最近手里事情不太多，随意看了看源码，在学习缓存技术的时候，都少不了使用各种缓存调度算法（FIFO,LRU,LFU），今天总结一下LRU算法。
LinkedHashMap已经为我们自己实现LRU算法提供了便利。
LinkedHashMap继承了HashMap底层是通过Hash表+单向链表实现Hash算法，内部自己维护了一套元素访问顺序的列表。

/** * The head of the doubly linked list. */ private transient Entry<K,V> header; ..... /** * LinkedHashMap entry. */ private static class Entry<K,V> extends HashMap.Entry<K,V> { // These fields comprise the doubly linked list used for iteration. Entry<K,V> before, after;

HashMap构造函数中回调了子类的init方法实现对元素初始化

void init() { header = new Entry<K,V>(-1, null, null, null); header.before = header.after = header; }

LinkedHashMap中有一个属性可以执行列表元素的排序算法

/** * The iteration ordering method for this linked hash map: <tt>true</tt> * for access-order, <tt>false</tt> for insertion-order. * * @serial */ private final boolean accessOrder;

注释已经写的很明白，accessOrder为true使用访问顺序排序，false使用插入顺序排序那么在哪里可以设置这个值。

/** * Constructs an empty <tt>LinkedHashMap</tt> instance with the * specified initial capacity, load factor and ordering mode. * * @param initialCapacity the initial capacity. * @param loadFactor the load factor. * @param accessOrder the ordering mode - <tt>true</tt> for * access-order, <tt>false</tt> for insertion-order. * @throws IllegalArgumentException if the initial capacity is negative * or the load factor is nonpositive. */ public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) { super(initialCapacity, loadFactor); this.accessOrder = accessOrder; }

那么我们就行有访问顺序排序方式实现LRU,那么哪里LinkedHashMap是如何实现LRU的呢？

//LinkedHashMap方法 public V get(Object key) { Entry<K,V> e = (Entry<K,V>)getEntry(key); if (e == null) return null; e.recordAccess(this); return e.value; } //HashMap方法 public V put(K key, V value) { if (key == null) return putForNullKey(value); int hash = hash(key.hashCode()); int i = indexFor(hash, table.length); for (Entry<K,V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, key, value, i); return null; }

当调用get或者put方法的时候，如果K-V已经存在，会回调Entry.recordAccess()方法
我们再看一下LinkedHashMap的Entry实现

/** * This method is invoked by the superclass whenever the value * of a pre-existing entry is read by Map.get or modified by Map.set. * If the enclosing Map is access-ordered, it moves the entry * to the end of the list; otherwise, it does nothing. */ void recordAccess(HashMap<K,V> m) { LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m; if (lm.accessOrder) { lm.modCount++; remove(); addBefore(lm.header); } } /** * Remove this entry from the linked list. */ private void remove() { before.after = after; after.before = before; } /** * Insert this entry before the specified existing entry in the list. */ private void addBefore(Entry<K,V> existingEntry) { after = existingEntry; before = existingEntry.before; before.after = this; after.before = this; }

recordAccess方法会accessOrder为true会先调用remove清楚的当前首尾元素的指向关系，之后调用addBefore方法，将当前元素加入header之前。

当有新元素加入Map的时候会调用Entry的addEntry方法，会调用removeEldestEntry方法，这里就是实现LRU元素过期机制的地方，默认的情况下removeEldestEntry方法只返回false表示元素永远不过期。

/** * This override alters behavior of superclass put method. It causes newly * alcate.html' target='_blank'>located entry to get inserted at the end of the linked list and * removes the eldest entry if appropriate. */ void addEntry(int hash, K key, V value, int bucketIndex) { createEntry(hash, key, value, bucketIndex); // Remove eldest entry if instructed, else grow capacity if appropriate Entry<K,V> eldest = header.after; if (removeEldestEntry(eldest)) { removeEntryForKey(eldest.key); } else { if (size >= threshold) resize(2 * table.length); } } /** * This override differs from addEntry in that it doesn't resize the * table or remove the eldest entry. */ void createEntry(int hash, K key, V value, int bucketIndex) { HashMap.Entry<K,V> old = table[bucketIndex]; Entry<K,V> e = new Entry<K,V>(hash, key, value, old); table[bucketIndex] = e; e.addBefore(header); size++; } protected boolean removeEldestEntry(Map.Entry<K,V> eldest) { return false; }

基本的原理已经介绍完了，那基于LinkedHashMap我们看一下是该如何实现呢？

public static class LRULinkedHashMap<K, V> extends LinkedHashMap<K, V> { /** serialVersionUID */ private static final long serialVersionUID = -5933045562735378538L; /** 最大数据存储容量 */ private static final int LRU_MAX_CAPACITY = 1024; /** 存储数据容量 */ private int capacity; /** * 默认构造方法 */ public LRULinkedHashMap() { super(); } /** * 带参数构造方法 * @param initialCapacity 容量 * @param loadFactor 装载因子 * @param isLRU 是否使用lru算法，true：使用（按方案顺序排序）;false：不使用（按存储顺序排序） */ public LRULinkedHashMap(int initialCapacity, float loadFactor, boolean isLRU) { super(initialCapacity, loadFactor, true); capacity = LRU_MAX_CAPACITY; } /** * 带参数构造方法 * @param initialCapacity 容量 * @param loadFactor 装载因子 * @param isLRU 是否使用lru算法，true：使用（按方案顺序排序）;false：不使用（按存储顺序排序） * @param lruCapacity lru存储数据容量 */ public LRULinkedHashMap(int initialCapacity, float loadFactor, boolean isLRU, int lruCapacity) { super(initialCapacity, loadFactor, true); this.capacity = lruCapacity; } /** * @see java.util.LinkedHashMap#removeEldestEntry(java.util.Map.Entry) */ @Override protected boolean removeEldestEntry(Entry<K, V> eldest) { System.out.println(eldest.getKey() + "=" + eldest.getValue()); if(size() > capacity) { return true; } return false; } }

测试代码：

public static void main(String[] args) { LinkedHashMap<String, String> map = new LRULinkedHashMap<String, String>(16, 0.75f, true); map.put("a", "a"); //a a map.put("b", "b"); //a a b map.put("c", "c"); //a a b c map.put("a", "a"); // b c a map.put("d", "d"); //b b c a d map.put("a", "a"); // b c d a map.put("b", "b"); // c d a b map.put("f", "f"); //c c d a b f map.put("g", "g"); //c c d a b f g map.get("d"); //c a b f g d for (Entry<String, String> entry : map.entrySet()) { System.out.print(entry.getValue() + ", "); } System.out.println(); map.get("a"); //c b f g d a for (Entry<String, String> entry : map.entrySet()) { System.out.print(entry.getValue() + ", "); } System.out.println(); map.get("c"); //b f g d a c for (Entry<String, String> entry : map.entrySet()) { System.out.print(entry.getValue() + ", "); } System.out.println(); map.get("b"); //f g d a c b for (Entry<String, String> entry : map.entrySet()) { System.out.print(entry.getValue() + ", "); } System.out.println(); map.put("h", "h"); //f f g d a c b h for (Entry<String, String> entry : map.entrySet()) { System.out.print(entry.getValue() + ", "); } System.out.println(); }

运行结果：
a=a
a=a
a=a
b=b
c=c
c=c
c, a, b, f, g, d,
c, b, f, g, d, a,
b, f, g, d, a, c,
f, g, d, a, c, b,
f=f
f, g, d, a, c, b, h,