java 线程池keepAliveTime的含义说明

电玩女神 2022-10-30 07:23 123阅读 0赞

之前对线程池中属性：keepAliveTime比较模糊，而且看过之后过一段时间就会忘掉，于是就在此记录一下。

keepAliveTime的jdk中的解释为：  
当线程数大于核心时，此为终止前多余的空闲线程等待新任务的最长时间。

说的让人感觉比较模糊，总结一下大概意思为：比如说线程池中最大的线程数为50，而其中只有40个线程任务在跑，相当于有10个空闲线程，这10个空闲线程不能让他一直在开着，因为线程的存在也会特别好资源的，所有就需要设置一个这个空闲线程的存活时间，这么解释应该就很清楚了。

这样以后忘记了就过来看看就OK了。

补充：线程池的状态及KeepAliveTime参数

五个状态  
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// runState is stored in the high-order bits  
private static final int RUNNING = -1 << COUNT\_BITS;  
private static final int SHUTDOWN = 0 << COUNT\_BITS;  
private static final int STOP = 1 << COUNT\_BITS;  
private static final int TIDYING = 2 << COUNT\_BITS;  
private static final int TERMINATED = 3 << COUNT\_BITS;  
循环getTask方法  
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/\*\*  
\* Performs blocking or timed wait for a task, depending on  
\* current configuration settings, or returns null if this worker  
\* must exit because of any of:  
\* 1. There are more than maximumPoolSize workers (due to  
\* a call to setMaximumPoolSize).  
\* 2. The pool is stopped.  
\* 3. The pool is shutdown and the queue is empty.  
\* 4. This worker timed out waiting for a task, and timed-out  
\* workers are subject to termination (that is,  
\* \{@code allowCoreThreadTimeOut || workerCount > corePoolSize\})  
\* both before and after the timed wait.  
\*  
\* @return task, or null if the worker must exit, in which case  
\* workerCount is decremented  
\*/  
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;  
//默认allowCoreThreadTimeOut为false，除非程序指定  
//（1）当没有超过核心线程时，默认allowCoreThreadTimeOut为false时  
//timed值为false，始终break掉，不会销毁线程  
//（2）当超过核心线程数，默认allowCoreThreadTimeOut为false时  
//timed值为true，如果超过最大值，则销毁；如果timeout过，则销毁  
// 如果allowCoreThreadTimeOut为true，则timed始终为true  
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  
\}  
try \{  
Runnable r = timed ?  
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :  
workQueue.take();  
if (r != null)  
return r;  
timedOut = true;  
\} catch (InterruptedException retry) \{  
timedOut = false;  
\}  
\}  
\}  
线程池状态大于SHUTDOWN值的两种情况  
1、调用shutdown方法  
当线程池调用了shutdown方法，线程池的状态会首先被设置为SHUTDOWN，然后遍历线程池中所有线程，调用一次interrupt方法，如果在休眠中的线程将会激活，激活后的线程以及调用shutdown方法本身的线程都会尝试去调用tryTerminate方法，该方法将判定如果线程池中所有记录的线程数为0，则将线程状态改为TERMINATED，这个值为3，将大于SHUTDOWN状态值。

2、调用shutdownNow方法  
当线程调用了shutdownNow方法后，首先将线程的状态修改为STOP，这个状态是大于SHUTDOWN值的，接下来它也会通过中断激活线程，只是它来的更暴力一些，连加锁和一些基本判断都没有，直接中断；在调用tryTerminate之前会先清空阻塞队列中所有的元素，这些元素被组装为一个List列表作为shutdownNow方法的返回值。换句话说，没有执行的任务在shutdownNow执行后的返回值中可以得到。在程序某些必要的情况下，可以通过线程池的isTerminating，isTerminated，isStopped，isShutdown来对线程做一些状态判定。

KeepAliveTime参数

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workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS)  
当阻塞队列中没有任务时，等待时间达到keepAliveTime毫秒值时就会被自动唤醒，而不会永远地沉睡下去。

keepAliveTime，如果是通过newCachedThreadPool的话，默认是1分钟超时，如果遇到前面所提到的瞬间冲击，那么线程池数量将瞬间快速膨胀，而且这些瞬间膨胀的线程的生命周期最少在1分钟以上。

如果设置了该参数，那么当timeout的时候，就return null，就会跳出循环，回收线程。

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if (wc <= maximumPoolSize && ! (timedOut && timed))  
break;  
if (compareAndDecrementWorkerCount(c))  
return null;  
allowCoreThreadTimeout : 默认情况下核心线程不会退出，可通过将该参数设置为true，让核心线程也退出。

默认的Executors工厂，只有newCachedThreadPool，timeout为60秒，出现timeout情况下，而且线程数超过了核心线程数，会销毁销毁线程。保持在corePoolSize数（如果是cached的，corePoolSize为0）。

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/\*\*  
\* Timeout in nanoseconds for idle threads waiting for work.  
\* Threads use this timeout when there are more than corePoolSize  
\* present or if allowCoreThreadTimeOut. Otherwise they wait  
\* forever for new work.  
\*/  
private volatile long keepAliveTime;  
/\*\*  
\* If false (default), core threads stay alive even when idle.  
\* If true, core threads use keepAliveTime to time out waiting  
\* for work.  
\*/  
private volatile boolean allowCoreThreadTimeOut;  
线程池最小是corePoolSize，最大是maximumPoolSize，除非设置了allowCoreThreadTimeOut和超时时间，这种情况线程数可能减少到0，最大可能是Integer.MAX\_VALUE。

Core pool size is the minimum number of workers to keep alive(and not allow to time out etc) unless allowCoreThreadTimeOut is set, in which case the minimum is zero.

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/\*\*  
\* Creates a thread pool that creates new threads as needed, but  
\* will reuse previously constructed threads when they are  
\* available. These pools will typically improve the performance  
\* of programs that execute many short-lived asynchronous tasks.  
\* Calls to <tt>execute</tt> will reuse previously constructed  
\* threads if available. If no existing thread is available, a new  
\* thread will be created and added to the pool. Threads that have  
\* not been used for sixty seconds are terminated and removed from  
\* the cache. Thus, a pool that remains idle for long enough will  
\* not consume any resources. Note that pools with similar  
\* properties but different details (for example, timeout parameters)  
\* may be created using \{@link ThreadPoolExecutor\} constructors.  
\*  
\* @return the newly created thread pool  
\*/  
public static ExecutorService newCachedThreadPool() \{  
return new ThreadPoolExecutor(0, Integer.MAX\_VALUE,  
60L, TimeUnit.SECONDS,  
new SynchronousQueue<Runnable>());  
\}  
/\*\*  
\* Creates a thread pool that creates new threads as needed, but  
\* will reuse previously constructed threads when they are  
\* available, and uses the provided  
\* ThreadFactory to create new threads when needed.  
\* @param threadFactory the factory to use when creating new threads  
\* @return the newly created thread pool  
\* @throws NullPointerException if threadFactory is null  
\*/  
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) \{  
return new ThreadPoolExecutor(0, Integer.MAX\_VALUE,  
60L, TimeUnit.SECONDS,  
new SynchronousQueue<Runnable>(),  
threadFactory);  
\}  
超时timeout设置为0的话，表示不等待

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public E poll(long timeout, TimeUnit unit) throws InterruptedException \{  
return pollFirst(timeout, unit);  
\}  
具体如下  
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public E pollFirst(long timeout, TimeUnit unit)  
throws InterruptedException \{  
long nanos = unit.toNanos(timeout);  
final ReentrantLock lock = this.lock;  
lock.lockInterruptibly();  
try \{  
E x;  
while ( (x = unlinkFirst()) == null) \{  
if (nanos <= 0)  
return null;  
nanos = notEmpty.awaitNanos(nanos);  
\}  
return x;  
\} finally \{  
lock.unlock();  
\}  
\}  
以上为个人经验，希望能给大家一个参考