Worker是ThreadPoolExecutor的一个内部类，Worker本身实现了Runnable接口，并封装了一个Thread对象，最后在构造方法中获取了一个Runnable对象，这个对象就是ThreadPoolExecutor通过execute提交过来的目标任务。
跟踪runWorker(this)方法：
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock();
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
w.lock();
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();//在这里直接调用了目标任务的run方法，并没有将它传给Thread对象。
} 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);
}
}
回过头来在看看Worker的构造方法：
Worker(Runnable firstTask) {
setState(-1);
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
它将自己传给了自己的成员变量thread。目标任务被执行的步骤可能就是：Worker的成员变量thread启动后调用worker的run方法，worker的run方法中将自己传给runWorker，runWorker在调用目标执行对象的run方法。
那么thread是何时被执行的呢？
下面看看ThreadPoolExecutor中的一个其他方法：
private boolean addWorker(Runnable firstTask, boolean core) {
......
try {
final ReentrantLock mainLock = this.mainLock;
w = new Worker(firstTask);
final Thread t = w.thread;//这里初始化一个Worker对象w，在将w的成员变量thread付给t
if (t != null) {
mainLock.lock();
try {
int c = ctl.get();
int rs = runStateOf(c);
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive())
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();//在这里调用t的start方法。
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
这里为什么会设计的这么绕，我想主要是Worker不仅封装了一个thread，而且对目标任务进行了封装，在运行封装过后的目标任务前，addWorker可以做一些相关操作。
这里仅仅介绍了ThreadPoolExecutor的线程池，那么这个线程池是如何被维护的，下面介绍几个关键的参数。
private volatile int corePoolSize;
private volatile int maximumPoolSize;
private final BlockingQueue<Runnable> workQueue;