我要评论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;
}
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