数组+链表改成了数组 + 链表/红黑树
链表插入由头插法改为尾插法
扩容时1.7对原数组中的元素重新hash定位,1.8是位置不变或者是索引+旧容量大小
插入与扩容的顺序。1.8是先插入再扩容。
hashmap有数据覆盖的问题。不是线程安全。
例子:putval 比如线程A符合判断条件 if ((p = tab[i = (n - 1) & hash]) == null)
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获取table数组的索引下标 index 和链表的头结点,进入条件判断后正好挂起;而线程B也符合条件判断语句,并且获取的table数组的索引下标也是index和链表的头结点,B的数据会写入table[index]。
之后A线程恢复,持有过期的链表的头结点,A的数据会写入table[index]中,覆盖B的数据。
这是A恢复现场,赋值操作。还有重复扩容。
putval的步骤:
一、若数组为空,则通过resize()扩容 二、计算数组索引,若为空则直接插入 三、 否则说明索引对应的位置已有元素,分类讨论
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; //第一步:判断table是否为空,则调用resize()函数创建一个 if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; //第二步:计算元素的储存位置index,如果为空则直接插入 if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); //若不为空,说明要添加的位置上已经有元素,需要分类讨论 else { Node<K,V> e; K k; //第一种情况:key值相同,直接覆盖 if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; else if (p instanceof TreeNode) //第二种情况:判断是否是红黑树 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { //第三种情况:链表 for (int binCount = 0; ; ++binCount) { // 如果是链表末尾,新增节点 if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st treeifyBin(tab, hash); break; } // 如果链表中存在key,则覆盖 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } if (e != null) { // existing mapping for key V oldValue = e.value; //若链表长度>=7,转红黑树 if (!onlyIfAbsent || oldValue == null) e.value = value;//放入值 afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; } 复制代码
直接在操作方法上加上synchronized,锁住整个数组,粗粒度。
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, java.io.Serializable { ...................................... public Hashtable(Map<? extends K, ? extends V> t) { this(Math.max(2*t.size(), 11), 0.75f); putAll(t); } public synchronized int size() { return count; } public synchronized boolean isEmpty() { return count == 0; } public synchronized Enumeration<K> keys() { return this.<K>getEnumeration(KEYS); } public synchronized Enumeration<V> elements() { return this.<V>getEnumeration(VALUES); } public synchronized boolean contains(Object value) { if (value == null) { throw new NullPointerException(); } Entry<?,?> tab[] = table; for (int i = tab.length ; i-- > 0 ;) { for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) { if (e.value.equals(value)) { return true; } } } return false; } } ....................................... } 复制代码
内部定义一个对象锁mutex
public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) { return new SynchronizedMap<>(m); } /** * @serial include */ private static class SynchronizedMap<K,V> implements Map<K,V>, Serializable { private static final long serialVersionUID = 1978198479659022715L; private final Map<K,V> m; // Backing Map final Object mutex; // Object on which to synchronize SynchronizedMap(Map<K,V> m) { this.m = Objects.requireNonNull(m); mutex = this; } SynchronizedMap(Map<K,V> m, Object mutex) { this.m = m; this.mutex = mutex; } public int size() { synchronized (mutex) {return m.size();} } public boolean isEmpty() { synchronized (mutex) {return m.isEmpty();} } public boolean containsKey(Object key) { synchronized (mutex) {return m.containsKey(key);} } public boolean containsValue(Object value) { synchronized (mutex) {return m.containsValue(value);} } public V get(Object key) { synchronized (mutex) {return m.get(key);} } public V put(K key, V value) { synchronized (mutex) {return m.put(key, value);} } public V remove(Object key) { synchronized (mutex) {return m.remove(key);} } public void putAll(Map<? extends K, ? extends V> map) { synchronized (mutex) {m.putAll(map);} } public void clear() { synchronized (mutex) {m.clear();} } private transient Set<K> keySet; private transient Set<Map.Entry<K,V>> entrySet; private transient Collection<V> values; public Set<K> keySet() { synchronized (mutex) { if (keySet==null) keySet = new SynchronizedSet<>(m.keySet(), mutex); return keySet; } } public Set<Map.Entry<K,V>> entrySet() { synchronized (mutex) { if (entrySet==null) entrySet = new SynchronizedSet<>(m.entrySet(), mutex); return entrySet; } } public Collection<V> values() { synchronized (mutex) { if (values==null) values = new SynchronizedCollection<>(m.values(), mutex); return values; } } public boolean equals(Object o) { if (this == o) return true; synchronized (mutex) {return m.equals(o);} } public int hashCode() { synchronized (mutex) {return m.hashCode();} } public String toString() { synchronized (mutex) {return m.toString();} } // Override default methods in Map @Override public V getOrDefault(Object k, V defaultValue) { synchronized (mutex) {return m.getOrDefault(k, defaultValue);} } @Override public void forEach(BiConsumer<? super K, ? super V> action) { synchronized (mutex) {m.forEach(action);} } @Override public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) { synchronized (mutex) {m.replaceAll(function);} } @Override public V putIfAbsent(K key, V value) { synchronized (mutex) {return m.putIfAbsent(key, value);} } @Override public boolean remove(Object key, Object value) { synchronized (mutex) {return m.remove(key, value);} } @Override public boolean replace(K key, V oldValue, V newValue) { synchronized (mutex) {return m.replace(key, oldValue, newValue);} } @Override public V replace(K key, V value) { synchronized (mutex) {return m.replace(key, value);} } ........................... } 复制代码
使用分段锁,降低锁的粒度。
ConcurrentHashMap成员变量 使用volatile 修饰 。
使用CAS操作和synchronized结合实现赋值操作,多线程操作只会锁住当前操作索引的节点。
public class ConcurrentHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Serializable { ............................................. transient volatile Node<K,V>[] table; /** * The next table to use; non-null only while resizing. */ private transient volatile Node<K,V>[] nextTable; /** * Base counter value, used mainly when there is no contention, * but also as a fallback during table initialization * races. Updated via CAS. */ private transient volatile long baseCount; /** * Table initialization and resizing control. When negative, the * table is being initialized or resized: -1 for initialization, * else -(1 + the number of active resizing threads). Otherwise, * when table is null, holds the initial table size to use upon * creation, or 0 for default. After initialization, holds the * next element count value upon which to resize the table. */ private transient volatile int sizeCtl; /** * The next table index (plus one) to split while resizing. */ private transient volatile int transferIndex; /** * Spinlock (locked via CAS) used when resizing and/or creating CounterCells. */ private transient volatile int cellsBusy; /** * Table of counter cells. When non-null, size is a power of 2. */ private transient volatile CounterCell[] counterCells; ................................................... } 复制代码
JDK1.8版本的,内部使用数组 + 链表/红黑树。 如果自己传入初始大小k,初始化大小为大于k的2的整数次方,例如如果传10,大小为16。默认初始值是16,最大容量是2^31次方。
/** * The default initial capacity - MUST be a power of two. */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 /** * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. */ static final int MAXIMUM_CAPACITY = 1 << 30; 复制代码
先拿到key的hashcode,然后让hashcode的前16位与后16位进行异或
尽可能降低hash碰撞,越分散越好
尽可能高效,这是高频操作,所以采用位运算
不直接使用hashcode的原因是hashcode函数的返回类型是int型散列值。初始化数组只有16,容易出现哈希冲突
static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); } 复制代码
Node
final Node<K,V> getNode(int hash, Object key) { Node<K,V>[] tab; Node<K,V> first, e; int n; K k; if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) { if (first.hash == hash && // always check first node ((k = first.key) == key || (key != null && key.equals(k)))) return first; if ((e = first.next) != null) { if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } while ((e = e.next) != null); } } return null; } 复制代码
get函数
public V get(Object key) { Node<K,V> e; return (e = getNode(hash(key), key)) == null ? null : e.value; } /** * Implements Map.get and related methods * * @param hash hash for key * @param key the key * @return the node, or null if none */ final Node<K,V> getNode(int hash, Object key) { Node<K,V>[] tab; Node<K,V> first, e; int n; K k; if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) { if (first.hash == hash && // always check first node ((k = first.key) == key || (key != null && key.equals(k)))) return first; if ((e = first.next) != null) { if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } while ((e = e.next) != null); } } return null; } 复制代码
判断数组是否为空,为空进行初始化;
不为空,计算 k 的 hash 值,通过(n - 1) & hash计算应当存放在数组中的下标 index; 查看 table[index] 是否存在数据,没有数据就构造一个Node节点存放在 table[index] 中; 存在数据,说明发生了hash冲突(存在二个节点key的hash值一样), 继续判断key是否相等,相等,用新的value替换原数据(onlyIfAbsent为false);
如果不相等,判断当前节点类型是不是树型节点,如果是树型节点,创造树型节点插入红黑树中;
如果不是树型节点,创建普通Node加入链表中;判断链表长度是否大于 8, 大于的话链表转换为红黑树;
插入完成之后判断当前节点数是否大于阈值,如果大于开始扩容为原数组的二倍。
LinkedHashMap内部维护了一个单链表,有头尾节点。
LinkedHashMap节点Entry内部除了继承HashMap的Node属性, before 和 after用于标识前置节点和后置节点 。
实现按插入的顺序或访问顺序排序。
public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V> { /** * HashMap.Node subclass for normal LinkedHashMap entries. */ static class Entry<K,V> extends HashMap.Node<K,V> { Entry<K,V> before, after; Entry(int hash, K key, V value, Node<K,V> next) { super(hash, key, value, next); } } ..................................................... } 复制代码
默认是自然排序。 key所属的类实现Comparable接口进行比较。
public class TreeMap<K,V> extends AbstractMap<K,V> implements NavigableMap<K,V>, Cloneable, java.io.Serializable { /** * The comparator used to maintain order in this tree map, or * null if it uses the natural ordering of its keys. * * @serial */ private final Comparator<? super K> comparator; ........................................... public TreeMap(Comparator<? super K> comparator) { this.comparator = comparator; } } 复制代码