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
- 如何辨别外围线程和非核心线程
- 存活工夫达到时如何敞开线程
线程池的提交
线程池的提交有3种形式：
<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);能够看到别离是2个Runnable和1个Callable的提交，那为什么有两个Runnable的提交呢。通常咱们都是认为Runnable和Callable的区别是Callable是能够返回一个后果。其实Runnable也是能够返回后果的，依据参数T result能够看出T result就是返回后果，如果不传，那么默认返回NULL。
接下来进入submit办法
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
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;
}
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}这三个办法是先调用newTaskFor失去一个RunnableFuture，再提交执行。
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);
}newTaskFor内结构的FutureTask就是咱们用来获取执行状态和后果的适配类了。在FutureTask类内，则是对立适配为Callable，只不过Runnable返回NULL或传入的固定值。
咱们再看看ThreadPoolExecutor的执行过程：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
//1、判断是否在外围线程内，如果是则尝试创立外围线程
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//2、超出外围线程数则尝试间接退出队列
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、退出队列失败则尝试创立非核心线程，留神第二个参数，在增加工作前要做检测
//，ture示意检测小于外围线程数，false示意检测小于最大线程数
else if (!addWorker(command, false))
//4、执行回绝策略
reject(command);
}很显著，在提交工作的过程中，最重要的就是addWorker这个办法。
private boolean addWorker(Runnable firstTask, boolean core) {
//1、做各种检测，并且尝试CAS线程数，胜利则持续，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来治理工作的执行，并将Worker增加到workers中治理
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，间接运行start执行
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}后面说到，参数boolean core管制增加外围/非核心线程，然而在该办法中只有判断时用到了core，所以如何辨别外围/非核心线程呢？答案是不辨别，这两兄弟在线程池看起来没有区别，只须要思考线程数和外围数，最大数就能够。
间接运行start，那么队列里的工作怎么办呢？这在Worker外部解决。Worker是一个实现了Runnable的外部类，它的执行逻辑全副在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、循环获取工作，直到没有工作
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、前置解决，默认空实现
beforeExecute(wt, task);
Throwable thrown = null;
try {
//3、间接执行run，这样就能够保障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、后置解决，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);
}
}咱们能够猜到，getTask就是从队列内获取工作，当获取不到工作，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 {
//如果须要判断超时，那么就在keepAliveTime工夫后拉取工作，否则就阻塞拉取工作
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代表是否须要判断超时，timedOut代表是否超时。如果以后线程数在外围线程数内并且外围线程容许超时，那么timed就是false，这种状况下只有空队列阻塞和获取到工作返回两种状况；而如果是以后线程数超过外围线程，那么也有两种状况，队列有工作间接获取，队列没有工作在keepAliveTime后再获取一次工作，没有工作就完结执行。
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