ThreadPoolExecutor以内部线程池的形式对外提供管理任务执行,线程调度,线程池管理等等服务。
ThreadPoolExecutor构造方法解析
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {}
参数名 | 作用 |
---|---|
corePoolSize | 核心线程池大小 |
maximumPoolSize | 最大线程池大小 |
keepAliveTime | 线程池中超过corePoolSize数目的空闲线程最大存活时间 |
TimeUnit | keepAliveTime时间单位 |
workQueue | 阻塞任务队列 |
threadFactory | 新建线程工厂 |
RejectedExecutionHandler | 当提交任务数超过maximumPoolSize+workQueue之和时,任务会交给RejectedExecutionHandler来处理 |
重点讲解:
- 当线程池小于corePoolSize时,新提交任务将创建一个新线程执行任务,即使此时线程池中存在空闲线程。
- 当线程池达到corePoolSize时,新提交任务将被放入workQueue中,等待线程池中任务调度执行
- 当workQueue已满,且maximumPoolSize>corePoolSize时,新提交任务会创建新线程执行任务
- 当提交任务数超过maximumPoolSize时,新提交任务由RejectedExecutionHandler处理
- 当线程池中超过corePoolSize线程,空闲时间达到keepAliveTime时,关闭空闲线程
- 当设置allowCoreThreadTimeOut(true)时,线程池中corePoolSize线程空闲时间达到keepAliveTime也将关闭
Executors提供静态方法方便我们构造出线程池,不建议使用Executors,而直接使用ThreadPoolExecutor,更能加深这些参数是什么意思,也能更好的构造出我们想要的线程池。
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
构造一个固定线程数目的线程池,配置的corePoolSize与maximumPoolSize大小相同,同时使用了一个无界LinkedBlockingQueue存放阻塞任务,因此多余的任务将存在再阻塞队列,不会由RejectedExecutionHandler处理
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
构造一个缓冲功能的线程池,配置corePoolSize=0,maximumPoolSize=Integer.MAX_VALUE,keepAliveTime=60s,以及一个无容量的阻塞队列 SynchronousQueue,因此任务提交之后,将会创建新的线程执行;线程空闲超过60s将会销毁
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
构造一个只支持一个线程的线程池,配置corePoolSize=maximumPoolSize=1,无界阻塞队列LinkedBlockingQueue;保证任务由一个线程串行执行
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}
public ScheduledThreadPoolExecutor(int corePoolSize,
ThreadFactory threadFactory) {
super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
new DelayedWorkQueue(), threadFactory);
}
构造有定时功能的线程池,配置corePoolSize,无界延迟阻塞队列DelayedWorkQueue;有意思的是:maximumPoolSize=Integer.MAX_VALUE,由于DelayedWorkQueue是无界队列,所以这个值是没有意义的。
说ScheduledThreadPoolExecutor之前,先来看看Timer。
package com.koma.demo;
import java.util.Timer;
import java.util.TimerTask;
/**
* @author koma
* @version 2017年12月8日 下午1:55:06
*/
public class OutOfTime {
public static void main(String[] args) throws InterruptedException {
Timer timer=new Timer();
timer.schedule(new ThrowTask(), 1);
Thread.sleep(1000L);
timer.schedule(new ThrowTask(), 1);
Thread.sleep(5000L);
}
}
class ThrowTask extends TimerTask{
@Override
public void run() {
throw new RuntimeException();
}
}
结果:
Exception in thread "Timer-0" java.lang.RuntimeException
at com.koma.demo.ThrowTask.run(OutOfTime.java:24)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)
Exception in thread "main" java.lang.IllegalStateException: Timer already cancelled.
at java.util.Timer.sched(Unknown Source)
at java.util.Timer.schedule(Unknown Source)
at com.koma.demo.OutOfTime.main(OutOfTime.java:15)
你可能认为程序运行6秒后退出,实际上并不是,并且抛出异常消息:Timer already cancelled。
package com.koma.demo;
import java.util.Timer;
import java.util.TimerTask;
/**
* @author koma
* @version 2017年12月8日 下午2:15:23
*/
public class TimerTest {
public static void main(String[] args) throws Exception {
TimerTask task1 = new TimerTask() {
@Override
public void run() {
System.out.println("task1: " + System.currentTimeMillis());
}
};
TimerTask task2 = new TimerTask() {
@Override
public void run() {
System.out.println("task2: " + System.currentTimeMillis());
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
Timer timer = new Timer();
// 一秒钟执行一次
timer.schedule(task1, 1, 1000);
timer.schedule(task2, 1);
}
}
结果:
task1: 1512714371122
task2: 1512714371122
task1: 1512714375122
task1: 1512714376122
task1: 1512714377122
task1: 1512714378122
从结果可看,task1本意是一秒钟一次,但是第二次在四秒后才运行,丢失了三次。
Timer缺点:
- TimerTask抛出了一个未检查的异常,Timer线程并不会捕获异常,并且终止了定时线程。这种情况下,Timer也不会恢复线程的执行,而是错误的认为整个Timer都被取消了。
- 某个周期TimerTask需要没1000ms执行一次,而另一个Timertask需要执行4000ms,那么周期任务或者在40ms任务执行完成后快速连续的调用3次,或者彻底丢失4次调用,取决于它是基于固定速率还是固定延时来调度。
ScheduledThreadPoolExecutor调度:
package com.koma.demo;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
/**
* @author koma
* @version 2017年12月8日 下午2:11:39
*/
public class ScheduledThreadPoolExecutorTest {
public static void main(String[] args) {
ScheduledExecutorService service = Executors.newScheduledThreadPool(2);
service.scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
System.out.println("task1: " + System.currentTimeMillis());
}
}, 1, 1000, TimeUnit.MILLISECONDS);
service.schedule(new Runnable() {
@Override
public void run() {
System.out.println("task2: " + System.currentTimeMillis());
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}, 1, TimeUnit.MILLISECONDS);
}
}
结果:
task1: 1512715487934
task2: 1512715487934
task1: 1512715488934
task1: 1512715489934
task1: 1512715490934
task1: 1512715491934
由此可见task1就是1秒执行一次。
package com.koma.demo;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
/**
* @author koma
* @version 2017年12月8日 下午2:11:39
*/
public class ScheduledThreadPoolExecutorTest {
public static void main(String[] args) throws InterruptedException {
ScheduledExecutorService service = Executors.newScheduledThreadPool(2);
ScheduledThreadDemo t=new ScheduledThreadDemo();
Thread.sleep(1000L);
System.out.println(System.currentTimeMillis());
service.schedule(t, 1, TimeUnit.MILLISECONDS);
Thread.sleep(5000L);
service.schedule(t, 1, TimeUnit.MILLISECONDS);
System.out.println(System.currentTimeMillis());
service.shutdown();
}
}
class ScheduledThreadDemo implements Runnable{
@Override
public void run() {
throw new RuntimeException();
}
}
结果:
1512715983935
1512715988938
并没有抛出异常。