OSCache源码阅读(二)

前文LRU Cache 暨LinkedHashMap源码阅读提到了如何使用LinkedHashMap来实现一个LRU数据结构，今天在看OSCache代码算法部分的时候，就用到了该知识，what was done contributes what is done now。

algorithm包是包含下列缓存过期策略的类：

下面重点介绍LRU和FIFO。

LRU

private Collection list = new LinkedHashSet();

使用一个LinkedHashSet对象来实现LRU，而LinkedHashSet的父类是HashSet，

public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, java.io.Serializable {

    private static final long serialVersionUID = -2851667679971038690L;

    /**
     * Constructs a new, empty linked hash set with the specified initial
     * capacity and load factor.
     *
     * @param      initialCapacity the initial capacity of the linked hash set
     * @param      loadFactor      the load factor of the linked hash set
     * @throws     IllegalArgumentException  if the initial capacity is less
     *               than zero, or if the load factor is nonpositive
     */
    public LinkedHashSet(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor, true);
    }

    /**
     * Constructs a new, empty linked hash set with the specified initial
     * capacity and the default load factor (0.75).
     *
     * @param   initialCapacity   the initial capacity of the LinkedHashSet
     * @throws  IllegalArgumentException if the initial capacity is less
     *              than zero
     */
    public LinkedHashSet(int initialCapacity) {
        super(initialCapacity, .75f, true);
    }

    /**
     * Constructs a new, empty linked hash set with the default initial
     * capacity (16) and load factor (0.75).
     */
    public LinkedHashSet() {
        super(16, .75f, true);
    }

    /**
     * Constructs a new linked hash set with the same elements as the
     * specified collection.  The linked hash set is created with an initial
     * capacity sufficient to hold the elements in the specified collection
     * and the default load factor (0.75).
     *
     * @param c  the collection whose elements are to be placed into
     *           this set
     * @throws NullPointerException if the specified collection is null
     */
    public LinkedHashSet(Collection<? extends E> c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }
}

其中，HaseSet的构造器之一是：

HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

通过LinkedHashMap来实现的。

所以LRU的实现就容易理解了。

FIFO

public class FIFOCache extends AbstractConcurrentReadCache {

    private static final long serialVersionUID = -10333778645392679L;

    /**
     * A queue containing all cache keys
     */
    private Collection list = new LinkedHashSet();

底层也是用LinkedHashSet来存储key的……也比较好理解了。