Java并发(五):并发,迭代器和容器

　　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

Java并发(五):并发,迭代器和容器

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