阅读ConcurrentLinkedQueue 源码的过程中发现了奇怪问题。现象及重复步骤如下:
1.执行代码
ConcurrentLinkedQueue<Integer> cq = new ConcurrentLinkedQueue();
cq.offer(1);2.进入到offer方法,看334行 if (p.casNext(null, newNode)):
public boolean offer(E e) {
checkNotNull(e);
final Node<E> newNode = new Node<E>(e); for (Node<E> t = tail, p = t;;) {
Node<E> q = p.next;
if (q == null) {
// p is last node
if (p.casNext(null, newNode)) { // 334行
// Successful CAS is the linearization point
// for e to become an element of this queue,
// and for newNode to become "live".
if (p != t) // hop two nodes at a time
casTail(t, newNode); // Failure is OK.
return true;
}
// Lost CAS race to another thread; re-read next
}
else if (p == q)
// We have fallen off list. If tail is unchanged, it
// will also be off-list, in which case we need to
// jump to head, from which all live nodes are always
// reachable. Else the new tail is a better bet.
p = (t != (t = tail)) ? t : head;
else
// Check for tail updates after two hops.
p = (p != t && t != (t = tail)) ? t : q;
}
}此时head == tail == p,head的对象如图所示:2.进入casNext方法,casNext方法代码如下:
boolean casNext(Node<E> cmp, Node<E> val) {
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
执行了 UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val)后,this为:
此时只有一点奇怪。3.casNext返回后,各变量如图所示:此时就比较奇怪了,其一本来p和casNext里面的this指向同一个对象,出栈后却不一样了,其二本来要设置p的next,结果设置为不是newNode而是它自己,其三head节点忽然就指向了newNode了,没有看到有这样的操作,求大神解释,多谢!
1.执行代码
ConcurrentLinkedQueue<Integer> cq = new ConcurrentLinkedQueue();
cq.offer(1);2.进入到offer方法,看334行 if (p.casNext(null, newNode)):
public boolean offer(E e) {
checkNotNull(e);
final Node<E> newNode = new Node<E>(e); for (Node<E> t = tail, p = t;;) {
Node<E> q = p.next;
if (q == null) {
// p is last node
if (p.casNext(null, newNode)) { // 334行
// Successful CAS is the linearization point
// for e to become an element of this queue,
// and for newNode to become "live".
if (p != t) // hop two nodes at a time
casTail(t, newNode); // Failure is OK.
return true;
}
// Lost CAS race to another thread; re-read next
}
else if (p == q)
// We have fallen off list. If tail is unchanged, it
// will also be off-list, in which case we need to
// jump to head, from which all live nodes are always
// reachable. Else the new tail is a better bet.
p = (t != (t = tail)) ? t : head;
else
// Check for tail updates after two hops.
p = (p != t && t != (t = tail)) ? t : q;
}
}此时head == tail == p,head的对象如图所示:2.进入casNext方法,casNext方法代码如下:
boolean casNext(Node<E> cmp, Node<E> val) {
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
执行了 UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val)后,this为:
此时只有一点奇怪。3.casNext返回后,各变量如图所示:此时就比较奇怪了,其一本来p和casNext里面的this指向同一个对象,出栈后却不一样了,其二本来要设置p的next,结果设置为不是newNode而是它自己,其三head节点忽然就指向了newNode了,没有看到有这样的操作,求大神解释,多谢!
https://blog.csdn.net/u010880076/article/details/88878926