线程池类图
本篇文章我们先从左边这条线
Executor==>ExcutorService==>AbstractExecutorService==>ThreadPoolExecutor来分析一下。
- 上面url继承类图,线程池的最顶层的接口是Executor,这个接口只有一个方法void execute(Runnable command)
- ExecutorService继承Executor,新增了submit(Runnable(Callable)),shutDown,shutDownNow等几个主要方法
- AbstractExecutorService实现了上面的ExecutorService接口的若干个方法。
- ThreadPoolExecutor继承AbstractExecutorService,实现了线程池的一些主要的方法execute(Runnable)。
AbstractExecutorService
AbstractExecutorService实现了submit方法,代码如下:
- submit(Callable task)方法
public <T> Future<T> submit(Callable<T> task) { if (task == null) throw new NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task); execute(ftask); return ftask; }
- newTaskFor(Callable callable)方法
上面的FutureTask实现了RunnableFuture接口,RunnableFuture继承了protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) { return new FutureTask<T>(callable); }
Runnable和Future接口。Runnable接口只有一个void run方法,Future接口有cancel(boolean),V get(),V get(long timeout, TimeUnit unit),boolean isCancelled(),boolean isDone()方法。
ThreadPoolExecutor
接着上面的AbstractExecutorService.submit方法,会调用到execute(ftask),这个execute方法就是ThreadPoolExecutor中的。我们接下来就以execute方法作为起点来分析。
execute
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
- 首先检查当前工作线程数是否小于corePoolSize,若小于,则添加一个worker来处理这个任务(commadn),添加任务成功则返回.
- 如果线程还处于running状态,并且任务成功添加到queue中,重新检查一次线程池的状态,若线程池非running,则从queue中删除任务,成功则调用reject,这里根据拒绝策略来执行;若当前工作的线程数为0,则添加一个worker(addWorker(null, false),这里要注意,这次的addWorker的参数和上面第一次的不一样)
- 如果添加worker失败,也执行reject方法。
addWorker(Runnable firstTask, boolean core)
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;
}
(1)这个判断逻辑比较复杂,我们先来看下
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
若当前状态大于SHUTDOWN,显然if判断条件为ture,直接returnfalse。(很好理解,线程池处于关闭状态,肯定不让新添加worker了)
若当前状态小于SHUTDOWN,if判断条件为false,接着往下走(线程池为RUNNING状态,很好理解)
若当前状态等于SHUTDOWN:若firstTask等于null并且工作队列有任务,则if判断条件为false,代码不会return,会继续往下运行;若firstTask不等于null或者工作队列为空,则判断条件为true,会return false(这个也好理解,我们知道SHUTDOWN状态,线程池不再接受新的任务,但是已经在工作队列中的任务还是要完成才行。所以若first等于null,并且工作队列有任务,还要继续往下走。若相反,则不会往下走)
(2)判断当前工作线程数
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
}
当前工作线程数没有超过线程池设置的参数的限制,则利用CAS添加一个worker,并跳出外层的for循环,继续向下运行。否则返回false,添加worker失败。
(3) 完成了上述1 2步骤后,会执行new Worker(firstTask),Thread t = w.thread并再次检查线程池的状态,若合法,则向工作线程池HashSet中添加当前worker,并执行t.start。此时才开启了子线程来执行任务。
子线程run方法
上面步骤3调用了t.start,会开启一个子线程来运行Worker中的run方法。
public void run() {
runWorker(this);
}
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();
}finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
上述worker不断通过getTask()方法,从workQueue中获取任务;若没有获取到任务,则调用processWorkerExit方法。
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;
}
}
}
getTask方法是一个无限的for循环方法,它首先判断当前线程池的状态
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
这个判断也很好理解,若rs==SHUTDOWN,workQueue为空,显然应该直接返回null,并提前是工作的worker减一。(getTask返回null,runWorker方法会调用processWorkerExit从HashSet中remove当前worker);若rs>大于SHUTDOWN(这个对应线程池的shutDownNow方法,工作队列中等待的任务不再执行);其他情况,说明线程池处于运行状态,继续往下运行。然后根据当前线程池设置的最大线程数,以及是否允许线coreThread超时间以及workQueue的状态来判断是否通过CAS操作来是线程数减一并return null。最后我们要关注下下面这个从工作队列中取得任务的三目运算。
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
若timed为ture(设置allowCoreThreadTimeOut为true),则超过了等待的时间还没有从workQueue中取得任务则r = null,此时就有可能造成即使workerCount小于corePoolSize,当前的worker也可能被回收。
若timed为false,则调用阻塞方法从workQueue中获取任务,newFixedThreadPool就会一直调用这个阻塞方法,从而达到不显示关闭线程池的情况下,即使workQueue为空,也能维持固定的工作线程的个数。
shutDown(shutDownNow)方法
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
//shutDwonNow为STOP,shutDown为SHUTDOWN
advanceRunState(STOP);(advanceRunState(SHUTDOWN);)
interruptWorkers();(interruptIdleWorkers)
//shutDownNow专用
tasks = drainQueue();
//shutDown专用 ScheduledThreadPoolExecutor回调
onShutdown();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
shutDown和shutDownnNow方法区别(代码层面):
- shutDownNow:advanceRunState(STOP),interruptWorkers
shutDown:advanceRunState(shutDown),interruptIdleWorkers - shutDown多了个onShutdown();ScheduledThreadPoolExecutor复写了onShutDown方法。
- shutDownNow方法工作队列中还未完成的任务。
- interruptIdleWorkers
interruptIdleWorkers与interruptWorkers
(1)shutDownNow
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfStarted();
} finally {
mainLock.unlock();
}
}
显然这个是中断所有的线程
(2)shutDown
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
Thread t = w.thread;
if (!t.isInterrupted() && w.tryLock()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
注意onlyOne参数,这个只有在调用tryTerminate()方法里面,会调用interruptIdleWorkers(true),其他情况都是interruptIdleWorkers(false),所以对于shutDown方法,也是尝试中断所有还没有被中断的线程。
3)tryTerminate
上面(2)中提到了tryTerminate方法,接下来就来看下这个方法
final void tryTerminate() {
for (;;) {
int c = ctl.get();
if (isRunning(c) ||
runStateAtLeast(c, TIDYING) ||
(runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
return;
if (workerCountOf(c) != 0) { // Eligible to terminate
interruptIdleWorkers(ONLY_ONE);
return;
}
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
try {
terminated();
} finally {
ctl.set(ctlOf(TERMINATED, 0));
termination.signalAll();
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
}
从上述代码可以看出,若线程池状态为SHUTDOWN,workQueue为空,工作线程数为0或者线程池状态为STOP,工作线程数为0,都最终会把线程池状态设置为TERMINATED,并且唤醒所有因为调用awaitTermination()方法阻塞在termination.awaitNanos(nanos)还未醒过来的线程。
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (;;) {
if (runStateAtLeast(ctl.get(), TERMINATED))
return true;
if (nanos <= 0)
return false;
nanos = termination.awaitNanos(nanos);
}
} finally {
mainLock.unlock();
}
}
上述tryTerminate方法,在addWorkerFailed(),processWorkerExit(),shutDown(),shutDownNow(),remove(Runnable task)方法中都会调用到。
线程池5种状态解释
上面经常提到线程池的运行状态,这里稍作解释一下。
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
种状态的定义
- RUNNING: 接受新的任务,处理workQueue中的任务。
- SHUTDOWN: 不接受新的任务,但是会继续完成workQueue中的任务
- STOP: 不接受新的任务,也不处理workQueue中未完成的任务,尝试中断所有运行中的任务
- TIDYING: 所有任务已经完成, 工作线程数为0,线程池状态变成TIDYING随之将会调用terminated()方法。
- TERMINATED: terminated()方法已经完成
5种状态相互转换
- RUNNING -> SHUTDOWN: 调用shutdown()方法,也许隐式在finalize()方法
- (RUNNING or SHUTDOWN) -> STOP: 调用shutdownNow()方法
- SHUTDOWN -> TIDYING: workQueue和pool都为空
- STOP -> TIDYING: pool为空
- TIDYING -> TERMINATED: terminated()方法完成