用了mip的网站,爱客crm登陆,广州番禺怎么样,数字营销技术应用网站四种线程池拒绝策略#xff08;handler#xff09; 当线程池的线程数达到最大线程数时#xff0c;需要执行拒绝策略。拒绝策略需要实现 RejectedExecutionHandler 接口#xff0c;并实现 rejectedExecution(Runnable r, ThreadPoolExecutor executor) 方法。不过… 四种线程池拒绝策略handler 当线程池的线程数达到最大线程数时需要执行拒绝策略。拒绝策略需要实现 RejectedExecutionHandler 接口并实现 rejectedExecution(Runnable r, ThreadPoolExecutor executor) 方法。不过 Executors 框架已经为我们实现了 4 种拒绝策略
AbortPolicy默认丢弃任务并抛出 RejectedExecutionException 异常。 CallerRunsPolicy由调用线程处理该任务。 DiscardPolicy丢弃任务但是不抛出异常。可以配合这种模式进行自定义的处理方式。 DiscardOldestPolicy丢弃队列最早的未处理任务然后重新尝试执行任务。 线程池默认的拒绝策略 查看java.util.concurrent.ThreadPoolExecutor类的源码我们可以看到
/*** The default rejected execution handler*/
private static final RejectedExecutionHandler defaultHandler new AbortPolicy();
线程池的默认拒绝策略为AbortPolicy即丢弃任务并抛出RejectedExecutionException异常。我们可以通过代码来验证这一点现有如下代码
public class ThreadPoolTest {public static void main(String[] args) {BlockingQueueRunnable queue new ArrayBlockingQueue(100);ThreadFactory factory r - new Thread(r, TestThreadPool);ThreadPoolExecutor executor new ThreadPoolExecutor(5, 5,0L, TimeUnit.SECONDS, queue, factory);while (true) {executor.submit(() - {try {System.out.println(queue.size());Thread.sleep(10000);} catch (InterruptedException e) {e.printStackTrace();}});}}}
这里是一个默认的线程池没有设置拒绝策略设置了最大线程队列是100。运行代码结果如下 结果是符合预期的这也证明了线程池的默认拒绝策略是ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常。
拒绝策略场景分析 1.AbortPolicy ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常。源码解释如下: /*** Creates an {code AbortPolicy}.*/public AbortPolicy() { }/*** Always throws RejectedExecutionException.** param r the runnable task requested to be executed* param e the executor attempting to execute this task* throws RejectedExecutionException always*/
这是线程池默认的拒绝策略在任务不能再提交的时候抛出异常及时反馈程序运行状态。如果是比较关键的业务推荐使用此拒绝策略这样子在系统不能承载更大的并发量的时候能够及时的通过异常发现。 2.DiscardPolicy ThreadPoolExecutor.DiscardPolicy丢弃任务但是不抛出异常。如果线程队列已满则后续提交的任务都会被丢弃且是静默丢弃。源码解释如下: /*** Creates a {code DiscardOldestPolicy} for the given executor.*/public DiscardOldestPolicy() { }/*** Obtains and ignores the next task that the executor* would otherwise execute, if one is immediately available,* and then retries execution of task r, unless the executor* is shut down, in which case task r is instead discarded.** param r the runnable task requested to be executed* param e the executor attempting to execute this task*/
使用此策略可能会使我们无法发现系统的异常状态。建议是一些无关紧要的业务采用此策略。例如某些视频网站统计视频的播放量就是采用的这种拒绝策略。
3.DiscardOldestPolicy ThreadPoolExecutor.DiscardOldestPolicy丢弃队列最前面的任务然后重新提交被拒绝的任务。源码解释如下: /*** Creates a {code DiscardOldestPolicy} for the given executor.*/public DiscardOldestPolicy() { }/*** Obtains and ignores the next task that the executor* would otherwise execute, if one is immediately available,* and then retries execution of task r, unless the executor* is shut down, in which case task r is instead discarded.** param r the runnable task requested to be executed* param e the executor attempting to execute this task*/
此拒绝策略是一种喜新厌旧的拒绝策略。是否要采用此种拒绝策略还得根据实际业务是否允许丢弃老任务来认真衡量。
4.CallerRunsPolicy ThreadPoolExecutor.CallerRunsPolicy由调用线程处理该任务 源码解释如下: /*** Creates a {code CallerRunsPolicy}.*/public CallerRunsPolicy() { }/*** Executes task r in the callers thread, unless the executor* has been shut down, in which case the task is discarded.** param r the runnable task requested to be executed* param e the executor attempting to execute this task*/
如果任务被拒绝了则由调用线程提交任务的线程直接执行此任务我们可以通过代码来验证这一点
public static void main(String[] args) {BlockingQueueRunnable queue new ArrayBlockingQueue(10);ThreadFactory factory r - new Thread(r, TestThreadPool);ThreadPoolExecutor executor new ThreadPoolExecutor(5, 5,0L, TimeUnit.SECONDS, queue, factory, new ThreadPoolExecutor.CallerRunsPolicy());for (int i 0; i 1000; i) {executor.submit(() - {try {System.out.println(Thread.currentThread().getName() :执行任务);Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}});}
}
把队列最大值改为10打印输出线程的名称。执行结果如下 通过结果可以看到主线程main也执行了任务这正说明了此拒绝策略由调用线程提交任务的线程直接执行被丢弃的任务的。
知识来源
【23版面试突击】你知道线程池有哪几种拒绝策略吗_哔哩哔哩_bilibili
线程池的拒绝策略_线程池拒绝策略_小赵在练琴的博客-CSDN博客
