今天对五种常见的java内置线程池进行讲解。
public static void cache() {
ExecutorService pool = Executors.newCachedThreadPool();
long start = System.currentTimeMillis();
pool.execute(() -> {
int sum = 0;
for (int i = 0; i < 10; i++) {
sum = (int) Math.sqrt(i * i - 1 + i);
System.out.println(sum);
}
});
System.out.println("cache: " + (System.currentTimeMillis() - start));
}
源码:
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
通过源码可以看出底层调用的是ThreadPoolExecutor方法,传入一个同步的阻塞队列实现缓存。
下面说一下ThreadPoolExecutor
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
通过源码可以看出,我们可以传入线程池的核心线程数(最小线程数),最大线程数量,保持时间,时间单位,阻塞队列这些参数,最大线程数设置为jvm可用的cpu数量为最佳实践
源码:
public static ExecutorService newWorkStealingPool() {
return new ForkJoinPool
(Runtime.getRuntime().availableProcessors(),
ForkJoinPool.defaultForkJoinWorkerThreadFactory,
null, true);
}
通过源码可以看出底层调用的是ForkJoinPool线程池
下面说一下ForkJoinPool
public ForkJoinPool(int parallelism,
ForkJoinWorkerThreadFactory factory,
UncaughtExceptionHandler handler,
boolean asyncMode) {
this(checkParallelism(parallelism),
checkFactory(factory),
handler,
asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
"ForkJoinPool-" + nextPoolId() + "-worker-");
checkPermission();
}
使用一个无限队列来保存需要执行的任务,可以传入线程的数量,不传入,则默认使用当前计算机中可用的cpu数量,使用分治法来解决问题,使用fork()和join()来进行调用
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
源码:
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
源码:
return new ScheduledThreadPoolExecutor(corePoolSize);
通过源码可以看出底层调用的是一个ScheduledThreadPoolExecutor,然后传入线程数量
下面来介绍一下ScheduledThreadPoolExecutor
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}
通过源码可以看出底层调用了ThreadPoolExecutor,维护了一个延迟队列,可以传入线程数量,传入延时的时间等参数,下面给出一个demo
public static void main(String[] args) {
ScheduledExecutorService pool = Executors.newScheduledThreadPool(5);
for (int i = 0; i < 15; i = i + 5) {
pool.schedule(() -> System.out.println("我被执行了,当前时间" + new Date()), i, TimeUnit.SECONDS);
}
pool.shutdown();
}
执行结果
我被执行了,当前时间Fri Jan 12 11:20:41 CST 2018
我被执行了,当前时间Fri Jan 12 11:20:46 CST 2018
我被执行了,当前时间Fri Jan 12 11:20:51 CST 2018
有的小伙伴可能会用疑问,为什么使用schedule()而不使用submit()或者execute()呢,下面通过源码来分析
public void execute(Runnable command) {
schedule(command, 0, NANOSECONDS);
}
public Future<?> submit(Runnable task) {
return schedule(task, 0, NANOSECONDS);
}
通过源码可以发现这两个方法都是调用的schedule(),而且将延时时间设置为了0,所以想要实现延时操作,需要直接调用schedule()
下面我们再来分析一下submit()和execute()的以及shutdown()和shutdownNow()的区别
以上就是今天所想要分享的内容,由于并发接触并不深入,如有错误,请联系博主
浩瀚星辰中渺小的我们,却有着改变世界的梦想
作者:Stalary 链接:https://www.jianshu.com/p/135c89001b61