喜歡的話,勞煩關注一下微信公衆號《深夜裏的程序猿》噢~html
曾幾什麼時候,本身開始研究jdk源碼,許久以後又記憶模糊,因此仍是決定寫個博客記錄一下學習過程。爲何標題要寫着java7呢?由於J8的Hashmap改動不小,因此先從本身稍微熟悉點的J7源碼開始,廢話很少說,開整~java
宏觀角度觀察HashMap算法
首先這玩意是 【數組+鏈表】的結構,能夠接受null做爲鍵值,是非線程安全的類 ...數組
HashMap查詢速度快主要是由於它是經過計算散列碼來決定存儲的位置,HashMap中主要是經過key的hashCode來計算hash值的,只要hashCode相同,計算出來的hash值就同樣。若是存儲的對象對多了,就有可能不一樣的對象所算出來的hash值是相同的,這就出現了所謂的hash衝突,HashMap底層是經過鏈表來解決hash衝突的。安全
源碼分析微信
一、HashMap的本質多線程
static final Entry<?,?>[] EMPTY_TABLE = {}; transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
HashMap實際上是一個Entry數組,Entry對象中包含了鍵和值,其中next也是一個Entry對象,它就是用來處理hash衝突的,造成一個鏈表,關於Entry的的實現,你們可查看下面完整的源碼。app
二、何爲負載因子?dom
static final float DEFAULT_LOAD_FACTOR = 0.75f;
默認的負載因子大小爲0.75,也就是說,當HashMap填滿了75%的bucket(也就是數組)之後,將會建立原來HashMap大小的兩倍的bucket數組,來從新調整map的大小,並將原來的對象放入新的bucket數組中。這個過程叫做rehashing,由於它調用hash方法找到新的bucket位置。ide
三、如何存放數據?
public V put(K key, V value) { //判斷當前數組是否等於{},如果則初始化數組 if (table == EMPTY_TABLE) { inflateTable(threshold); } //判斷 key 是否等於 null,是則將把當前鍵值對添加進table[0]中,遍歷table[0]鏈表 //若是已經有null爲key的Entry,則修改值,返回舊值,若無則直接添加。 if (key == null) return putForNullKey(value); //key不爲null則計算hash int hash = hash(key); //搜索對應hash所在的table中的索引 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; } } //修改次數 modCount++; addEntry(hash, key, value, i); return null; }
這裏注意一下addEntry();這個方法
void addEntry(int hash, K key, V value, int bucketIndex) { //判斷當前的元素數量是否大於等於臨界值 //如果則對數組進行2倍擴容,並從新得到新的bucketindex if ((size >= threshold) && (null != table[bucketIndex])) { resize(2 * table.length); hash = (null != key) ? hash(key) : 0; bucketIndex = indexFor(hash, table.length); } //真正的添加方法 createEntry(hash, key, value, bucketIndex); } 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++; }
四、數據的獲取
public V get(Object key) { if (key == null) return getForNullKey(); Entry<K,V> entry = getEntry(key); return null == entry ? null : entry.getValue(); } final Entry<K,V> getEntry(Object key) { if (size == 0) { return null; } int hash = (key == null) ? 0 : hash(key); 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 != null && key.equals(k)))) return e; } }
五、關於數組調整大小
void resize(int newCapacity) { Entry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; //將舊數組移動至新數組 transfer(newTable, initHashSeedAsNeeded(newCapacity)); table = newTable; //從新計算臨界值 threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); }
HashMap擴展
一、到達臨界值,從新調整HashMap大小有什麼問題嗎?
在多線程環境下,可能會產生條件競爭。因此這時候能夠用ConcurrentHashMap或者使用Conllections.synchronizedMap(map)包裝一下。
二、Conllections.synchronizedMap(map)幹了什麼事?
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);} }
這裏截取了裏面的部分方法,變量 m 是咱們傳進去的map,如今你們應該知道這個方法幹了什麼事了吧?其實就是對之前的老map包裹了一層 帶synchrinized的同名方法。
三、HashTable和HashMap區別?
簡單的說有三點:
1 HashTable方法上帶有synchrinized,是線程安全的,而HashMap是線程不安全的
2 HashTable key,value不能爲null
3 HashTable 繼承Dictionary ,HashMap繼承AbstractMap
四、HashSet和HashMap區別?
HashSet底層其實就是一個固定值的HashMap
private transient HashMap<E,Object> map; private static final Object PRESENT = new Object(); /* *HashSet的add()方法實際是調用了HashMap的put()方法,值是惟一的 */ public boolean add(E e) { return map.put(e, PRESENT)==null; }
四、使用自定義對象做爲鍵
必須遵循equals()和hashCode()方法的定義規則,插入Map之後必須不可變,不然沒法取出數據。
結尾
本片博客只做爲本身簡單記錄一下,hashmap裏面還有不少值得研究的地方,例如計算hash的方法,計算存儲索引的方法,因爲本身是算法渣,就不去深究了。
文中有什麼說的不對的地方歡迎你們指正、交流,謝謝。
參考:
http://www.importnew.com/7099.html
http://www.cnblogs.com/ITtangtang/p/3948406.html
HashMap完整源碼
package java.util; import java.io.*; public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable { static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 static final int MAXIMUM_CAPACITY = 1 << 30; static final float DEFAULT_LOAD_FACTOR = 0.75f;//默認加載因子 static final Entry<?,?>[] EMPTY_TABLE = {}; transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;//存放元素數組 transient int size;//存放元素個數 int threshold;//臨界值 加載因子 * 容量 final float loadFactor;//加載因子 transient int modCount;//修改次數 static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; private static class Holder { static final int ALTERNATIVE_HASHING_THRESHOLD; static { String altThreshold = java.security.AccessController.doPrivileged( new sun.security.action.GetPropertyAction( "jdk.map.althashing.threshold")); int threshold; try { threshold = (null != altThreshold) ? Integer.parseInt(altThreshold) : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT; // disable alternative hashing if -1 if (threshold == -1) { threshold = Integer.MAX_VALUE; } if (threshold < 0) { throw new IllegalArgumentException("value must be positive integer."); } } catch(IllegalArgumentException failed) { throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed); } ALTERNATIVE_HASHING_THRESHOLD = threshold; } } transient int hashSeed = 0; public HashMap(int initialCapacity, float loadFactor) { if (initialCapacity < 0) throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity); if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal load factor: " + loadFactor); this.loadFactor = loadFactor; threshold = initialCapacity; init(); } public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } public HashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); } public HashMap(Map<? extends K, ? extends V> m) { this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR); inflateTable(threshold); putAllForCreate(m); } private static int roundUpToPowerOf2(int number) { // assert number >= 0 : "number must be non-negative"; int rounded = number >= MAXIMUM_CAPACITY ? MAXIMUM_CAPACITY : (rounded = Integer.highestOneBit(number)) != 0 ? (Integer.bitCount(number) > 1) ? rounded << 1 : rounded : 1; return rounded; } private void inflateTable(int toSize) { // Find a power of 2 >= toSize int capacity = roundUpToPowerOf2(toSize); threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); table = new Entry[capacity]; initHashSeedAsNeeded(capacity); } // internal utilities void init() { } final boolean initHashSeedAsNeeded(int capacity) { boolean currentAltHashing = hashSeed != 0; boolean useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); boolean switching = currentAltHashing ^ useAltHashing; if (switching) { hashSeed = useAltHashing ? sun.misc.Hashing.randomHashSeed(this) : 0; } return switching; } final int hash(Object k) { int h = hashSeed; if (0 != h && k instanceof String) { return sun.misc.Hashing.stringHash32((String) k); } h ^= k.hashCode(); // 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); } static int indexFor(int h, int length) { // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2"; return h & (length-1); } public int size() { return size; } public boolean isEmpty() { return size == 0; } public V get(Object key) { if (key == null) return getForNullKey(); Entry<K,V> entry = getEntry(key); return null == entry ? null : entry.getValue(); } private V getForNullKey() { if (size == 0) { return null; } 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; } final Entry<K,V> getEntry(Object key) { if (size == 0) { return null; } int hash = (key == null) ? 0 : hash(key); 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 != null && key.equals(k)))) return e; } return null; } public V put(K key, V value) { if (table == EMPTY_TABLE) { inflateTable(threshold); } if (key == null) return putForNullKey(value); int hash = hash(key); 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; } private V putForNullKey(V value) { for (Entry<K,V> e = table[0]; e != null; e = e.next) { if (e.key == null) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(0, null, value, 0); return null; } private void putForCreate(K key, V value) { int hash = null == key ? 0 : hash(key); int i = indexFor(hash, table.length); /** * Look for preexisting entry for key. This will never happen for * clone or deserialize. It will only happen for construction if the * input Map is a sorted map whose ordering is inconsistent w/ equals. */ 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); } private void putAllForCreate(Map<? extends K, ? extends V> m) { for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) putForCreate(e.getKey(), e.getValue()); } void resize(int newCapacity) { Entry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; transfer(newTable, initHashSeedAsNeeded(newCapacity)); table = newTable; threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); } void transfer(Entry[] newTable, boolean rehash) { int newCapacity = newTable.length; for (Entry<K,V> e : table) { while(null != e) { Entry<K,V> next = e.next; if (rehash) { e.hash = null == e.key ? 0 : hash(e.key); } int i = indexFor(e.hash, newCapacity); e.next = newTable[i]; newTable[i] = e; e = next; } } } public void putAll(Map<? extends K, ? extends V> m) { int numKeysToBeAdded = m.size(); if (numKeysToBeAdded == 0) return; if (table == EMPTY_TABLE) { inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold)); } if (numKeysToBeAdded > threshold) { int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1); if (targetCapacity > MAXIMUM_CAPACITY) targetCapacity = MAXIMUM_CAPACITY; int newCapacity = table.length; while (newCapacity < targetCapacity) newCapacity <<= 1; if (newCapacity > table.length) 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); } final Entry<K,V> removeEntryForKey(Object key) { if (size == 0) { return null; } int hash = (key == null) ? 0 : hash(key); 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; e.recordRemoval(this); return e; } prev = e; e = next; } return e; } final Entry<K,V> removeMapping(Object o) { if (size == 0 || !(o instanceof Map.Entry)) return null; Map.Entry<K,V> entry = (Map.Entry<K,V>) o; Object key = entry.getKey(); int hash = (key == null) ? 0 : hash(key); 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)) { modCount++; size--; if (prev == e) table[i] = next; else prev.next = next; e.recordRemoval(this); return e; } prev = e; e = next; } return e; } public void clear() { modCount++; Arrays.fill(table, null); size = 0; } 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; } private boolean containsNullValue() { Entry[] tab = table; 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; } if (result.table != EMPTY_TABLE) { result.inflateTable(Math.min( (int) Math.min( size * Math.min(1 / loadFactor, 4.0f), // we have limits... HashMap.MAXIMUM_CAPACITY), table.length)); } result.entrySet = null; result.modCount = 0; result.size = 0; result.init(); result.putAllForCreate(this); return result; } static class Entry<K,V> implements Map.Entry<K,V> { final K key; V value; Entry<K,V> next; int hash; /** * Creates new entry. */ 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 Objects.hashCode(getKey()) ^ Objects.hashCode(getValue()); } public final String toString() { return getKey() + "=" + getValue(); } /** * This method is invoked whenever the value in an entry is * overwritten by an invocation of put(k,v) for a key k that's already * in the HashMap. */ void recordAccess(HashMap<K,V> m) { } /** * This method is invoked whenever the entry is * removed from the table. */ void recordRemoval(HashMap<K,V> m) { } } void addEntry(int hash, K key, V value, int bucketIndex) { if ((size >= threshold) && (null != table[bucketIndex])) { resize(2 * table.length); hash = (null != key) ? hash(key) : 0; bucketIndex = indexFor(hash, table.length); } createEntry(hash, key, value, bucketIndex); } 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++; } private abstract class HashIterator<E> implements Iterator<E> { Entry<K,V> next; // next entry to return 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 private transient Set<Map.Entry<K,V>> entrySet = null; 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())); } 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(); } } private void writeObject(java.io.ObjectOutputStream s) throws IOException { // Write out the threshold, loadfactor, and any hidden stuff s.defaultWriteObject(); // Write out number of buckets if (table==EMPTY_TABLE) { s.writeInt(roundUpToPowerOf2(threshold)); } else { s.writeInt(table.length); } // Write out size (number of Mappings) s.writeInt(size); // Write out keys and values (alternating) if (size > 0) { for(Map.Entry<K,V> e : entrySet0()) { s.writeObject(e.getKey()); s.writeObject(e.getValue()); } } } private static final long serialVersionUID = 362498820763181265L; private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // Read in the threshold (ignored), loadfactor, and any hidden stuff s.defaultReadObject(); if (loadFactor <= 0 || Float.isNaN(loadFactor)) { throw new InvalidObjectException("Illegal load factor: " + loadFactor); } // set other fields that need values table = (Entry<K,V>[]) EMPTY_TABLE; // Read in number of buckets s.readInt(); // ignored. // Read number of mappings int mappings = s.readInt(); if (mappings < 0) throw new InvalidObjectException("Illegal mappings count: " + mappings); // capacity chosen by number of mappings and desired load (if >= 0.25) int capacity = (int) Math.min( mappings * Math.min(1 / loadFactor, 4.0f), // we have limits... HashMap.MAXIMUM_CAPACITY); // allocate the bucket array; if (mappings > 0) { inflateTable(capacity); } else { threshold = capacity; } init(); // Give subclass a chance to do its thing. // Read the keys and values, and put the mappings in the HashMap for (int i = 0; i < mappings; 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; } }