撸一下ThreadPoolExecutor核心思路
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ThreadPoolExecutor中知识点很多，本文只是从7个构造参数入手，看看其运转的核心思路。重点不是扣代码，是体会设计思想哈！
欢迎纠错和沟通。
ThreadPoolExecutor
以下是构造ThreadPoolExecutor的7大参数。
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
corePoolSize 和 maximumPoolSize 以及 workQueue（BlockingQueue）共同决定了线程池中线程的数量。 下面几条是我总结的观点：
- 核心线程数小于等corePoolSize。
- 普通线程在workQueue满了后才会创建。
- 普通线程在任务结束后存活时长为：
keepAliveTime*unit。 - 任务总数如果超过了workQueue的容量+普通线程数，会触发
RejectedExecutionHandler。 - 最好自定义
ThreadFactory来创建线程，方便标识线程名等。 ThreadPoolExecutor提供了hook的方法beforeExecute()、afterExecute()shutdown()不会立即停止线程池中的未完成的任务，shutdownNow()会。
这里有个设计上的亮点：它使用了一个AtomicInteger类型的ctl来同时记录线程池状态和线程池中线程数。ctl中高3位记录状态，低29位代表线程数量。 这种方法值得学习，除了节省变量，也减少了线程池状态和当前线程数量同步问题。
@ReachabilitySensitive
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; }
核心代码区域execute()：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
//如果当前worker数量小于corePoolSize则创建新的核心Worker
if (addWorker(command, true))
return;
c = ctl.get();
}
//线程池正在运行且可以正常添加任务(即workQueue还没有满)，此时等待任务执行即可。注意此处已经将新的Runnable存储到了workQueue里了。
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
//二次确认，此时线程池不属于运行状态，且刚添加进去的任务还没有执行，则reject
reject(command);
else if (workerCountOf(recheck) == 0)
//线程池showdown中，后续应该什么都不会做直接return
addWorker(null, false);
}
//workerQueue中满了，创建非核心worker，如果不成功则reject
else if (!addWorker(command, false))
reject(command);
}
看下addWorker()到底干了啥
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
//如果当前线程池停止了，或者 当前task为空且workQueue中没有任务了。这些情况可以直接退出该方法。
// 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;
}
上面代码，除了线程池状态校验以及保证代码的异步安全，核心就是：
w = new Worker(firstTask);
final Thread t = w.thread;
t.start()；
创建一个Worker并启动其中的线程。
Worker
Worker是什么？Worker是ThreadPoolExecutor的内部类，ThreadPoolExecutor中把excute()传递进来的Runnable认为是一个Work，那么执行Work的就是Worker了。
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/** 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);
}
public void run() {
runWorker(this);
}
}
Worker的构造函数中持有了传入的Task,并通过ThreadFactory创建了个新的线程。
note: 新创建的线程持有了Worker对象自己，而Worker本身又实现了Runnable接口。所以当该线程启动时，run()就会被执行。
看下Worker的run()方法具体干了什么？
首先里面有个while循环，主要是获取task的，如果task一直能获取到，则就能执行到while内部。
- 当前task非空，即核心线程创建的时候，自带了一个task。会触发task.run()方法，并在它前后分别又
beforeExecute(wt, task)和afterExecute(task, thrown)hook点。 - 当前task==null，就需要到
getTask()中获取。
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);
}
}
getTask()就跟ThreadPoolExecutor构造函数中的BlockQueue关联上了。当前线程如果执行完第一个Task后，应该怎么办呢？如果还不满足结束条件(比如说存活时间超过keepAlive*unit)，则会向worksQueue（BlockQueue）中所要任务，然后执行。
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;
}
}
}
上面前段代码就是判断当前线程池是否还能继续存在，如果能存在并且未超时，那么就从workQueue中取task()。Runnable r = timed ?workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS):workQueue.take();
/**
* Retrieves and removes the head of this queue, waiting if necessary
* until an element becomes available.
*
* @return the head of this queue
* @throws InterruptedException if interrupted while waiting
*/
E take() throws InterruptedException;
是个阻塞的行为。所以TheadPoolExecutor中的workQueue参数传入哪种类型的BlockQueue直接影响了未执行任务的执行顺序。 不过需要注意的是即使使用了优先级队列，高优先级的任务也不一定比低优先级任务先执行，因为任务是由线程发起的，workQueue没法影响线程的执行顺序。