CocurrentHashMap 作用
HashTable通过对整张表加锁的方式实现并发hash查找与储存，CocurrentHashMapt通过Segment的方式可以实现相同的功能，不过效率更加高，在jdk1.6的时候，CocuentHashMap有弱一致性的问题，不过在jdk1.7的时候，这个问题已经修复了。所以是并发安全性还是性能都是非常高的。接下来我尝试基于jdk1.7的源码去分析CocurrentHashMap。
cocurrentHashMap 初始化预处理
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long SBASE;
private static final int SSHIFT;
private static final long TBASE;
private static final int TSHIFT;
static {
int ss, ts;
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> tc = HashEntry[].class;
Class<?> sc = Segment[].class;
TBASE = UNSAFE.arrayBaseOffset(tc);
SBASE = UNSAFE.arrayBaseOffset(sc);
ts = UNSAFE.arrayIndexScale(tc);
ss = UNSAFE.arrayIndexScale(sc);
} catch (Exception e) {
throw new Error(e);
}
if ((ss & (ss-1)) != 0 || (ts & (ts-1)) != 0)
throw new Error("data type scale not a power of two");
SSHIFT = 31 - Integer.numberOfLeadingZeros(ss);
TSHIFT = 31 - Integer.numberOfLeadingZeros(ts);
}
代码解析：首先获取Unsafe提供cas操作，java底层多线程并发都是通过cas完成的，不过cas操作对于高精度的并发还是存在一定问题。【至于这个问题，以后再分析】。UNSAFE.arrayBaseOffset(tc)和UNSAFE.arrayBaseOffset(sc)这两个都是用于计算HashEntry和Segment实体对象相对于数组对象的内存偏移值。这是cas操作必须要获取的值。
注释：
//获取数组中第一个元素的偏移量(get offset of a first element in the array)
public native int arrayBaseOffset(java.lang.Class aClass);
//获取数组中一个元素的大小(get size of an element in the array)
public native int arrayIndexScale(java.lang.Class aClass);
cocurrentHashMap 初始化
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;
}
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]);
Segment<K,V>[] ss = (Segment<K,V>[])new Segment<?,?>[ssize];
UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
this.segments = ss;
}代码解析:以上代码主要的作用是初始化Segment[]数组对象以及Segment对象。解析来分析最重要的put和get方法。
put方法
<span style="font-size:18px;"> public V put(K key, V value) {
Segment<K,V> s;
if (value == null)
throw new NullPointerException();
int hash = hash(key.hashCode());
int j = (hash >>> segmentShift) & segmentMask;
if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck
(segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment
s = ensureSegment(j);
return s.put(key, hash, value, false);
}</span>
代码解析：大体的意思是先计算出key的hash值，然后利用这个hash值得到Segment对象。然后Segment对象执行put方法。这样就完成了put操作。由于这个过程非常重要，我们肯定想要知道它是如何处理并发以及 内部实现。
ensureSegment
private Segment<K,V> ensureSegment(int k) {
final Segment<K,V>[] ss = this.segments;
long u = (k << SSHIFT) + SBASE; // raw offset
Segment<K,V> seg;
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
Segment<K,V> proto = ss[0]; // use segment 0 as prototype
int cap = proto.table.length;
float lf = proto.loadFactor;
int threshold = (int)(cap * lf);
HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry<?,?>[cap];
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
== null) { // recheck
Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
== null) {
if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
break;
}
}
}
return seg;
} 首先计算出偏移量，然后利用UnSafe去获取对象。在这里有可能大家对这个偏移值的获取有点疑惑，在这里我也分析一下这个偏移量获取既long u=(k<<SSHIFT)+SBASE; 大家有可能会提出为什么会这样计算呢。一般人都会这样计算偏移值：设k为index,size为对象的大小，_offset为第一个元素的偏移值，那偏移值的大小应该为offset=index*size+offset。没错，理论上这么来说是对的。但是在java内存模型中内存遵循8字节对齐。所以在java内存模型中你这样计算是错误的。有前面的初始化可以知道：SSHIFT为对象大小对应的二进制数对应的位数。所以k<<SSHIFT也就实现了8字节对齐。
segment->put
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
HashEntry<K,V> node = tryLock() ? null :
scanAndLockForPut(key, hash, value);
V oldValue;
try {
HashEntry<K,V>[] tab = table;
int index = (tab.length - 1) & hash;
HashEntry<K,V> first = entryAt(tab, index);
for (HashEntry<K,V> e = first;;) {
if (e != null) {
K k;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) {
oldValue = e.value;
if (!onlyIfAbsent) {
e.value = value;
++modCount;
}
break;
}
e = e.next;
}
else {
if (node != null)
node.setNext(first);
else
node = new HashEntry<K,V>(hash, key, value, first);
int c = count + 1;
if (c > threshold && tab.length < MAXIMUM_CAPACITY)
rehash(node);
else
setEntryAt(tab, index, node);
++modCount;
count = c;
oldValue = null;
break;
}
}
} finally {
unlock();
}
return oldValue;
}以下画一个流程图：
get()
public V get(Object key) {
Segment<K,V> s; // manually integrate access methods to reduce overhead
HashEntry<K,V>[] tab;
int h = hash(key.hashCode());
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;
} 原理很简单先定位到segment，然后定位到实体。并且通过getObjectVolatie保证能够读到最新的数据。
总结:concurrentHashMap实现涉及到很多多线程的知识和java内存模型这方面的知识，如果没有足够的能力，介意不要模仿，但是我们可以学习它的思想以及是如何实现的。
