JDK容器类Map源码解读
- 2019 年 10 月 7 日
- 筆記
java.util.Map接口是JDK1.2开始提供的一个基于键值对的散列表接口，其设计的初衷是为了替换JDK1.0中的java.util.Dictionary抽象类。Dictionary是JDK最初的键值对类，它不可以存储null作为key和value，目前这个类早已不被使用了。目前都是在使用Map接口，它是可以存储null值作为key和value，但Map的key是不可以重复的。其常用的实现类主要有HashMap，TreeMap，ConcurrentHashMap等
HashMap源码解读
目前JDK已经发布到JDK12，主流的JDK版本是JDK8， 但是如果阅读HashMap的源码建议先看JDK7的源码。JDK7和JDK8的源码中HashMap的实现原理大体相同，只不过是在JDK8中做了部分优化。但是JDK8的源码可读性非常差。
HashMap 是一个存储键值对(key-value)映射的散列表，继承于AbstractMap，实现了Map、Cloneable、java.io.Serializable接口，HashMap是线程不安全的，它存储的映射也是无序的。 HashMap的底层主要是基于数组和链表来实现的（JDK8之后又引入了红黑树），数据存储时会通过对key进行哈希操作取到哈希值，然后将哈希值对数组长度取模，得到的值就是该键值对在数组中的索引index值，如果数组该位置没有值则直接将该键值对放在该位置，如果该位置已经有值则将其插入相应链表的位置，JDK8开始为优化链表长度过长导致的性能问题从而引入了红黑树，当链表的长度大于8时会自动将链表转成红黑树。
JDK7中HashMap的源码解读
JDK7中HashMap采用Entry数组来存储键值对，每一个键值对组成了一个Entry实体，Entry类实际上是一个单向的链表结构，它具有Next指针，可以连接下一个Entry实体组成链表。
JDK7中HashMap源码中的主要字段
// 数组默认的大小 // 1 << 4，表示1，左移4位，变成10000，即16，以二进制形式运行，效率更高 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 数组最大值 static final int MAXIMUM_CAPACITY = 1 << 30; // 默认的负载因子 static final float DEFAULT_LOAD_FACTOR = 0.75f; // 真正存放数据的数组 transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
HashMap中默认的数组容量为 16，负载因子为 0.75。Map 在使用过程中不断的往里面存放数据，当数量达到了 16 * 0.75 = 12 就需要将当前 16 的容量进行扩容，而扩容这个过程涉及到 rehash、复制数据等操作，所以非常消耗性能。因此通常建议能提前预估 HashMap 的大小最好，尽量的减少扩容带来的性能损耗。
JDK7中HashMap源码中的构造器
/** 默认的初始化容量、默认的加载因子 * 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); } /** 做了两件事：1、为threshold、loadFactor赋值 2、调用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)) //检查 loadFactor throw new IllegalArgumentException("Illegal load factor: " + loadFactor); //真正在做的，只是记录下loadFactor、initialCpacity的值 this.loadFactor = loadFactor; //记录下loadFactor threshold = initialCapacity; //初始的 阈值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源码中的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、扩容 合并为了一个方法） } if (key == null) //对key为null做特殊处理 return putForNullKey(value); int hash = hash(key); //计算hash值 int i = indexFor(hash, table.length); //根据hash值计算出index下标 for (Entry<K,V> e = table[i]; e != null; e = e.next) { //遍历下标为i处的链表 Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //如果key值相同，覆盖旧值，返回新值 V oldValue = e.value; e.value = value; //新值 覆盖 旧值 e.recordAccess(this); //do nothing return oldValue; //返回旧值 } } modCount++; //修改次数+1，类似于一个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大于threshold && table在下标为index的地方已经有entry了 resize(2 * table.length); //扩容，将数组长度变为原来两倍 hash = (null != key) ? hash(key) : 0; //重新计算 hash 值 bucketIndex = indexFor(hash, table.length); //重新计算下标 } createEntry(hash, key, value, bucketIndex); //创建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) { //状态检查 threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; //实例化新的table transfer(newTable, initHashSeedAsNeeded(newCapacity)); //赋值数组元素到新的数组 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); //对key进行hash } int i = indexFor(e.hash, newCapacity); //用新的index来取模 e.next = newTable[i]; newTable[i] = e; //把元素存入新table新的新的index处 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]; //获取table中存的entry table[bucketIndex] = new Entry<>(hash, key, value, e); //将新的entry放到数组中，next指向旧的table[i] size++; //修改map中元素个数 }
JDK7中HashMap源码中的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的源码解读
JDK8中HashMap采用Node数组来存储键值对，Node其实就是JDK7中的Entry，只不过是换了一个名字，同样每一个键值对组成了一个Node实体，然后组成链表。当 Hash 冲突严重时，链表会变的越来越长，这样在查询时的效率就会越来越低，JDK8所做的优化就是，当链表的长度达到8的时候会转变成红黑树TreeNode。
JDK8中HashMap源码中的主要字段
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; static final int MAXIMUM_CAPACITY = 1 << 30; static final float DEFAULT_LOAD_FACTOR = 0.75f; // 用于判断是否需要将链表转换为红黑树的阈值 static final int TREEIFY_THRESHOLD = 8; // 用于判断是否需要将红黑树转换为链表的阈值 static final int UNTREEIFY_THRESHOLD = 6; static final int MIN_TREEIFY_CAPACITY = 64; // 存放数据的数组 transient Node<K,V>[] table;
JDK8中HashMap源码中的构造器
/** * 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源码中的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为null n = (tab = resize()).length; //resize if ((p = tab[i = (n - 1) & hash]) == null) //计算下标i，取出i处的元素为p，如果p为null tab[i] = newNode(hash, key, value, null); //创建新的node，放到数组中 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; //直接覆盖 else if (p instanceof TreeNode) //如果为 树节点 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //放到树中 else { //如果key不相同，也不是treeNode for (int binCount = 0; ; ++binCount) { //遍历i处的链表 if ((e = p.next) == null) { //找到尾部 p.next = newNode(hash, key, value, null); //在末尾添加一个node if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //如果链表长度 >= 8 treeifyBin(tab, hash); //将链表转成共黑树 break; } if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) //若果key相同，直接退出循环 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 旧数组为null就讲旧的容量看做是0，否则用旧的table长度当做容量 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]; //创建新的数组 table = newTab; //赋值给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源码中的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源码解读
ConcurrentHashMap是一个线程安全的HashMap实现，ConcurrentHashMap在JDK7和JDK8中的实现差别比较大，JDK7中ConcurrentHashMap是使用Segment数组来存放数据，一个Segment就相当于一个HashMap的数据结构，每个Segment使用一个锁。JDK8之后Segment虽保留，但仅是为了兼容旧版本，已经不再使用，JDK8中ConcurrentHashMap使用和HashMap一样的数据结构Node数组来存储数据，每个数组位置使用一个锁。
JDK7中的ConcurrentHashMap源码解读
JDK7中ConcurrentHashMap的底层Segment组，而Segment其实就是特殊的HashMap，Segment的数据结构跟HashMap一样，同时它继承了ReentrantLock，通过ReentrantLock提供的锁实现了线程的安全。ConcurrentHashMap使用分段锁技术，将数据分成一段一段的存储，每个Segment就是一段，然后给每一段数据配一把锁，当一个线程占用锁访问其中一个段数据的时候，其他段的数据也能被其他线程访问，能够实现并发访问，Segment数组的长度就是ConcurrentHashMap的线程并行级别，Segment数组默认的长度为16，也就是说最多同时可以有16个线程去访问ConcurrentHashMap。segment 数组不能扩容，而是对 segment 数组某个位置的segmen内部的数组HashEntry[] 进行扩容，扩容后容量为原来的 2 倍，该方法没有考虑并发，因为执行该方法之前已经获取了锁。
JDK7中的ConcurrentHashMap源码中的主要字段
// 数组默认大小 static final int DEFAULT_INITIAL_CAPACITY = 16; // 默认的负载因子 static final float DEFAULT_LOAD_FACTOR = 0.75f; // 默认线程并发度 static final int DEFAULT_CONCURRENCY_LEVEL = 16; static final int MIN_SEGMENT_TABLE_CAPACITY = 2; static final int MAX_SEGMENTS = 1 << 16; // 数组最大大小 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源码中的构造器
/** * 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) //参数检查 throw new IllegalArgumentException(); if (concurrencyLevel > MAX_SEGMENTS) //ConcurrentcyLevel实际上就是最大并发数 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]); //创建一个segment Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; //创建一个segment数组 UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] //将s0设置为ss的第一个元素 this.segments = ss; //将ss作为segments }
JDK7中的ConcurrentHashMap源码中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); // 计算Hash值 int j = (hash >>> segmentShift) & segmentMask; //计算下标j if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment s = ensureSegment(j); //若j处有segment就返回，若没有就创建并返回 return s.put(key, hash, value, false); //将值put到segment中去 } // Segment 中put数据的方法 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，否则。。。V oldValue; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; //根据table数组的长度 和 hash值计算index小标 HashEntry<K,V> first = entryAt(tab, index); //找到table数组在 index处链表的头部 for (HashEntry<K,V> e = first;;) { //从first开始遍历链表 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; //获取旧值 if (!onlyIfAbsent) { //若absent=false e.value = value; //覆盖旧值 ++modCount; // } break; //若已经找到，就退出链表遍历 } e = e.next; //若key不相同，继续遍历 } else { //直到e为null if (node != null) //将元素放到链表头部 node.setNext(first); else node = new HashEntry<K,V>(hash, key, value, first); //创建新的Entry int c = count + 1; //count 用来记录元素个数 if (c > threshold && tab.length < MAXIMUM_CAPACITY) //如果hashmap元素个数超过threshold，并且table长度小于最大容量 rehash(node); //rehash跟resize的功能差不多,将table的长度变为原来的两倍，重新打包entries，并将给定的node添加到新的table else //如果还有容量 setEntryAt(tab, index, node); //就在index处添加链表节点 ++modCount; //修改操作数 count = c; //将count+1 oldValue = null; // break; } } } finally { unlock(); //执行完操作后，释放锁 } return oldValue; //返回oldValue } /** 将table的长度变为原来的两倍，重新打包entries，并将给定的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源码中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源码解读
JDK8中的ConcurrentHashMap取消了基于 Segment 的分段锁思想，改用 CAS + synchronized 控制并发操作，锁的粒度变得更小，并发度更高。并且追随JDK8的HashMap底层实现，使用数组+链表+红黑树进行数据存储。
JDK8中的ConcurrentHashMap源码中的主要字段
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 //转发节点的hash值 static final int TREEBIN = -2; // hash for roots of trees //树的根节点的hash值 static final int RESERVED = -3; // hash for transient reservations //临时节点的 hash值 static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash //正常节点的hash值
JDK8中的ConcurrentHashMap源码中构造器
/** * 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源码中的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()); //计算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) { //若下标 i 处的元素为null if (casTabAt(tab, i, null, //直接用CAS操作，i处的元素 new Node<K,V>(hash, key, value, null))) break; // no lock when adding to empty bin 想emptybin中假如元素的时候，不需要加锁 } else if ((fh = f.hash) == MOVED) //若下标 i 处的元素不为null，且f.hash==MOVED MOVED为常量值-1 tab = helpTransfer(tab, f); // else { //如果是一般的节点 V oldVal = null; synchronized (f) { //当头部元素不为null，且不需要转换成树时，需要进行同步操作 if (tabAt(tab, i) == f) { if (fh >= 0) { //若 链表头部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) //且不为absent e.val = value; //旧值覆盖新值 break; } Node<K,V> pred = e; if ((e = e.next) == null), { //如果链表遍历完成，还没退出，说明没有相同的key存在，在尾部添加节点 pred.next = new Node<K,V>(hash, key, value, null); break; } } } else if (f instanceof TreeBin) { //如果f是Tree的节点 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. *///通过CAS抢sizeCtl，来抢占initTable的资格，其他线程自旋等待，直到table不为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 //线程让步，让其他线程优先执行 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]; //初始化数组 table = tab = nt; //将nt赋值给table sc = n - (n >>> 2); } } finally { sizeCtl = sc; } break; } } return tab; }
JDK8中的ConcurrentHashMap源码中的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; }
