Java中有界隊列的飽和策略(reject policy)原理解析
我們在使用ExecutorService的時候知道,在ExecutorService中有個一個Queue來保存提交的任務,通過不同的構造函數,我們可以創建無界的隊列(ExecutorService.newCachedThreadPool)和有界的隊列(ExecutorService newFixedThreadPool(int nThreads))。
無界隊列很好理解,我們可以無限制的向ExecutorService提交任務。那么對于有界隊列來說,如果隊列滿了該怎么處理呢?
今天我們要介紹一下java中ExecutorService的飽和策略(reject policy)。
以ExecutorService的具體實現ThreadPoolExecutor來說,它定義了4種飽和策略。分別是AbortPolicy,DiscardPolicy,DiscardOldestPolicy和CallerRunsPolicy。
如果要在ThreadPoolExecutor中設定飽和策略可以調用setRejectedExecutionHandler方法,如下所示:
ThreadPoolExecutor threadPoolExecutor= new ThreadPoolExecutor(5, 10, 10, TimeUnit.SECONDS, new LinkedBlockingDeque<Runnable>(20)); threadPoolExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy() );
上面的例子中我們定義了一個初始5個,最大10個工作線程的Thread Pool,并且定義其中的Queue的容量是20。如果提交的任務超出了容量,則會使用AbortPolicy策略。
AbortPolicy
AbortPolicy意思是如果隊列滿了,最新的提交任務將會被拒絕,并拋出RejectedExecutionException異常:
public static class AbortPolicy implements RejectedExecutionHandler { /** * 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 */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { throw new RejectedExecutionException('Task ' + r.toString() + ' rejected from ' + e.toString()); } }
上面的代碼中,rejectedExecution方法中我們直接拋出了RejectedExecutionException異常。
DiscardPolicy
DiscardPolicy將會悄悄的丟棄提交的任務,而不報任何異常。
public static class DiscardPolicy implements RejectedExecutionHandler { /** * Creates a {@code DiscardPolicy}. */ public DiscardPolicy() { } /** * Does nothing, which has the effect of discarding task r. * * @param r the runnable task requested to be executed * @param e the executor attempting to execute this task */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { } }
DiscardOldestPolicy
DiscardOldestPolicy將會丟棄最老的任務,保存最新插入的任務。
public static class DiscardOldestPolicy implements RejectedExecutionHandler { /** * 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 */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) {e.getQueue().poll();e.execute(r); } } }
我們看到在rejectedExecution方法中,poll了最老的一個任務,然后使用ThreadPoolExecutor提交了一個最新的任務。
CallerRunsPolicy
CallerRunsPolicy和其他的幾個策略不同,它既不會拋棄任務,也不會拋出異常,而是將任務回退給調用者,使用調用者的線程來執行任務,從而降低調用者的調用速度。我們看下是怎么實現的:
public static class CallerRunsPolicy implements RejectedExecutionHandler { /** * Creates a {@code CallerRunsPolicy}. */ public CallerRunsPolicy() { } /** * Executes task r in the caller’s 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 */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) {r.run(); } } }
在rejectedExecution方法中,直接調用了 r.run()方法,這會導致該方法直接在調用者的主線程中執行,而不是在線程池中執行。從而導致主線程在該任務執行結束之前不能提交任何任務。從而有效的阻止了任務的提交。
使用Semaphore
如果我們并沒有定義飽和策略,那么有沒有什么方法來控制任務的提交速度呢?考慮下之前我們講到的Semaphore,我們可以指定一定的資源信號量來控制任務的提交,如下所示:
public class SemaphoreUsage { private final Executor executor; private final Semaphore semaphore; public SemaphoreUsage(Executor executor, int count) { this.executor = executor; this.semaphore = new Semaphore(count); } public void submitTask(final Runnable command) throws InterruptedException { semaphore.acquire(); try { executor.execute(() -> { try { command.run(); } finally { semaphore.release(); } } ); } catch (RejectedExecutionException e) { semaphore.release(); } }}
本文的例子可參考https://github.com/ddean2009/learn-java-concurrency/tree/master/rejectPolicy
以上就是本文的全部內容,希望對大家的學習有所幫助,也希望大家多多支持好吧啦網。
相關文章:
1. 使用.net core 自帶DI框架實現延遲加載功能2. php網絡安全中命令執行漏洞的產生及本質探究3. Angular獲取ngIf渲染的Dom元素示例4. php面向對象程序設計介紹5. ASP調用WebService轉化成JSON數據,附json.min.asp6. 無線標記語言(WML)基礎之WMLScript 基礎第1/2頁7. 三個不常見的 HTML5 實用新特性簡介8. php測試程序運行速度和頁面執行速度的代碼9. Warning: require(): open_basedir restriction in effect,目錄配置open_basedir報錯問題分析10. ASP.NET Core 5.0中的Host.CreateDefaultBuilder執行過程解析
