Java线程池原理浅析
一、线程池工厂Executors
我们平时在使用线程池的时候一般都是通过Executors的newXxxxxPool()静态方法来获得不同功能的线程池对象。我们来看一下这些方法都是怎么创建线程池的：
/**
* 固定线程数的线程池
*/
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
/**
* 只有一个线程的线程池
*/
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
/**
* 可变大小线程池
*/
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
/**
*定时执行的线程池
*/
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
我们可以看到这newFixedThreadPool()、newSingleThreadExecutor()、newCachedThreadPool()都是创建了一个ThreadPoolExecutor对象，而newScheduledThreadPool()则是创建了一个ScheduledThreadPoolExecutor对象，其实ScheduledThreadPoolExecutor也是继承了ThreadPoolExecutor这个类，通过在ThreadPoolExecutor上扩展实现了定时执行线程的功能。
ThreadPoolExecutor
我们先来看一下ThreadPoolExecutor的构造方法：
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
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:线程池里最小线程数
maximumPoolSize:线程池里最大线程数
keepAliveTime:空闲线程存活的时间，也就是线程池的线程数超过corePoolSize后，空闲线程可以存活的时间，超过这个时间就会被销毁。
unit: keepAliveTime的单位
workQueue:用来存放等待任务的队列。这个队列是个阻塞队列
threadFactory:用来产生线程池里的线程的工厂
handler:当任务超过最大允许的任务数量后，新来任务的拒绝策略。
知道了上面几个参数，我们对ThreadPoolExecutor应该有所了解，对Executors产生的不同功能的线程池也应该有所了解。我们接下来讨论一下ThreadPoolExecutor实现线程池的原理。
首先从提交任务的方法开始：
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*执行给定的任务，这个任务可能在一个新的线程里执行，也可能在一个已经存在的线程里执行
*如果任务不能被提交，不管是因为executor被shutdown还是因为容量到达界限，任务都会被RejectedExecutionHandler（拒绝策略）处理。
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
* {@code RejectedExecutionHandler}, if the task
* cannot be accepted for execution
* @throws NullPointerException if {@code command} is null
*/
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);
}
首先看到这一行代码：int c = ctl.get(); ctl是什么呢？我们来看一下关于ctl的定义:
/**
* The main pool control state, ctl, is an atomic integer packing
* two conceptual fields
* workerCount, indicating the effective number of threads
* runState, indicating whether running, shutting down etc
*
* In order to pack them into one int, we limit workerCount to
* (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
* billion) otherwise representable. If this is ever an issue in
* the future, the variable can be changed to be an AtomicLong,
* and the shift/mask constants below adjusted. But until the need
* arises, this code is a bit faster and simpler using an int.
*
* The workerCount is the number of workers that have been
* permitted to start and not permitted to stop. The value may be
* transiently different from the actual number of live threads,
* for example when a ThreadFactory fails to create a thread when
* asked, and when exiting threads are still performing
* bookkeeping before terminating. The user-visible pool size is
* reported as the current size of the workers set.
*
* The runState provides the main lifecycle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
*
* 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
*
* Threads waiting in awaitTermination() will return when the
* state reaches TERMINATED.
*
* Detecting the transition from SHUTDOWN to TIDYING is less
* straightforward than you'd like because the queue may become
* empty after non-empty and vice versa during SHUTDOWN state, but
* we can only terminate if, after seeing that it is empty, we see
* that workerCount is 0 (which sometimes entails a recheck -- see
* below).
*/
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
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;
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
注释已经很清楚告诉我们ctl是workerCount和runSate的结合。我们可以看到，线程池的容量是CAPACITY（线程池中允许的最大线程数是CAPACITY），也就是2的Integer.SIZE-3次方减一。ctl用低29位表示线程池中的线程数，用剩下的高3位表示线程池的运行状态。这一点大家要理解清楚。这下面三个方法是对ctl的操作
private static int runStateOf(int c) { return c & ~CAPACITY; } //获取高三位，也就是线程池的运行状态
private static int workerCountOf(int c) { return c & CAPACITY; } //获取低29位，也就是线程池线程的数量
private static int ctlOf(int rs, int wc) { return rs | wc; } //生成ctl
理解了这个之后，我们继续回到execute方法，当一个任务被提交给线程池后，分三种情况：
1、当前线程池中线程的数量小于corePoolSize，这个时候我们直接创建一个新线程来执行提交的任务。
2、当线程池中的线程数大于corePoolSize时，如果线程池的状态是RUNNING状态，并且任务加到任务队列成功，我们仍然要再次检查一下线程池的状态，防止任务在添加到任务队列的过程中线程池被停止。如果线程池没有被停止，则调用addWorker方法尝试再创建一个线程去处理任务队列。这里只是去尝试创建，并不一定能创建成功，具体addWorker的实现我们接下来会讨论。
3、如果任务添加到任务队列失败，这个时候我们再次调用addWorker方法尝试创建一个新线程来处理当前任务，如果失败，则说明线程池被shutdown或者线程池的任务队列已经满了。
知道了一个任务被提交到线程池的处理流程之后，我们来看一下每个步骤的具体实现。首先是addWorker方法，我们来看一下具体实现：
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;
}
首先进入retry循环体，这个循环体的功能是去判断线程池是否可以新创建线程。首先线程池的状态如果大于SHUTDOWN状态，就不允许新创建线程（STOP状态：不再接受新任务也不处理任务队列里的任务，中断正在进行的任务；TIDYING：所有的任务被结束，workerCount被设置为0，线程状态被转变成TIDYING将会调用terminated()钩子方法；TERMINATED：线程调用完terminated()方法）。如果线程池的状态是SHUTDOWN状态，因为我们通过executor方法传进来的任务不是空，所以，这个时候会返回false，不回去了创建新的线程了。也就是说，只有线程池处于RUNNING的时候才有创建新线程的机会。然后判断当前线程数是否超过了线程池的最大容量，如果是则返回false不允许创建。然后通过CAS操作将workerCount加一，如果成功则跳出循环创建线程池，如果失败，再次判断线程池的状态和进入方法时的状态是否一致，如果不一致则重新执行retry循环体，如果一致，则重新判断线程池容量，决定是否能够创建新的线程。
如果通过以上判断，允许创建新的线程，则新创建一个Worker对象。Worker是个什么东西呢？我们来看一下：
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;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
protected boolean isHeldExclusively() {
return getState() != 0;
}
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
我们先看一下Worker的构造方法：当创建Worker对象的时候，会通过我们之前设置的ThreadFactory的newThread方法来创建一个线程，并交给Worker对象持有。我们来看一下默认的线程池的实现：
public Thread newThread(Runnable r) {
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
在调用该方法的时候，会把Worker对象本身传入，我们可以看到Worker实现了Runnable接口。所以当线程启动的时候会调用的是Worker的run()方法。而Worker的run()方法调用了外部类的runWorker方法，我们看一下这个方法:
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);
}
}
这个方法才是线程池处理任务的整个核心内容，进入方法后，会进入一个循环体：首先获取要执行的任务，如果当前Worker持有的任务不是空，获取的就是该任务，如果是空，就调用getTask()方法来获取任务队列里的任务。这个方法也是实现线程池中空闲线程销毁的关键。我们来看一下它的内部实现：
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;
}
}
}
进入方法的时候先定义一个标识位timedOut，这个标识位用来表示从任务队列中获取任务是否超时。如果超时，说明这段时间没有新任务过来，这个线程也就是空闲的，如果当前线程数大于corePoolSize，这个线程就会被销毁。我们来看一下这个过程是怎么实现的：当设置标识位之后，进入一个循环体，来判断当前的线程池的状态，如果当前线程池的状态大于等于STOP，方法直接返回null，返回nulll是什么概念呢？我们回到runWorker方法看一下，当getTask()返回null的时候，while循环结束，执行finall语句块里的processWorkerExit方法。执行完这个方法后线程就会结束，也就是这个线程会被销毁。我们继续回到getTask()方法，当前线程池的状态大于等于STOP时，不管任务队列里是否有任务都不会获取到任务，线程会被销毁。当线程池状态是RUNNING状态的时候会继续接下来的判断，当线程池状态是SHUTDOWN的时候要去判断任务队列是否为空，如果是空就返回null，销毁线程，如果不是空继续接下来的操作。
当进行完上面的判断后，在设置一个标识位timed，这个标识位用来表示当获取任务超时后是否需要销毁线程。然后进入if ((wc > maximumPoolSize || (timed && timedOut))这个判断，如果当前线程数大于maximumPoolSize，说明线程被创建多了，这个时候要销毁线程，直接返回null。如果获取任务超时（第一次进入这个循环的时候肯定不存在这种情况，因为timedOut标识位被设置成了false），并且当前线程池里面的线程数大于1（因为要保证线程池里必须至少有一个线程）、任务队列是空的时候，返回null销毁线程。
结束以上判断的时候就要去任务队列取任务，如果timed标识位（表示当获取任务超时后是否需要销毁线程）是ture，就需要在给定时间内获取任务，不然就会返回null，如果返回null，就设置timedOut标识位为ture，表示获取任务超时，当前线程是空闲线程。等到下次循环的时候就会结束方法返回null。如果正常获取任务就讲任务返回。到此getTask()的分析结束，我们做一个小小的总结：如果线程池状态大于STOP，直接返回null销毁线程；如果当前线程池状态是SHUTDOWN并且任务队列是空，返回null销毁线程；如果不是以上两种情况，再判断线程池是否设置了空闲线程销毁，如果是的话，并且从任务队列中获取任务超时，就返回null销毁线程；如果不是就返回获取的线程。
当获取到任务之后，就去判断当前线程池是否被stop，如果是，中断当前线程，如果不是，就调用interrupted()方法取消中断标志。这一步是用来防止成功获取任务之后线程池被中断。
当做完 以上检查之后，调用beforeExecute(wt, task)方法，来执行前置操作，这个方法是个模板方法，交由子类实现。之后会执行任务的run方法，真正的执行任务。执行完任务之后会调用 afterExecute(task, thrown)方法来执行后置操作，这个方法也是模板方法。执行完之后，会再次去获取任务执行以上操作。getTask()方法返回null的时候，会调用processWorkerExit(w, completedAbruptly)方法，这个方法做了讲当前的worker对象从线程池中去除等操作（有可能还会重新创建一个线程）。有兴趣的同学可以看一下。
到此Worker分析结束，我们继续回到addWorker方法，当我们创建一个Worker对象后，讲worker对象添加到workers容器里。然后启动worker对象持有的线程。也就是用来处理任务的线程。
到此，线程池添加任务、处理任务的分析结束。
