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1. 策略模式
提供一组算法 / 策略, 调用时遵循 DIP 准则, 调用形象的策略接口, 实现类可自定义, 新增策略, 只须要新增扩大, 调用者能够抉择自在切换
juc 中线程池 TPE(ThreadPoolExecutor) 的回绝策略就是用了策略模式: 拒绝执行处理器
在 TPE(ThreadPoolExecutor 类中提供的 4 个策略实现):
2. 线程池的回绝策略
RejectedExecutionHandler
2.1 AbortPolicy: 抛出异样
AbortPolicy // 抛出异样: RejectedExecutionException (拒绝执行异样)
/** * A handler for rejected tasks that throws a 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()); } }
2.2. CallerRunsPolicy: 调用者代解决
/**
* A handler for rejected tasks that runs the rejected task directly in the calling thread of the 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();
}
}
}
2.3. DiscardPolicy: 默默抛弃
静默抛弃被回绝的工作
/**
* 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) {}}
2.4. DiscardOldestPolicy: 抛弃最早的未解决工作
抛弃最早的未解决申请,而后重试执行. 若 executor 曾经被敞开,在这种状况下,该工作也被抛弃。
/**
* A handler for rejected tasks that discards the oldest unhandled request and then retries 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);
}
}
}
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