDao;

    @Resource
    private TaskService taskService;

    private static ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(THREAD_COUNT);

    public TaskProcessSchedule() { 
   
        for (int i = 0; i < THREAD_COUNT; i++) { 
   
            scheduledExecutorService.scheduleAtFixedRate(
                    new TaskWorker(i * ROWS_STEP, ROWS_STEP),
                    10,
                    2,
                    TimeUnit.SECONDS
            );
        }
        log.info("TaskProcessSchedule scheduleAtFixedRate start success.");
    }
 
    class TaskWorker implements Runnable { 
   
        private int offset;
        private int rows;

        TaskWorker(int offset, int rows) { 
   
            this.offset = offset;
            this.rows = rows;
        }

        @Override
        public void run() { 
   
            List<Task> taskList = taskDao.selectProcessingTaskByLimitRange(offset, rows);
            if (CollectionUtils.isEmpty(taskList)) { 
   
                return;
            }
            log.info("TaskWorker: current schedule thread name is {}, taskList is {}", Thread.currentThread().getName(), JsonUtil.toJson(taskList));
            taskService.processTask(taskList);         
        }
    }
}

如上述代码所示，启动 10 个调度线程，延迟 10 秒，开始执行定时逻辑，然后每隔 2 秒执行一次定时任务。定时任务类为TaskWorker，其要做的事就是根据offset和rows参数，到数据库捞取指定范围的待处理记录，然后送到TaskService的processTask方法中进行处理。从逻辑上来看，该定时没有什么毛病，但是在执行定时任务的时候，却经常出现卡顿的问题，表现出来的现象就是：定时任务不执行了。

问题定位

既然已经知道问题的现象了，现在我们就来看看如果定位问题。

• 使用jps命令，查询当前服务器运行的 Java 进程PID

当然，也可以直接使用jps | grep "ServerName"查询指定服务的PID，其中ServerName为服务名称。

• 使用jstack PID | grep "schedule"命令，查询调度线程的状态

如上图所示，发现我们启动的 10 个调度线程均处于WAITING状态。

• 使用jstack PID | grep "schedule-task-10" -A 50命令，查询指定线程的详细信息

如上图所示，我们可以知道调度线程在执行DelayedWorkQueue的take()方法的时候被卡主了。

深入分析

通过上面的问题定位，我们已经知道了代码卡在了这里：

at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1088)

那么接下来，我们就详细分析一下出问题的代码。

        public RunnableScheduledFuture<?> take() throws InterruptedException { 
   
            final ReentrantLock lock = this.lock;
            lock.lockInterruptibly();
            try { 
   
                for (;;) { 
   
                    RunnableScheduledFuture<?> first = queue[0];
                    if (first == null)
                        available.await();
                    else { 
   
                        long delay = first.getDelay(NANOSECONDS);
                        if (delay <= 0)
                            return finishPoll(first);
                        first = null; // don't retain ref while waiting
                        if (leader != null)
                            available.await(); // 1088 行代码
                        else { 
   
                            Thread thisThread = Thread.currentThread();
                            leader = thisThread;
                            try { 
   
                                available.awaitNanos(delay);
                            } finally { 
   
                                if (leader == thisThread)
                                    leader = null;
                            }
                        }
                    }
                }
            } finally { 
   
                if (leader == null && queue[0] != null)
                    available.signal();
                lock.unlock();
            }
        }

由于上述代码可知，当延迟队列的任务为空，或者当任务不为空且leader线程不为null的时候，都会调用await方法；而且，就算leader为null，后续也会调用awaitNanos方法进行延迟设置。下面， 我们再来看看提交任务的方法scheduleAtFixedRate：

    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit) { 
   
        if (command == null || unit == null)
            throw new NullPointerException();
        if (period <= 0)
            throw new IllegalArgumentException();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(period));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        delayedExecute(t);
        return t;
    }

在scheduleAtFixedRate方法中会调用decorateTask方法装饰任务t，然后再将该任务扔到delayedExecute方法中进行处理。

    private void delayedExecute(RunnableScheduledFuture<?> task) { 
   
        if (isShutdown())
            reject(task);
        else { 
   
            super.getQueue().add(task);
            if (isShutdown() &&
                !canRunInCurrentRunState(task.isPeriodic()) &&
                remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }

在delayedExecute方法中，主要是检查线程池中是否可以创建线程，如果不可以，则拒绝任务；否则，向任务队列中添加任务并调用ensurePrestart方法。

    void ensurePrestart() { 
   
        int wc = workerCountOf(ctl.get());
        if (wc < corePoolSize)
            addWorker(null, true);
        else if (wc == 0)
            addWorker(null, false);
    }

在ensurePrestart方法中，主要就是判断工作线程数量是否大于核心线程数，然后根据判断的结果，使用不同的参数调用addWorker方法。

    private boolean addWorker(Runnable firstTask, boolean core) { 

retry:
for (;;) { 

int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) { 

int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get();  // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try { 

w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) { 

final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try { 

// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) { 

if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally { 

mainLock.unlock();
}
if (workerAdded) { 

t.start();
workerStarted = true;
}
}
} finally { 

if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}

在addWorker方法中，主要目的就是将任务添加到workers工作线程池并启动工作线程。接下来，我们再来看看Worker的执行逻辑，也就是run方法：

        public void run() { 

runWorker(this);
}

在run方法中，主要就是将调用转发到外部的runWorker方法：

    final void runWorker(Worker w) { 

Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try { 

while (task != null || (task = getTask()) != null) { 

w.lock();
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try { 

beforeExecute(wt, task);
Throwable thrown = null;
try { 

task.run(); // 执行调度任务
} catch (RuntimeException x) { 

thrown = x; throw x;
} catch (Error x) { 

thrown = x; throw x;
} catch (Throwable x) { 

thrown = x; throw new Error(x);
} finally { 

afterExecute(task, thrown);
}
} finally { 

task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally { 

processWorkerExit(w, completedAbruptly);
}
}

在runWorker方法中，核心操作就是调用task.run()，其中task为Runnable类型，其实现类为ScheduledFutureTask，而ScheduledFutureTask继承了FutureTask类。对于FutureTask类，如果在执行run方法的过程中抛出异常，则这个异常并不会显示抛出，而是需要我们调用FutureTask的get方法来获取，因此如果我们在执行调度任务的时候没有进行异常处理，则异常会被吞噬。

特别地，在FutureTask类中，大量操作了sun.misc.Unsafe LockSupport类，而这个类的park方法，正是上面我们排查问题时定位到调度任务卡住的地方。除此之外，如果我们详细阅读了ScheduledExecutorService的scheduleAtFixedRate的 doc 文档，如下所示：

/** * Creates and executes a periodic action that becomes enabled first * after the given initial delay, and subsequently with the given * period; that is executions will commence after * {@code initialDelay} then {@code initialDelay+period}, then * {@code initialDelay + 2 * period}, and so on. * If any execution of the task * encounters an exception, subsequent executions are suppressed. * Otherwise, the task will only terminate via cancellation or * termination of the executor. If any execution of this task * takes longer than its period, then subsequent executions * may start late, but will not concurrently execute. * * @param command the task to execute * @param initialDelay the time to delay first execution * @param period the period between successive executions * @param unit the time unit of the initialDelay and period parameters * @return a ScheduledFuture representing pending completion of * the task, and whose {@code get()} method will throw an * exception upon cancellation * @throws RejectedExecutionException if the task cannot be * scheduled for execution * @throws NullPointerException if command is null * @throws IllegalArgumentException if period less than or equal to zero */
public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
long initialDelay,
long period,
TimeUnit unit);

我们会发现这样一句话：

If any execution of the task encounters an exception, subsequent executions are suppressed.

翻译过来，就是：

如果任务的任何执行遇到异常，则禁止后续的执行。

说白了，就是在执行调度任务的时候，如果遇到了（未捕获）的异常，则后续的任务都不会执行了。

解决方法

到这里，我们已经知道了问题产生的原因。下面，我们就修改开篇的示例代码，进行优化：

@Slf4j
@Component
public class TaskProcessSchedule { 

// 核心线程数
private static final int THREAD_COUNT = 10;
// 查询数据步长
private static final int ROWS_STEP = 30;
@Resource
private TaskDao taskDao;
@Resource
private TaskService taskService;
private static ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(THREAD_COUNT);
public TaskProcessSchedule() { 

for (int i = 0; i < THREAD_COUNT; i++) { 

scheduledExecutorService.scheduleAtFixedRate(
new TaskWorker(i * ROWS_STEP, ROWS_STEP),
10,
2,
TimeUnit.SECONDS
);
}
log.info("TaskProcessSchedule scheduleAtFixedRate start success.");
}
class TaskWorker implements Runnable { 

private int offset;
private int rows;
TaskWorker(int offset, int rows) { 

this.offset = offset;
this.rows = rows;
}
@Override
public void run() { 

List<Task> taskList = taskDao.selectProcessingTaskByLimitRange(offset, rows);
if (CollectionUtils.isEmpty(taskList)) { 

return;
}
log.info("TaskWorker: current schedule thread name is {}, taskList is {}", Thread.currentThread().getName(), JsonUtil.toJson(taskList));
try { 
 // 新增异常处理
taskService.processTask(taskList);         
} catch (Throwable e) { 

log.error("TaskWorker come across a error {}", e);
}
}
}
}

如上述代码所示，我们对任务的核心逻辑进行了try-catch处理，这样当任务再抛出异常的时候，仅会忽略抛出异常的任务，而不会影响后续的任务。这也说明一件事，那就是：我们在编码的时候，要特别注意对异常情况的处理。