java ConcurrentHashMap的读操作不要加锁 java中ConcurrentHashMap的读操作为什么不需要加锁

//会发现源码中没有一处加了锁
public V get(Object key) {
 Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
 int h = spread(key.hashCode()); //计算hash
 if ((tab = table) != null && (n = tab.length) > 0 &&
 (e = tabAt(tab, (n - 1) & h)) != null) {//读取首节点的Node元素
 if ((eh = e.hash) == h) { //如果该节点就是首节点就返回
  if ((ek = e.key) == key || (ek != null && key.equals(ek)))
  return e.val;
 }
 //hash值为负值表示正在扩容，这个时候查的是ForwardingNode的find方法来定位到nextTable来
 //eh=-1，说明该节点是一个ForwardingNode，正在迁移，此时调用ForwardingNode的find方法去nextTable里找。
 //eh=-2，说明该节点是一个TreeBin，此时调用TreeBin的find方法遍历红黑树，由于红黑树有可能正在旋转变色，所以find里会有读写锁。
 //eh>=0，说明该节点下挂的是一个链表，直接遍历该链表即可。
 else if (eh < 0)
  return (p = e.find(h, key)) != null ? p.val : null;
 while ((e = e.next) != null) {//既不是首节点也不是ForwardingNode，那就往下遍历
  if (e.hash == h &&
  ((ek = e.key) == key || (ek != null && key.equals(ek))))
  return e.val;
 }
 }
 return null;
}

 /**
  * The array of bins. Lazily initialized upon first insertion.
  * Size is always a power of two. Accessed directly by iterators.
  */
 transient volatile Node<K,V>[] table;

static class Node<K,V> implements Map.Entry<K,V> {
 final int hash;
 final K key;
 //可以看到这些都用了volatile修饰
 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;
 }
}