java多執行緒與並發面試題目（1~3題，附答案）

DelayQueue=BlockingQueue+PriorityQueue+Delayed

public interface Comparable { public int compareTo(T o); } public interface Delayed extends Comparable { long getDelay(TimeUnit unit); } public class DelayQueue implements BlockingQueue { private final PriorityQueue q = new PriorityQueue(); }

public boolean offer(E e) { final ReentrantLock lock = this.lock; lock.lock(); try { E first = q.peek(); q.offer(e); if (first == null || e.compareTo(first) < 0) available.signalAll(); return true; } finally { lock.unlock(); } }

public E take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { for (; ; ) { E first = q.peek(); if (first == null) { available.await(); } else { long delay = first.getDelay(TimeUnit.NANOSECONDS); if (delay > 0) { long tl = available.awaitNanos(delay); } else { E x = q.poll(); assert x != null; if (q.size() != 0) available.signalAll(); //wake up other takers return x; } } } } finally { lock.unlock(); } }

public class Pair { public K first; public V second; public Pair() { } public Pair(K first, V second) { this.first = first; this.second = second; } }

import java.util.concurrent.Delayed; import java.util.concurrent.TimeUnit; import java.util.concurrent.atomic.AtomicLong; public class DelayItem implements Delayed { /** * Base of nanosecond timings， to avoid wrapping */ private static final long NANO_ORIGIN = System.nanoTime(); /** * Returns nanosecond time offset by origin */ final static long now() { return System.nanoTime() - NANO_ORIGIN; } /** * Sequence number to break scheduling ties， and in turn to guarantee FIFO order among tied * entries. */ private static final AtomicLong sequencer = new AtomicLong(0); /** * Sequence number to break ties FIFO */ private final long sequenceNumber; /** * The time the task is enabled to execute in nanoTime units */ private final long time; private final T item; public DelayItem(T submit， long timeout) { this.time = now() + timeout; this.item = submit; this.sequenceNumber = sequencer.getAndIncrement(); } public T getItem() { return this.item; } public long getDelay(TimeUnit unit) { long d = unit.convert(time - now()， TimeUnit.NANOSECONDS); return d; } public int compareTo(Delayed other) { if (other == this) // compare zero ONLY if same object return 0; if (other instanceof DelayItem) { DelayItem x = (DelayItem) other; long diff = time - x.time; if (diff < 0) return -1; else if (diff > 0) return 1; else if (sequenceNumber < x.sequenceNumber) return -1; else return 1; } long d = (getDelay(TimeUnit.NANOSECONDS) - other.getDelay(TimeUnit.NANOSECONDS)); return (d == 0) ?0 :((d < 0) ?-1 :1); } }

import javafx.util.Pair; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; import java.util.concurrent.DelayQueue; import java.util.concurrent.TimeUnit; import java.util.logging.Level; import java.util.logging.Logger; public class Cache { private static final Logger LOG = Logger.getLogger(Cache.class.getName()); private ConcurrentMap cacheObjMap = new ConcurrentHashMap(); private DelayQueue>> q = new DelayQueue>>(); private Thread daemonThread; public Cache() { Runnable daemonTask = new Runnable() { public void run() { daemonCheck(); } }; daemonThread = new Thread(daemonTask); daemonThread.setDaemon(true); daemonThread.setName("Cache Daemon"); daemonThread.start(); } private void daemonCheck() { if (LOG.isLoggable(Level.INFO)) LOG.info("cache service started."); for (; ; ) { try { DelayItem> delayItem = q.take(); if (delayItem != null) { // 超时对象处理 Pair pair = delayItem.getItem(); cacheObjMap.remove(pair.first, pair.second); // compare and remove } } catch (InterruptedException e) { if (LOG.isLoggable(Level.SEVERE)) LOG.log(Level.SEVERE, e.getMessage(), e); break; } } if (LOG.isLoggable(Level.INFO)) LOG.info("cache service stopped."); } // 添加缓存对象 public void put(K key, V value, long time, TimeUnit unit) { V oldValue = cacheObjMap.put(key, value); if (oldValue != null) q.remove(key); long nanoTime = TimeUnit.NANOSECONDS.convert(time, unit); q.put(new DelayItem>(new Pair(key, value), nanoTime)); } public V get(K key) { return cacheObjMap.get(key); } }

// 测试入口函数 public static void main(String[] args) throws Exception { Cache cache = new Cache(); cache.put(1, "aaaa", 3, TimeUnit.SECONDS); Thread.sleep(1000 * 2); { String str = cache.get(1); System.out.println(str); } Thread.sleep(1000 * 2); { String str = cache.get(1); System.out.println(str); } }

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