LRU(least recently used) : 最近最少使用
LRU就是一种经典的算法，在容器中，对元素定义一个最后使用时间，当新的元素写入的时候，如果容器已满，则淘汰最近最少使用的元素，把新的元素写入。
比如redis，如何自己实现简易版的redis缓存。
那么我们需要一种数据结构，支持set和get操作。
1） get操作时间复杂度O(1);
2）需要支持RLU算法，空间不足时，需要将使用最少的元素移除，为新元素让空间；
3）时间失效remove(这个先不谈，比较麻烦)。
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-collections4</artifactId>
<version>4.2</version>
</dependency>
LRUMap<String, String> map = new LRUMap<>(3);
map.put("1", "1");
map.put("2", "2");
map.put("3", "3");
map.get("2");
System.out.println("---------------------------------");
map.forEach((k,v)->
System.out.println(k+"\t"+v)
);
map.put("4", "4");
map.put("5", "5");
System.out.println("---------------------------------");
map.forEach((k,v)->
System.out.println(k+"\t"+v)
);
map.put("6", "6");
System.out.println("---------------------------------");
map.forEach((k,v)->
System.out.println(k+"\t"+v)
);
结果如下：
---------------------------------
1 1
3 3
2 2
---------------------------------
2 2
4 4
5 5
---------------------------------
4 4
5 5
6 6
可以看出在get("2")，2的位置挪后，然后移除的顺序就延后。
容量不足时，总是移除，使用最少的，时间最远的。
public class LRUMap<K, V>
extends AbstractLinkedMap<K, V> implements BoundedMap<K, V>, Serializable, Cloneable {
进一步查看AbstractLinkedMap，AbstractHashedMap
public abstract class AbstractLinkedMap<K, V> extends AbstractHashedMap<K, V> implements OrderedMap<K, V> {
public class AbstractHashedMap<K, V> extends AbstractMap<K, V> implements IterableMap<K, V> {
本质是自定义AbstractMap
我们看看HashMap LinkedHashMap
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
可以看出AbstractMap，AbstractHashedMap，LRUMap的本质其实也是HashMap。
protected static final int DEFAULT_MAX_SIZE = 100;
public LRUMap() {
this(DEFAULT_MAX_SIZE, DEFAULT_LOAD_FACTOR, false);
}
可以看出默认初始化容量100，最大容量也是100.
进一步跟踪
public LRUMap(final int maxSize, final float loadFactor, final boolean scanUntilRemovable) {
this(maxSize, maxSize, loadFactor, scanUntilRemovable);
}
/**
* Constructs a new, empty map with the specified max / initial capacity and load factor.
*
* @param maxSize the maximum size of the map
* @param initialSize the initial size of the map
* @param loadFactor the load factor
* @param scanUntilRemovable scan until a removeable entry is found, default false
* @throws IllegalArgumentException if the maximum size is less than one
* @throws IllegalArgumentException if the initial size is negative or larger than the maximum size
* @throws IllegalArgumentException if the load factor is less than zero
* @since 4.1
*/
public LRUMap(final int maxSize,
final int initialSize,
final float loadFactor,
final boolean scanUntilRemovable) {
super(initialSize, loadFactor);
if (maxSize < 1) {
throw new IllegalArgumentException("LRUMap max size must be greater than 0");
}
if (initialSize > maxSize) {
throw new IllegalArgumentException("LRUMap initial size must not be greather than max size");
}
this.maxSize = maxSize;
this.scanUntilRemovable = scanUntilRemovable;
}
跟踪super(initialSize, loadFactor);
public abstract class AbstractLinkedMap<K, V> extends AbstractHashedMap<K, V> implements OrderedMap<K, V> {
protected AbstractLinkedMap(final int initialCapacity, final float loadFactor) {
super(initialCapacity, loadFactor);
}
//又super，再上一层追踪
public class AbstractHashedMap<K, V> extends AbstractMap<K, V> implements IterableMap<K, V> {
//定义一些基本初始化数据
/** The default capacity to use */
protected static final int DEFAULT_CAPACITY = 16;
/** The default threshold to use */
protected static final int DEFAULT_THRESHOLD = 12;
/** The default load factor to use */
protected static final float DEFAULT_LOAD_FACTOR = 0.75f;
/** The maximum capacity allowed */
protected static final int MAXIMUM_CAPACITY = 1 << 30;
/** Load factor, normally 0.75 */
transient float loadFactor;
/** The size of the map */
transient int size;
/** Map entries */
transient HashEntry<K, V>[] data;
/** Size at which to rehash */
transient int threshold;
/** Modification count for iterators */
transient int modCount;
/** Entry set */
transient EntrySet<K, V> entrySet;
/** Key set */
transient KeySet<K> keySet;
/** Values */
transient Values<V> values;
protected AbstractHashedMap(int initialCapacity, final float loadFactor) {
super();
if (initialCapacity < 0) {
throw new IllegalArgumentException("Initial capacity must be a non negative number");
}
if (loadFactor <= 0.0f || Float.isNaN(loadFactor)) {
throw new IllegalArgumentException("Load factor must be greater than 0");
}
this.loadFactor = loadFactor;
initialCapacity = calculateNewCapacity(initialCapacity);
this.threshold = calculateThreshold(initialCapacity, loadFactor);
this.data = new HashEntry[initialCapacity];
init();
}
/**
* Initialise subclasses during construction, cloning or deserialization.
*/
protected void init() {
//没有任何逻辑，仅用于子类构造
}
DEFAULT_LOAD_FACTOR = 0.75f; 负载因子0.75
可以看出LRUMap的本质，HashEntry数组。
上面的init方法没有实现逻辑，但是在他的子类中AbstractLinkedMap有相关的定义。
/** Header in the linked list */
transient LinkEntry<K, V> header;
/**
* Creates an entry to store the data.
* <p>
* This implementation creates a new LinkEntry instance.
*
* @param next the next entry in sequence
* @param hashCode the hash code to use
* @param key the key to store
* @param value the value to store
* @return the newly created entry
*/
@Override
protected LinkEntry<K, V> createEntry(final HashEntry<K, V> next, final int hashCode, final K key, final V value) {
return new LinkEntry<>(next, hashCode, convertKey(key), value);
}
protected static class LinkEntry<K, V> extends HashEntry<K, V> {
/** The entry before this one in the order */
protected LinkEntry<K, V> before;
/** The entry after this one in the order */
protected LinkEntry<K, V> after;
/**
* Constructs a new entry.
*
* @param next the next entry in the hash bucket sequence
* @param hashCode the hash code
* @param key the key
* @param value the value
*/
protected LinkEntry(final HashEntry<K, V> next, final int hashCode, final Object key, final V value) {
super(next, hashCode, key, value);
}
}
/**
* Initialise this subclass during construction.
* <p>
* NOTE: As from v3.2 this method calls
* {@link #createEntry(HashEntry, int, Object, Object)} to create
* the map entry object.
*/
@Override
protected void init() {
header = createEntry(null, -1, null, null);
header.before = header.after = header;
}
这个很关键。可以看出LRUMap是持有LinkEntry header，的双链表结构，初始header为null，前后节点都是自身。将HashEntry转成LinkEntry。
解析HashEntry
transient HashEntry<K, V>[] data; //构造初始化 this.data = new HashEntry[initialCapacity];
再跟踪
protected static class HashEntry<K, V> implements Map.Entry<K, V>, KeyValue<K, V> {
/** The next entry in the hash chain */
protected HashEntry<K, V> next;
/** The hash code of the key */
protected int hashCode;
/** The key */
protected Object key;
/** The value */
protected Object value;
key，value，next单链表。
public int hashCode() {
return (getKey() == null ? 0 : getKey().hashCode()) ^
(getValue() == null ? 0 : getValue().hashCode());
}
hashCode方法可以看出是key的hash与value的hash按位^运算。
在此我们看透LRU的本质了，数组+单链表。同时是持有头结点的双链表结构（怎么看就是LinkedHashMap的结构，只是有尾节点）。
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>
{
/**
* The head (eldest) of the doubly linked list.
*/
transient LinkedHashMap.Entry<K,V> head;
/**
* The tail (youngest) of the doubly linked list.
*/
transient LinkedHashMap.Entry<K,V> tail;
那么LRUMap是如何实现LRU算法的？
public V get(final Object key) {
return get(key, true);
}
public V get(final Object key, final boolean updateToMRU) {
final LinkEntry<K, V> entry = getEntry(key);
if (entry == null) {
return null;
}
if (updateToMRU) {
moveToMRU(entry);
}
return entry.getValue();
}
//父类方法获取值entry
protected HashEntry<K, V> getEntry(Object key) {
key = convertKey(key);
final int hashCode = hash(key);
HashEntry<K, V> entry = data[hashIndex(hashCode, data.length)]; // no local for hash index
while (entry != null) {
if (entry.hashCode == hashCode && isEqualKey(key, entry.key)) {
return entry;
}
entry = entry.next;
}
return null;
}
下面看不一样的moveToMRU(entry);
/**
* Moves an entry to the MRU position at the end of the list.
* <p>
* This implementation moves the updated entry to the end of the list.
*
* @param entry the entry to update
*/
protected void moveToMRU(final LinkEntry<K, V> entry) {
if (entry.after != header) {
modCount++;
// remove
if(entry.before == null) {
throw new IllegalStateException("Entry.before is null." +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to dev@commons.apache.org as a bug.");
}
entry.before.after = entry.after;
entry.after.before = entry.before;
// add first
entry.after = header;
entry.before = header.before;
header.before.after = entry;
header.before = entry;
} else if (entry == header) {
throw new IllegalStateException("Can't move header to MRU" +
" (please report this to dev@commons.apache.org)");
}
}
看出LRU的一个本质，每次get方法拨动指针，将get的元素移动到header的前一个位置。
remove方法使用的父类的方法
/**
* Removes the specified mapping from this map.
*
* @param key the mapping to remove
* @return the value mapped to the removed key, null if key not in map
*/
@Override
public V remove(Object key) {
key = convertKey(key);
final int hashCode = hash(key);
final int index = hashIndex(hashCode, data.length);
HashEntry<K, V> entry = data[index];
HashEntry<K, V> previous = null;
while (entry != null) {
if (entry.hashCode == hashCode && isEqualKey(key, entry.key)) {
final V oldValue = entry.getValue();
removeMapping(entry, index, previous);
return oldValue;
}
previous = entry;
entry = entry.next;
}
return null;
}
/**
* Removes a mapping from the map.
* <p>
* This implementation calls <code>removeEntry()</code> and <code>destroyEntry()</code>.
* It also handles changes to <code>modCount</code> and <code>size</code>.
* Subclasses could override to fully control removals from the map.
*
* @param entry the entry to remove
* @param hashIndex the index into the data structure
* @param previous the previous entry in the chain
*/
protected void removeMapping(final HashEntry<K, V> entry, final int hashIndex, final HashEntry<K, V> previous) {
modCount++;
removeEntry(entry, hashIndex, previous);
size--;
destroyEntry(entry);
}
protected void removeEntry(final HashEntry<K, V> entry, final int hashIndex, final HashEntry<K, V> previous) {
if (previous == null) {
data[hashIndex] = entry.next;
} else {
previous.next = entry.next;
}
}
protected void destroyEntry(final HashEntry<K, V> entry) {
entry.next = null;
entry.key = null;
entry.value = null;
}
这里并没有移除header双链表的数据。
/**
* Puts a key-value mapping into this map.
*
* @param key the key to add
* @param value the value to add
* @return the value previously mapped to this key, null if none
*/
@Override
public V put(final K key, final V value) {
final Object convertedKey = convertKey(key);
final int hashCode = hash(convertedKey);
final int index = hashIndex(hashCode, data.length);
HashEntry<K, V> entry = data[index];
//仅在元素存在才循环，更新updateEntry，header前一个位置
while (entry != null) {
if (entry.hashCode == hashCode && isEqualKey(convertedKey, entry.key)) {
final V oldValue = entry.getValue();
updateEntry(entry, value);
return oldValue;
}
entry = entry.next;
}
addMapping(index, hashCode, key, value);
return null;
}
updateEntry(entry, value);
/**
* Updates an existing key-value mapping.
* <p>
* This implementation moves the updated entry to the end of the list
* using {@link #moveToMRU(AbstractLinkedMap.LinkEntry)}.
*
* @param entry the entry to update
* @param newValue the new value to store
*/
@Override
protected void updateEntry(final HashEntry<K, V> entry, final V newValue) {
moveToMRU((LinkEntry<K, V>) entry); // handles modCount
entry.setValue(newValue);
}
moveToMRU((LinkEntry<K, V>) entry); // handles modCount
上面get方法有讲，更新了链表的指针，新添加的元素在双链表的header前一个位置，仅在元素存在的时候，while循环才生效。
那么新增的元素呢？
下面看重点 addMapping(index, hashCode, key, value); 这句代码定义了，容量满了的处理策略。
/**
* Adds a new key-value mapping into this map.
* <p>
* This implementation checks the LRU size and determines whether to
* discard an entry or not using {@link #removeLRU(AbstractLinkedMap.LinkEntry)}.
* <p>
* From Commons Collections 3.1 this method uses {@link #isFull()} rather
* than accessing <code>size</code> and <code>maxSize</code> directly.
* It also handles the scanUntilRemovable functionality.
*
* @param hashIndex the index into the data array to store at
* @param hashCode the hash code of the key to add
* @param key the key to add
* @param value the value to add
*/
@Override
protected void addMapping(final int hashIndex, final int hashCode, final K key, final V value) {
//容量是否已满
if (isFull()) {
LinkEntry<K, V> reuse = header.after;
boolean removeLRUEntry = false;
//默认是false
if (scanUntilRemovable) {
//这里不知道干啥，难道是以后扩展？
while (reuse != header && reuse != null) {
//removeLRU一定返回true，很奇怪，估计以后扩展用
if (removeLRU(reuse)) {
removeLRUEntry = true;
break;
}
reuse = reuse.after;
}
if (reuse == null) {
throw new IllegalStateException(
"Entry.after=null, header.after" + header.after + " header.before" + header.before +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to dev@commons.apache.org as a bug.");
}
} else {
//一定返回true
removeLRUEntry = removeLRU(reuse);
}
if (removeLRUEntry) {
if (reuse == null) {
throw new IllegalStateException(
"reuse=null, header.after=" + header.after + " header.before" + header.before +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to dev@commons.apache.org as a bug.");
}
reuseMapping(reuse, hashIndex, hashCode, key, value);
} else {
super.addMapping(hashIndex, hashCode, key, value);
}
} else {
super.addMapping(hashIndex, hashCode, key, value);
}
}
protected boolean removeLRU(final LinkEntry<K, V> entry) {
return true;
}
先判断容量
public boolean isFull() {
return size >= maxSize;
}
未满就直接添加
super.addMapping(hashIndex, hashCode, key, value);
protected void addMapping(final int hashIndex, final int hashCode, final K key, final V value) {
modCount++;
final HashEntry<K, V> entry = createEntry(data[hashIndex], hashCode, key, value);
addEntry(entry, hashIndex);
size++;
checkCapacity();
}
//这里调用了AbstractLinkedMap的方法
addEntry(entry, hashIndex);
/**
* Adds an entry into this map, maintaining insertion order.
* <p>
* This implementation adds the entry to the data storage table and
* to the end of the linked list.
*
* @param entry the entry to add
* @param hashIndex the index into the data array to store at
*/
@Override
protected void addEntry(final HashEntry<K, V> entry, final int hashIndex) {
final LinkEntry<K, V> link = (LinkEntry<K, V>) entry;
link.after = header;
link.before = header.before;
header.before.after = link;
header.before = link;
data[hashIndex] = link;
}
放在header的前一个位置，最早的元素链接到header。
双向环回链表。
如果容量满了，执行LRU算法 reuseMapping(reuse, hashIndex, hashCode, key, value);
/**
* Reuses an entry by removing it and moving it to a new place in the map.
* <p>
* This method uses {@link #removeEntry}, {@link #reuseEntry} and {@link #addEntry}.
*
* @param entry the entry to reuse
* @param hashIndex the index into the data array to store at
* @param hashCode the hash code of the key to add
* @param key the key to add
* @param value the value to add
*/
protected void reuseMapping(final LinkEntry<K, V> entry, final int hashIndex, final int hashCode,
final K key, final V value) {
// find the entry before the entry specified in the hash table
// remember that the parameters (except the first) refer to the new entry,
// not the old one
try {
//要干掉的元素下标
final int removeIndex = hashIndex(entry.hashCode, data.length);
final HashEntry<K, V>[] tmp = data; // may protect against some sync issues
HashEntry<K, V> loop = tmp[removeIndex];
HashEntry<K, V> previous = null;
//避免已经被删除
while (loop != entry && loop != null) {
previous = loop;
loop = loop.next;
}
//如果被其他线程删除，抛异常
if (loop == null) {
throw new IllegalStateException(
"Entry.next=null, data[removeIndex]=" + data[removeIndex] + " previous=" + previous +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to dev@commons.apache.org as a bug.");
}
// reuse the entry
modCount++;
//双链表移除旧元素,AbstractHashedMap移除旧元素
removeEntry(entry, removeIndex, previous);
//复用移除的对象，减少创建对象和GC；增加AbstractHashedMap单链表next指向
reuseEntry(entry, hashIndex, hashCode, key, value);
//复用的元素加AbstractLinkedMap双链表和AbstractHashedMap单链表
addEntry(entry, hashIndex);
} catch (final NullPointerException ex) {
throw new IllegalStateException(
"NPE, entry=" + entry + " entryIsHeader=" + (entry==header) +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to dev@commons.apache.org as a bug.");
}
}
/**
* Removes an entry from the map and the linked list.
* <p>
* This implementation removes the entry from the linked list chain, then
* calls the superclass implementation.
*
* @param entry the entry to remove
* @param hashIndex the index into the data structure
* @param previous the previous entry in the chain
*/
@Override
protected void removeEntry(final HashEntry<K, V> entry, final int hashIndex, final HashEntry<K, V> previous) {
final LinkEntry<K, V> link = (LinkEntry<K, V>) entry;
link.before.after = link.after;
link.after.before = link.before;
link.after = null;
link.before = null;
super.removeEntry(entry, hashIndex, previous);
}
/**
* Removes an entry from the chain stored in a particular index.
* <p>
* This implementation removes the entry from the data storage table.
* The size is not updated.
* Subclasses could override to handle changes to the map.
*
* @param entry the entry to remove
* @param hashIndex the index into the data structure
* @param previous the previous entry in the chain
*/
protected void removeEntry(final HashEntry<K, V> entry, final int hashIndex, final HashEntry<K, V> previous) {
if (previous == null) {
data[hashIndex] = entry.next;
} else {
previous.next = entry.next;
}
}
/**
* Reuses an existing key-value mapping, storing completely new data.
* <p>
* This implementation sets all the data fields on the entry.
* Subclasses could populate additional entry fields.
*
* @param entry the entry to update, not null
* @param hashIndex the index in the data array
* @param hashCode the hash code of the key to add
* @param key the key to add
* @param value the value to add
*/
protected void reuseEntry(final HashEntry<K, V> entry, final int hashIndex, final int hashCode,
final K key, final V value) {
entry.next = data[hashIndex];
entry.hashCode = hashCode;
entry.key = key;
entry.value = value;
}
/**
* Adds an entry into this map, maintaining insertion order.
* <p>
* This implementation adds the entry to the data storage table and
* to the end of the linked list.
*
* @param entry the entry to add
* @param hashIndex the index into the data array to store at
*/
@Override
protected void addEntry(final HashEntry<K, V> entry, final int hashIndex) {
final LinkEntry<K, V> link = (LinkEntry<K, V>) entry;
link.after = header;
link.before = header.before;
header.before.after = link;
header.before = link;
data[hashIndex] = link;
}
LRU的本质了，数组+单链表。同时是持有头结点的环回双链表结构
LRU最新使用的元素放在双链表的header的前一个位置，如果，新增元素容量已满就会移除header的后一个元素。
