Java 線程池的拒絕策略 RejectedExecutorHandler

當線程池被關閉以後，在execute()方法中提交新任務將被拒絕（線程池被關閉或線程數達到maximumPoolSize），此時會採取定義的拒絕策略。在ThreadPoolExecutor中定義了四個靜態內部類，均實現了RejectedExecutorService接口，並對接口中的rejectedExecution方法給出具體實現。

一、CallerRunsPolicy

如果線程池沒有被關閉，則由提交該任務的線程來執行

public static class CallerRunsPolicy implements RejectedExecutionHandler {
	public CallerRunsPolicy() { }

	public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
		if (!e.isShutdown()) {
			r.run();
		}
	}
}

二、AbortPolicy

默認的拒絕策略，直接拋出RejectedExecutionException異常。

public static class AbortPolicy implements RejectedExecutionHandler {
	public AbortPolicy() { }

	public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
		throw new RejectedExecutionException("Task " + r.toString() +
											 " rejected from " +
											 e.toString());
	}
}

三、DiscardPolicy

什麼都不做，直接丟棄被拒絕的任務

public static class DiscardPolicy implements RejectedExecutionHandler {
	public DiscardPolicy() { }

	public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
	}
}

四、DiscardOldestPolicy

如果線程池沒有被關閉，則丟棄隊列首部的任務，提交這個新的任務

public static class DiscardOldestPolicy implements RejectedExecutionHandler {
	public DiscardOldestPolicy() { }

	public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
		if (!e.isShutdown()) {
			e.getQueue().poll();
			e.execute(r);
		}
	}
}