JDK容器類Map源碼解讀
- 2019 年 10 月 7 日
- 筆記
java.util.Map介面是JDK1.2開始提供的一個基於鍵值對的散列表介面，其設計的初衷是為了替換JDK1.0中的java.util.Dictionary抽象類。Dictionary是JDK最初的鍵值對類，它不可以存儲null作為key和value，目前這個類早已不被使用了。目前都是在使用Map介面，它是可以存儲null值作為key和value，但Map的key是不可以重複的。其常用的實現類主要有HashMap，TreeMap，ConcurrentHashMap等
HashMap源碼解讀
目前JDK已經發布到JDK12，主流的JDK版本是JDK8， 但是如果閱讀HashMap的源碼建議先看JDK7的源碼。JDK7和JDK8的源碼中HashMap的實現原理大體相同，只不過是在JDK8中做了部分優化。但是JDK8的源碼可讀性非常差。
HashMap 是一個存儲鍵值對(key-value)映射的散列表，繼承於AbstractMap，實現了Map、Cloneable、java.io.Serializable介面，HashMap是執行緒不安全的，它存儲的映射也是無序的。 HashMap的底層主要是基於數組和鏈表來實現的（JDK8之後又引入了紅黑樹），數據存儲時會通過對key進行哈希操作取到哈希值，然後將哈希值對數組長度取模，得到的值就是該鍵值對在數組中的索引index值，如果數組該位置沒有值則直接將該鍵值對放在該位置，如果該位置已經有值則將其插入相應鏈表的位置，JDK8開始為優化鏈表長度過長導致的性能問題從而引入了紅黑樹，當鏈表的長度大於8時會自動將鏈錶轉成紅黑樹。
JDK7中HashMap的源碼解讀
JDK7中HashMap採用Entry數組來存儲鍵值對，每一個鍵值對組成了一個Entry實體，Entry類實際上是一個單向的鏈表結構，它具有Next指針，可以連接下一個Entry實體組成鏈表。
JDK7中HashMap源碼中的主要欄位
// 數組默認的大小 // 1 << 4，表示1，左移4位，變成10000，即16，以二進位形式運行，效率更高 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 數組最大值 static final int MAXIMUM_CAPACITY = 1 << 30; // 默認的負載因子 static final float DEFAULT_LOAD_FACTOR = 0.75f; // 真正存放數據的數組 transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
HashMap中默認的數組容量為 16，負載因子為 0.75。Map 在使用過程中不斷的往裡面存放數據，當數量達到了 16 * 0.75 = 12 就需要將當前 16 的容量進行擴容，而擴容這個過程涉及到 rehash、複製數據等操作，所以非常消耗性能。因此通常建議能提前預估 HashMap 的大小最好，盡量的減少擴容帶來的性能損耗。
JDK7中HashMap源碼中的構造器
/** 默認的初始化容量、默認的載入因子 * Constructs an empty <tt>HashMap</tt> with the default initial capacity * (16) and the default load factor (0.75). */ public HashMap() { //16 0.75 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); } /** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and the default load factor (0.75). * * @param initialCapacity the initial capacity. * @throws IllegalArgumentException if the initial capacity is negative. */ public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** 做了兩件事：1、為threshold、loadFactor賦值 2、調用init() * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and load factor. * * @param initialCapacity the initial capacity * @param loadFactor the load factor * @throws IllegalArgumentException if the initial capacity is negative * or the load factor is nonpositive */ public HashMap(int initialCapacity, float loadFactor) { if (initialCapacity < 0) throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity); if (initialCapacity > MAXIMUM_CAPACITY) //限制最大容量 initialCapacity = MAXIMUM_CAPACITY; if (loadFactor <= 0 || Float.isNaN(loadFactor)) //檢查 loadFactor throw new IllegalArgumentException("Illegal load factor: " + loadFactor); //真正在做的，只是記錄下loadFactor、initialCpacity的值 this.loadFactor = loadFactor; //記錄下loadFactor threshold = initialCapacity; //初始的 閾值threshold=initialCapacity=16 init(); } /** * Constructs a new <tt>HashMap</tt> with the same mappings as the * specified <tt>Map</tt>. The <tt>HashMap</tt> is created with * default load factor (0.75) and an initial capacity sufficient to * hold the mappings in the specified <tt>Map</tt>. * * @param m the map whose mappings are to be placed in this map * @throws NullPointerException if the specified map is null */ 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); }
JDK7中HashMap源碼中的put方法
/** * Associates the specified value with the specified key in this map. * If the map previously contained a mapping for the key, the old * value is replaced. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt>. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with <tt>key</tt>.) */ public V put(K key, V value) { if (table == EMPTY_TABLE) { inflateTable(threshold); //初始化表 （初始化、擴容 合併為了一個方法） } if (key == null) //對key為null做特殊處理 return putForNullKey(value); int hash = hash(key); //計算hash值 int i = indexFor(hash, table.length); //根據hash值計算出index下標 for (Entry<K,V> e = table[i]; e != null; e = e.next) { //遍歷下標為i處的鏈表 Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //如果key值相同，覆蓋舊值，返回新值 V oldValue = e.value; e.value = value; //新值 覆蓋 舊值 e.recordAccess(this); //do nothing return oldValue; //返回舊值 } } modCount++; //修改次數+1，類似於一個version number addEntry(hash, key, value, i); return null; } /** * Adds a new entry with the specified key, value and hash code to * the specified bucket. It is the responsibility of this * method to resize the table if appropriate. * * Subclass overrides this to alter the behavior of put method. */ void addEntry(int hash, K key, V value, int bucketIndex) { if ((size >= threshold) && (null != table[bucketIndex])) { //如果size大於threshold && table在下標為index的地方已經有entry了 resize(2 * table.length); //擴容，將數組長度變為原來兩倍 hash = (null != key) ? hash(key) : 0; //重新計算 hash 值 bucketIndex = indexFor(hash, table.length); //重新計算下標 } createEntry(hash, key, value, bucketIndex); //創建entry } /** * Rehashes the contents of this map into a new array with a * larger capacity. This method is called automatically when the * number of keys in this map reaches its threshold. * * If current capacity is MAXIMUM_CAPACITY, this method does not * resize the map, but sets threshold to Integer.MAX_VALUE. * This has the effect of preventing future calls. * * @param newCapacity the new capacity, MUST be a power of two; * must be greater than current capacity unless current * capacity is MAXIMUM_CAPACITY (in which case value * is irrelevant). */ void resize(int newCapacity) { Entry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { //狀態檢查 threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; //實例化新的table transfer(newTable, initHashSeedAsNeeded(newCapacity)); //賦值數組元素到新的數組 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); //對key進行hash } int i = indexFor(e.hash, newCapacity); //用新的index來取模 e.next = newTable[i]; newTable[i] = e; //把元素存入新table新的新的index處 e = next; } } } /** * Like addEntry except that this version is used when creating entries * as part of Map construction or "pseudo-construction" (cloning, * deserialization). This version needn't worry about resizing the table. * * Subclass overrides this to alter the behavior of HashMap(Map), * clone, and readObject. */ void createEntry(int hash, K key, V value, int bucketIndex) { Entry<K,V> e = table[bucketIndex]; //獲取table中存的entry table[bucketIndex] = new Entry<>(hash, key, value, e); //將新的entry放到數組中，next指向舊的table[i] size++; //修改map中元素個數 }
JDK7中HashMap源碼中的put方法
/** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code (key==null ? k==null : * key.equals(k))}, then this method returns {@code v}; otherwise * it returns {@code null}. (There can be at most one such mapping.) * * <p>A return value of {@code null} does not <i>necessarily</i> * indicate that the map contains no mapping for the key; it's also * possible that the map explicitly maps the key to {@code null}. * The {@link #containsKey containsKey} operation may be used to * distinguish these two cases. * * @see #put(Object, Object) */ public V get(Object key) { if (key == null) return getForNullKey(); Entry<K,V> entry = getEntry(key); return null == entry ? null : entry.getValue(); } /** * Returns the entry associated with the specified key in the * HashMap. Returns null if the HashMap contains no mapping * for the key. */ 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; }
JDK8中HashMap的源碼解讀
JDK8中HashMap採用Node數組來存儲鍵值對，Node其實就是JDK7中的Entry，只不過是換了一個名字，同樣每一個鍵值對組成了一個Node實體，然後組成鏈表。當 Hash 衝突嚴重時，鏈表會變的越來越長，這樣在查詢時的效率就會越來越低，JDK8所做的優化就是，當鏈表的長度達到8的時候會轉變成紅黑樹TreeNode。
JDK8中HashMap源碼中的主要欄位
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; static final int MAXIMUM_CAPACITY = 1 << 30; static final float DEFAULT_LOAD_FACTOR = 0.75f; // 用於判斷是否需要將鏈錶轉換為紅黑樹的閾值 static final int TREEIFY_THRESHOLD = 8; // 用於判斷是否需要將紅黑樹轉換為鏈表的閾值 static final int UNTREEIFY_THRESHOLD = 6; static final int MIN_TREEIFY_CAPACITY = 64; // 存放數據的數組 transient Node<K,V>[] table;
JDK8中HashMap源碼中的構造器
/** * Constructs an empty <tt>HashMap</tt> with the default initial capacity * (16) and the default load factor (0.75). */ public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted } /** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and the default load factor (0.75). * * @param initialCapacity the initial capacity. * @throws IllegalArgumentException if the initial capacity is negative. */ public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and load factor. * * @param initialCapacity the initial capacity * @param loadFactor the load factor * @throws IllegalArgumentException if the initial capacity is negative * or the load factor is nonpositive */ 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; this.threshold = tableSizeFor(initialCapacity); } /** * Constructs a new <tt>HashMap</tt> with the same mappings as the * specified <tt>Map</tt>. The <tt>HashMap</tt> is created with * default load factor (0.75) and an initial capacity sufficient to * hold the mappings in the specified <tt>Map</tt>. * * @param m the map whose mappings are to be placed in this map * @throws NullPointerException if the specified map is null */ public HashMap(Map<? extends K, ? extends V> m) { this.loadFactor = DEFAULT_LOAD_FACTOR; putMapEntries(m, false); }
JDK8中HashMap源碼中的put方法
/** * Associates the specified value with the specified key in this map. * If the map previously contained a mapping for the key, the old * value is replaced. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt>. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with <tt>key</tt>.) */ public V put(K key, V value) { return putVal(hash(key), key, value, false, true); } /** * Implements Map.put and related methods. 添加元素 * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don't change existing value * @param evict if false, the table is in creation mode. * @return previous value, or null if none */ final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; if ((tab = table) == null || (n = tab.length) == 0) //若table為null n = (tab = resize()).length; //resize if ((p = tab[i = (n - 1) & hash]) == null) //計算下標i，取出i處的元素為p，如果p為null tab[i] = newNode(hash, key, value, null); //創建新的node，放到數組中 else { //若 p!=null Node<K,V> e; K k; if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) //若key相同 e = p; //直接覆蓋 else if (p instanceof TreeNode) //如果為 樹節點 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //放到樹中 else { //如果key不相同，也不是treeNode for (int binCount = 0; ; ++binCount) { //遍歷i處的鏈表 if ((e = p.next) == null) { //找到尾部 p.next = newNode(hash, key, value, null); //在末尾添加一個node if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //如果鏈表長度 >= 8 treeifyBin(tab, hash); //將鏈錶轉成共黑樹 break; } if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) //若果key相同，直接退出循環 break; p = e; } } if (e != null) { // existing mapping for key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; } /** * Replaces all linked nodes in bin at index for given hash unless * table is too small, in which case resizes instead. */ final void treeifyBin(Node<K,V>[] tab, int hash) { int n, index; Node<K,V> e; if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY) resize(); else if ((e = tab[index = (n - 1) & hash]) != null) { TreeNode<K,V> hd = null, tl = null; do { TreeNode<K,V> p = replacementTreeNode(e, null); if (tl == null) hd = p; else { p.prev = tl; tl.next = p; } tl = p; } while ((e = e.next) != null); if ((tab[index] = hd) != null) hd.treeify(tab); } } /** * Initializes or doubles table size. If null, allocates in * accord with initial capacity target held in field threshold. * Otherwise, because we are using power-of-two expansion, the * elements from each bin must either stay at same index, or move * with a power of two offset in the new table. * * @return the table */ final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; // 如果 舊數組為null就講舊的容量看做是0，否則用舊的table長度當做容量 int oldThr = threshold; int newCap, newThr = 0; if (oldCap > 0) { if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; else { // zero initial threshold signifies using defaults newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } threshold = newThr; @SuppressWarnings({"rawtypes","unchecked"}) Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; //創建新的數組 table = newTab; //賦值給table if (oldTab != null) { for (int j = 0; j < oldCap; ++j) { Node<K,V> e; if ((e = oldTab[j]) != null) { oldTab[j] = null; if (e.next == null) newTab[e.hash & (newCap - 1)] = e; else if (e instanceof TreeNode) ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); else { // preserve order Node<K,V> loHead = null, loTail = null; Node<K,V> hiHead = null, hiTail = null; Node<K,V> next; do { next = e.next; if ((e.hash & oldCap) == 0) { if (loTail == null) loHead = e; else loTail.next = e; loTail = e; } else { if (hiTail == null) hiHead = e; else hiTail.next = e; hiTail = e; } } while ((e = next) != null); if (loTail != null) { loTail.next = null; newTab[j] = loHead; } if (hiTail != null) { hiTail.next = null; newTab[j + oldCap] = hiHead; } } } } } return newTab; }
JDK8中HashMap源碼中的get方法
/** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code (key==null ? k==null : * key.equals(k))}, then this method returns {@code v}; otherwise * it returns {@code null}. (There can be at most one such mapping.) * * <p>A return value of {@code null} does not <i>necessarily</i> * indicate that the map contains no mapping for the key; it's also * possible that the map explicitly maps the key to {@code null}. * The {@link #containsKey containsKey} operation may be used to * distinguish these two cases. * * @see #put(Object, Object) */ 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; }
ConcurrentHashMap源碼解讀
ConcurrentHashMap是一個執行緒安全的HashMap實現，ConcurrentHashMap在JDK7和JDK8中的實現差別比較大，JDK7中ConcurrentHashMap是使用Segment數組來存放數據，一個Segment就相當於一個HashMap的數據結構，每個Segment使用一個鎖。JDK8之後Segment雖保留，但僅是為了兼容舊版本，已經不再使用，JDK8中ConcurrentHashMap使用和HashMap一樣的數據結構Node數組來存儲數據，每個數組位置使用一個鎖。
JDK7中的ConcurrentHashMap源碼解讀
JDK7中ConcurrentHashMap的底層Segment組，而Segment其實就是特殊的HashMap，Segment的數據結構跟HashMap一樣，同時它繼承了ReentrantLock，通過ReentrantLock提供的鎖實現了執行緒的安全。ConcurrentHashMap使用分段鎖技術，將數據分成一段一段的存儲，每個Segment就是一段，然後給每一段數據配一把鎖，當一個執行緒佔用鎖訪問其中一個段數據的時候，其他段的數據也能被其他執行緒訪問，能夠實現並發訪問，Segment數組的長度就是ConcurrentHashMap的執行緒並行級別，Segment數組默認的長度為16，也就是說最多同時可以有16個執行緒去訪問ConcurrentHashMap。segment 數組不能擴容，而是對 segment 數組某個位置的segmen內部的數組HashEntry[] 進行擴容，擴容後容量為原來的 2 倍，該方法沒有考慮並發，因為執行該方法之前已經獲取了鎖。
JDK7中的ConcurrentHashMap源碼中的主要欄位
// 數組默認大小 static final int DEFAULT_INITIAL_CAPACITY = 16; // 默認的負載因子 static final float DEFAULT_LOAD_FACTOR = 0.75f; // 默認執行緒並發度 static final int DEFAULT_CONCURRENCY_LEVEL = 16; static final int MIN_SEGMENT_TABLE_CAPACITY = 2; static final int MAX_SEGMENTS = 1 << 16; // 數組最大大小 static final int MAXIMUM_CAPACITY = 1 << 30; static final int MAXIMUM_CAPACITY = 1 << 30; static final int RETRIES_BEFORE_LOCK = 2;
JDK7中的ConcurrentHashMap源碼中的構造器
/** * Creates a new, empty map with a default initial capacity (16), * load factor (0.75) and concurrencyLevel (16). */ public ConcurrentHashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new, empty map with the specified initial capacity, * and with default load factor (0.75) and concurrencyLevel (16). * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @throws IllegalArgumentException if the initial capacity of * elements is negative. */ public ConcurrentHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new, empty map with the specified initial capacity * and load factor and with the default concurrencyLevel (16). * * @param initialCapacity The implementation performs internal * sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @throws IllegalArgumentException if the initial capacity of * elements is negative or the load factor is nonpositive * * @since 1.6 */ public ConcurrentHashMap(int initialCapacity, float loadFactor) { this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new, empty map with the specified initial * capacity, load factor and concurrency level. * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @param concurrencyLevel the estimated number of concurrently * updating threads. The implementation performs internal sizing * to try to accommodate this many threads. * @throws IllegalArgumentException if the initial capacity is * negative or the load factor or concurrencyLevel are * nonpositive. */ @SuppressWarnings("unchecked") public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) { if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) //參數檢查 throw new IllegalArgumentException(); if (concurrencyLevel > MAX_SEGMENTS) //ConcurrentcyLevel實際上就是最大並發數 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]); //創建一個segment Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; //創建一個segment數組 UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] //將s0設置為ss的第一個元素 this.segments = ss; //將ss作為segments }
JDK7中的ConcurrentHashMap源碼中put方法
/** * Maps the specified key to the specified value in this table. * Neither the key nor the value can be null. * * <p> The value can be retrieved by calling the <tt>get</tt> method * with a key that is equal to the original key. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key or value is null */ @SuppressWarnings("unchecked") public V put(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key); // 計算Hash值 int j = (hash >>> segmentShift) & segmentMask; //計算下標j if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment s = ensureSegment(j); //若j處有segment就返回，若沒有就創建並返回 return s.put(key, hash, value, false); //將值put到segment中去 } // Segment 中put數據的方法 final V put(K key, int hash, V value, boolean onlyIfAbsent) { HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value); //如果tryLock成功，就返回null，否則。。。V oldValue; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; //根據table數組的長度 和 hash值計算index小標 HashEntry<K,V> first = entryAt(tab, index); //找到table數組在 index處鏈表的頭部 for (HashEntry<K,V> e = first;;) { //從first開始遍歷鏈表 if (e != null) { //若e!=null K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { //如果key相同 oldValue = e.value; //獲取舊值 if (!onlyIfAbsent) { //若absent=false e.value = value; //覆蓋舊值 ++modCount; // } break; //若已經找到，就退出鏈表遍歷 } e = e.next; //若key不相同，繼續遍歷 } else { //直到e為null if (node != null) //將元素放到鏈表頭部 node.setNext(first); else node = new HashEntry<K,V>(hash, key, value, first); //創建新的Entry int c = count + 1; //count 用來記錄元素個數 if (c > threshold && tab.length < MAXIMUM_CAPACITY) //如果hashmap元素個數超過threshold，並且table長度小於最大容量 rehash(node); //rehash跟resize的功能差不多,將table的長度變為原來的兩倍，重新打包entries，並將給定的node添加到新的table else //如果還有容量 setEntryAt(tab, index, node); //就在index處添加鏈表節點 ++modCount; //修改操作數 count = c; //將count+1 oldValue = null; // break; } } } finally { unlock(); //執行完操作後，釋放鎖 } return oldValue; //返回oldValue } /** 將table的長度變為原來的兩倍，重新打包entries，並將給定的node添加到新的table * Doubles size of table and repacks entries, also adding the * given node to new table */ @SuppressWarnings("unchecked") private void rehash(HashEntry<K,V> node) { /* * Reclassify nodes in each list to new table. Because we * are using power-of-two expansion, the elements from * each bin must either stay at same index, or move with a * power of two offset. We eliminate unnecessary node * creation by catching cases where old nodes can be * reused because their next fields won't change. * Statistically, at the default threshold, only about * one-sixth of them need cloning when a table * doubles. The nodes they replace will be garbage * collectable as soon as they are no longer referenced by * any reader thread that may be in the midst of * concurrently traversing table. Entry accesses use plain * array indexing because they are followed by volatile * table write. */ HashEntry<K,V>[] oldTable = table; int oldCapacity = oldTable.length; int newCapacity = oldCapacity << 1; threshold = (int)(newCapacity * loadFactor); HashEntry<K,V>[] newTable = (HashEntry<K,V>[]) new HashEntry[newCapacity]; int sizeMask = newCapacity - 1; for (int i = 0; i < oldCapacity ; i++) { HashEntry<K,V> e = oldTable[i]; if (e != null) { HashEntry<K,V> next = e.next; int idx = e.hash & sizeMask; if (next == null) // Single node on list newTable[idx] = e; else { // Reuse consecutive sequence at same slot HashEntry<K,V> lastRun = e; int lastIdx = idx; for (HashEntry<K,V> last = next; last != null; last = last.next) { int k = last.hash & sizeMask; if (k != lastIdx) { lastIdx = k; lastRun = last; } } newTable[lastIdx] = lastRun; // Clone remaining nodes for (HashEntry<K,V> p = e; p != lastRun; p = p.next) { V v = p.value; int h = p.hash; int k = h & sizeMask; HashEntry<K,V> n = newTable[k]; newTable[k] = new HashEntry<K,V>(h, p.key, v, n); } } } } int nodeIndex = node.hash & sizeMask; // add the new node node.setNext(newTable[nodeIndex]); newTable[nodeIndex] = node; table = newTable; }
JDK7中的ConcurrentHashMap源碼中get方法
/** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code key.equals(k)}, * then this method returns {@code v}; otherwise it returns * {@code null}. (There can be at most one such mapping.) * * @throws NullPointerException if the specified key is null */ 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; }
JDK8中的ConcurrentHashMap源碼解讀
JDK8中的ConcurrentHashMap取消了基於 Segment 的分段鎖思想，改用 CAS + synchronized 控制並發操作，鎖的粒度變得更小，並發度更高。並且追隨JDK8的HashMap底層實現，使用數組+鏈表+紅黑樹進行數據存儲。
JDK8中的ConcurrentHashMap源碼中的主要欄位
private static final int MAXIMUM_CAPACITY = 1 << 30; private static final int DEFAULT_CAPACITY = 16; private static final float LOAD_FACTOR = 0.75f; static final int TREEIFY_THRESHOLD = 8; static final int UNTREEIFY_THRESHOLD = 6; static final int MIN_TREEIFY_CAPACITY = 64; private static final int MIN_TRANSFER_STRIDE = 16; static final int MOVED = -1; // hash for forwarding nodes //轉發節點的hash值 static final int TREEBIN = -2; // hash for roots of trees //樹的根節點的hash值 static final int RESERVED = -3; // hash for transient reservations //臨時節點的 hash值 static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash //正常節點的hash值
JDK8中的ConcurrentHashMap源碼中構造器
/** * Creates a new, empty map with the default initial table size (16). */ public ConcurrentHashMap() { } /** * Creates a new, empty map with an initial table size * accommodating the specified number of elements without the need * to dynamically resize. * * @param initialCapacity The implementation performs internal * sizing to accommodate this many elements. * @throws IllegalArgumentException if the initial capacity of * elements is negative */ public ConcurrentHashMap(int initialCapacity) { if (initialCapacity < 0) throw new IllegalArgumentException(); int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY : tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1)); this.sizeCtl = cap; } /** * Creates a new, empty map with an initial table size based on * the given number of elements ({@code initialCapacity}) and * initial table density ({@code loadFactor}). * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements, * given the specified load factor. * @param loadFactor the load factor (table density) for * establishing the initial table size * @throws IllegalArgumentException if the initial capacity of * elements is negative or the load factor is nonpositive * * @since 1.6 */ public ConcurrentHashMap(int initialCapacity, float loadFactor) { this(initialCapacity, loadFactor, 1); } /** * Creates a new, empty map with an initial table size based on * the given number of elements ({@code initialCapacity}), table * density ({@code loadFactor}), and number of concurrently * updating threads ({@code concurrencyLevel}). * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements, * given the specified load factor. * @param loadFactor the load factor (table density) for * establishing the initial table size * @param concurrencyLevel the estimated number of concurrently * updating threads. The implementation may use this value as * a sizing hint. * @throws IllegalArgumentException if the initial capacity is * negative or the load factor or concurrencyLevel are * nonpositive */ public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) { if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0) throw new IllegalArgumentException(); if (initialCapacity < concurrencyLevel) // Use at least as many bins initialCapacity = concurrencyLevel; // as estimated threads long size = (long)(1.0 + (long)initialCapacity / loadFactor); int cap = (size >= (long)MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : tableSizeFor((int)size); this.sizeCtl = cap; }
JDK8中的ConcurrentHashMap源碼中的put方法
/** * Maps the specified key to the specified value in this table. * Neither the key nor the value can be null. * * <p>The value can be retrieved by calling the {@code get} method * with a key that is equal to the original key. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with {@code key}, or * {@code null} if there was no mapping for {@code key} * @throws NullPointerException if the specified key or value is null */ public V put(K key, V value) { return putVal(key, value, false); } /** Implementation for put and putIfAbsent */ final V putVal(K key, V value, boolean onlyIfAbsent) { if (key == null || value == null) throw new NullPointerException(); int hash = spread(key.hashCode()); //計算hash值 int binCount = 0; for (Node<K,V>[] tab = table;;) { //自旋 Node<K,V> f; int n, i, fh; if (tab == null || (n = tab.length) == 0) //table==null || table.length==0 tab = initTable(); //就initTable else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //若下標 i 處的元素為null if (casTabAt(tab, i, null, //直接用CAS操作，i處的元素 new Node<K,V>(hash, key, value, null))) break; // no lock when adding to empty bin 想emptybin中假如元素的時候，不需要加鎖 } else if ((fh = f.hash) == MOVED) //若下標 i 處的元素不為null，且f.hash==MOVED MOVED為常量值-1 tab = helpTransfer(tab, f); // else { //如果是一般的節點 V oldVal = null; synchronized (f) { //當頭部元素不為null，且不需要轉換成樹時，需要進行同步操作 if (tabAt(tab, i) == f) { if (fh >= 0) { //若 鏈表頭部hash值 >=0 binCount = 1; for (Node<K,V> e = f;; ++binCount) { K ek; if (e.hash == hash && ((ek = e.key) == key || (ek != null && key.equals(ek)))) { //如果key相同 oldVal = e.val; if (!onlyIfAbsent) //且不為absent e.val = value; //舊值覆蓋新值 break; } Node<K,V> pred = e; if ((e = e.next) == null), { //如果鏈表遍歷完成，還沒退出，說明沒有相同的key存在，在尾部添加節點 pred.next = new Node<K,V>(hash, key, value, null); break; } } } else if (f instanceof TreeBin) { //如果f是Tree的節點 Node<K,V> p; binCount = 2; if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key, value)) != null) { oldVal = p.val; if (!onlyIfAbsent) p.val = value; } } } } if (binCount != 0) { if (binCount >= TREEIFY_THRESHOLD) treeifyBin(tab, i); if (oldVal != null) return oldVal; break; } } } addCount(1L, binCount); return null; } /** * Initializes table, using the size recorded in sizeCtl. *///通過CAS搶sizeCtl，來搶佔initTable的資格，其他執行緒自旋等待，直到table不為null private final Node<K,V>[] initTable() { Node<K,V>[] tab; int sc; while ((tab = table) == null || tab.length == 0) { if ((sc = sizeCtl) < 0) Thread.yield(); // lost initialization race; just spin //執行緒讓步，讓其他執行緒優先執行 else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { try { if ((tab = table) == null || tab.length == 0) { int n = (sc > 0) ? sc : DEFAULT_CAPACITY; @SuppressWarnings("unchecked") Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n]; //初始化數組 table = tab = nt; //將nt賦值給table sc = n - (n >>> 2); } } finally { sizeCtl = sc; } break; } } return tab; }
JDK8中的ConcurrentHashMap源碼中的get方法
/** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code key.equals(k)}, * then this method returns {@code v}; otherwise it returns * {@code null}. (There can be at most one such mapping.) * * @throws NullPointerException if the specified key is null */ public V get(Object key) { Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek; int h = spread(key.hashCode()); if ((tab = table) != null && (n = tab.length) > 0 && (e = tabAt(tab, (n - 1) & h)) != null) { if ((eh = e.hash) == h) { if ((ek = e.key) == key || (ek != null && key.equals(ek))) return e.val; } else if (eh < 0) return (p = e.find(h, key)) != null ? p.val : null; while ((e = e.next) != null) { if (e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) return e.val; } } return null; }
