我寻找一个队列，该队列最多可存储 N 元素一段时间（即 10 秒），或者如果已满则应处置最旧的值。
我在 Apache Collections (CircularFifoQueueJavaDoc) 中发现了一个类似的队列，它忽略了生存时间。对于我的小项目来说，一个完整的消息代理似乎是一种开销。
你介意给我一个提示我应该如何实现吗？轮询元素时是否应过滤掉旧值？
【问题讨论】：
标签： java collections message-queue java.util.concurrent
我习惯于跟随队列实现。该代码主要基于 Apaches CircularFifoQueue，并且仅经过弱测试。此外，该实现不是线程安全的并且不可序列化。
如果您发现错误，请发表评论。
import java.util.AbstractCollection;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.concurrent.TimeUnit;
/**
* TimedQueue is a first-in first-out queue with a fixed size that
* replaces its oldest element if full.
* <p>
* The removal order of a {@link TimedQueue} is based on the
* insertion order; elements are removed in the same order in which they
* were added. The iteration order is the same as the removal order.
* <p>
* The {@link #add(Object)}, {@link #remove()}, {@link #peek()}, {@link #poll},
* {@link #offer(Object)} operations all perform in constant time.
* All other operations perform in linear time or worse.
* <p>
* This queue prevents null objects from being added and it is not thread-safe and not serializable.
*
* The majority of this source code was copied from Apaches {@link CircularFifoQueue}: http://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/queue/CircularFifoQueue.html
*
* @version 1.0
*/
public class TimedQueue<E> extends AbstractCollection<E>
implements Queue<E> {
/** Underlying storage array. */
private Item<E>[] elements;
/** Array index of first (oldest) queue element. */
private int start = 0;
/**
* Index mod maxElements of the array position following the last queue
* element. Queue elements start at elements[start] and "wrap around"
* elements[maxElements-1], ending at elements[decrement(end)].
* For example, elements = {c,a,b}, start=1, end=1 corresponds to
* the queue [a,b,c].
*/
private transient int end = 0;
/** Flag to indicate if the queue is currently full. */
private transient boolean full = false;
/** Capacity of the queue. */
private final int maxElements;
private TimeUnit unit;
private int delay;
/**
* Constructor that creates a queue with the default size of 32.
*/
public TimedQueue() {
this(32);
}
/**
* Constructor that creates a queue with the specified size.
*
* @param size the size of the queue (cannot be changed)
* @throws IllegalArgumentException if the size is < 1
*/
public TimedQueue(final int size) {
this(size, 3, TimeUnit.SECONDS);
}
@SuppressWarnings("unchecked")
public TimedQueue(final int size, int delay, TimeUnit unit) {
if (size <= 0) {
throw new IllegalArgumentException("The size must be greater than 0");
}
elements = new Item[size];
maxElements = elements.length;
this.unit = unit;
this.delay = delay;
}
/**
* Constructor that creates a queue from the specified collection.
* The collection size also sets the queue size.
*
* @param coll the collection to copy into the queue, may not be null
* @throws NullPointerException if the collection is null
*/
public TimedQueue(final Collection<? extends E> coll) {
this(coll.size());
addAll(coll);
}
/**
* Returns the number of elements stored in the queue.
*
* @return this queue's size
*/
@Override
public int size() {
int size = 0;
for(int i = 0; i < elements.length; i++) {
if(validElement(i) != null) {
size++;
}
}
return size;
}
/**
* Returns true if this queue is empty; false otherwise.
*
* @return true if this queue is empty
*/
@Override
public boolean isEmpty() {
return size() == 0;
}
private boolean isAtFullCapacity() {
return size() == maxElements;
}
/**
* Clears this queue.
*/
@Override
public void clear() {
full = false;
start = 0;
end = 0;
Arrays.fill(elements, null);
}
/**
* Adds the given element to this queue. If the queue is full, the least recently added
* element is discarded so that a new element can be inserted.
*
* @param element the element to add
* @return true, always
* @throws NullPointerException if the given element is null
*/
@Override
public boolean add(final E element) {
if (null == element) {
throw new NullPointerException("Attempted to add null object to queue");
}
if (isAtFullCapacity()) {
remove();
}
elements[end++] = new Item<E>(element);
if (end >= maxElements) {
end = 0;
}
if (end == start) {
full = true;
}
return true;
}
/**
* Returns the element at the specified position in this queue.
*
* @param index the position of the element in the queue
* @return the element at position {@code index}
* @throws NoSuchElementException if the requested position is outside the range [0, size)
*/
public E get(final int index) {
final int sz = size();
if (sz == 0) {
throw new NoSuchElementException(
String.format("The specified index (%1$d) is outside the available range because the queue is empty.", Integer.valueOf(index)));
}
if (index < 0 || index >= sz) {
throw new NoSuchElementException(
String.format("The specified index (%1$d) is outside the available range [0, %2$d]",
Integer.valueOf(index), Integer.valueOf(sz-1)));
}
final int idx = (start + index) % maxElements;
return validElement(idx);
}
private E validElement(int idx) {
if(elements[idx] == null){
return null;
}
long diff = System.currentTimeMillis() - elements[idx].getCreationTime();
if(diff < unit.toMillis(delay)) {
return (E) elements[idx].getValue();
} else {
elements[idx] = null;
return null;
}
}
//-----------------------------------------------------------------------
/**
* Adds the given element to this queue. If the queue is full, the least recently added
* element is discarded so that a new element can be inserted.
*
* @param element the element to add
* @return true, always
* @throws NullPointerException if the given element is null
*/
public boolean offer(E element) {
return add(element);
}
public E poll() {
if (isEmpty()) {
return null;
}
return remove();
}
public E element() {
if (isEmpty()) {
throw new NoSuchElementException("queue is empty");
}
return peek();
}
public E peek() {
if (isEmpty()) {
return null;
}
return (E) elements[start].getValue();
}
public E remove() {
if (isEmpty()) {
throw new NoSuchElementException("queue is empty");
}
final E element = validElement(start);
if (null != element) {
elements[start++] = null;
if (start >= maxElements) {
start = 0;
}
full = false;
}
return element;
}
/**
* Increments the internal index.
*
* @param index the index to increment
* @return the updated index
*/
private int increment(int index) {
index++;
if (index >= maxElements) {
index = 0;
}
return index;
}
/**
* Decrements the internal index.
*
* @param index the index to decrement
* @return the updated index
*/
private int decrement(int index) {
index--;
if (index < 0) {
index = maxElements - 1;
}
return index;
}
/**
* Returns an iterator over this queue's elements.
*
* @return an iterator over this queue's elements
*/
@Override
public Iterator<E> iterator() {
return new Iterator<E>() {
private int index = start;
private int lastReturnedIndex = -1;
private boolean isFirst = full;
public boolean hasNext() {
return (isFirst || index != end) && size() > 0;
}
public E next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
isFirst = false;
lastReturnedIndex = index;
index = increment(index);
if(validElement(lastReturnedIndex) == null) {
return next();
} else {
return validElement(lastReturnedIndex);
}
}
public void remove() {
if (lastReturnedIndex == -1) {
throw new IllegalStateException();
}
// First element can be removed quickly
if (lastReturnedIndex == start) {
TimedQueue.this.remove();
lastReturnedIndex = -1;
return;
}
int pos = lastReturnedIndex + 1;
if (start < lastReturnedIndex && pos < end) {
// shift in one part
System.arraycopy(elements, pos, elements, lastReturnedIndex, end - pos);
} else {
// Other elements require us to shift the subsequent elements
while (pos != end) {
if (pos >= maxElements) {
elements[pos - 1] = elements[0];
pos = 0;
} else {
elements[decrement(pos)] = elements[pos];
pos = increment(pos);
}
}
}
lastReturnedIndex = -1;
end = decrement(end);
elements[end] = null;
full = false;
index = decrement(index);
}
};
}
private static final class Item<E> {
private long creationTime;
private E in;
public Item(E in) {
this.in = in;
creationTime = System.currentTimeMillis();
}
public E getValue() {
return in;
}
public long getCreationTime() {
return creationTime;
}
}
}
【讨论】：
我认为java.util.LinkedHashMap 是适合您的解决方案。它有一个removeEldest() 方法，只要将条目放入地图中就会调用该方法。您可以覆盖它以指示是否应删除最旧的条目。
JavaDoc 给出了一个很好的例子：
private static final int MAX_ENTRIES = 100;
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > MAX_ENTRIES;
}
如果地图有超过 100 个条目，这将删除最旧的条目。
10 秒后主动移除项目需要单独的线程来检查年龄并移除旧项目。从你的描述来看，我猜这不是你想要的。
【讨论】：
有一个名为LinkedHashMap 的类，它有一个特殊的方法来删除过时的数据。来自文档：
protected boolean removeEldestEntry(Map.Entry eldest)
如果此映射应删除其最旧的条目，则返回 true。
只要将任何内容添加到列表（队列）中，就会调用方法removeEldestEntry。如果它返回true，则删除最旧的条目以为新条目腾出空间，否则不会删除任何内容。您可以添加自己的实现来检查最旧条目上的时间戳，如果它早于过期阈值（例如 10 秒），则返回 true。所以你的实现可能看起来像这样：
protected boolean removeEldestEntry(Map.Entry eldest) {
long currTimeMillis = System.currentTimeMillis();
long entryTimeMillis = eldest.getValue().getTimeCreation();
return (currTimeMillis - entryTimeMillis) > (1000*10*60);
}
【讨论】：