我们平时大部分大部分业务都会用到线程池，首先用线程池的一个好处在于它能复用线程，之前一直有疑问它是如何复用的，今天我们来看看里面的具体实现。
首先我们来看下线程池提交任务的方法ThreadPoolExecutor execute(Runnable command)
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//判断当前的线程数是否小于核心线程数
if (workerCountOf(c) < corePoolSize) {
//新增线程执行command
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
//如果线程池正常运行 并且成功加入任务队列
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)//当现在没有线程的时候
addWorker(null, false);//注意这里方法第一个参数是null ??? 下面我们看看里面用这个参数的意义
}
else if (!addWorker(command, false))
reject(command);
}
在上面的代码中我们可以看到addWorker方法的传参出现了两种形式，一种是直接传任务 一种是传null 我们下面看看里面到底是怎么运行的。
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
}
}
------------------上面可以省略·------------------
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
final ReentrantLock mainLock = this.mainLock;
//这里会new一个工作线程 我们可以进去看下new Worker的实现其实就是开启了一个线程
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int c = ctl.get();
int rs = runStateOf(c);
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) {
//工作线程添加成功后 开始跑线程
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
下面我们看下Worker里面的具体实现
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;//这个就是外面传进来的任务
this.thread = getThreadFactory().newThread(this);//new了一个自己作为线程本身
}
//t.start()执行的方法
public void run() {
runWorker(this);
}
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();//获取当前线程对象
Runnable task = w.firstTask;//获取线程里面任务
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//这里就是关键所在了 当我们传了firstTask的时候 会直接执行下面的逻辑
//我们可以看到下面其实就是run方法
//那当我们前面没传firstTask的时候它会做什么处理呢 我们看下getTask方法
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?
retry:
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;
}
boolean timed; // Are workers subject to culling?
for (;;) {
int wc = workerCountOf(c);
timed = allowCoreThreadTimeOut || wc > corePoolSize;
if (wc <= maximumPoolSize && ! (timedOut && timed))
break;
if (compareAndDecrementWorkerCount(c))
return null;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
````````````上面是一些筛选条件········
///我们可以看到下面是从工作队列里面获取任务 返回出去
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
所以当线程池里没有线程的时候 addWorker(null, false) 会new一个线程从任务队列里面一个一个获取执行。
