ThreadPoolExecutor 的理解 原
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公司有位大佬写下这么一段话
①,FixedThreadPool任务队列的无边界会导致内存溢出以及高延迟
②,CachedThreadPool线程数的无边界会导致并发高的时候创建的线程数不可控然后我就找了个机会了解下,这是怎么回事
查看ThreadPoolExecutor#execute 方法
简介说,这可能启动新的线程或原有线程开启任务或拒绝任务,代码如下
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//如何工作的线程数小于核心线程数
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))//新起一个线程并添加到工作队列,然后return
return;
c = ctl.get();
}
//workQueue.offer将任务添加到工作队列中
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);
}
else if (!addWorker(command, false))
reject(command);添加到工作队列失败,使用拒绝策略
}
work工作时会运行runWorker方法,如下
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//不断从工作队列中获取task,并执行,这就是所谓的线程池和工作窃取机制
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);
}
}