java - Java中 `insert()`和 `size()`并发执行会出现死锁吗？

Question

代码如下所示(link):

/***
 * Excerpted from "Seven Concurrency Models in Seven Weeks",
***/
import java.util.concurrent.locks.ReentrantLock;

class ConcurrentSortedList {

  private class Node {
    int value;
    Node prev;
    Node next;
    ReentrantLock lock = new ReentrantLock();

    Node() {}

    Node(int value, Node prev, Node next) {
      this.value = value; this.prev = prev; this.next = next;
    }
  }

  private final Node head;
  private final Node tail;

  public ConcurrentSortedList() {
    head = new Node(); tail = new Node();
    head.next = tail; tail.prev = head;
  }

  public void insert(int value) {
    Node current = head;
    current.lock.lock(); 
    Node next = current.next;
    try {
      while (true) {
        next.lock.lock(); 
        try {
          if (next == tail || next.value < value) { 
            Node node = new Node(value, current, next); 
            next.prev = node;
            current.next = node;
            return; 
          }
        } finally { current.lock.unlock(); } 
        current = next;
        next = current.next;
      }
    } finally { next.lock.unlock(); } 
  }

  public int size() {
    Node current = tail;
    int count = 0;

    while (current.prev != head) {
      ReentrantLock lock = current.lock;
      lock.lock();
      try {
        ++count;
        current = current.prev;
      } finally { lock.unlock(); }
    }

    return count;
  }

}

它说它使用手对手锁定。 insert() 获取从列表头到列表尾的锁，size() 获取从列表尾到列表头的锁。 size() 和 insert() 可以并发执行。

但我认为size()和insert()不能并发执行。因为如果 insert 持有 aNode 上的锁并请求 aNode.next 上的锁，而 size 持有锁定aNode.next并请求锁定aNode，将会出现死锁。

有人对此有什么想法吗？谢谢!

Answer 1

我明白了.. size() 将在请求新锁之前释放当前锁.. 因此不会出现死锁..