線程池的拒絕策略

若在線程池當中的核心線程數已被用完且阻塞隊列已排滿，則此時線程池的線程資源已耗盡，線程池沒有足夠的線程資源執行新的任務。

所以為了保證操作系統的安全性，線程池將通過拒絕策略來處理新添加的線程任務。

JDK 中內置的拒絕策略有 AbortPolicy，CallerRunsPolicy、DiscardOldestPolicy、DiscardPolicy 這4種，默認的拒絕策略在 ThreadPoolExecutor 中作為內部類來進行提供的，在默認的拒絕策略都不能滿足應用的需求時，也可以自定義拒絕策略。

AbortPolicy拒絕策略：

該策略會直接拋出異常，阻止系統正常工作。

jdk源碼：

    /*** A handler for rejected tasks that throws a* {@code RejectedExecutionException}.*/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());}}

CallerRunsPolicy拒絕策略：

如果線程池的線程數量達到上限，該策略會把任務隊列中的任務放在調用者線程（如main函數）當中運行。

jdk源碼：

    /*** A handler for rejected tasks that runs the rejected task* directly in the calling thread of the {@code execute} method,* unless the executor has been shut down, in which case the task* is discarded.*/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();}}}

DiscardOldestPolicy拒絕策略：

該策略將移除最早的一個請求，也就是即將被執 行的任務，然后并嘗試再次提交當前的任務。

jdk源碼：

    /*** A handler for rejected tasks that discards the oldest unhandled* request and then retries {@code execute}, unless the executor* is shut down, in which case the task is discarded.*/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);}}}

DiscardPolicy拒絕策略：

丟棄當前線程任務而不做任何處理。如果系統允許在資源不足的情況下丟棄部分任務，則這將是保障系統安全，穩定的一種很好的方案。

jdk源碼：

    /*** A handler for rejected tasks that silently discards the* rejected task.*/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) {}}

以上4種拒絕策略均是實現的 RejectedExecutionHandler 接口，來實現拒絕策略，若無法滿足實際需要，則用戶就可以自己自定義來實現拒絕策略。