在随后的博文中我会继续分析并发包源码,在这里,得分别谈谈容器类和迭代器及其源码,虽然很突兀,但我认为这对于学习Java并发很重要;
ConcurrentModificationException:
JavaAPI中的解释:当不允许这样的修改时，可以通过检测到对象的并发修改的方法来抛出此异常。一个线程通常不允许修改集合，而另一个线程正在遍历它。 一般来说，在这种情况下，迭代的结果是未定义的。 某些迭代器实现（包括由JRE提供的所有通用集合实现的实现）可能会选择在检测到此行为时抛出此异常。 这样做的迭代器被称为"及时失败"迭代器，当他们发现容器在迭代时被修改时,就会报异常;它称不上时一种处理机制,而是一种预防机制,只能作为并发问题的预警指示器.
迭代器与容器:
Vector这个"古老"的容器类相信大家很熟悉了,他是线程安全的没错,我们利用elements()方法遍历,不会出现线程安全的问题:
1 /**
2 * JDK1.8源码
3 */
4
5 /**
6 * Returns an enumeration of the components of this vector. The
7 * returned {@code Enumeration} object will generate all items in
8 * this vector. The first item generated is the item at index {@code 0},
9 * then the item at index {@code 1}, and so on.
10 *
11 * @return an enumeration of the components of this vector
12 * @see Iterator
13 */
14 public Enumeration<E> elements() {
15 return new Enumeration<E>() {
16 int count = 0;
17
18 public boolean hasMoreElements() {
19 return count < elementCount;
20 }
21
22 public E nextElement() {
23 synchronized (Vector.this) {
24 if (count < elementCount) {
25 return elementData(count++);
26 }
27 }
28 throw new NoSuchElementException("Vector Enumeration");
29 }
30 };
31 }
JDK1.8源码 from AbstractCollection (been extends by Vector)
但当我们利用foreach进行迭代时,底层自动调用了iterator(),若我们不锁住容器,可能会报ConcurrentModificationException
1 /**
2 * Returns an iterator over the elements in this list in proper sequence.
3 *
4 * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
5 *
6 * @return an iterator over the elements in this list in proper sequence
7 */
8 public Iterator<E> iterator() {
9 return new Itr();
10 //JDK1.8写成了一个内部类的形式返回迭代器
11 }
12
13 /**
14 * An optimized version of AbstractList.Itr
15 */
16 private class Itr implements Iterator<E> {
17 int cursor; // index of next element to return
18 int lastRet = -1; // index of last element returned; -1 if no such
19 int expectedModCount = modCount;
20
21 Itr() {}
22
23 public boolean hasNext() {
24 return cursor != size;
25 }
26
27 //观察下面的代码可以发现,每次迭代一次都要检查容器长度是否改变
28 @SuppressWarnings("unchecked")
29 public E next() {
30 checkForComodification();
31 int i = cursor;
32 if (i >= size)
33 throw new NoSuchElementException();
34 Object[] elementData = ArrayList.this.elementData;
35 if (i >= elementData.length)
36 throw new ConcurrentModificationException();
37 cursor = i + 1;
38 return (E) elementData[lastRet = i];
39 }
40
41 public void remove() {
42 if (lastRet < 0)
43 throw new IllegalStateException();
44 checkForComodification();
45
46 try {
47 ArrayList.this.remove(lastRet);
48 cursor = lastRet;
49 lastRet = -1;
50 expectedModCount = modCount;
51 } catch (IndexOutOfBoundsException ex) {
52 throw new ConcurrentModificationException();
53 }
54 }
55
56 @Override
57 @SuppressWarnings("unchecked")
58 public void forEachRemaining(Consumer<? super E> consumer) {
59 Objects.requireNonNull(consumer);
60 final int size = ArrayList.this.size;
61 int i = cursor;
62 if (i >= size) {
63 return;
64 }
65 final Object[] elementData = ArrayList.this.elementData;
66 if (i >= elementData.length) {
67 throw new ConcurrentModificationException();
68 }
69 while (i != size && modCount == expectedModCount) {
70 consumer.accept((E) elementData[i++]);
71 }
72 // update once at end of iteration to reduce heap write traffic
73 cursor = i;
74 lastRet = i - 1;
75 checkForComodification();
76 }
77
78 final void checkForComodification() {
79 if (modCount != expectedModCount)
80 throw new ConcurrentModificationException();
81 }
82 }
JDK1.8源码 from Vector
查看代码时可以发现,iterator()的内部类中提供的next,remove等方法都进行了迭代操作,这样的迭代被称为"隐藏迭代器";
部分容器类的toString()方法会在调用StringBuilder的append()同时迭代容器,例如继承了AbstractSet的HashSet类,而AbstractSet又继承于AbstractCollection:
// String conversion
/**
* Returns a string representation of this collection. The string
* representation consists of a list of the collection‘s elements in the
* order they are returned by its iterator, enclosed in square brackets
* (<tt>"[]"</tt>). Adjacent elements are separated by the characters
* <tt>", "</tt> (comma and space). Elements are converted to strings as
* by {@link String#valueOf(Object)}.
*
* @return a string representation of this collection
*/
public String toString() {
Iterator<E> it = iterator();
if (! it.hasNext())
return "[]";
StringBuilder sb = new StringBuilder();
sb.append(‘[‘);
for (;;) {
E e = it.next();
sb.append(e == this ? "(this Collection)" : e);
if (! it.hasNext())
return sb.append(‘]‘).toString();
sb.append(‘,‘).append(‘ ‘);
}
}
JDK1.8源码: from AbstractCollection
隐藏的迭代器普遍存在,甚至出现在hashCode,equals,contains,remove等方法中,此处不再赘述;
解决办法:
考虑到并发安全性,我们不得不对容器类单独加锁,但同时我们又不希望加锁,那样太损耗性能了,还存在线程饥饿和死锁的风险,极大降低吞吐量和CPU利用率;
作为有限的替代,我们可以考虑"克隆"容器,并在将其副本封闭起来进行迭代,避免了抛出ConcurrentModificationException,但在克隆过程中仍然要对容器加锁;显然,克隆容器存在系统开销,我们在选择这种方案时必须考虑的因素有:容器大小,每个元素上执行的工作,迭代操作相对于其它操作的调用频率,以及相应时间和系统吞吐率的需求.
参考材料:
Java并发编程实战
原文地址：https://www.cnblogs.com/Joey44/p/10347732.html