ConcurrentHashMap 中putIfAbsent 和put的区别

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，区别在于最后一句 　

return s.put(key, hash, value, false); 　

return s.put(key, hash, value, true);

putIfAbsent下不会进入修改e.value

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;

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();
System.out.println(map.put("userName", "1"));
System.out.println(map.put("userName", "2"));
System.out.println(map.get("1"));                            

打印出来是

2

putIfAbsent方法，则打印出来是1，
因此可以知道onlyIfAbsent，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