ConcurrentHashMap原始碼解析(JDK1.8)
阿新 • • 發佈:2018-12-09
package java.util.concurrent;
import java.io.ObjectStreamField;
import java.io.Serializable;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.*;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.*;
import java.util.stream.Stream;
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
implements ConcurrentMap<K, V>, Serializable {
private static final long serialVersionUID = 7249069246763182397L;
/* ---------------- Constants -------------- */
/**
* node陣列最大容量
*/
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* 預設初始值,必須是2的冪數
*/
private static final int DEFAULT_CAPACITY = 16;
/**
* 陣列可能最大值,需要與toArray()相關方法關聯
*/
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 併發級別,遺留下來的,為相容以前的版本
*/
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
/**
* 負載因子
*/
private static final float LOAD_FACTOR = 0.75f;
/**
* 連結串列轉樹的閥值,如果table[i]下面的連結串列長度大於8時就轉化為數
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* 樹轉連結串列的閥值,小於等於6是轉為連結串列,僅在擴容tranfer時才可能樹轉連結串列
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* 在轉變成樹之前,還會有一次判斷,只有鍵值對數量大於 64 才會發生轉換。
* 這是為了避免在雜湊表建立初期,多個鍵值對恰好被放入了同一個連結串列中而導致不必要的轉化。
*/
static final int MIN_TREEIFY_CAPACITY = 64;
/**
* Minimum number of rebinnings per transfer step. Ranges are
* subdivided to allow multiple resizer threads. This value
* serves as a lower bound to avoid resizers encountering
* excessive memory contention. The value should be at least
* DEFAULT_CAPACITY.
*/
private static final int MIN_TRANSFER_STRIDE = 16;
/**
* The number of bits used for generation stamp in sizeCtl.
* Must be at least 6 for 32bit arrays.
*/
private static int RESIZE_STAMP_BITS = 16;
/**
* 2^15-1,help resize的最大執行緒數
*/
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
/**
* 32-16=16,sizeCtl中記錄size大小的偏移量
*/
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
/*
* Encodings for Node hash fields. See above for explanation.
*/
static final int MOVED = -1; // hash for forwarding nodes (forwarding nodes的hash值)、標示位
static final int TREEBIN = -2; // hash值是-2 表示這是一個TreeBin節點
static final int RESERVED = -3; // hash for transient reservations
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash (ReservationNode的hash值)
/**
* 可用處理器數量
*/
static final int NCPU = Runtime.getRuntime().availableProcessors();
/**
* For serialization compatibility.
*/
private static final ObjectStreamField[] serialPersistentFields = {
new ObjectStreamField("segments", Segment[].class),
new ObjectStreamField("segmentMask", Integer.TYPE),
new ObjectStreamField("segmentShift", Integer.TYPE)
};
/* ---------------- Nodes -------------- */
/**
* Node是最核心的內部類,它包裝了key-value鍵值對,所有插入ConcurrentHashMap的資料都包裝在這裡面。
* 它與HashMap中的定義很相似,但是但是有一些差別,它對value和next屬性設定了volatile同步鎖,
* 它不允許呼叫setValue方法直接改變Node的value域,它增加了find方法輔助map.get()方法。
*/
static class Node<K, V> implements Map.Entry<K, V> {
final int hash;
final K key;
//val和next都會在擴容時發生變化,所以加上volatile來保持可見性和禁止重排序
volatile V val;
volatile Node<K, V> next;
Node(int hash, K key, V val, Node<K, V> next) {
this.hash = hash;
this.key = key;
this.val = val;
this.next = next;
}
public final K getKey() {
return key;
}
public final V getValue() {
return val;
}
/**
* HashMap中Node類的hashCode()方法中的程式碼為:Objects.hashCode(key) ^ Objects.hashCode(value)
* 而Objects.hashCode(key)最終也是呼叫了 key.hashCode(),但是效果一樣
*/
public final int hashCode() {
return key.hashCode() ^ val.hashCode();
}
public final String toString() {
return key + "=" + val;
}
//不允許直接改變value的值
public final V setValue(V value) {
throw new UnsupportedOperationException();
}
/**
* HashMap使用if (o == this),且巢狀if;ConcurrentHashMap使用&&
*/
public final boolean equals(Object o) {
Object k, v, u;
Map.Entry<?, ?> e;
return ((o instanceof Map.Entry) &&
(k = (e = (Map.Entry<?, ?>) o).getKey()) != null &&
(v = e.getValue()) != null &&
(k == key || k.equals(key)) &&
(v == (u = val) || v.equals(u)));
}
/**
* 增加find方法輔助get方法 ,HashMap中的Node類中沒有此方法
*/
Node<K, V> find(int h, Object k) {
Node<K, V> e = this;
if (k != null) {
do {
K ek;
if (e.hash == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
return e;
} while ((e = e.next) != null);
}
return null;
}
}
/* ---------------- Static utilities -------------- */
/**
* 對hashCode進行再雜湊,演算法為(h ^ (h >>> 16)) & HASH_BITS
*/
static final int spread(int h) {
return (h ^ (h >>> 16)) & HASH_BITS;
}
/**
* 返回大於等於count的最小的2的冪次方
*/
private static final int tableSizeFor(int c) {
int n = c - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
/**
* Returns x's Class if it is of the form "class C implements
* Comparable<C>", else null.
*/
static Class<?> comparableClassFor(Object x) {
if (x instanceof Comparable) {
Class<?> c;
Type[] ts, as;
Type t;
ParameterizedType p;
if ((c = x.getClass()) == String.class) // bypass checks
return c;
if ((ts = c.getGenericInterfaces()) != null) {
for (int i = 0; i < ts.length; ++i) {
if (((t = ts[i]) instanceof ParameterizedType) &&
((p = (ParameterizedType) t).getRawType() ==
Comparable.class) &&
(as = p.getActualTypeArguments()) != null &&
as.length == 1 && as[0] == c) // type arg is c
return c;
}
}
}
return null;
}
/**
* Returns k.compareTo(x) if x matches kc (k's screened comparable
* class), else 0.
*/
@SuppressWarnings({"rawtypes", "unchecked"}) // for cast to Comparable
static int compareComparables(Class<?> kc, Object k, Object x) {
return (x == null || x.getClass() != kc ? 0 :
((Comparable) k).compareTo(x));
}
/* ---------------- Table element access -------------- */
/*
* Volatile access methods are used for table elements as well as
* elements of in-progress next table while resizing. All uses of
* the tab arguments must be null checked by callers. All callers
* also paranoically precheck that tab's length is not zero (or an
* equivalent check), thus ensuring that any index argument taking
* the form of a hash value anded with (length - 1) is a valid
* index. Note that, to be correct wrt arbitrary concurrency
* errors by users, these checks must operate on local variables,
* which accounts for some odd-looking inline assignments below.
* Note that calls to setTabAt always occur within locked regions,
* and so in principle require only release ordering, not
* full volatile semantics, but are currently coded as volatile
* writes to be conservative.
*/
/**
* 獲得在i位置上的Node節點
*/
@SuppressWarnings("unchecked")
static final <K, V> Node<K, V> tabAt(Node<K, V>[] tab, int i) {
return (Node<K, V>) U.getObjectVolatile(tab, ((long) i << ASHIFT) + ABASE);
}
/**
* 利用CAS演算法設定i位置上的Node節點(將c和table[i]比較,相同則插入v)。
*/
static final <K, V> boolean casTabAt(Node<K, V>[] tab, int i,
Node<K, V> c, Node<K, V> v) {
return U.compareAndSwapObject(tab, ((long) i << ASHIFT) + ABASE, c, v);
}
/**
* 利用volatile方法設定第i個節點的值,這個操作一定是成功的。
*/
static final <K, V> void setTabAt(Node<K, V>[] tab, int i, Node<K, V> v) {
U.putObjectVolatile(tab, ((long) i << ASHIFT) + ABASE, v);
}
/* ---------------- Fields -------------- */
/**
* 存放node的陣列,大小是2的冪次方
*/
transient volatile Node<K, V>[] table;
/**
* 擴容時用於存放資料的變數,擴容完成後會置為null。
*/
private transient volatile Node<K, V>[] nextTable;
/**
* 記錄容器的容量大小,通過CAS更新
*/
private transient volatile long baseCount;
/**
* 負數代表正在進行初始化或擴容操作 ,其中-1代表正在初始化 ,-N 表示有N-1個執行緒正在進行擴容操作
* 正數或0代表hash表還沒有被初始化,這個數值表示初始化或下一次進行擴容的大小,類似於擴容閾值。
* 它的值始終是當前ConcurrentHashMap容量的0.75倍,這與loadfactor是對應的。實際容量>=sizeCtl,則擴容。
*/
private transient volatile int sizeCtl;//控制識別符號
/**
* The next table index (plus one) to split while resizing.
*/
private transient volatile int transferIndex;
/**
* 自旋鎖 (鎖定通過 CAS) 在調整大小和/或建立 CounterCells 時使用。
* 在CounterCell類更新value中會使用,功能類似顯示鎖和內建鎖,效能更好
*/
private transient volatile int cellsBusy;
/**
* counter cell表,長度總為2的冪次
*/
private transient volatile CounterCell[] counterCells;
// views
private transient KeySetView<K, V> keySet;
private transient ValuesView<K, V> values;
private transient EntrySetView<K, V> entrySet;
/* ---------------- Public operations -------------- */
/**
* 預設的建構函式
*/
public ConcurrentHashMap() {
}
/**
* 指定容量的建構函式
*
* @param initialCapacity 初始化容量
* @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;//初始化sizeCtl
}
/**
* 建立與給定map具有相同對映的新map
*
* @param m the map
*/
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
this.sizeCtl = DEFAULT_CAPACITY;
putAll(m);
}
/**
* 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 初始容量
* @param loadFactor 負載因子,當容量達到initialCapacity*loadFactor時,執行擴容
* @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 初始容量
* @param loadFactor 負載因子,當容量達到initialCapacity*loadFactor時,執行擴容
* @param concurrencyLevel 預估的併發更新執行緒數
* @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;
}
// Original (since JDK1.2) Map methods
/**
* {@inheritDoc}
*/
public int size() {
long n = sumCount();
return ((n < 0L) ? 0 :
(n > (long) Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int) n);
}
/**
* {@inheritDoc}
*/
public boolean isEmpty() {
return sumCount() <= 0L; // ignore transient negative values
}
/**
* 根據key在Map中找出其對應的value,如果不存在key,則返回null,
* 其中key不允許為null,否則拋異常
* 對於節點可能在連結串列或樹上的情況,需要分別去查詢
*
* @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());//兩次hash計算出hash值
//根據hash值確定節點位置
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
// 搜尋到的節點key與傳入的key相同且不為null,直接返回這個節點
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
} else if (eh < 0)//如果eh<0 說明這個節點在樹上 直接尋找
return (p = e.find(h, key)) != null ? p.val : null;
//否則遍歷連結串列 找到對應的值並返回
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
/**
* 檢查table中是否含有key
*
* @param key possible key
* @return {@code true} if and only if the specified object
* is a key in this table, as determined by the
* {@code equals} method; {@code false} otherwise
* @throws NullPointerException if the specified key is null
*/
public boolean containsKey(Object key) {
//直接呼叫get(int key)方法即可,如果有返回值,則說明是包含key的
return get(key) != null;
}
/**
* 檢查在所有對映(k,v)中只要出現一次及以上的v==value,返回true
* 這個方法可能需要完全遍歷Map,因此比containsKey要慢的多
*
* @param value value whose presence in this map is to be tested
* @return {@code true} if this map maps one or more keys to the
* specified value
* @throws NullPointerException if the specified value is null
*/
public boolean containsValue(Object value) {
if (value == null)
throw new NullPointerException();
Node<K, V>[] t;
if ((t = table) != null) {
Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
for (Node<K, V> p; (p = it.advance()) != null; ) {
V v;
if ((v = p.val) == value || (v != null && value.equals(v)))
return true;
}
}
return false;
}
/**
* 直接呼叫putVal(key, value, false)方法
*
* @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);
}
/**
* putVal方法可以分為以下幾步:
* 1、檢查key/value是否為空,如果為空,則拋異常,否則進行2
* 2、進入for死迴圈,進行3
* 3、檢查table是否初始化了,如果沒有,則呼叫initTable()進行初始化然後進行 2,否則進行4
* 4、根據key的hash值計算出其應該在table中儲存的位置i,取出table[i]的節點用f表示。
* 根據f的不同有如下三種情況:
* 1)如果table[i]==null(即該位置的節點為空,沒有發生碰撞),則利用CAS操作直接儲存在該位置,如果CAS操作成功則退出死迴圈。
* 2)如果table[i]!=null(即該位置已經有其它節點,發生碰撞),碰撞處理也有兩種情況
* 2.1)檢查table[i]的節點的hash是否等於MOVED,如果等於,則檢測到正在擴容,則幫助其擴容
* 2.2)說明table[i]的節點的hash值不等於MOVED,如果table[i]為連結串列節點,則將此節點插入連結串列中即可
* 如果table[i]為樹節點,則將此節點插入樹中即可。插入成功後,進行 5
* 5、如果table[i]的節點是連結串列節點,則檢查table的第i個位置的連結串列是否需要轉化為數,如果需要則呼叫treeifyBin函式進行轉化
*/
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();// key和value不允許null
int hash = spread(key.hashCode());//兩次hash,減少hash衝突,可以均勻分佈
int binCount = 0;//i處結點標誌,0: 未加入新結點, 2: TreeBin或連結串列結點數, 其它:連結串列結點數。主要用於每次加入結點後檢視是否要由連結串列轉為紅黑樹
for (Node<K, V>[] tab = table; ; ) {//CAS經典寫法,不成功無限重試
Node<K, V> f;
int n, i, fh;
//檢查是否初始化了,如果沒有,則初始化
if (tab == null || (n = tab.length) == 0)
tab = initTable();
/**
* i=(n-1)&hash 等價於i=hash%n(前提是n為2的冪次方).即取出table中位置的節點用f表示。 有如下兩種情況:
* 1、如果table[i]==null(即該位置的節點為空,沒有發生碰撞),則利用CAS操作直接儲存在該位置, 如果CAS操作成功則退出死迴圈。
* 2、如果table[i]!=null(即該位置已經有其它節點,發生碰撞)
*/
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node<K, V>(hash, key, value, null)))
break; // no lock when adding to empty bin
} else if ((fh = f.hash) == MOVED)//檢查table[i]的節點的hash是否等於MOVED,如果等於,則檢測到正在擴容,則幫助其擴容
tab = helpTransfer(tab, f);
else {//table[i]的節點的hash值不等於MOVED。
V oldVal = null;
// 針對首個節點進行加鎖操作,而不是segment,進一步減少執行緒衝突
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K, V> e = f; ; ++binCount) {
K ek;
// 如果在連結串列中找到值為key的節點e,直接設定e.val = value即可
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
// 如果沒有找到值為key的節點,直接新建Node並加入連結串列即可
Node<K, V> pred = e;
if ((e = e.next) == null) {//插入到連結串列末尾並跳出迴圈
pred.next = new Node<K, V>(hash, key,
value, null);
break;
}
}
} else if (f instanceof TreeBin) {// 如果首節點為TreeBin型別,說明為紅黑樹結構,執行putTreeVal操作
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) {
// 如果節點數>=8,那麼轉換連結串列結構為紅黑樹結構
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);//若length<64,直接tryPresize,兩倍table.length;不轉紅黑樹
if (oldVal != null)
return oldVal;
break;
}
}
}
// 計數增加1,有可能觸發transfer操作(擴容)
addCount(1L, binCount);
return null;
}
/**
* Copies all of the mappings from the specified map to this one.
* These mappings replace any mappings that this map had for any of the
* keys currently in the specified map.
*
* @param m mappings to be stored in this map
*/
public void putAll(Map<? extends K, ? extends V> m) {
tryPresize(m.size());
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
putVal(e.getKey(), e.getValue(), false);
}
/**
* Removes the key (and its corresponding value) from this map.
* This method does nothing if the key is not in the map.
*
* @param key the key that needs to be removed
* @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 is null
*/
public V remove(Object key) {
return replaceNode(key, null, null);
}
/**
* Implementation for the four public remove/replace methods:
* Replaces node value with v, conditional upon match of cv if
* non-null. If resulting value is null, delete.
*/
final V replaceNode(Object key, V value, Object cv) {
int hash = spread(key.hashCode());
for (Node<K, V>[] tab = table; ; ) {
Node<K, V> f;
int n, i, fh;
if (tab == null || (n = tab.length) == 0 ||
(f = tabAt(tab, i = (n - 1) & hash)) == null)
break;
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
boolean validated = false;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
validated = true;
for (Node<K, V> e = f, pred = null; ; ) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
V ev = e.val;
if (cv == null || cv == ev ||
(ev != null && cv.equals(ev))) {
oldVal = ev;
if (value != null)
e.val = value;
else if (pred != null)
pred.next = e.next;
else
setTabAt(tab, i, e.next);
}
break;
}
pred = e;
if ((e = e.next) == null)
break;
}
} else if (f instanceof TreeBin) {
validated = true;
TreeBin<K, V> t = (TreeBin<K, V>) f;
TreeNode<K, V> r, p;
if ((r = t.root) != null &&
(p = r.findTreeNode(hash, key, null)) != null) {
V pv = p.val;
if (cv == null || cv == pv ||
(pv != null && cv.equals(pv))) {
oldVal = pv;
if (value != null)
p.val = value;
else if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
}
if (validated) {
if (oldVal != null) {
if (value == null)
addCount(-1L, -1);
return oldVal;
}
break;
}
}
}
return null;
}
/**
* Removes all of the mappings from this map.
*/
public void clear() {
long delta = 0L; // negative number of deletions
int i = 0;
Node<K, V>[] tab = table;
while (tab != null && i < tab.length) {
int fh;
Node<K, V> f = tabAt(tab, i);
if (f == null)
++i;
else if ((fh = f.hash) == MOVED) {
tab = helpTransfer(tab, f);
i = 0; // restart
} else {
synchronized (f) {
if (tabAt(tab, i) == f) {
Node<K, V> p = (fh >= 0 ? f :
(f instanceof TreeBin) ?
((TreeBin<K, V>) f).first : null);
while (p != null) {
--delta;
p = p.next;
}
setTabAt(tab, i++, null);
}
}
}
}
if (delta != 0L)
addCount(delta, -1);
}
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. The set supports element
* removal, which removes the corresponding mapping from this map,
* via the {@code Iterator.remove}, {@code Set.remove},
* {@code removeAll}, {@code retainAll}, and {@code clear}
* operations. It does not support the {@code add} or
* {@code addAll} operations.
* <p>
* <p>The view's iterators and spliterators are
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
* <p>
* <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT},
* {@link Spliterator#DISTINCT}, and {@link Spliterator#NONNULL}.
*
* @return the set view
*/
public KeySetView<K, V> keySet() {
KeySetView<K, V> ks;
return (ks = keySet) != null ? ks : (keySet = new KeySetView<K, V>(this, null));
}
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. The collection
* supports element removal, which removes the corresponding
* mapping from this map, via the {@code Iterator.remove},
* {@code Collection.remove}, {@code removeAll},
* {@code retainAll}, and {@code clear} operations. It does not
* support the {@code add} or {@code addAll} operations.
* <p>
* <p>The view's iterators and spliterators are
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
* <p>
* <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT}
* and {@link Spliterator#NONNULL}.
*
* @return the collection view
*/
public Collection<V> values() {
ValuesView<K, V> vs;
return (vs = values) != null ? vs : (values = new ValuesView<K, V>(this));
}
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. The set supports element
* removal, which removes the corresponding mapping from the map,
* via the {@code Iterator.remove}, {@code Set.remove},
* {@code removeAll}, {@code retainAll}, and {@code clear}
* operations.
* <p>
* <p>The view's iterators and spliterators are
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
* <p>
* <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT},
* {@link Spliterator#DISTINCT}, and {@link Spliterator#NONNULL}.
*
* @return the set view
*/
public Set<Map.Entry<K, V>> entrySet() {
EntrySetView<K, V> es;
return (es = entrySet) != null ? es : (entrySet = new EntrySetView<K, V>(this));
}
/**
* Returns the hash code value for this {@link Map}, i.e.,
* the sum of, for each key-value pair in the map,
* {@code key.hashCode() ^ value.hashCode()}.
*
* @return the hash code value for this map
*/
public int hashCode() {
int h = 0;
Node<K, V>[] t;
if ((t = table) != null) {
Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
for (Node<K, V> p; (p = it.advance()) != null; )
h += p.key.hashCode() ^ p.val.hashCode();
}
return h;
}
/**
* Returns a string representation of this map. The string
* representation consists of a list of key-value mappings (in no
* particular order) enclosed in braces ("{@code {}}"). Adjacent
* mappings are separated by the characters {@code ", "} (comma
* and space). Each key-value mapping is rendered as the key
* followed by an equals sign ("{@code =}") followed by the
* associated value.
*
* @return a string representation of this map
*/
public String toString() {
Node<K, V>[] t;
int f = (t = table) == null ? 0 : t.length;
Traverser<K, V> it = new Traverser<K, V>(t, f, 0, f);
StringBuilder sb = new StringBuilder();
sb.append('{');
Node<K, V> p;
if ((p = it.advance()) != null) {
for (; ; ) {
K k = p.key;
V v = p.val;
sb.append(k == this ? "(this Map)" : k);
sb.append('=');
sb.append(v == this ? "(this Map)" : v);
if ((p = it.advance()) == null)
break;
sb.append(',').append(' ');
}
}
return sb.append('}').toString();
}
/**
* Compares the specified object with this map for equality.
* Returns {@code true} if the given object is a map with the same
* mappings as this map. This operation may return misleading
* results if either map is concurrently modified during execution
* of this method.
*
* @param o object to be compared for equality with this map
* @return {@code true} if the specified object is equal to this map
*/
public boolean equals(Object o) {
if (o != this) {
if (!(o instanceof Map))
return false;
Map<?, ?> m = (Map<?, ?>) o;
Node<K, V>[] t;
int f = (t = table) == null ? 0 : t.length;
Traverser<K, V> it = new Traverser<K, V>(t, f, 0, f);
for (Node<K, V> p; (p = it.advance()) != null; ) {
V val = p.val;
Object v = m.get(p.key);
if (v == null || (v != val && !v.equals(val)))
return false;
}
for (Map.Entry<?, ?> e : m.entrySet()) {
Object mk, mv, v;
if ((mk = e.getKey()) == null ||
(mv = e.getValue()) == null ||
(v = get(mk)) == null ||
(mv != v && !mv.equals(v)))
return false;
}
}
return true;
}
/**
* Stripped-down version of helper class used in previous version,
* declared for the sake of serialization compatibility
*/
static class Segment<K, V> extends ReentrantLock implements Serializable {
private static final long serialVersionUID = 2249069246763182397L;
final float loadFactor;
Segment(float lf) {
this.loadFactor = lf;
}
}
/**
* Saves the state of the {@code ConcurrentHashMap} instance to a
* stream (i.e., serializes it).
*
* @param s the stream
* @throws java.io.IOException if an I/O error occurs
* @serialData the key (Object) and value (Object)
* for each key-value mapping, followed by a null pair.
* The key-value mappings are emitted in no particular order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// For serialization compatibility
// Emulate segment calculation from previous version of this class
int sshift = 0;
int ssize = 1;
while (ssize < DEFAULT_CONCURRENCY_LEVEL) {
++sshift;
ssize <<= 1;
}
int segmentShift = 32 - sshift;
int segmentMask = ssize - 1;
@SuppressWarnings("unchecked")
Segment<K, V>[] segments = (Segment<K, V>[])
new Segment<?, ?>[DEFAULT_CONCURRENCY_LEVEL];
for (int i = 0; i < segments.length; ++i)
segments[i] = new Segment<K, V>(LOAD_FACTOR);
s.putFields().put("segments", segments);
s.putFields().put("segmentShift", segmentShift);
s.putFields().put("segmentMask", segmentMask);
s.writeFields();
Node<K, V>[] t;
if ((t = table) != null) {
Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
for (Node<K, V> p; (p = it.advance()) != null; ) {
s.writeObject(p.key);
s.writeObject(p.val);
}
}
s.writeObject(null);
s.writeObject(null);
segments = null; // throw away
}
/**
* Reconstitutes the instance from a stream (that is, deserializes it).
*
* @param s the stream
* @throws ClassNotFoundException if the class of a serialized object
* could not be found
* @throws java.io.IOException if an I/O error occurs
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
/*
* To improve performance in typical cases, we create nodes
* while reading, then place in table once size is known.
* However, we must also validate uniqueness and deal with
* overpopulated bins while doing so, which requires
* specialized versions of putVal mechanics.
*/
sizeCtl = -1; // force exclusion for table construction
s.defaultReadObject();
long size = 0L;
Node<K, V> p = null;
for (; ; ) {
@SuppressWarnings("unchecked")
K k = (K) s.readObject();
@SuppressWarnings("unchecked")
V v = (V) s.readObject();
if (k != null && v != null) {
p = new Node<K, V>(spread(k.hashCode()), k, v, p);
++size;
} else
break;
}
if (size == 0L)
sizeCtl = 0;
else {
int n;
if (size >= (long) (MAXIMUM_CAPACITY >>> 1))
n = MAXIMUM_CAPACITY;
else {
int sz = (int) size;
n = tableSizeFor(sz + (sz >>> 1) + 1);
}
@SuppressWarnings("unchecked")
Node<K, V>[] tab = (Node<K, V>[]) new Node<?, ?>[n];
int mask = n - 1;
long added = 0L;
while (p != null) {
boolean insertAtFront;
Node<K, V> next = p.next, first;
int h = p.hash, j = h & mask;
if ((first = tabAt(tab, j)) == null)
insertAtFront = true;
else {
K k = p.key;
if (first.hash < 0) {
TreeBin<K, V> t = (TreeBin<K, V>) first;
if (t.putTreeVal(h, k, p.val) == null)
++added;
insertAtFront = false;
} else {
int binCount = 0;
insertAtFront = true;
Node<K, V> q;
K qk;
for (q = first; q != null; q = q.next) {
if (q.hash == h &&
((qk = q.key) == k ||
(qk != null && k.equals(qk)))) {
insertAtFront = false;
break;
}
++binCount;
}
if (insertAtFront && binCount >= TREEIFY_THRESHOLD) {
insertAtFront = false;
++added;
p.next = first;
TreeNode<K, V> hd = null, tl = null;
for (q = p; q != null; q = q.next) {
TreeNode<K, V> t = new TreeNode<K, V>
(q.hash, q.key, q.val, null, null);
if ((t.prev = tl) == null)
hd = t;
else
tl.next = t;
tl = t;
}
setTabAt(tab, j, new TreeBin<K, V>(hd));
}
}
}
if (insertAtFront) {
++added;
p.next = first;
setTabAt(tab, j, p);
}
p = next;
}
table = tab;
sizeCtl = n - (n >>> 2);
baseCount = added;
}
}
// ConcurrentMap methods
/**
* {@inheritDoc}
*
* @return the previous value associated with the specified key,
* or {@code null} if there was no mapping for the key
* @throws NullPointerException if the specified key or value is null
*/
public V putIfAbsent(K key, V value) {
return putVal(key, value, true);
}
/**
* {@inheritDoc}
*
* @throws NullPointerException if the specified key is null
*/
public boolean remove(Object key, Object value) {
if (key == null)
throw new NullPointerException();
return value != null && replaceNode(key, null, value) != null;
}
/**
* {@inheritDoc}
*
* @throws NullPointerException if any of the arguments are null
*/
public boolean replace(K key, V oldValue, V newValue) {
if (key == null || oldValue == null || newValue == null)
throw new NullPointerException();
return replaceNode(key, newValue, oldValue) != null;
}
/**
* {@inheritDoc}
*
* @return the previous value associated with the specified key,
* or {@code null} if there was no mapping for the key
* @throws NullPointerException if the specified key or value is null
*/
public V replace(K key, V value) {
if (key == null || value == null)
throw new NullPointerException();
return replaceNode(key, value, null);
}
// Overrides of JDK8+ Map extension method defaults
/**
* Returns the value to which the specified key is mapped, or the
* given default value if this map contains no mapping for the
* key.
*
* @param key the key whose associated value is to be returned
* @param defaultValue the value to return if this map contains
* no mapping for the given key
* @return the mapping for the key, if present; else the default value
* @throws NullPointerException if the specified key is null
*/
public V getOrDefault(Object key, V defaultValue) {
V v;
return (v = get(key)) == null ? defaultValue : v;
}
public void forEach(BiConsumer<? super K, ? super V> action) {
if (action == null) throw new NullPointerException();
Node<K, V>[] t;
if ((t = table) != null) {
Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
for (Node<K, V> p; (p = it.advance()) != null; ) {
action.accept(p.key, p.val);
}
}
}
public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
if (function == null) throw new NullPointerException();
Node<K, V>[] t;
if ((t = table) != null) {
Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
for (Node<K, V> p; (p = it.advance()) != null; ) {
V oldValue = p.val;
for (K key = p.key; ; ) {
V newValue = function.apply(key, oldValue);
if (newValue == null)
throw new NullPointerException();
if (replaceNode(key, newValue, oldValue) != null ||
(oldValue = get(key)) == null)
break;
}
}
}
}
/**
* If the specified key is not already associated with a value,
* attempts to compute its value using the given mapping function
* and enters it into this map unless {@code null}. The entire
* method invocation is performed atomically, so the function is
* applied at most once per key. Some attempted update operations
* on this map by other threads may be blocked while computation
* is in progress, so the computation should be short and simple,
* and must not attempt to update any other mappings of this map.
*
* @param key key with which the specified value is to be associated
* @param mappingFunction the function to compute a value
* @return the current (existing or computed) value associated with
* the specified key, or null if the computed value is null
* @throws NullPointerException if the specified key or mappingFunction
* is null
* @throws IllegalStateException if the computation detectably
* attempts a recursive update to this map that would
* otherwise never complete
* @throws RuntimeException or Error if the mappingFunction does so,
* in which case the mapping is left unestablished
*/
public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
if (key == null || mappingFunction == null)
throw new NullPointerException();
int h = spread(key.hashCode());
V val = null;
int binCount = 0;
for (Node<K, V>[] tab = table; ; ) {
Node<K, V> f;
int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
Node<K, V> r = new ReservationNode<K, V>();
synchronized (r) {
if (casTabAt(tab, i, null, r)) {
binCount = 1;
Node<K, V> node = null;
try {
if ((val = mappingFunction.apply(key)) != null)
node = new Node<K, V>(h, key, val, null);
} finally {
setTabAt(tab, i, node);
}
}
}
if (binCount != 0)
break;
} else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
boolean added = false;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K, V> e = f; ; ++binCount) {
K ek;
V ev;
if (e.hash == h &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
val = e.val;
break;
}
Node<K, V> pred = e;
if ((e = e.next) == null) {
if ((val = mappingFunction.apply(key)) != null) {
added = true;
pred.next = new Node<K, V>(h, key, val, null);
}
break;
}
}
} else if (f instanceof TreeBin) {
binCount = 2;
TreeBin<K, V> t = (TreeBin<K, V>) f;
TreeNode<K, V> r, p;
if ((r = t.root) != null &&
(p = r.findTreeNode(h, key, null)) != null)
val = p.val;
else if ((val = mappingFunction.apply(key)) != null) {
added = true;
t.putTreeVal(h, key, val);
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (!added)
return val;
break;
}
}
}
if (val != null)
addCount(1L, binCount);
return val;
}
/**
* If the value for the specified key is present, attempts to
* compute a new mapping given the key and its current mapped
* value. The entire method invocation is performed atomically.
* Some attempted update operations on this map by other threads
* may be blocked while computation is in progress, so the
* computation should be short and simple, and must not attempt to
* update any other mappings of this map.
*
* @param key key with which a value may be associated
* @param remappingFunction the function to compute a value
* @return the new value associated with the specified key, or null if none
* @throws NullPointerException if the specified key or remappingFunction
* is null
* @throws IllegalStateException if the computation detectably
* attempts a recursive update to this map that would
* otherwise never complete
* @throws RuntimeException or Error if the remappingFunction does so,
* in which case the mapping is unchanged
*/
public V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
if (key == null || remappingFunction == null)
throw new NullPointerException();
int h = spread(key.hashCode());
V val = null;
int delta = 0;
int binCount = 0;
for (Node<K, V>[] tab = table; ; ) {
Node<K, V> f;
int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & h)) == null)
break;
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K, V> e = f, pred = null; ; ++binCount) {
K ek;
if (e.hash == h &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
val = remappingFunction.apply(key, e.val);
if (val != null)
e.val = val;
else {
delta = -1;
Node<K, V> en = e.next;
if (pred != null)
pred.next = en;
else
setTabAt(tab, i, en);
}
break;
}
pred = e;
if ((e = e.next) == null)
break;
}
} else if (f instanceof TreeBin) {
binCount = 2;
TreeBin<K, V> t = (TreeBin<K, V>) f;
TreeNode<K, V> r, p;
if ((r = t.root) != null &&
(p = r.findTreeNode(h, key, null)) != null) {
val = remappingFunction.apply(key, p.val);
if (val != null)
p.val = val;
else {
delta = -1;
if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
}
if (binCount != 0)
break;
}
}
if (delta != 0)
addCount((long) delta, binCount);
return val;
}
/**
* Attempts to compute a mapping for the specified key and its
* current mapped value (or {@code null} if there is no current
* mapping). The entire method invocation is performed atomically.
* Some attempted update operations on this map by other threads
* may be blocked while computation is in progress, so the
* computation should be short and simple, and must not attempt to
* update any other mappings of this Map.
*
* @param key key with which the specified value is to be associated
* @param remappingFunction the function to compute a value
* @return the new value associated with the specified key, or null if none
* @throws NullPointerException if the specified key or remappingFunction
* is null
* @throws IllegalStateException if the computation detectably
* attempts a recursive update to this map that would
* otherwise never complete
* @throws RuntimeException or Error if the remappingFunction does so,
* in which case the mapping is unchanged
*/
public V compute(K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
if (key == null || remappingFunction == null)
throw new NullPointerException();
int h = spread(key.hashCode());
V val = null;
int delta = 0;
int binCount = 0;
for (Node<K, V>[] tab = table; ; ) {
Node<K, V> f;
int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
Node<K, V> r = new ReservationNode<K, V>();
synchronized (r) {
if (casTabAt(tab, i, null, r)) {
binCount = 1;
Node<K, V> node = null;
try {
if ((val = remappingFunction.apply(key, null)) != null) {
delta = 1;
node = new Node<K, V>(h, key, val, null);
}
} finally {
setTabAt(tab, i, node);
}
}
}
if (binCount != 0)
break;
} else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K, V> e = f, pred = null; ; ++binCount) {
K ek;
if (e.hash == h &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
val = remappingFunction.apply(key, e.val);
if (val != null)
e.val = val;
else {
delta = -1;
Node<K, V> en = e.next;
if (pred != null)
pred.next = en;
else
setTabAt(tab, i, en);
}
break;
}
pred = e;
if ((e = e.next) == null) {
val = remappingFunction.apply(key, null);
if (val != null) {
delta = 1;
pred.next =
new Node<K, V>(h, key, val, null);
}
break;
}
}
} else if (f instanceof TreeBin) {
binCount = 1;
TreeBin<K, V> t = (TreeBin<K, V>) f;
TreeNode<K, V> r, p;
if ((r = t.root) != null)
p = r.findTreeNode(h, key, null);
else
p = null;
V pv = (p == null) ? null : p.val;
val = remappingFunction.apply(key, pv);
if (val != null) {
if (p != null)
p.val = val;
else {
delta = 1;
t.putTreeVal(h, key, val);
}
} else if (p != null) {
delta = -1;
if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
break;
}
}
}
if (delta != 0)
addCount((long) delta, binCount);
return val;
}
/**
* If the specified key is not already associated with a
* (non-null) value, associates it with the given value.
* Otherwise, replaces the value with the results of the given
* remapping function, or removes if {@code null}. The entire
* method invocation is performed atomically. Some attempted
* update operations on this map by other threads may be blocked
* while computation is in progress, so the computation should be
* short and simple, and must not attempt to update any other
* mappings of this Map.
*
* @param key key with which the specified value is to be associated
* @param value the value to use if absent
* @param remappingFunction the function to recompute a value if present
* @return the new value associated with the specified key, or null if none
* @throws NullPointerException if the specified key or the
* remappingFunction is null
* @throws RuntimeException or Error if the remappingFunction does so,
* in which case the mapping is unchanged
*/
public V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
if (key == null || value == null || remappingFunction == null)
throw new NullPointerException();
int h = spread(key.hashCode());
V val = null;
int delta = 0;
int binCount = 0;
for (Node<K, V>[] tab = table; ; ) {
Node<K, V> f;
int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
if (casTabAt(tab, i, null, new Node<K, V>(h, key, value, null))) {
delta = 1;
val = value;
break;
}
} else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K, V> e = f, pred = null; ; ++binCount) {
K ek;
if (e.hash == h &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
val = remappingFunction.apply(e.val, value);
if (val != null)
e.val = val;
else {
delta = -1;
Node<K, V> en = e.next;
if (pred != null)
pred.next = en;
else
setTabAt(tab, i, en);
}
break;
}
pred = e;
if ((e = e.next) == null) {
delta = 1;
val = value;
pred.next =
new Node<K, V>(h, key, val, null);
break;