线程池流程：
线程池核心类：
ThreadPoolExecutor：普通的线程池
ScheduledThreadPoolExecutor： 延时重复执行线程池。暂不研究
ForkJoinPool：fork join分片合并线程池。暂不研究
Executors：线程池工具类，提供了6中线程池工具方法：
1.newFixedThreadPool
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue());
}
返回ThreadPoolExecutor对象，核心线程数和最大线程数相等，堵塞队列为LinkedBlockingQueue，大小没有限制，意味着，当核心线程数满了之后，一直往堵塞队列添加任务，知道Integer.MAX_VALUE。故在一定条件下，会占用很多内存。
2.newCachedThreadPool
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue());
}
返回ThreadPoolExecutor对象，核心线程数为0， 最大线程数为Integer.MAX_VALUE， 线程空闲时间为60秒，堵塞队列为同步队列，SynchronousQueue队列的put和take是互相唤醒的，但是线程池execute提交任务调用的是offer方法，offer方法不会堵塞，能添加进去则添加进去（只有其他线程正在poll堵塞在那，才可能offer进去），不能则马上返回然后启动一个线程执行，线程获取任务调用的是poll方法，因为poll方法可以设定超时时间，来达到超时60秒就返回线程结束的机制，而take方法没有超时机制。
再一定条件下，可能会产生很多个线程，占用大量的资源。
3.newScheduledThreadPool
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
暂不研究
4.newSingleThreadExecutor
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue()));
}
newFixedThreadPool的单线程版
5.newSingleThreadScheduledExecutor
public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1));
}
暂不研究
6.newWorkStealingPool
public static ExecutorService newWorkStealingPool() {
return new ForkJoinPool
(Runtime.getRuntime().availableProcessors(),
ForkJoinPool.defaultForkJoinWorkerThreadFactory,
null, true);
}
暂不研究。
ThreadPoolExecutor源码分析
内部类
image.png
总共有5个内部类，前面4个是拒绝任务策略，最后一个是实际执行任务的线程子类。
4个拒绝任务策略类都很简单：
AbortPolicy:直接报错。
CallerRunsPolicy：如果线程池正在运行，则执行掉抛弃的任务。
DiscardOldestPolicy：如果线程池正在运行，则抛弃堵塞队列的头部最先进堵塞队列的任务，重新提交任务。
DiscardPolicy： 抛弃任务，什么都不做
真正执行任务的类：
work:
/** 父类位AQS类，实现了tryAcquire和tryRelease方法，能够同步锁住资源。
* 实现了runnable接口，所以可以直接new Thread(worker)进行线程执行。
**/
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. */
// 执行自身work任务的线程引用
final Thread thread;
/** Initial task to run. Possibly null. */
// 线程当前执行的任务，firstTask任务，会从堵塞队列中不停的取
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;
// 指定的线程factory创建线程，传入了work本身，
//所以这个线程start的时候，执行的就是work的run方法
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
// 真正开启线程，循环从堵塞队列中取任务执行
runWorker(this);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
protected boolean isHeldExclusively() {
return getState() != 0;
}
// 最简单粗暴的锁资源，没有读写锁，可重入锁等等
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) {
}
}
}
}
关键成员变量
// ctl变量保存了两个元素：workcount线程数量和runstate线程池运行状态
// runstate只有5中情况，3个字节可以表示完。int类型占32个字节，还剩29个字节，这29个字节表示workCount。
//组成就是：高3位表示runstate，低29位表示workcount
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
// 正常运行
private static final int RUNNING = -1 << COUNT_BITS;
// 线程池关闭，不接受新任务，已经进入队列的任务会执行完
private static final int SHUTDOWN = 0 << COUNT_BITS;
// 线程池停止，不接受新任务，也不执行队列任务
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// 堵塞队列
private final BlockingQueue workQueue;
// 可重入锁，锁住一些并发操作，比如，添加工作线程
private final ReentrantLock mainLock = new ReentrantLock();
/**
* Set containing all worker threads in pool. Accessed only when
* holding mainLock.
*/
private final HashSet workers = new HashSet();
/**
* Wait condition to support awaitTermination
*/
private final Condition termination = mainLock.newCondition();
/**
* Tracks largest attained pool size. Accessed only under
* mainLock.
*/
// 最大线程数容量，实际最大开启过的线程数
private int largestPoolSize;
/**
* Counter for completed tasks. Updated only on termination of
* worker threads. Accessed only under mainLock.
*/
private long completedTaskCount;
/*
* All user control parameters are declared as volatiles so that
* ongoing actions are based on freshest values, but without need
* for locking, since no internal invariants depend on them
* changing synchronously with respect to other actions.
*/
private volatile ThreadFactory threadFactory;
/**
* Handler called when saturated or shutdown in execute.
*/
private volatile RejectedExecutionHandler handler;
/**
* Timeout in nanoseconds for idle threads waiting for work.
* Threads use this timeout when there are more than corePoolSize
* present or if allowCoreThreadTimeOut. Otherwise they wait
* forever for new work.
*/
private volatile long keepAliveTime;
/**
* If false (default), core threads stay alive even when idle.
* If true, core threads use keepAliveTime to time out waiting
* for work.
*/
private volatile boolean allowCoreThreadTimeOut;
// 核心线程数
private volatile int corePoolSize;
/**
* Maximum pool size. Note that the actual maximum is internally
* bounded by CAPACITY.
*/
// 最大线程数
private volatile int maximumPoolSize;
/**
* The default rejected execution handler
*/
// 默认抛弃策略是报错
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
构造函数
初始化一些成员变量
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue 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;
}
关键方法
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();
// 正在运行的工作线程数小于核心线程数，则addWorker方法开启新的工作线程
// 如果已开启的工作线程数大于或等于核心线程数，则把任务添加入堵塞队列
// 添加失败，则继续开启工作线程，如果已经开启线程数小于最大线程数，则添加成功，否则，使用指定的拒绝策略拒绝
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);
}
// 添加失败，则继续开启非核心线程执行任务
else if (!addWorker(command, false))
reject(command);
}
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);
// core为true，则和核心线程数比较，core为false，则和最大线程数比较
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 {
// 构建新workker工作线程，firstTask为传入的runnable，thread为根据传入的runnable构建的线程
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
// 开启工作线程，是同步造作，不能并发
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();
}
// 添加成功，则启动新工作线程，新线程会执行worker类的run方法
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
// 开启线程后，第一次执行的是本身的worker任务，而不是从队列中取任务。
this.firstTask = firstTask;
// worker的thread为根据传入的runnable构建的线程，故开启线程，则会运行run方法
this.thread = getThreadFactory().newThread(this);
}
启动工作线程之后
/** Delegates main run loop to outer runWorker */
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 {
// 循环从堵塞队列中取任务执行，直到取不到了
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为空方法，用户可以重写它，在执行任务前，
// 可以执行一些用户自定义操作
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为空方法，用户可以重写它，在执行任务后，
// 可以执行一些用户自定义操作
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
// 直到到这里，说明这个工作线程要死了。要不线程池非runtime状态，要不堵塞队列空了，做一些资源收尾操作
processWorkerExit(w, completedAbruptly);
}
}
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?
// 这个线程是否有超时关闭，还是一直运行。当前线程数大于核心线程数，则需要超时关闭
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
// 需要超时关闭，则堵塞队列为空，poll方法超时则返回不会一直堵塞，不需要超时关闭，则take方法会一直堵塞
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
再看一个方法getActiveCount方法,
不能用executorService.getActiveCount()方法查看线程数，因为，这个方法查看的并非是真正的线程数，而是正在执行任务的线程数，看源码就会发现，只会对没有unlock的work技术，而work只要执行完了task.run方法就会释放lock，也就不会计数。
public int getActiveCount() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
int n = 0;
for (Worker w : workers)
if (w.isLocked())
++n;
return n;
} finally {
mainLock.unlock();
}
}
总结：ThreadPoolExecutor大概的运行流程是：指定核心线程数，最大线程数，堵塞队列类型，大小，抛弃策略，非核心线程空闲时间。execute提交任务的时候，如果已经开启的工作线程数小于核心线程数，则开启工作线程处理这个任务，并且会一直从堵塞队列取任务执行，堵塞队列为空，则堵塞这个线程。如果已经开启的工作线程数大于核心线程数，则将任务放入堵塞队列，直到堵塞队列放不下之后，开启非核心线程数执行任务，如果总线程数小于最大线程数，则开启成功，否则，开启失败，走指定的拒绝策略。非核心线程数，空闲指定时间后会关闭，就是当堵塞队列没有任务了一段时间后，非核心线程会关闭，核心线程会一直堵塞，不会关闭。