线程池（ThreadPoolExecutor）的几个状态
- RUNNING：Accept new tasks and process queued tasks（可接受新的任务并且执行已经加入队列的任务）
- SHUTDOWN：Don't accept new tasks, but process queued tasks（不再接受新的任务，但是可以继续把已经加入队列的任务执行完成）
- STOP：Don't accept new tasks, don't process queued tasks,and interrupt in-progress tasks（不接收新任务，不执行已经加入到队列中的任务，中断当前正在执行的任务）
- TIDYING：All tasks have terminated, workerCount is zero,the thread transitioning to state TIDYING will run the terminated() hook method（所有的任务都结束了，workerCount 变为0，线程状态变为TIDYING ，然后会执行terminated()方法）
- TERMINATED：terminated() has completed（terminated()方法已经执行完成）
线程池的状态切换过程
- RUNNING -> SHUTDOWN：On invocation of shutdown(), perhaps implicitly in finalize()
- (RUNNING or SHUTDOWN) -> STOP：On invocation of shutdownNow()
- SHUTDOWN -> TIDYING：When both queue and pool are empty
- STOP -> TIDYING：When pool is empty
- TIDYING -> TERMINATED：When the terminated() hook method has completed
线程池的构造函数参数
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
- corePoolSize：核心线程数，即当前线程池中维持运行状态的线程数量，可以为0，也可以大于0，如果大于0就代表当前线程允许多个线程并发执行，如果等于0就代表线程池中的线程任务运行结束后就不在有线程处于运行状态，如果等于1就代表当前线程池在同一时间只允许一个线程在运行，类似于：Executors.newSingleThreadExecutor()
- maximumPoolSize：线程池中最大可接受的线程数量，或者说任务数量
- keepAliveTime：如果当前线程中中接受的线程数量已经超过和核心线程数，次数空闲线程在等待接收新线程可最多维持keepAliveTime的单位时间，超过这个时间，空闲线程将会终止
- workQueue：任务队列，是阻塞队列，BlockingQueue
- handler：当新put进来的任务被reject之后就会转给handler去处理，默认实现为抛出RejectedExecutionException异常
线程池的execute方法
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
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);
}
- isRunning方法判断当前的线程池是否存在已经在运行的线程，workQueue是任务队列，负责存放execute进来的command任务对象
- 这里有两个地方会被线程池reject，也就是会抛出RejectedExecutionException异常，一个是当线程池已经被shutdown掉了，另一个当workQueue已经满了，而且当前线程池中正在工作的线程数量已经超过maximumPoolSize限制的数量了，这时候也会拒绝接受任务了
- 如果当前线程池正在运行，并且线程池中的活跃线程数量还小于核心线程数corePoolSize，此时会直接start一个线程作为核心线程，线程池中运行的核心线程都是并发的
- 如果当前线程池中的核心线程数已经达到限制，这时候会尝试吧任务加到任务队列中，等核心线程去遍历取出任务队列中的任务来执行
- 线程池之所以说能够合理的管理线程资源，不乱开线程的原因就是控制核心线程数的运行数量，在线程池中并发的核心线程是有一个最大上线的（corePoolSize），所有外部execute进来的任务在核心线程数达到上线之后都会放进workQueue，这样做的好处就是开启的线程数最多只有corePoolSize，而其他多余的线程任务将以对象的方式存放到workQueue等待执行，所以申请了线程创建的只有corePoolSize个线程，而其他的任务只是占用了对象内存而已，避免了每一个线程都去start，既占用了对象内存，又申请了线程资源，对于java来说运行一个线程需要去申请线程的运行内存的，在底层是用pthread_create去创建一个linux线程，代价相对来说比较大，所以由线程池管理线程的这种方式限制了线程start的个数，避免造成过于的并发线程占用了大量的内存资源而导致OOM的问题
下面继续分析线程池如何管理运行线程，其实就一句话，维护一个线程队列，然后对这个线程队列进行存取操作
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;
}
每一个运行起来的核心线程都会被封装成一个Worker对象然后保存到workers列表中，核心线程start之后会执行Worker中的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 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);
}
}
这里面是一个while循环，反复的去workQueue队列里面去取任务出来执行，在同一个核心线程中，只有等上一个任务执行完成之后才能继续去执行下一个任务，所以同一个核心线程中的任务执行并不是并发的，但是不同核心线程中的任务执行是并发的，继续分析getTask方法：
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方法就是去workQueue任务队列中去取任务，这个方法里面还会对当前线程池的状态做判断，如果线程池被shutdown并且任务队列已经空了，或者线程池被shutdownNow了，这时候都会返回null，要是线程池还是在运行状态，那么这时候就会去判断如果当前核心线程数已经达到上线，或者限制核心线程数不能无限等待（allowCoreThreadTimeOut = true），会从workQueue中以poll的方式取任务，并且等待keepAliveTime时间，如果超过这个时间还没取出任务，此时poll方法返回null，然后进入下一次循环，但是最后如果是核心线程数已经达到上线，workQueue已经为空，那么此时getTask也会返回null，然后线程池会结束所有的核心线程，不再维持任何核心线程；这边举一个例子，假设keepAliveTime为0，那么当线程池中执行完所有的任务后，核心线程逐个马上就会结束并且释放，这时候要是有新的任务进来需要再重新开一个核心线程去执行，如果keepAliveTime为60秒，那么在线程池中完成所有任务后，核心线程会再等待60秒后才会结束运行，在这60秒之内如果还有新的任务进来就直接取出来接着执行，所以关于核心线程的维持活跃状态的实现是通过阻塞队列去实现的
shutdown和shutdownNow方法
shutdown方法在调用完了之后，线程池会把workQueue中剩下的任务都执行完了之后才会完全结束，而shutdownNow执行完了之后，二者都不允许再接收新的任务，workQueue中剩下的任务将不会再执行，具体的区别实现在一下几个实现
- getTask方法中：
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
shutdown之后线程池的状态是SHUTDOWN，shutdownNow后线程池的状态是STOP，所以shutdownNow之后getTask会直接返回null，而shutdown之后如果workQueue不为空getTask还会取任务
- processWorkerExit方法：
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
if (runStateLessThan(c, STOP))这个条件判断就是说要是只调用了shutdown，那就是满足的，次数会继续执行addWorker(null, false)来再次出发workQueue的任务存取执行
- shutDown方法
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(SHUTDOWN);
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
}
private void interruptIdleWorkers() {
interruptIdleWorkers(false);
}
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();
}
}
在shutdown调用后会去workers遍历里面的Worker对象，上文已经提到了，workers里面维护的Worker对象是核心线程的包装对象，所以这里是去中断核心线程，但是只是中断空闲的核心线程，什么叫空闲？w.tryLock()可以获取到锁的核心线程就是空闲的，因为在Worker运行的时候会先lock获取锁，在执行完成之后才unlock，所以要是Worker对象还在执行任务，这时候就不中断
- shutdownNow
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(STOP);
interruptWorkers();
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfStarted();
} finally {
mainLock.unlock();
}
}
shutdownNow方法就是直接遍历workers中的Worker对象并且进行中断操作，不管Worker是否是空闲状态，这里的线程中断就是调用Thread的interrupt方法，而在Worker的runWorker会去判断该Thread的的中断状态，根据中断状态来判断是否接着执行任务
总结
- 线程池的好处就是合理的管理线程，避免造成过多的并发线程占用了大量内存资源，从何导致OOM
- 线程池所谓的并发执行并不一定是并发，因为同一个核心线程中执行的任务是按照顺序执行的，并不是并发的，只有不同的核心线程的任务执行才是并发的
- 多线程的实现可以用队列加上一个核心线程的方式去实现，可以开启一个核心线程负责遍历队列中的任务，然后在核心线程中去run，注意。如果核心线程只有一个的话，那么就无法达到并发效果，类似newSingleThreadExecutor方法创建出来的ThreadPoolExecutor，这时候execute多次，一定要等上一次的任务执行完成之后才能执行下一次的任务
- Java线程start起来之后会有两个部分的内存占用，一个是JVM部分的线程对象资源，而真正开线程的其实是用linux的pthread_create去开启了一个linux线程实现异步的，所以这里还占用了系统底层的线程资源，因此开启一个Java线程的代价比直接用native方法去开启一个linux线程要大，占用的内存资源要多，所以要谨慎的使用java多线程，避免造成线程泄漏，或者OOM
声明
- 我的博客即将搬运同步至腾讯云+社区，邀请大家一同入驻：https://cloud.tencent.com/developer/support-plan?invite_code=2h71iay2bd6o8
