// 创建一个线程
var es = Executors.newSingleThreadExecutor();
// 创建固定长度的线程池,比如4个
var expool = Executors.newFixedThreadPool(4);
// 创建弹性可伸缩的线程池
Executors.newCachedThreadPool();
// 关闭线程池
es.shutdown();
package com.starry.service.starrythreads;
import java.util.concurrent.Executors;
public class ThreadPoolDemo {
public static void main(String[] args) {
// 创建一个线程
var es = Executors.newSingleThreadExecutor();
// 创建固定长度的线程池,比如4个
var expool = Executors.newFixedThreadPool(4);
// 创建弹性可伸缩的线程池
Executors.newCachedThreadPool();
try {
for (int i = 0; i < 10; i++) {
final int temp = i;
es.execute(() -> {
System.out.printf(Thread.currentThread().getName() + String.valueOf(temp)); // 永远是同一个线程执行
});
}
for (int i = 0; i < 10; i++) {
final int temp = i;
expool.execute(() -> {
System.out.printf(Thread.currentThread().getName() + String.valueOf(temp)); // 最多4个线程协作执行
});
}
} finally {
// 关闭线程池
es.shutdown();
// 关闭线程池
expool.shutdown();
}
}
}
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
ThreadPoolExecutor将根据corePoolSize和maximumPoolSize设置的值调整线程池大小。当新任务调用方法execute(Runnable)提交时,如果运行的线程少于corePoolSize,则创建新线程来处理请求。如果正在运行的线程数等于corePoolSize时,则新任务被添加到队列中,直到队列满。当队列满了后,会继续开辟新线程来处理任务,但不超过最大线程数。当任务队列满了并且已开辟了最大线程数,此时又来了新任务,ThreadPoolExecutor会拒绝服务。
阿里巴巴规范只能用ThreadPoolExecutor类,==不可以使用Executors创建== 为啥不允许呢? public LinkedBlockingQueue() { this(Integer.MAX_VALUE); }
结论:
1)FixedThreadPool 和 SingleThreadPool:
允许的请求队列长度为 Integer.MAX_VALUE,可能会堆积大量的请求,从而导致 OOM。
2)CachedThreadPool 和 ScheduledThreadPool:
允许的创建线程数量为 Integer.MAX_VALUE,可能会创建大量的线程,从而导致 OOM。
==先看源码:ThreadPoolExecutor类==
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
当线程空闲超过keepAliveTime,非核心线程会被回收,若allowCoreThreadTimeOut为true则核心线程也会被回收。 ==默认情况下,核心线程不会被回收;当allowCoreThreadTimeOut为true,核心线程也会被回收。==
线程池共包括4种拒绝策略,它们分别是:AbortPolicy, CallerRunsPolicy, DiscardOldestPolicy和DiscardPolicy。
AbortPolicy(jdk默认策略)
AbortPolicy -- 当任务添加到线程池中被拒绝时,它将抛出 RejectedExecutionException 异常。(jdk默认策略)
CallerRunsPolicy -- 当任务添加到线程池中被拒绝时,会在线程池当前正在运行的Thread线程池中处理被拒绝的任务。
DiscardOldestPolicy -- 当任务添加到线程池中被拒绝时,线程池会放弃等待队列中最旧的未处理任务,然后将被拒绝的任务添加到等待队列中。
DiscardPolicy -- 当任务添加到线程池中被拒绝时,线程池将丢弃被拒绝的任务。
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本文链接:https://cloud.tencent.com/developer/article/2019224
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