ConcurrentHashMap1.7和ConcurrentHashMap1.8的异同
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ConcurrentHashMap
ConcurrentHashMap在与HashMap的不同之处在于多线程下更安全，而针对于Hashtable的基础上进行了一次升级。Hashtable的安全机制过于粗暴，导致性能很低。根据不同版本采用的方式也有不同。
public class ConcurrentHashMap<K,V> extends AbstractMap<K,V>
implements ConcurrentMap<K,V>, Serializable
1.7版本的ConcurrentHashMap
主要采用的是分段锁机制，并且读取时不进行加锁操作。底层的结构还是和1.7版本的hashmap类似，只是多了一个Segments数组，一个Segment可以看作类似的hashtable，里面包含了一个table数组用来存取hashentry，table里每个元素代表着链表的头节点，transient表示该变量不进行序列化，继承了ReentrantLock 重入锁。
static final class Segment<K,V> extends ReentrantLock implements Serializable
transient volatile int count;
transient int modCount;
transient int threshold;
transient volatile HashEntry<K,V>[] table;
final float loadFactor;
HashEntry的声明类，用于数据存取的对象
static final class HashEntry<K,V>
final K key; //键
final int hash; //hash值hashcode(k)^hashcode(v)
volatile V value;
final HashEntry<K,V> next; //final修饰的变量只能被初始化一次
ConcurrentHashMap的实现：
public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel)
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
if (concurrencyLevel > MAX_SEGMENTS)
concurrencyLevel = MAX_SEGMENTS;
// Find power-of-two sizes best matching arguments
int sshift = 0;
int ssize = 1;
while (ssize < concurrencyLevel)
++sshift;
ssize <<= 1;
segmentShift = 32 - sshift;
segmentMask = ssize - 1;
this.segments = Segment.newArray(ssize);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
int c = initialCapacity / ssize;
if (c * ssize < initialCapacity)
++c;
int cap = 1;
while (cap < c)
cap <<= 1;
for (int i = 0; i < this.segments.length; ++i)
this.segments[i] = new Segment<K,V>(cap, loadFactor);
获取数据V不需要进行加锁，首先找到哪一个segment，然后确定table[i]数组中的位置，在进行遍历链表，取值。
1 public V get(Object key)
2 int hash = hash(key.hashCode()); //采用二次hash减少hash冲突
3 return segmentFor(hash).get(key, hash);
4
final Segment<K,V> segmentFor(int hash)
return segments[(hash >>> segmentShift) & segmentMask];
1 V get(Object key, int hash) //get方法
2 if (count != 0) // read-volatile // ①
3 HashEntry<K,V> e = getFirst(hash); 获取table里的元素
4 while (e != null)
5 if (e.hash == hash && key.equals(e.key))
6 V v = e.value;
7 if (v != null) // ② 注意这里
8 return v;
9 return readValueUnderLock(e); // recheck
10
11 e = e.next;
12
13
14 return null;
15
在这里需要注意的是next指向下一个结点被final关键字修饰，也就是说不能在直接进行插入或者删除的工作，而且添加元素也只能在头部进行插入。当我们正在查询时，此时进行了添加工作，应该优先进行添加工作，happen-before原则。这样能保证我们读取的值是最新的。在创建entry时会发现很多变量都被volatile和final修饰，volatile主要是保证线程的有序性和可见性。
put操作源码
1 V put(K key, int hash, V value, boolean onlyIfAbsent)
2 lock(); //获取锁的对象
3 try
4 int c = count;
5 if (c++ > threshold) // ensure capacity 扩容
6 rehash();
7 HashEntry<K,V>[] tab = table;
8 int index = hash & (tab.length - 1); //获取数组下标值
9 HashEntry<K,V> first = tab[index]; //数组中第一个entry对象
10 HashEntry<K,V> e = first;
11 while (e != null && (e.hash != hash || !key.equals(e.key)))
12 e = e.next;
13
14 V oldValue;
15 if (e != null) //表示数组已经存在该KEY
16 oldValue = e.value;
17 if (!onlyIfAbsent) //表示是否要覆盖
18 e.value = value;
19
20 else //不存在key，
21 oldValue = null;
22 ++modCount;
23 tab[index] = new HashEntry<K,V>(key, hash, first, value);
24 count = c; // write-volatile
25
26 return oldValue;
27 finally
28 unlock();
29
30
remove操作详情
1 V remove(Object key, int hash, Object value)
2 lock(); //获取锁对象
3 try
4 int c = count - 1;
5 HashEntry<K,V>[] tab = table;
6 int index = hash & (tab.length - 1);
7 HashEntry<K,V> first = tab[index];
8 HashEntry<K,V> e = first;
9 while (e != null && (e.hash != hash || !key.equals(e.key)))
10 e = e.next;
11
12 V oldValue = null;
13 if (e != null)
14 V v = e.value;
15 if (value == null || value.equals(v))
16 oldValue = v;
17 // All entries following removed node can stay
18 // in list, but all preceding ones need to be
19 // cloned.
20 ++modCount;
21 HashEntry<K,V> newFirst = e.next;
22 for (HashEntry<K,V> p = first; p != e; p = p.next)
23 newFirst = new HashEntry<K,V>(p.key, p.hash,
24 newFirst, p.value); //方法里的newFirst表示next指针。
25 tab[index] = newFirst;
26 count = c; // write-volatile
27
28
29 return oldValue;
30 finally
31 unlock(); //解锁释放锁的对象
32
33
由于删除之后表结构会发生该表，会将删除结点之前的元素进行一个倒叙排列，最后再将结点之后的元素连接起来。
ConcurrentHashMap1.8
ConcurrentHashMap1.8底层采用的数据结构是数组+链表+红黑树，采用的synchronization+CAS实现多线程安全机制。
Node结点代码里只有find并没有其他方法！
static class Node<K,V> implements Map.Entry<K,V>
final int hash;
final K key;
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;
public final int hashCode() return key.hashCode() ^ val.hashCode();
public final String toString() return key + "=" + val;
public final V setValue(V value)
throw new UnsupportedOperationException();
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)));
/**
* Virtualized support for map.get(); overridden in subclasses.
*/
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;
一些compareAndSwapObject（CAS）操作,保证了原子性
@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);
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);
static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v)
U.putObjectVolatile(tab, ((long)i << ASHIFT) + ABASE, v);
定义了一些变量
transient volatile Node<K,V>[] table;
/**
* The next table to use; non-null only while resizing.
*/
private transient volatile Node<K,V>[] nextTable;
/**
* Base counter value, used mainly when there is no contention,
* but also as a fallback during table initialization
* races. Updated via CAS.
*/
private transient volatile long baseCount;
/**
* Table initialization and resizing control. When negative, the
* table is being initialized or resized: -1 for initialization,
* else -(1 + the number of active resizing threads). Otherwise,
* when table is null, holds the initial table size to use upon
* creation, or 0 for default. After initialization, holds the
* next element count value upon which to resize the table.
*/
private transient volatile int sizeCtl;
/**
* The next table index (plus one) to split while resizing.
*/
private transient volatile int transferIndex;
/**
* Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
*/
private transient volatile int cellsBusy;
/**
* Table of counter cells. When non-null, size is a power of 2.
*/
private transient volatile CounterCell[] counterCells;
// views
private transient KeySetView<K,V> keySet;
private transient ValuesView<K,V> values;
private transient EntrySetView<K,V> entrySet;
get方法，并没有进行加锁，支持高并发
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) ConcurrentHashMap1.7和ConcurrentHashMap1.8的异同
ConcurrentHashMap1.7和ConcurrentHashMap1.8的异同