Mastering Java ThreadPool Rejection Policies: When and How to Use Them

Thread pools provide four standard rejection policies.

AbortPolicy (throws exception, handled by caller) : the default policy that throws a RejectedExecutionException, discarding the task; suitable when immediate awareness of rejection is needed.

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

CallerRunsPolicy (caller runs the task) : does not discard or throw; the calling thread executes the task, slowing submission speed; suitable when tasks must not be lost.

public static class CallerRunsPolicy implements RejectedExecutionHandler {
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            r.run();
        }
    }
}

DiscardOldestPolicy (discard oldest task) : discards the oldest task in the queue and resubmits the new one; useful when only the latest tasks matter, such as real‑time data processing.

public static class DiscardOldestPolicy implements RejectedExecutionHandler {
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            e.getQueue().poll(); // discard oldest
            e.execute(r); // submit new task
        }
    }
}

DiscardPolicy (silently discard) : simply discards the task without throwing an exception; appropriate when loss is acceptable, e.g., logging.

public static class DiscardPolicy implements RejectedExecutionHandler {
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        // do nothing, discard task
    }
}

In addition, you can implement a custom rejection policy by implementing the RejectedExecutionHandler interface.

public class CustomRejectedExecutionHandler implements RejectedExecutionHandler {
    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        // custom handling logic
    }
}
ThreadPoolExecutor executor = new ThreadPoolExecutor(
    2, 4, 60L, TimeUnit.SECONDS,
    new LinkedBlockingQueue<>(2),
    new CustomRejectedExecutionHandler()
);