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线程池源码解读的示例分析，针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。
一、execute
ctl作为AtomicInteger类存放了类中的两种信息，在其中由高3位来保存线程池的状态，后29位来保存此时线程池中的Woker类线程数量(由此可知，线程池中的线程数量最高可以接受大约在五亿左右)。由此可见给出的runStateOf()和workerCountOf()方法分别给出了查看线程状态和线程数量的方法。
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//如果运行的线程数小于corePoolSize，尝试创建一个新线程（Worker），并执行它的第一个command
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//线程数大于corePoolSize，将线程放入任务队列
//第一次校验线程池在运行状态
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//第二次校验，防止在第一次校验通过后线程池关闭。如果线程池关闭，在队列中删除task并拒绝task
if (! isRunning(recheck) && remove(command))
reject(command);
//如果线程数=0（线程都死掉了，比如：corePoolSize=0），新建线程且未指定firstTask，仅轮询任务队列
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//任务队列已满，尝试创建新线程执行task，创建失败后拒绝task
//创建失败原因：1.线程池关闭；2.线程数已经达到maxPoolSize
else if (!addWorker(command, false))
reject(command);
}private boolean addWorker(Runnable firstTask, boolean core) {
retry:
//外层循环判断线程池的状态
for (;;) {
int c = ctl.get();
//线程池状态
int rs = runStateOf(c);
//线程池状态:RUNNING = -1、SHUTDOWN = 0、STOP = 1、TIDYING = 2、TERMINATED
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//用CAS的方式对线程数量进行+1操作
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 {
//worker实现了Runable接口
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是一个worker数组
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
//启动线程，执行的就是worker中的run()
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}Worker类：
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable//Worker是一个线程
{
private static final long serialVersionUID = 6138294804551838833L;
final Thread thread;
Runnable firstTask;
volatile long completedTasks;
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
//把当前Worker包装成一个thread
this.thread = getThreadFactory().newThread(this);
}
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 {
//如果这个worker还没有执行过在构造方法就传入的任务，那么在这个方法中，会直接执行这一任务，如果没有，则会
//尝试去从任务队列当中去取的新的任务。
//在执行完毕后，工作线程的使命并没有真正宣告段落。在while部分worker仍旧会通过getTask()方法试图取得新
//的任务。下面是getTask()的实现。
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();
} 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);
}
}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 {
//从工作队列中取出线程
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}二、submit
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
} protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}可见，submit将普通的runnable包装成FutureTask并返回，再调用execute去执行。
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