伪代码
static List<Runnable> rList =new ArrayList<>();//用来保存task的队列
public static void main(String[] args) throws Exception {
for( int a=0;a<10;a++) {
rList.add(new Test().new Run(a));
}
Thread thread =new Thread(new Test().new MyRun()); //创建线程，线程池当中是进行了一次封装将 有个worker 类，该类实现了Runnable 接口，同时持有Thread 对象，在内部实例化Thread 的时候，将自身当做Runnable 传给了内部的Thread 对象，后续的工作就是在这个类的run()中进行了
thread.start();;
}
class MyRun implements Runnable{ //thread 传入的Runnable
@Override
public void run() {// 当thread.start() 后这个run()就会得到执行，在这个方法的内部循环task队列，直接调用task 的run()
// TODO Auto-generated method stub
for(int i=0;i<rList.size();i++) {
rList.get(i).run();
}
}
}
class Run implements Runnable{//相当于我们自己传入 的runnable
private int i ;
public Run(int i) {
super();
this.i = i;
}
@Override
public void run() {
// TODO Auto-generated method stub
System.err.println(i);
}
}
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {//小于核心线程数
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {//加入到队列
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);
}
addWorker()
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
}
}
//创建新的线程
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask); //实例化 Worker 类
final Thread t = w.thread; //创建Thread 对象
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();//启动线程
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
看下Worker 类，worker 类本身是一个Runnable
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this); // 这里创建了Thread ,并将this 传入，上面说过，Worker 本身就是一个Runnable 对象
}
//既然是一个Runnable ，必然复写了run()
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 {
while (task != null || (task = getTask()) != null) {// 这里会阻塞，不停的循环任务队列
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();//执行task 队列中每个队列的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()
// 为分析而简化后的代码
private Runnable getTask() {
boolean timedOut = false;
for (;;) {
int c = ctl.get();
int wc = workerCountOf(c);
// timed变量用于判断是否需要进行超时控制。
// allowCoreThreadTimeOut默认是false，也就是核心线程不允许进行超时；
// wc > corePoolSize，表示当前线程池中的线程数量大于核心线程数量；
// 对于超过核心线程数量的这些线程，需要进行超时控制
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if (timed && timedOut) {
// 如果需要进行超时控制，且上次从缓存队列中获取任务时发生了超时，那么尝试将workerCount减1,即当前活动线程数减1，
// 如果减1成功，则返回null，这就意味着runWorker()方法中的while循环会被退出，其对应的线程就要销毁了，也就是线
程池中少了一个线程了
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
// 注意workQueue中的poll()方法与take()方法的区别
//poll方式取任务的特点是从缓存队列中取任务,最长等待keepAliveTime的时长，取不到返回null（当timed 为true的时候，就说明当前线程数大于核心线程数，这个时候需要考虑是否需要销毁线程，等keepAliveTime时间后如果返回null就表明需要销毁该线程了）
//take方式取任务的特点是从缓存队列中取任务，若队列为空,则进入阻塞状态，直到能取出对象为止（线程池中的线程数小于核心线程数，说明很闲，只需要获取任务即可）
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
总结：Thread.start() 会执行传入Runnable 的run(）,同时执行完后，当前THread 就结束了，这怎么实现复用呢？多线可能会有多个Runnable 啊,每次执行完就结束了那岂不是每一个Runnable 都要重新new Thread（runable），这跟自己new Thread 有什么区别？其实线程池创建的Thead 传入的Runnable 并不是我们传入的task Runanble, 而是其内部一个封装的一个Worker 类，我们创建的Runnable 并没有被当做参数传入到Runnable 中，而是被加入到对比当中，在worker 这个runnable 的run()中去循环取出，就是让worker的run（)内部执行while循环（循环任务队列，这样就能保证一直复用了），具体分 poll（）,和take()两种形式，来区分该线程是否需要被销毁，同时也说明核心线程的其实就是最后活下来的那几个线程
参考：https://blog.csdn.net/MingHuang2017/article/details/79571529
