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前言: 本渣渣想分析分析
Doug Lea大佬对高并发代码编写思路, 于是找到了我们今天的小主角ConcurrentLinkedQueue进行鞭打, 说实话草稿我都打好了, 就差临门一脚, 给踢折了
直接看问题, idea在 Debug 和非 Debug模式下运行结果不同, vscode 复现, eclipse 毫无鸭梨
怎么发现的问题?
从这段代码开始
public static void main(String[] args) throws InterruptedException, NoSuchFieldException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, InstantiationException {ConcurrentLinkedQueue<String> queue = new ConcurrentLinkedQueue<>();
queue.add("zhazha");
// 在下面这行下断点
Field headField = queue.getClass().getDeclaredField("head");
headField.setAccessible(true);
Object head = headField.get(queue);
Field itemField = queue.getClass().getDeclaredField("ITEM");
itemField.setAccessible(true);
VarHandle ITEM = (VarHandle) itemField.get(head);
Object o = ITEM.get(head);
System.out.println(o);
}你会发现一个神奇的现象, 如果我们下断点在 Field headField = queue.getClass().getDeclaredField("head"); 这一行代码, 单步执行下来会发现 System.out.println(o); 打印出了zhazha, 但是如果不下断点, 直接运行打印null
为了防止是
WARNING: An illegal reflective access operation has occurred警告的影响, 我改了改源码, 用 unsafe 获取试试
private static Unsafe unsafe;
static {
Class<Unsafe> unsafeClass = Unsafe.class;
Unsafe unsafe = null;
try {Field unsafeField = unsafeClass.getDeclaredField("theUnsafe");
unsafeField.setAccessible(true);
ConcurrentLinkedQueueDemo.unsafe = (Unsafe) unsafeField.get(null);
} catch (NoSuchFieldException | IllegalAccessException e) {e.printStackTrace();
}
}
public static void main(String[] args) throws InterruptedException, NoSuchFieldException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, InstantiationException {ConcurrentLinkedQueue<String> queue = new ConcurrentLinkedQueue<>();
queue.add("zhazha");
// 在下面这行下断点
long headOffset = unsafe.objectFieldOffset(queue.getClass().getDeclaredField("head"));
Object head = unsafe.getObject(queue, headOffset);
long itemOffset = unsafe.staticFieldOffset(ConcurrentLinkedQueue.class.getDeclaredField("ITEM"));
Object base = unsafe.staticFieldBase(ConcurrentLinkedQueue.class.getDeclaredField("ITEM"));
VarHandle ITEM = (VarHandle) unsafe.getObject(base, itemOffset);
Object o = ITEM.get(head);
System.out.println(o);
}完美复现
第一反应我的问题
去源码里看看怎么回事. 但 ……. 这 ………..
仔细看红箭头的地址, t、p、head和 tail 都是同一个地址, 看上面的代码发现全是 tail 赋值给这三个变量的
而NEXT源码
他的接收类是Node, 接收字段是next, 接收字段类型Node
看这源码的势头, NEXT修改的是 p 对象, 如果该对象的 next 节点为 null, 则把newNode 设置到节点上, 此时 p 对象指向的是 tail, 同时head 也是指向的 tail 节点, 所以这句话执行完毕, head.next和 tail.next 同样都是 newNode 节点
但 ………………… 这 …………………
head节点被直接替换掉, tail保持不变
此时我的表情应该是这样
怀疑猫生
private static Unsafe unsafe;
static {
Class<Unsafe> unsafeClass = Unsafe.class;
Unsafe unsafe = null;
try {Field unsafeField = unsafeClass.getDeclaredField("theUnsafe");
unsafeField.setAccessible(true);
ConcurrentLinkedQueueDemo.unsafe = (Unsafe) unsafeField.get(null);
} catch (NoSuchFieldException | IllegalAccessException e) {e.printStackTrace();
}
}
public static void main(String[] args) throws InterruptedException, NoSuchFieldException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, InstantiationException {ConcurrentLinkedQueue<String> queue = new ConcurrentLinkedQueue<>();
queue.add("zhazha");
// 在这里下断点
Class<? extends ConcurrentLinkedQueue> queueClass = queue.getClass();
Object head = unsafe.getObject(queue, unsafe.objectFieldOffset(queueClass.getDeclaredField("head")));
Field itemField = queueClass.getDeclaredField("ITEM");
itemField.setAccessible(true);
VarHandle ITEM = (VarHandle) itemField.get(queue);
Object item = ITEM.get(head);
System.out.println(item); // zhazha
long itemOffset = unsafe.staticFieldOffset(queueClass.getDeclaredField("ITEM"));
Object base = unsafe.staticFieldBase(queueClass.getDeclaredField("ITEM"));
VarHandle ITEM2 = (VarHandle) unsafe.getObject(base, itemOffset);
Object item2 = ITEM2.get(head);
System.out.println(item2); // zhazha
}单步调试出来还是 zhazha, 而且为了防止反射出了问题, 我同时用了Unsafe 和反射两种方法
copy 源码添加自己的调试函数再次测试
得了得了, 放终极大招试试, copy ConcurrentLinkedQueue源码出来改成 MyConcurrentLinkedQueue
在offer方法添加几个输出
public boolean offer(E e) {final Node<E> newNode = new Node<E>(Objects.requireNonNull(e));
for (Node<E> t = tail, p = t; ;) {
Node<E> q = p.next;
if (q == null) {if (NEXT.compareAndSet(p, null, newNode)) {System.out.println("this.head.item =" + this.head.item);
System.out.println("this.tail.item =" + this.tail.item);
System.out.println("this.head.next.item =" + this.head.next.item);
System.out.println("this.tail.next.item =" + this.tail.next.item);
if (p != t) {TAIL.weakCompareAndSet(this, t, newNode);
}
return true;
}
}
else if (p == q) {p = (t != (t = tail)) ? t : head;
}
else {p = (p != t && t != (t = tail)) ? t : q;
}
}
}主函数就比较简单了直接
public static void main(String[] args) {MyConcurrentLinkedQueue<String> queue = new MyConcurrentLinkedQueue<String>();
queue.add("zhazha");
}直接在非 Debug 模式下运行, 发现打印出来的是
this.head.item = null
this.tail.item = null
this.head.next.item = zhazha
this.tail.next.item = zhazha
Process finished with exit code 0在 Debug 模式下单步运行发现
this.head.item = zhazha
this.tail.item = null
Exception in thread "main" java.lang.NullPointerException
at com.zhazha.juc.MyConcurrentLinkedQueue.offer(MyConcurrentLinkedQueue.java:117)
at com.zhazha.juc.MyConcurrentLinkedQueue.add(MyConcurrentLinkedQueue.java:67)
at com.zhazha.juc.MyConcurrentLinkedQueueDemo.main(MyConcurrentLinkedQueueDemo.java:13)
Process finished with exit code 1纳尼?
不信邪的我在 NEXT cas 操作的前后增加了 sleep 方法, 以 非 Debug模式下运行
this.head.item = null
this.tail.item = null
this.head.next.item = zhazha
this.tail.next.item = zhazha还是不一样
多环境 IDE 测试
放终极终极终极 SVIP 大招 ===> 放在 eclipse 上试试??? 或者 vscode 上???
在 vscode 上以 Debug 模式单步运行输出
this.head.item = zhazha
this.tail.item = null
Exception in thread "main" java.lang.NullPointerException
at MyConcurrentLinkedQueue.offer(MyConcurrentLinkedQueue.java:116)
at MyConcurrentLinkedQueue.add(MyConcurrentLinkedQueue.java:66)
at MyConcurrentLinkedQueueDemo.main(MyConcurrentLinkedQueueDemo.java:11)
非 Debug模式直接输出
this.head.item = null
this.tail.item = null
this.head.next.item = zhazha
this.tail.next.item = zhazha在 eclipse 上以 Debug 模式单步运行输出
this.head.item = null
this.tail.item = null
this.head.next.item = zhazha
this.tail.next.item = zhazha
非 Debug运行输出
this.head.item = null
this.tail.item = null
this.head.next.item = zhazha
this.tail.next.item = zhazha发现了没有? 还是我大 eclipse 坚挺住了