putIfAbsent 源代码
public V putIfAbsent(K key, V value) {
Segment<K,V> s;
if (value == null)
throw new NullPointerException();
int hash = hash(key);
int j = (hash >>> segmentShift) & segmentMask;
if ((s = (Segment<K,V>)UNSAFE.getObject
(segments, (j << SSHIFT) + SBASE)) == null)
s = ensureSegment(j);
return s.put(key, hash, value, true);
}
put源代码
public V put(K key, V value) {
Segment<K,V> s;
if (value == null)
throw new NullPointerException();
int hash = hash(key);
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);
}
前面一段都是一样的，都是先计算hash再同步取值，区别在于
s.put(key, hash, value, true); 和
return s.put(key, hash, value, false);
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
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; //putIfAbsent下不会进入修改e.value, 在key已经存在的情况下
++modCount; } break; } e = e.next; }
onlyIfAbsent 参数，如果key存在的情况下，在putIfAbsent下不会修改,而put下则会修改成新的值
测试put
ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();
System.out.println(map.put("1", "1"));
System.out.println(map.put("1", "2"));
System.out.println(map.get("1"));
结果为：
null
1
2
测试putIfAbsent
ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();
System.out.println(map.putIfAbsent("1", "1"));
System.out.println(map.putIfAbsent("1", "2"));
System.out.println(map.get("1"));
结果为:
null
1
1
ConcurrentHashMap 中putIfAbsent 和put的区别
原文：http://www.cnblogs.com/chengxin1982/p/6489672.html