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 = nullthis.tail.item = nullthis.head.next.item = zhazhathis.tail.next.item = zhazhaProcess finished with exit code 0在Debug模式下单步运行发现
this.head.item = zhazhathis.tail.item = nullException 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 = nullthis.tail.item = nullthis.head.next.item = zhazhathis.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 = zhazhathis.tail.item = nullException 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 = nullthis.tail.item = nullthis.head.next.item = zhazhathis.tail.next.item = zhazha在eclipse上以Debug模式单步运行输出
this.head.item = nullthis.tail.item = nullthis.head.next.item = zhazhathis.tail.next.item = zhazha非Debug运行输出
this.head.item = nullthis.tail.item = nullthis.head.next.item = zhazhathis.tail.next.item = zhazha发现了没有? 还是我大eclipse坚挺住了