1 一切需要从HashMap属性字段说起：
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/** The default initial capacity - MUST be a power of two. 初始容量 */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 /** * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. 最大容量 */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * The load factor used when none specified in constructor. * 默认的负载因子，当map的size>=负载因子*capacity时候并且插入元素时候的table[i]!=null进行扩容 * 扩容判断逻辑：java.util.HashMap#addEntry函数中 * */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * An empty table instance to share when the table is not inflated. */ static final Entry<?,?>[] EMPTY_TABLE = {}; /** * The table, resized as necessary. Length MUST Always be a power of two. 哈希表 */ transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE; /** * The number of key-value mappings contained in this map. map的大小 */ transient int size; /** * The next size value at which to resize (capacity * load factor). * @serial */ // If table == EMPTY_TABLE then this is the initial capacity at which the // table will be created when inflated. 扩容的阈值 = capacity * 负载因子 int threshold; /** * The load factor for the hash table. 负载因子，默认是0.75，可以在创建HashMap时候通过构造函数指定 * * @serial */ final float loadFactor; /** * The number of times this HashMap has been structurally modified * Structural modifications are those that change the number of mappings in * the HashMap or otherwise modify its internal structure (e.g., * rehash). This field is used to make iterators on Collection-views of * the HashMap fail-fast. (See ConcurrentModificationException). * 修改次数：例如进行rehash或者返回hashMap视图时候如果发生修改可以fast-fail */ transient int modCount; /** * The default threshold of map capacity above which alternative hashing is * used for String keys. Alternative hashing reduces the incidence of * collisions due to weak hash code calculation for String keys. * <p/> * This value may be overridden by defining the system property * {@code jdk.map.althashing.threshold}. A property value of {@code 1} * forces alternative hashing to be used at all times whereas * {@code -1} value ensures that alternative hashing is never used. * rehash时候判断的一个阈值 */ static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; |
2: 接下来查看一下HashMap的put方法：
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/** * 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) {//初始化哈希表 inflateTable(threshold); } if (key == null) //如果key 为null 存储到table[0]位置 return putForNullKey(value); int hash = hash(key); //计算hash值 int i = indexFor(hash, table.length);//计算entry在table中的位置 //for循环逻辑用于修改key对应的value的 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;//如果是更新返回旧值 } } //修改次数++ modCount++; //添加元素到哈希表中 addEntry(hash, key, value, i); // 如果是添加元素则返回null return null; } |
3 put中调用的inflateTable方法：
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/** * Inflates the table. */ private void inflateTable(int toSize) { // Find a power of 2 >= toSize //计算大于等于toSize的最小的2的整数次幂的值 int capacity = roundUpToPowerOf2(toSize); //计算扩容阈值 threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); //初始化哈希表 table = new Entry[capacity]; //更新一下rehash的判断条件，便于以后判断是否rehash initHashSeedAsNeeded(capacity); } |
4 put方法中调用的indexFor方法：
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/** * Returns index for hash code h. 返回哈希值对应的哈希表索引 */ static int indexFor(int h, int length) { // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2"; //使用&操作，而不使用取余原因：均匀分布在哈希表中 。length-1目的是：由于table的长度都是2的整数次幂进行扩容，length-1的二进制全是1，计算效率高 return h & (length-1); } |
5 put方法中调用的addEntry方法：
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/** * 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) { //判断是否扩容，只有size大于等于阈值而且当前插入table[i]!=null(就是able[i]已经被占用则扩容) if ((size >= threshold) && (null != table[bucketIndex])) { resize(2 * table.length); hash = (null != key) ? hash(key) : 0; //如果需要扩容的话则需要更新再次重新计算哈希表位置 bucketIndex = indexFor(hash, table.length); } //将值插入到哈希表中 createEntry(hash, key, value, bucketIndex); } |
6 addEntry方法中调用的createEntry方法：
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/** * 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) { // 获取到哈希表指定位置 Entry<K,V> e = table[bucketIndex]; // 链表的头插入方式进行插入，插入逻辑在Entry的构造器中。然后将新节点存储到 table[bucketIndex]中 table[bucketIndex] = new Entry<>(hash, key, value, e); size++;//更新size即可 } |
Entry构造器：
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/** * * @param h hash值 * @param k key * @param v value * @param n 原始链表 */ Entry(int h, K k, V v, Entry<K,V> n) { value = v; //将原始链表接该节点后面 next = n; key = k; hash = h; } |
7 接下来看一下java.util.HashMap#addEntry扩容机制：
当进行扩容时候需要重新计算哈希值和在哈希表中的位置。
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void addEntry(int hash, K key, V value, int bucketIndex) { //满足扩容条件进行扩容 if ((size >= threshold) && (null != table[bucketIndex])) { //扩容，2倍进行扩容 resize(2 * table.length); //重新计算哈数值 hash = (null != key) ? hash(key) : 0; //重新计算哈希表中的位置 bucketIndex = indexFor(hash, table.length); } createEntry(hash, key, value, bucketIndex); } |
接下来看一下java.util.HashMap#resize方法：
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/** * 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) {//判断当前old容量是否最最大容量，是的话更新阈值 threshold = Integer.MAX_VALUE; return; } //创建新的表 Entry[] newTable = new Entry[newCapacity]; //元素转移，根据initHashSeedAsNeeded结果判断是否进行rehash transfer(newTable, initHashSeedAsNeeded(newCapacity)); // 新表赋给table table = newTable; //更新阈值 threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); } |
关于HashMap在并发情况下的常见问题，其实在多线程环境下使用HashMap本来就是有风险错误的，但是一般面试却喜欢这么问，下面列举一下自己印象中的常见问题：
1：在进行扩容时候，其他线程是否可以进行进行插入操作（多线程环境下可能会导致HashMap进入死循环，此处暂不考虑）？
答：首先HashMap就不是一个线程安全的容器，所以在多线程环境下使用就是错误的。其次在扩容时候可以进行插入的，但是不安全。例如：
当主线程在调用transfer方法进行复制元素：
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/** * 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; } } } |
此时另一个线程在添加新元素是可以的，新元素添加到table中。如果子线程需要扩容的话可以进行扩容，然后将新容器赋给table。而此时主线程转移元素的工作就是将table中元素转移到newTable中。注意main线程的transfer方法：
如果main线程刚进入transfer方法时候newTable大小是32的话，由于子线程的添加操作导致table此时元素如果有128的话。则128个元素就会存储到大小为32的newTable中（此处不会扩容）。这就会导致HashMap性能下降！！！
可以使用多线程环境进行debug查看即可确定（推荐Idea的debug，的确强大，尤其是Evaluate Expression功能）。
2：进行扩容时候元素是否需要重新Hash？
这个需要具体情况判断，调用initHashSeedAsNeeded方法判断（判断逻辑这里先不介绍）。
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/** * 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) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; //initHashSeedAsNeeded 判断是否需要重新Hash transfer(newTable, initHashSeedAsNeeded(newCapacity)); table = newTable; threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); } |
然后进行转移元素：
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/** * Transfers all entries from current table to newTable. */ void transfer(Entry[] newTable, boolean rehash) { int newCapacity = newTable.length; //多线程环境下，如果其他线程导致table快速扩大。newTable在此处无法扩容会导致性能下降。但是如果后面有再次调用put方法的话可以再次触发resize。 for (Entry<K,V> e : table) { while(null != e) { Entry<K,V> next = e.next; if (rehash) { //判断是否需要重新Hash e.hash = null == e.key ? 0 : hash(e.key); } int i = indexFor(e.hash, newCapacity); e.next = newTable[i]; newTable[i] = e; e = next; } } } |
3：如何判断是否需要重新Hash？
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/** * Initialize the hashing mask value. We defer initialization until we * really need it. */ final boolean initHashSeedAsNeeded(int capacity) { // hashSeed降低hash碰撞的hash种子，初始值为0 boolean currentAltHashing = hashSeed != 0; //ALTERNATIVE_HASHING_THRESHOLD： 当map的capacity容量大于这个值的时候并满足其他条件时候进行重新hash boolean useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); //TODO 异或操作，二者满足一个条件即可rehash boolean switching = currentAltHashing ^ useAltHashing; if (switching) { // 更新hashseed的值 hashSeed = useAltHashing ? sun.misc.Hashing.randomHashSeed(this) : 0; } return switching; } |
4：HashMap在多线程环境下进行put操作如何导致的死循环？
死循环产生时机：
当两个线程同时需要进行扩容，而且对哈希表同一个桶（table[i]）进行扩容时候，一个线程刚好确定e和next元素之后，线程被挂起。此时另一个线程得到cpu并顺利对该桶完成转移（需要要求被转移之后的线程1中的e和next指的元素在新哈希表的同一个桶中，此时e和next被逆序了）。接着线程从挂起恢复回来时候就会陷入死循环中。参考：https://coolshell.cn/articles/9606.html
产生原因：主要由于并发操作，对用一个桶的两个节点构成了环，导致对环进行无法转移完毕元素陷入死循环。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持服务器之家。
原文链接：https://www.cnblogs.com/leodaxin/p/7708618.html
