学习线程池过程中产生的问题
- 线程池的提交过程是怎么样的
- 线程池是如何复用的
- Runnable和Callable提交有什么区别
- 如何区分核心线程和非核心线程
- 存活时间到达时如何关闭线程
线程池的提交
线程池的提交有3种方式：
Future submit(Callable task);
Future submit(Runnable task, T result);
Future submit(Runnable task); 可以看到分别是2个Runnable和1个Callable的提交，那为什么有两个Runnable的提交呢。通常我们都是认为Runnable和Callable的区别是Callable是可以返回一个结果。其实Runnable也是可以返回结果的，根据参数T result可以看出T result就是返回结果，如果不传，那么默认返回NULL。
接下来进入submit方法
public Future submit(Callable task) {
if (task == null) throw new NullPointerException();
RunnableFuture ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
public Future submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
public Future submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
} 这三个方法是先调用newTaskFor得到一个RunnableFuture，再提交执行。
protected RunnableFuture newTaskFor(Runnable runnable, T value) {
return new FutureTask(runnable, value);
}
protected RunnableFuture newTaskFor(Callable callable) {
return new FutureTask(callable);
} newTaskFor内构造的FutureTask就是我们用来获取执行状态和结果的适配类了。在FutureTask类内，则是统一适配为Callable，只不过Runnable返回NULL或传入的固定值。
我们再看看ThreadPoolExecutor的执行过程：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
//1、判断是否在核心线程内，如果是则尝试创建核心线程
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//2、超出核心线程数则尝试直接加入队列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//3、加入队列失败则尝试创建非核心线程，注意第二个参数，在添加任务前要做检测
//，ture表示检测小于核心线程数，false表示检测小于最大线程数
else if (!addWorker(command, false))
//4、执行拒绝策略
reject(command);
}很明显，在提交任务的过程中，最重要的就是addWorker这个方法。
private boolean addWorker(Runnable firstTask, boolean core) {
//1、做各种检测，并且尝试CAS线程数，成功则继续，失败则重试或返回
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
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//2、新建Worker来管理任务的执行，并将Worker添加到workers中管理
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) {
//3、终于到执行这一步，直接运行start执行
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}前面说到，参数boolean core控制添加核心/非核心线程，但是在该方法中只有判断时用到了core，所以如何区分核心/非核心线程呢？答案是不区分，这两兄弟在线程池看起来没有区别，只需要考虑线程数和核心数，最大数就可以。
直接运行start，那么队列里的任务怎么办呢？这在Worker内部解决。Worker是一个实现了Runnable的内部类，它的执行逻辑全部在runWorker内：
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//1、循环获取任务，直到没有任务
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 {
//2、前置处理，默认空实现
beforeExecute(wt, task);
Throwable thrown = null;
try {
//3、直接执行run，这样就可以保证不同的逻辑在同一个线程内执行
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 {
//4、后置处理，默认空实现
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
5、移除Worker
processWorkerExit(w, completedAbruptly);
}
}我们可以猜到，getTask就是从队列内获取任务，当获取不到任务，退出循环后，就移除worker。
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 {
//如果需要判断超时，那么就在keepAliveTime时间后拉取任务，否则就阻塞拉取任务
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}参数timed代表是否需要判断超时，timedOut代表是否超时。如果当前线程数在核心线程数内并且核心线程允许超时，那么timed就是false，这种情况下只有空队列阻塞和获取到任务返回两种情况；而如果是当前线程数超过核心线程，那么也有两种情况，队列有任务直接获取，队列没有任务在keepAliveTime后再获取一次任务，没有任务就结束执行。