Java HashMap源码及并发环境常见问题解决

Java HashMap源码及并发环境常见问题解决

HashMap源码简单分析：

1 一切需要从HashMap属性字段说起：

/** 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[] table = (Entry[]) 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.

*

* 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：

/**

* 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 key, or

* null if there was no mapping for key.

* (A null return can also indicate that the map

* previously associated null with key.)

*/

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 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：

/**

* 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：

/**

* 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：

/**

* 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：

/**

* 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 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：

/**

*

* @param h hash值

* @param k key

* @param v value

* @param n 原始链表

*/

Entry(int h, K k, V v, Entry 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。

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：

/**

* 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：

/**

* Transfers all entries from current table to newTable.

*/

void transfer(Entry[] newTable, boolean rehash) {

int newCapacity = newTable.length;

for (Entry e : table) {

while(null != e) {

Entry 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;

}

}

}http://

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）。

/**

* 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：

/**

* 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 e : table) {

while(null != e) {

Entry 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？

/**

* 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。

版权声明：本文内容由网络用户投稿，版权归原作者所有，本站不拥有其著作权，亦不承担相应法律责任。如果您发现本站中有涉嫌抄袭或描述失实的内容，请联系我们jiasou666@gmail.com 处理，核实后本网站将在24小时内删除侵权内容。