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2022-09-13
LRU算法及Apache LRUMap源码实例解析
LRU算法及Apache LRUMap源码实例解析
目录1. 什么是LRU1.1 自定义实现LRU的要求1.2 Apache LRUMap示例1.2.1 pom依赖1.2.2 demo2. 源码解析2.1 设计2.2 数据结构2.3 方法解析put get remove2.3.1 get方法2.3.2 remove方法2.3.3 put方法3. 总结
1. 什么是LRU
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。
1.1 自定义实现LRU的要求
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)。
1.2 Apache LRUMap示例
1.2.1 pom依赖
1.2.2 demo
LRUMap
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。
2. 源码解析
2.1 设计
public class LRUMap
extends AbstractLinkedMap
进一步查看AbstractLinkedMap，AbstractHashedMap
public abstract class AbstractLinkedMap
public class AbstractHashedMap
本质是自定义AbstractMap
我们看看HashMap LinkedHashMap
public class LinkedHashMap
extends HashMap
implements Map
public class HashMap
implements Map
可以看出AbstractMap，AbstractHashedMap，LRUMap的本质其实也是HashMap。
2.2 数据结构
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
protected AbstractLinkedMap(final int initialCapacity, final float loadFactor) {
super(initialCapacity, loadFactor);
}
//又super，再上一层追踪
public class AbstractHashedMap
//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 */
protecthttp://ed 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
/** Size at which to rehash */
transient int threshold;
/** Modification count for iterators */
transient int modCount;
/** Entry set */
transient EntrySet
/** Key set */
transient KeySet
/** Values */
transient 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
/**
* Creates an entry to store the data.
*
* 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
return new LinkEntry<>(next, hashCode, convertKey(key), value);
}
protected static class LinkEntry
/** The entry before this one in the order */
protected LinkEntry
/** The entry after this one in the order */
protected LinkEntry
/**
* 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
super(next, hashCode, key, value);
}
}
/**
* Initialise this subclass during construction.
*
* 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
//构造初始化
this.data = new HashEntry[initialCapacity];
再跟踪
protected static class HashEntry
/** The next entry in the hash chain */
protected HashEntry
/** 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
extends HashMap
implements Map
{
/**
* The head (eldest) of the doubly linked list.
*/
transient LinkedHashMap.Entry
/**
* The tail (youngest) of the doubly linked list.
*/
transient LinkedHashMap.Entry
那么LRUMap是如何实现LRU算法的？
2.3 方法解析put get remove
2.3.1 get方法
public V get(final Object key) {
return get(key, true);
}
public V get(final Object key, final boolean updateToMRU) {
final LinkEntry
if (entry == null) {
return null;
}
if (updateToMRU) {
moveToMRU(entry);
}
return entry.getValue();
}
//父类方法获取值entry
protected HashEntry
key = convertKey(key);
final int hashCode = hash(key);
HashEntry
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.
*
* This implementation moves the updated entry to the end of the list.
*
* @param entry the entry to update
*/
protected void moveToMRU(final LinkEntry
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。
2.3.2 remove方法
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
HashEntry
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.
*
* This implementation calls removeEntry() and destroyEntry().
* It also handles changes to modCount and size.
* 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
modCount++;
removeEntry(entry, hashIndex, previous);
size--;
destroyEntry(entry);
}
protected void removeEntry(final HashEntry
if (previous == null) {
data[hashIndex] = entry.next;
} else {
previous.next = entry.next;
}
}
protected void destroyEntry(final HashEntry
entry.next = null;
entry.key = null;
entry.value = null;
}
这里并没有移除header双链表的数据。
2.3.3 put方法
/**
* 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
//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.
*
* 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
moveToMRU((LinkEntry
entry.setValue(newValue);
}
moveToMRU((LinkEntry
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.
*
* This implementation checks the LRU size and determines whether to
* discard an entry or not using {@link #removeLRU(AbstractLinkedMap.LinkEntry)}.
*
* From Commons Collections 3.1 this method uses {@link #isFull()} rather
* than accessing size and maxSize 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
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
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
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.
*
* 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
final LinkEntry
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.
*
* 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
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
HashEntry
HashEntry
//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.
*
* 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
final LinkEntry
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.
*
* 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
if (previous == null) {
data[hashIndex] = entry.next;
} else {
previous.next = entry.next;
}
}
/**
* Reuses an existing key-value mapping, storing completely new data.
*
* 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
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.
*
* 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
final LinkEntry
link.after = header;
link.before = header.before;
header.before.after = link;
header.before = link;
data[hashIndex] = link;
}
3. 总结
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。
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