Java ScheduledExecutorService源码分析
2024-08-25 04:05:45
Java 定时任务可以用Timer + TimerTask来做，或者使用ScheduledExecutorService，使用ScheduledExecutorService有两个好处：
1. 如果任务执行时间过长，TimerTask会出现延迟执行的情况。比如，第一任务在1000ms执行了4000ms，第二个任务定时在2000ms开始执行。这里由于第一个任务要执行4000，所以第二个任务实际在5000ms开始执行。这是由于Timer是单线程，且顺序执行提交的任务
2. 如果执行任务抛出异常，Timer是不会执行会后面的任务的
使用ScheduledExecutorService可以避免上面两种情况，因为ScheduledExecutorService是线程池，有多个线程执行。
下面是一个使用ScheduledExecutorService实现定时任务的Demo
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit; /**
* Created by gxf on 2017/6/26.
*/
public class TestSchedule {
public static void main(String[] args) {
ScheduledExecutorService scheduledExecutorService = new ScheduledThreadPoolExecutor(5);
Task task = new Task();
scheduledExecutorService.scheduleAtFixedRate(task, -10, 1, TimeUnit.SECONDS);
}
} class Task implements Runnable{
public void run(){
System.out.println("do task...");
}
}
这里每隔1秒，控制台会输出do task
ScheduledExecutorService是一个接口
public interface ScheduledExecutorService extends ExecutorService {
其中一个实现是
public class ScheduledThreadPoolExecutor
extends ThreadPoolExecutor
implements ScheduledExecutorService {
在Demo中
ScheduledExecutorService scheduledExecutorService = new ScheduledThreadPoolExecutor(5);
跟进ScheduledThreadPoolExecutor
/**
* Creates a new {@code ScheduledThreadPoolExecutor} with the
* given core pool size.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @throws IllegalArgumentException if {@code corePoolSize < 0}
*/
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}
这里Super调用
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
设定了线程池的各个参数，核心线程数，最大线程数，任务队列等。但这里还有线程创建，有任务提交了，才会创建线程池
继续Demo中
Task task = new Task();
这里只是创建了一个Runable对象
继续Demo
scheduledExecutorService.scheduleAtFixedRate(task, -10, 1, TimeUnit.SECONDS);
这里已经把任务提交给线程池了，进入方法
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
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;
}
前面都是一些条件检查和包装，看最后的delayedExecute(t)
/**
* Main execution method for delayed or periodic tasks. If pool
* is shut down, rejects the task. Otherwise adds task to queue
* and starts a thread, if necessary, to run it. (We cannot
* prestart the thread to run the task because the task (probably)
* shouldn't be run yet.) If the pool is shut down while the task
* is being added, cancel and remove it if required by state and
* run-after-shutdown parameters.
*
* @param task the task
*/
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();
}
}
ok，这里可以看到任务被添加到了一个队列里面。在看最后，ensurePrestart()
/**
* Same as prestartCoreThread except arranges that at least one
* thread is started even if corePoolSize is 0.
*/
void ensurePrestart() {
int wc = workerCountOf(ctl.get());
if (wc < corePoolSize)
addWorker(null, true);
else if (wc == 0)
addWorker(null, false);
}
这里可以看出，如果线程池里面的线程数，小于核心线程数，会继续添加线程。进入addWork(null, true)
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;
}
第32行和33行可以看出，任务已经提交给Work类了，第57行，t.start()这里启动线程，执行提交的任务
到这里，提交的任务已经开始执行了。这里，我们在看一下Worker这个包装类
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L; /** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
可以看到有Thread， task，这里，其实就是把task指向要执行的任务，thread作为作为线程执行任务。
在ThreadPoolExecutor中
private final HashSet<Worker> workers = new HashSet<Worker>();
这个保存我们生成的线程，有了这个就不用重新创建和销毁线程了
我们在看一下Worker的run()方法
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
继续跟进
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 pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
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();
} 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);
}
}
第8行getTask()取要执行的任务，第23行执行任务
继续跟进第8行getTask()
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
这里主要看第27-29，前面我们说过，任务是放到一个队列里面的。其实，是个阻塞队列，在队列为空的时候取，会阻塞线程。这里就用到了这个功能，在29行workQueue.take()如果没有任务，线程就会阻塞。有任务就会取任务进行执行。
简单点：线程池就是一个Set对象 + BlockingQueue对象， Workers + BlockingQueue
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