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);        }    }}