HashMap、ConcurrentHashMap和HashTable的區別
阿新 • • 發佈:2020-08-24
一:HashMap
HashMap底層實現繼承於Map介面。HashMap不是執行緒安全的。底層實現由1.8之前的 “陣列+連結串列” 改為 “陣列+連結串列+紅黑樹”。一下為jdk
HashMap的屬性:
/** * 預設的hashMap初始化大小. */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 /** * HashMap最大容量2的30次 */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * 負載因子0.75,當hashMap容量超過75%時,HashMap自動擴容 */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * 當表未膨脹時要共享的空表例項. */ static final Entry<?,?>[] EMPTY_TABLE = {}; /** * 陣列表,大小可以改變,且大小必須為2的冪. */ transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE; /** * hashMap的大小. */ transient int size; /** * 下一個需要擴容時的大小(容量 * 負載因子). * @serial */ int threshold; /** * 雜湊表的載入因子. * @serial */ final float loadFactor; /** *HashMap修改的次數. */ transient int modCount; /** * 最大擴容閾值. */ static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
HashMap擴容:
private static int roundUpToPowerOf2(int number) { // assert number >= 0 : "number must be non-negative"; // 斷言數量必須大於0 return number >= MAXIMUM_CAPACITY ? MAXIMUM_CAPACITY : (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1; } /** * 表膨脹. */ 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); }
HashMap的put方法
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; //當key已經存在時,新值替換久值,返回舊值 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; } 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++; }
當HashMap內容發生變化達到擴容值時,呼叫resize方法。將老table中資料賦值到給新的table中
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); } /** * Transfers all entries from current table to newTable. */ 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; } } }
二:ConcurrentHashMap
ConcurrentHashMap是java併發包下的一個執行緒安全的類。 底層實現繼承於Map介面。Segment 對將HashMap分成16(預設)段。分段加鎖實現(使用非公平鎖)。實現執行緒安全。Segment的建立使用 CAS操作。
ConcurrentHashMap的特殊屬性
/**
* 預設的併發級別,預設分成16段加鎖.
*/
static final int DEFAULT_CONCURRENCY_LEVEL = 16;
/** * 最小分段容量 */ static final int MIN_SEGMENT_TABLE_CAPACITY = 2; /** * 最大允許的分段數 */ static final int MAX_SEGMENTS = 1 << 16; // slightly conservative /** * 重新嘗試加鎖次數 為2 . */ static final int RETRIES_BEFORE_LOCK = 2;
有四種構造器 最後都呼叫public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) { if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) throw new IllegalArgumentException(); if (concurrencyLevel > MAX_SEGMENTS) concurrencyLevel = MAX_SEGMENTS; // Find power-of-two sizes best matching arguments int sshift = 0; int ssize = 1; while (ssize < concurrencyLevel) { ++sshift; ssize <<= 1; } this.segmentShift = 32 - sshift; this.segmentMask = ssize - 1; if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; int c = initialCapacity / ssize; if (c * ssize < initialCapacity) ++c; int cap = MIN_SEGMENT_TABLE_CAPACITY; while (cap < c) cap <<= 1; // create segments and segments[0] Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]); Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] this.segments = ss; } public ConcurrentHashMap(int initialCapacity, float loadFactor) { this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL); } public ConcurrentHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); } public ConcurrentHashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); }
put方法
public V put(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key); int j = (hash >>> segmentShift) & segmentMask; if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment s = ensureSegment(j); return s.put(key, hash, value, false); } final V put(K key, int hash, V value, boolean onlyIfAbsent) { HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value); V oldValue; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; HashEntry<K,V> first = entryAt(tab, index); for (HashEntry<K,V> e = first;;) { if (e != null) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; if (!onlyIfAbsent) { e.value = value; ++modCount; } break; } e = e.next; } else { if (node != null) node.setNext(first); else node = new HashEntry<K,V>(hash, key, value, first); int c = count + 1; if (c > threshold && tab.length < MAXIMUM_CAPACITY) rehash(node); else setEntryAt(tab, index, node); ++modCount; count = c; oldValue = null; break; } } } finally { unlock(); } return oldValue; }
remove方法
public V remove(Object key) { int hash = hash(key); Segment<K,V> s = segmentForHash(hash); return s == null ? null : s.remove(key, hash, null); } final V remove(Object key, int hash, Object value) { if (!tryLock()) scanAndLock(key, hash); V oldValue = null; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; HashEntry<K,V> e = entryAt(tab, index); HashEntry<K,V> pred = null; while (e != null) { K k; HashEntry<K,V> next = e.next; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { V v = e.value; if (value == null || value == v || value.equals(v)) { if (pred == null) setEntryAt(tab, index, next); else pred.setNext(next); ++modCount; --count; oldValue = v; } break; } pred = e; e = next; } } finally { unlock(); } return oldValue; }
get方法
public V get(Object key) { Segment<K,V> s; // manually integrate access methods to reduce overhead HashEntry<K,V>[] tab; int h = hash(key); long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value; } } return null; }
三:HashTable
HashMap底層實現沒有實現Map介面。