从HashMap到LruCache的源码分析
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HashMap的实现中主要维护一个数组,发生冲突通过链表来解决,链表插入类似于头插法
LinkedHashMap继承自HashMap,在hash的基础上,又维护了一个链表,这个链表是带头结点的双向循环链表,需要注意的链表的元素都是hash里面的元素,链表仅仅是在hash的基础上用指针将hash中的节点连接了起来
LruCache是android的utils包里面的一个类,用来实现缓存防止OOM的一个工具类,用途非常广泛。
关于LRU算法:Least Recently Used最近最少使用算法,在操作系统中,对内存的访问满足局部性原理,于是LRU用在缺页中断发生时的置换算法,将内存中的最近最长未使用的页面置换到磁盘,可以实现的方式可以为维护一个链表,当访问一个页面是,将该页面移动至表头(尾),发生缺页时取链表最后(前)的页面置换,这样存在问题是读取某个页面的复杂度太高,于是可以考虑将其进行hash,这样读取速度会提高,于是用到了LinkedHashMap这种数据结构。
android实现的LruCache类主要使用来进行内存缓存的,维护所用资源的强引用,当内存超过设定的缓存值时,将好久未使用的资源从内存删除。
LruCache的实现中在缓存的值达到最大值时采用的方法是,循环迭代从链表中取eldest的元素进行删除,知道占用的控件小于最大的缓存值。LinkedHashMap中提供的removeEldestEntry函数可以简单实现LRU的功能,但不能很好的满足一些场景,因为里面存放的元素的大小不总是大小一致的,或者说不仅仅是以缓存数据的个数来看的。下面基本上没有太多的文字,所有的解释都详细的列在代码里面
HashMap
http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/util/HashMap.java
- public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable{
- // 默认初始容量16
- static final int DEFAULT_INITIAL_CAPACITY = 16;
- // 最大容量2^30
- static final int MAXIMUM_CAPACITY = 1 << 30;
- // 默认加载因子
- static final float DEFAULT_LOAD_FACTOR = 0.75f;
- // hash映射的数组槽
- transient Entry[] table;
- // 元素个数
- transient int size;
- // 阈值 = 加载因子 * 容量
- int threshold;
- // 加载因子
- final float loadFactor;
- // 修改次数,判断迭代期间容器被修改,不然抛出ConcurrentModificationException
- transient int modCount;
- public HashMap(int initialCapacity, float loadFactor) {
- if (initialCapacity < 0)
- throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
- // 参数调整
- if (initialCapacity > MAXIMUM_CAPACITY)
- initialCapacity = MAXIMUM_CAPACITY;
- if (loadFactor <= 0 || Float.isNaN(loadFactor))
- throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
- // 找到大于initialCapacity的最小的2次幂
- int capacity = 1;
- while (capacity < initialCapacity)
- capacity <<= 1;
- this.loadFactor = loadFactor;
- // 设置阈值
- threshold = (int)(capacity * loadFactor);
- // 定义数组,大小为capacity
- table = new Entry[capacity];
- // 这里是空的实现,实际让其子类覆写该方法
- init();
- }
- public HashMap(int initialCapacity) {
- this(initialCapacity, DEFAULT_LOAD_FACTOR);
- }
- // 默认情况下默认的加载因子,默认的容量16
- public HashMap() {
- this.loadFactor = DEFAULT_LOAD_FACTOR;
- threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
- table = new Entry[DEFAULT_INITIAL_CAPACITY];
- init();
- }
- // 从已存在的Map创建HashMap
- public HashMap(Map<? extends K, ? extends V> m) {
- // 容量为Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,DEFAULT_INITIAL_CAPACITY),默认的加载因子
- this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
- // 遍历m将其元素添加到hashmap中
- putAllForCreate(m);
- }
- void init() {
- }
- // hash算法
- // 可以将1变的松散,可以减少冲突
- static int hash(int h) {
- // This function ensures that hashCodes that differ only by
- // constant multiples at each bit position have a bounded
- // number of collisions (approximately 8 at default load factor).
- h ^= (h >>> 20) ^ (h >>> 12);
- return h ^ (h >>> 7) ^ (h >>> 4);
- }
- // 根据hash值获得在我们维护的数组的索引
- // 即取hash值的小于length的部分,这样才能将其限定在数组大小的范围里面,这样的处理也会带来冲突
- static int indexFor(int h, int length) {
- return h & (length-1);
- }
- public int size() {
- return size;
- }
- public boolean isEmpty() {
- return size == 0;
- }
- // 根据键获取值
- public V get(Object key) {
- if (key == null)
- return getForNullKey();
- int hash = hash(key.hashCode());
- for (Entry<K,V> e = table[indexFor(hash, table.length)];
- e != null;
- e = e.next) {
- Object k;
- if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
- return e.value;
- }
- return null;
- }
- // 键为null的Entry都放在第0个槽中,相当于null经过hash后为0
- private V getForNullKey() {
- for (Entry<K,V> e = table[0]; e != null; e = e.next) {
- if (e.key == null)
- return e.value;
- }
- return null;
- }
- public boolean containsKey(Object key) {
- return getEntry(key) != null;
- }
- // 返回对应键的Entry,若不存在返回null
- final Entry<K,V> getEntry(Object key) {
- // 计算key的hash值
- int hash = (key == null) ? 0 : hash(key.hashCode());
- // 根据hash值获取其存放的槽,即indexFor函数的作用
- // 遍历这个槽上的链表
- for (Entry<K,V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) {
- Object k;
- // hash值一样且键一样(同一个内存地址或者值相同)即返回。
- if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))
- return e;
- }
- return null;
- }
- // 添加键值对
- public V put(K key, V value) {
- // 如果键为null,那么存放在第0个槽上
- if (key == null)
- return putForNullKey(value);
- // 获得键的hash值
- int hash = hash(key.hashCode());
- // 根据hash值得到保存在我们维护的数组中的那个下标处
- 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))) {
- // 已存在,修改
- V oldValue = e.value;
- e.value = value;
- e.recordAccess(this);
- // 将旧的值返回
- return oldValue;
- }
- }
- modCount++;
- // 不存在则添加
- addEntry(hash, key, value, i);
- return null;
- }
- // 添加键位null的键值对
- private V putForNullKey(V value) {
- // 键位null的放在第0个槽
- for (Entry<K,V> e = table[0]; e != null; e = e.next) {
- if (e.key == null) {
- // 已存在则替换
- V oldValue = e.value;
- e.value = value;
- // 被子类覆盖
- e.recordAccess(this);
- return oldValue;
- }
- }
- modCount++;
- // 不存在,添加
- addEntry(0, null, value, 0);
- return null;
- }
- // 和put类似,用在构造函数、clone
- private void putForCreate(K key, V value) {
- int hash = (key == null) ? 0 : 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 != null && key.equals(k)))) {
- e.value = value;
- return;
- }
- }
- createEntry(hash, key, value, i);
- }
- // 遍历map添加到新建的hashmap中
- private void putAllForCreate(Map<? extends K, ? extends V> m) {
- for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
- putForCreate(e.getKey(), e.getValue());
- }
- // 扩容
- void resize(int newCapacity) {
- Entry[] oldTable = table;
- int oldCapacity = oldTable.length;
- // 旧的容量已经达到最大了,调整阈值即可
- if (oldCapacity == MAXIMUM_CAPACITY) {
- threshold = Integer.MAX_VALUE;
- return;
- }
- // 用新的容量创建新数组
- Entry[] newTable = new Entry[newCapacity];
- // 并将原数组里面的hash表全部搬移到新的数组槽中
- transfer(newTable);
- // 将维护的数组引用重新赋值
- table = newTable;
- // 调整阈值
- threshold = (int)(newCapacity * loadFactor);
- }
- // 将原数组table里面的hash表全部搬移到新的数组槽中填充newTable
- void transfer(Entry[] newTable) {
- Entry[] src = table;
- int newCapacity = newTable.length;
- // 遍历数组的每个槽,每个槽中在一次遍历链表
- for (int j = 0; j < src.length; j++) {
- Entry<K,V> e = src[j];
- if (e != null) {
- src[j] = null;
- do {
- Entry<K,V> next = e.next;
- int i = indexFor(e.hash, newCapacity);
- e.next = newTable[i];
- newTable[i] = e;
- e = next;
- } while (e != null);
- }
- }
- }
- //
- public void putAll(Map<? extends K, ? extends V> m) {
- // map元素个数为0,什么也不用做
- int numKeysToBeAdded = m.size();
- if (numKeysToBeAdded == 0)
- return;
- // 如果待复制的元素个数大于阈值,需要扩容
- if (numKeysToBeAdded > threshold) {
- // 目标容量为满足当前设置的加载因子情况下的容量
- int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
- // 参数调整
- if (targetCapacity > MAXIMUM_CAPACITY)
- targetCapacity = MAXIMUM_CAPACITY;
- int newCapacity = table.length;
- // 找到大于targetCapacity的最小2的n次幂
- while (newCapacity < targetCapacity)
- newCapacity <<= 1;
- if (newCapacity > table.length)
- // 扩容为新的容量
- resize(newCapacity);
- }
- for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
- put(e.getKey(), e.getValue());
- }
- public V remove(Object key) {
- Entry<K,V> e = removeEntryForKey(key);
- return (e == null ? null : e.value);
- }
- // 移除key所对应的键值对
- // 和removeMapping类似,只是在判断相等时有点区别
- final Entry<K,V> removeEntryForKey(Object key) {
- int hash = (key == null) ? 0 : hash(key.hashCode());
- int i = indexFor(hash, table.length);
- Entry<K,V> prev = table[i];
- Entry<K,V> e = prev;
- while (e != null) {
- Entry<K,V> next = e.next;
- Object k;
- if (e.hash == hash &&
- ((k = e.key) == key || (key != null && key.equals(k)))) {
- modCount++;
- size--;
- if (prev == e)
- table[i] = next;
- else
- prev.next = next;
- // 依然在删除该键值对时调用,留给LinkedHashMap,因为可能会在访问hashmap时重新整理链表的指向关系
- e.recordRemoval(this);
- return e;
- }
- prev = e;
- e = next;
- }
- return e;
- }
- // 移除键值对
- final Entry<K,V> removeMapping(Object o) {
- // 传递参数不是Entry的子类,什么也不做
- if (!(o instanceof Map.Entry))
- return null;
- Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
- Object key = entry.getKey();
- // 获取要删除的键值对的键的哈希值
- int hash = (key == null) ? 0 : hash(key.hashCode());
- // 根据hash值得到保存在我们维护的数组中的那个下标处
- int i = indexFor(hash, table.length);
- Entry<K,V> prev = table[i];
- Entry<K,V> e = prev;
- while (e != null) {
- Entry<K,V> next = e.next;
- if (e.hash == hash && e.equals(entry)) {
- // hash值相同并且entry内容一样,即找到了
- modCount++;
- size--;
- if (prev == e)
- table[i] = next;
- else
- prev.next = next;
- // 空的实现,给LinkedHashMap实现,在删除键值对后执行
- e.recordRemoval(this);
- return e;
- }
- prev = e;
- e = next;
- }
- return e;
- }
- public void clear() {
- modCount++;
- Entry[] tab = table;
- for (int i = 0; i < tab.length; i++)
- tab[i] = null;
- size = 0;
- }
- // 判断是否包含值为value的键值对
- public boolean containsValue(Object value) {
- if (value == null)
- return containsNullValue();
- Entry[] tab = table;
- for (int i = 0; i < tab.length ; i++)
- for (Entry e = tab[i] ; e != null ; e = e.next)
- if (value.equals(e.value))
- return true;
- return false;
- }
- // 判断是否有值为null的键值对
- private boolean containsNullValue() {
- Entry[] tab = table;
- // 依次迭代数组和每个数组槽所对应的链表
- for (int i = 0; i < tab.length ; i++)
- for (Entry e = tab[i] ; e != null ; e = e.next)
- if (e.value == null)
- return true;
- return false;
- }
- public Object clone() {
- HashMap<K,V> result = null;
- try {
- result = (HashMap<K,V>)super.clone();
- } catch (CloneNotSupportedException e) {
- // assert false;
- }
- result.table = new Entry[table.length];
- result.entrySet = null;
- result.modCount = 0;
- result.size = 0;
- result.init();
- result.putAllForCreate(this);
- return result;
- }
- // hashmap的底层节点结构
- static class Entry<K,V> implements Map.Entry<K,V> {
- final K key;
- V value;
- Entry<K,V> next;
- final int hash;
- Entry(int h, K k, V v, Entry<K,V> n) {
- value = v;
- next = n;
- key = k;
- hash = h;
- }
- public final K getKey() {
- return key;
- }
- public final V getValue() {
- return value;
- }
- public final V setValue(V newValue) {
- V oldValue = value;
- value = newValue;
- return oldValue;
- }
- public final boolean equals(Object o) {
- if (!(o instanceof Map.Entry))
- return false;
- Map.Entry e = (Map.Entry)o;
- Object k1 = getKey();
- Object k2 = e.getKey();
- if (k1 == k2 || (k1 != null && k1.equals(k2))) {
- Object v1 = getValue();
- Object v2 = e.getValue();
- if (v1 == v2 || (v1 != null && v1.equals(v2)))
- return true;
- }
- return false;
- }
- public final int hashCode() {
- return (key==null ? 0 : key.hashCode()) ^
- (value==null ? 0 : value.hashCode());
- }
- public final String toString() {
- return getKey() + "=" + getValue();
- }
- /*******两个空的方法,分别在添加和删除时调用,用以子类实现访问该容器时做一些其他操作*******/
- void recordAccess(HashMap<K,V> m) {
- }
- void recordRemoval(HashMap<K,V> m) {
- }
- }
- // 添加一个Entry到bucketIndex槽的位置
- void addEntry(int hash, K key, V value, int bucketIndex) {
- Entry<K,V> e = table[bucketIndex];
- // 下面这句简单的表述实际上创建了一个Entry节点,下一个节点是e
- // 也就是说数组索引所在位置,然后在调整数组索引处为新创建的节点,即链表的头插法
- table[bucketIndex] = new Entry<>(hash, key, value, e);
- // 元素个数超过了阈值,进行扩容为原来的两倍
- if (size++ >= threshold)
- resize(2 * table.length);
- }
- // 逻辑和addEntry一模一样,只是少了扩容的判断,该函数用在构造函数里拷贝另一个map的值
- // 此前已经调整了容量,因此不会出现扩容的情况
- void createEntry(int hash, K key, V value, int bucketIndex) {
- Entry<K,V> e = table[bucketIndex];
- table[bucketIndex] = new Entry<>(hash, key, value, e);
- size++;
- }
- // 迭代器部分
- private abstract class HashIterator<E> implements Iterator<E> {
- Entry<K,V> next; // next entry to return
- // 迭代器的fast-fail机制,迭代期间不允许修改容器
- int expectedModCount; // For fast-fail
- int index; // current slot
- Entry<K,V> current; // current entry
- HashIterator() {
- expectedModCount = modCount;
- if (size > 0) { // advance to first entry
- Entry[] t = table;
- while (index < t.length && (next = t[index++]) == null)
- ;
- }
- }
- public final boolean hasNext() {
- return next != null;
- }
- final Entry<K,V> nextEntry() {
- if (modCount != expectedModCount)
- throw new ConcurrentModificationException();
- Entry<K,V> e = next;
- if (e == null)
- throw new NoSuchElementException();
- if ((next = e.next) == null) {
- Entry[] t = table;
- while (index < t.length && (next = t[index++]) == null)
- ;
- }
- current = e;
- return e;
- }
- public void remove() {
- if (current == null)
- throw new IllegalStateException();
- if (modCount != expectedModCount)
- throw new ConcurrentModificationException();
- Object k = current.key;
- current = null;
- HashMap.this.removeEntryForKey(k);
- expectedModCount = modCount;
- }
- }
- private final class ValueIterator extends HashIterator<V> {
- public V next() {
- return nextEntry().value;
- }
- }
- private final class KeyIterator extends HashIterator<K> {
- public K next() {
- return nextEntry().getKey();
- }
- }
- private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
- public Map.Entry<K,V> next() {
- return nextEntry();
- }
- }
- // Subclass overrides these to alter behavior of views' iterator() method
- Iterator<K> newKeyIterator() {
- return new KeyIterator();
- }
- Iterator<V> newValueIterator() {
- return new ValueIterator();
- }
- Iterator<Map.Entry<K,V>> newEntryIterator() {
- return new EntryIterator();
- }
- // Views
- // hasp里面的entry所对应的Set
- private transient Set<Map.Entry<K,V>> entrySet = null;
- // 键对应的Set
- public Set<K> keySet() {
- Set<K> ks = keySet;
- return (ks != null ? ks : (keySet = new KeySet()));
- }
- private final class KeySet extends AbstractSet<K> {
- public Iterator<K> iterator() {
- return newKeyIterator();
- }
- public int size() {
- return size;
- }
- public boolean contains(Object o) {
- return containsKey(o);
- }
- public boolean remove(Object o) {
- return HashMap.this.removeEntryForKey(o) != null;
- }
- public void clear() {
- HashMap.this.clear();
- }
- }
- public Collection<V> values() {
- Collection<V> vs = values;
- return (vs != null ? vs : (values = new Values()));
- }
- // 值的集合
- private final class Values extends AbstractCollection<V> {
- public Iterator<V> iterator() {
- return newValueIterator();
- }
- public int size() {
- return size;
- }
- public boolean contains(Object o) {
- return containsValue(o);
- }
- public void clear() {
- HashMap.this.clear();
- }
- }
- public Set<Map.Entry<K,V>> entrySet() {
- return entrySet0();
- }
- private Set<Map.Entry<K,V>> entrySet0() {
- Set<Map.Entry<K,V>> es = entrySet;
- return es != null ? es : (entrySet = new EntrySet());
- }
- private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
- public Iterator<Map.Entry<K,V>> iterator() {
- return newEntryIterator();
- }
- public boolean contains(Object o) {
- if (!(o instanceof Map.Entry))
- return false;
- Map.Entry<K,V> e = (Map.Entry<K,V>) o;
- Entry<K,V> candidate = getEntry(e.getKey());
- return candidate != null && candidate.equals(e);
- }
- public boolean remove(Object o) {
- return removeMapping(o) != null;
- }
- public int size() {
- return size;
- }
- public void clear() {
- HashMap.this.clear();
- }
- }
- // 序列化部分
- private void writeObject(java.io.ObjectOutputStream s)
- throws IOException
- {
- Iterator<Map.Entry<K,V>> i =
- (size > 0) ? entrySet0().iterator() : null;
- // Write out the threshold, loadfactor, and any hidden stuff
- s.defaultWriteObject();
- // Write out number of buckets
- s.writeInt(table.length);
- // Write out size (number of Mappings)
- s.writeInt(size);
- // Write out keys and values (alternating)
- if (i != null) {
- while (i.hasNext()) {
- Map.Entry<K,V> e = i.next();
- s.writeObject(e.getKey());
- s.writeObject(e.getValue());
- }
- }
- }
- private static final long serialVersionUID = 362498820763181265L;
- /**
- * Reconstitute the <tt>HashMap</tt> instance from a stream (i.e.,
- * deserialize it).
- */
- private void readObject(java.io.ObjectInputStream s)
- throws IOException, ClassNotFoundException
- {
- // Read in the threshold, loadfactor, and any hidden stuff
- s.defaultReadObject();
- // Read in number of buckets and allocate the bucket array;
- int numBuckets = s.readInt();
- table = new Entry[numBuckets];
- init(); // Give subclass a chance to do its thing.
- // Read in size (number of Mappings)
- int size = s.readInt();
- // Read the keys and values, and put the mappings in the HashMap
- for (int i=0; i<size; i++) {
- K key = (K) s.readObject();
- V value = (V) s.readObject();
- putForCreate(key, value);
- }
- }
- // These methods are used when serializing HashSets
- int capacity() { return table.length; }
- float loadFactor() { return loadFactor; }
- }
LinkedHashMap
http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/util/LinkedHashMap.java
- public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>{
- private static final long serialVersionUID = 3801124242820219131L;
- // 带头结点的双向循环链表 的头
- private transient Entry<K,V> header;
- // 取值代表使用的方式:false链表按照添加顺序组织,true按照使用顺序组织
- private final boolean accessOrder;
- // 在构造方法中accessOrder均被初始化为false
- public LinkedHashMap(int initialCapacity, float loadFactor) {
- super(initialCapacity, loadFactor);
- accessOrder = false;
- }
- public LinkedHashMap(int initialCapacity) {
- super(initialCapacity);
- accessOrder = false;
- }
- public LinkedHashMap() {
- super();
- accessOrder = false;
- }
- public LinkedHashMap(Map<? extends K, ? extends V> m) {
- super(m);
- accessOrder = false;
- }
- public LinkedHashMap(int initialCapacity,
- float loadFactor,
- boolean accessOrder) {
- super(initialCapacity, loadFactor);
- this.accessOrder = accessOrder;
- }
- // 复写父类的init方法,该方法在父类的构造方法里面调用
- void init() {
- // 初始化链表头结点header
- // 该头结点数字无意义。
- header = new Entry<>(-1, null, null, null);
- // 双向循环链表
- header.before = header.after = header;
- }
- // hashmap里面的该函数的意义是:将原数组table里面的hash表全部搬移到新的数组槽中填充newTable
- // 由于已经将所有元素用链表连起来了所以是用链表来赋值更加快速
- //
- void transfer(HashMap.Entry[] newTable) {
- int newCapacity = newTable.length;
- for (Entry<K,V> e = header.after; e != header; e = e.after) {
- int index = indexFor(e.hash, newCapacity);
- e.next = newTable[index];
- newTable[index] = e;
- }
- }
- // 判断是否含有某个value
- // 直接遍历链表会有更好的时间复杂度
- public boolean containsValue(Object value) {
- // Overridden to take advantage of faster iterator
- if (value==null) {
- for (Entry e = header.after; e != header; e = e.after)
- if (e.value==null)
- return true;
- } else {
- for (Entry e = header.after; e != header; e = e.after)
- if (value.equals(e.value))
- return true;
- }
- return false;
- }
- public V get(Object key) {
- Entry<K,V> e = (Entry<K,V>)getEntry(key);
- if (e == null)
- return null;
- // 访问即有可能要改变他在链表中的位置
- e.recordAccess(this);
- return e.value;
- }
- public void clear() {
- super.clear();
- header.before = header.after = header;
- }
- // linkedHashMap的节点
- private static class Entry<K,V> extends HashMap.Entry<K,V> {
- // 比起hashmap的节点多了两个指针,一个指向前一个节点一个指向后一个节点
- Entry<K,V> before, after;
- Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
- super(hash, key, value, next);
- }
- // 从链表中移除本身节点,仅仅指的是修改指针指向
- private void remove() {
- before.after = after;
- after.before = before;
- }
- // 从链表中添加本节点至existingEntry的前面
- private void addBefore(Entry<K,V> existingEntry) {
- after = existingEntry;
- before = existingEntry.before;
- before.after = this;
- after.before = this;
- }
- // 覆盖父类的方法
- void recordAccess(HashMap<K,V> m) {
- LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
- // 如果accessOrder为false什么都不做
- if (lm.accessOrder) {
- lm.modCount++;
- // 从链表中移除
- remove();
- // 将该节点添加到链表header的前面,也就是将其添加到链表末尾(header不变)
- addBefore(lm.header);
- //前两步其实就是移动该节点到连飙头,因为他刚被访问过
- }
- }
- // 覆盖父类的方法,删除键值对时同时从链表中移除
- void recordRemoval(HashMap<K,V> m) {
- remove();
- }
- }
- // 迭代器部分
- private abstract class LinkedHashIterator<T> implements Iterator<T> {
- Entry<K,V> nextEntry = header.after;
- Entry<K,V> lastReturned = null;
- int expectedModCount = modCount;
- public boolean hasNext() {
- return nextEntry != header;
- }
- public void remove() {
- if (lastReturned == null)
- throw new IllegalStateException();
- if (modCount != expectedModCount)
- throw new ConcurrentModificationException();
- LinkedHashMap.this.remove(lastReturned.key);
- lastReturned = null;
- expectedModCount = modCount;
- }
- Entry<K,V> nextEntry() {
- if (modCount != expectedModCount)
- throw new ConcurrentModificationException();
- if (nextEntry == header)
- throw new NoSuchElementException();
- Entry<K,V> e = lastReturned = nextEntry;
- nextEntry = e.after;
- return e;
- }
- }
- private class KeyIterator extends LinkedHashIterator<K> {
- public K next() { return nextEntry().getKey(); }
- }
- private class ValueIterator extends LinkedHashIterator<V> {
- public V next() { return nextEntry().value; }
- }
- private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {
- public Map.Entry<K,V> next() { return nextEntry(); }
- }
- // These Overrides alter the behavior of superclass view iterator() methods
- Iterator<K> newKeyIterator() { return new KeyIterator(); }
- Iterator<V> newValueIterator() { return new ValueIterator(); }
- Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }
- // 添加键值对
- 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;
- // 判断最旧的,也就是在链表头部的节点是否需要被删除
- if (removeEldestEntry(eldest)) {
- removeEntryForKey(eldest.key);
- } else {
- if (size >= threshold)
- resize(2 * table.length);
- }
- }
- // 比起hashmap中的createEntry方法,增加了修改链表
- void createEntry(int hash, K key, V value, int bucketIndex) {
- HashMap.Entry<K,V> old = table[bucketIndex];
- Entry<K,V> e = new Entry<>(hash, key, value, old);
- table[bucketIndex] = e;
- // 添加一个键值对时,总要将其链接到维护的链表结尾
- e.addBefore(header);
- size++;
- }
- /******LinkedHashMap暴露的方法,可以用起来实现LRU算法*****/
- protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
- return false;
- }
- }
LruCache
- public class LruCache<K, V> {
- // LRC算法底层由LinkedHashMap实现
- private final LinkedHashMap<K, V> map;
- // 缓存的数量大小,可以使元素个数、字节数等等任何想要的
- private int size;
- // 缓存的最大值
- private int maxSize;
- private int putCount;
- private int createCount;
- // 由于缓存空间满了被逐出的次数
- private int evictionCount;
- // 从缓存取命中次数
- private int hitCount;
- // 为在缓存中找到的次数,即失败次数
- private int missCount;
- public LruCache(int maxSize) {
- if (maxSize <= 0) {
- throw new IllegalArgumentException("maxSize <= 0");
- }
- this.maxSize = maxSize;
- this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
- }
- //
- public void resize(int maxSize) {
- if (maxSize <= 0) {
- throw new IllegalArgumentException("maxSize <= 0");
- }
- synchronized (this) {
- this.maxSize = maxSize;
- }
- trimToSize(maxSize);
- }
- //
- public final V get(K key) {
- if (key == null) {
- // 不允许出现null的键和HashMap不一样
- throw new NullPointerException("key == null");
- }
- V mapValue;
- synchronized (this) {
- mapValue = map.get(key);
- if (mapValue != null) {
- // 每get成功一次hitCount就自加一次,表示命中次数
- hitCount++;
- // 如果该键对应的值存在,返回之。
- return mapValue;
- }
- missCount++;
- }
- // 否则,创建该键值对,默认值为null
- V createdValue = create(key);
- if (createdValue == null) {
- return null;
- }
- synchronized (this) {
- createCount++;
- // 将创建的value添加到map
- mapValue = map.put(key, createdValue);
- if (mapValue != null) {
- // mapValue部位空,表示本线程在put之前已经被别的线程put了一个值,即产生了冲突
- // 此时我们扔掉刚创建的value,而是使用其他地方产生的value
- map.put(key, mapValue);
- } else {
- // 将其放进map中的同时缓存的size增加
- size += safeSizeOf(key, createdValue);
- }
- }
- if (mapValue != null) {
- entryRemoved(false, key, createdValue, mapValue);
- return mapValue;
- } else {
- // 根据maxSize修改map,因为有可能由于此次的put操作使得容量超过最大值,具体的修改方式在子函数中
- trimToSize(maxSize);
- return createdValue;
- }
- }
- // 和get基本一样
- public final V put(K key, V value) {
- if (key == null || value == null) {
- throw new NullPointerException("key == null || value == null");
- }
- V previous;
- synchronized (this) {
- putCount++;
- size += safeSizeOf(key, value);
- previous = map.put(key, value);
- if (previous != null) {
- // 返回值部位null,说明之前该键对应的有值,即使替换,因此占用空间减去之前元素
- size -= safeSizeOf(key, previous);
- }
- }
- if (previous != null) {
- // 移除元素时调用
- entryRemoved(false, key, previous, value);
- }
- trimToSize(maxSize);
- return previous;
- }
- // 根据maxSize增删map
- private void trimToSize(int maxSize) {
- while (true) {
- K key;
- V value;
- synchronized (this) {
- if (size < 0 || (map.isEmpty() && size != 0)) {
- throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!");
- }
- // 当前占用的空间小于最大空间时跳出
- if (size <= maxSize) {
- break;
- }
- // 否则,取出最近最长未使用的元素,也就是链表最前面的一个
- // v5.0.1版本的utils包提供的感觉有问题。
- /*Map.Entry<K, V> toEvict = null;
- for (Map.Entry<K, V> entry : map.entrySet()) {
- // 循环直到最后一个???
- toEvict = entry;
- }
- if (toEvict == null) {
- break;
- }
- */
- // V4 包里面的实现https://github.com/android/platform_frameworks_support/blob/master/v4/java/android/support/v4/util/LruCache.java
- Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
- // 然而google已经提供的LinkedHashMap中就有一个函数获得eldest的元素,于是有些版本()4.4.2的写法比较好理解
- key = toEvict.getKey();
- value = toEvict.getValue();
- // 移除该元素
- map.remove(key);
- // 并将占用空间减少
- size -= safeSizeOf(key, value);
- evictionCount++;
- }
- entryRemoved(true, key, value, null);
- }
- }
- /**
- * Removes the entry for {@code key} if it exists.
- *
- * @return the previous value mapped by {@code key}.
- */
- public final V remove(K key) {
- if (key == null) {
- throw new NullPointerException("key == null");
- }
- V previous;
- synchronized (this) {
- previous = map.remove(key);
- if (previous != null) {
- size -= safeSizeOf(key, previous);
- }
- }
- if (previous != null) {
- entryRemoved(false, key, previous, null);
- }
- return previous;
- }
- // true if the entry is being removed to make space, false if the removal was caused by a put or remove.
- /****** 可以覆盖进行其他操作 ******/
- protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
- // 当需要的元素不存在时执行,可以自行覆盖
- protected V create(K key) {
- return null;
- }
- // 返回一个值表示占用的空间,这里做了参数检查
- private int safeSizeOf(K key, V value) {
- int result = sizeOf(key, value);
- if (result < 0) {
- throw new IllegalStateException("Negative size: " + key + "=" + value);
- }
- return result;
- }
- // 返回一个值表示占用的空间
- /****** 需要覆盖对不同的元素(键值对)进行不同的处理 ******/
- protected int sizeOf(K key, V value) {
- return 1;
- }
- // 逐出所有的元素,参数为-1,只要里面还有元素就会大于-1,于是要全部移除
- public final void evictAll() {
- trimToSize(-1); // -1 will evict 0-sized elements
- }
- public synchronized final int size() {
- return size;
- }
- public synchronized final int maxSize() {
- return maxSize;
- }
- public synchronized final int hitCount() {
- return hitCount;
- }
- public synchronized final int missCount() {
- return missCount;
- }
- public synchronized final int createCount() {
- return createCount;
- }
- public synchronized final int putCount() {
- return putCount;
- }
- public synchronized final int evictionCount() {
- return evictionCount;
- }
- public synchronized final Map<K, V> snapshot() {
- return new LinkedHashMap<K, V>(map);
- }
- @Override
- public synchronized final String toString() {
- int accesses = hitCount + missCount;
- int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
- return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
- maxSize, hitCount, missCount, hitPercent);
- }
- }
android 4.4.2中的LinkedHashMap直接提供了获得最旧元素的方法
- /**
- * Returns the eldest entry in the map, or {@code null} if the map is empty.
- * @hide
- */
- public Entry<K, V> eldest() {
- LinkedEntry<K, V> eldest = header.nxt;
- return eldest != header ? eldest : null;
- }
上面提到的HashMap和LinkedHashMap在jdk的不同版本变化较大,并且和android包中的实现也有一些差异。
以上。
- 从HashMap到LruCache的源码分析
- 从HashMap到LruCache的源码分析
- 从源码分析 HashMap
- 从源码分析HashMap实现
- Android中LruCache的源码分析
- 从源码的角度分析Hashtable和HashMap的区别
- HashMap的源码分析
- HashMap的源码分析
- HashMap 的源码分析
- hashmap的源码分析
- android LRUCache源码分析
- LruCache源码分析
- Lrucache源码分析
- LruCache源码分析
- LruCache源码分析
- Android LruCache 源码分析
- LruCache源码分析
- LruCache 源码分析
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