ConcurrentHashMap中putIfAbsent和put的区别

    public V putIfAbsent(K key, V value) {
        Segment s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key);
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment)UNSAFE.getObject
             (segments, (j <            s = ensureSegment(j);
        return s.put(key, hash, value, true);
    }

  public V put(K key, V value) {
        Segment s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key);
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j <            s = ensureSegment(j);
        return s.put(key, hash, value, false);
    }

final V put(K key, int hash, V value, boolean onlyIfAbsent) {

for (HashEntry 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; }

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

	ConcurrentHashMap map = new ConcurrentHashMap();
	System.out.println(map.putIfAbsent("1", "1"));
	System.out.println(map.putIfAbsent("1", "2"));
	System.out.println(map.get("1"));