RejectedExecutionException产生的原因
引发java.util.concurrent.RejectedExecutionException主要有两种原因:
线程池显示的调用了shutdown()之后,再向线程池提交任务的时候,如果你配置的拒绝策略是ThreadPoolExecutor.AbortPolicy的话,这个异常就被会抛出来。
当你的排队策略为有界队列,并且配置的拒绝策略是ThreadPoolExecutor.AbortPolicy,当线程池的线程数量已经达到了maximumPoolSize并且超过队列数量的时候,你再向它提交任务,就会抛出ThreadPoolExecutor.AbortPolicy异常。
显示关闭掉线程池
这一点很好理解。比如说,你向一个仓库去存放货物,一开始,仓库管理员把门给你打开了,你放了第一件商品到仓库里,但是当你放好出去后,有人把仓库门关了,那你下次再来存放物品时,你就会被拒绝。示例代码如下:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TextExecutor {
public ExecutorService fixedExecutorService = Executors.newFixedThreadPool(5);
public ExecutorService cachedExecutorService = Executors.newCachedThreadPool();
public ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
public void testExecutorException() {
for (int i = 0; i < 10; i ++) {
fixedExecutorService.execute(new SayHelloRunnable());
fixedExecutorService.shutdown();
}
}
private class SayHelloRunnable implements Runnable {
@Override
public void run() {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
System.out.println("hello world!");
}
}
}
public static void main(String[] args) {
TextExecutor testExecutor = new TextExecutor();
testExecutor.testExecutorException();
}
}
解决方案
不要显示的调用shutdown方法,例如Android里,只有你在Destory方法里cancel掉AsyncTask,则线程池里没有活跃线程会自己回收自己。
调用线程池时,判断是否已经shutdown,通过API方法isShutDown方法判断,示例代码:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TextExecutor {
public ExecutorService fixedExecutorService = Executors.newFixedThreadPool(5);
public ExecutorService cachedExecutorService = Executors.newCachedThreadPool();
public ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
public void testExecutorException() {
for (int i = 0; i < 10; i ++) {
// 增加isShutdown()判断
if (!fixedExecutorService.isShutdown()) {
fixedExecutorService.execute(new SayHelloRunnable());
}
fixedExecutorService.shutdown();
}
}
private class SayHelloRunnable implements Runnable {
@Override
public void run() {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
System.out.println("hello world!");
}
}
}
public static void main(String[] args) {
TextExecutor testExecutor = new TextExecutor();
testExecutor.testExecutorException();
}
}
线程数量超过maximumPoolSize
示例代码里使用了自定义的ExecutorService,可以复现这种问题:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class TextExecutor {
public ExecutorService fixedExecutorService = Executors.newFixedThreadPool(5);
public ExecutorService cachedExecutorService = Executors.newCachedThreadPool();
public ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
public ExecutorService customerExecutorService = new ThreadPoolExecutor(3, 5, 0, TimeUnit.MILLISECONDS, new SynchronousQueue<Runnable>());
public void testExecutorException() {
for (int i = 0; i < 10; i ++) {
// 增加isShutdown()判断
if (!fixedExecutorService.isShutdown()) {
fixedExecutorService.execute(new SayHelloRunnable());
}
fixedExecutorService.shutdown();
}
}
public void testCustomerExecutorException() {
for (int i = 0; i < 100; i ++) {
customerExecutorService.execute(new SayHelloRunnable());
}
}
private class SayHelloRunnable implements Runnable {
@Override
public void run() {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
System.out.println("hello world!");
}
}
}
public static void main(String[] args) {
TextExecutor testExecutor = new TextExecutor();
testExecutor.testCustomerExecutorException();;
}
}
解决方案
尽量调大maximumPoolSize,例如设置为Integer.MAX_VALUE
public ExecutorService customerExecutorService = new ThreadPoolExecutor(3, Integer.MAX_VALUE, 0, TimeUnit.MILLISECONDS, new SynchronousQueue<Runnable>());
2.使用其他排队策略,例如LinkedBlockingQueue
public ExecutorService customerExecutorService = new ThreadPoolExecutor(3, 5, 0, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());
总结:
控制提交的任务数量,即提交的任务数量不要超过它当前能处理的能力 (这里可以用生产者消费者的模式来解决)
同一个线程池,确保不要在shutdown()之后在执行任务
可以用LinkedBlockingQueue代替ArrayBlockingQueue,因为LinkedBlockingQueue可以设成无界的,但是需要注意,设成无界后最终可能发生OOM(内存溢出),
所以要保证第一二点。
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