java.util.Collection的fail-fast机制

fail-fast 机制是java集合(Collection)中的一种错误机制。
fail-fast如何产生:
1.多个线程同时操作同一个collection

public static  List<String> list =new ArrayList<String>();public static void main( String[] args ){     new Thread(new Runnable() {         @Override         public void run() {             for (int i=0;i<10;i++ ){                 list.add("c"+i);                 printAll();             }         }     }).start();    new Thread(new Runnable() {        @Override        public void run() {            for (int i=0;i<10;i++ ){                list.add("d"+i);                printAll();            }        }    }).start();}public  static void printAll(){    for(String string: list){        System.out.println(string);    }}

2.通过增强循环for(Object obj:collection)遍历collection 时同时修改collection

public static  List<String> list =new ArrayList<String>();public static void main( String[] args ){    for (int i=0;i<10;i++ ){            list.add("d"+i);        }    printAll();}public  static void printAll(){    for(String string: list){        System.out.println(string);        list.remove(string);    }}

通过ArrayList 源码

 /** * An optimized version of AbstractList.Itr */private class Itr implements Iterator<E> {    int cursor;       // index of next element to return    int lastRet = -1; // index of last element returned; -1 if no such    int expectedModCount = modCount;    Itr() {}    public boolean hasNext() {        return cursor != size;    }    @SuppressWarnings("unchecked")    public E next() {        checkForComodification();        int i = cursor;        if (i >= size)            throw new NoSuchElementException();        Object[] elementData = ArrayList.this.elementData;        if (i >= elementData.length)            throw new ConcurrentModificationException();        cursor = i + 1;        return (E) elementData[lastRet = i];    }    public void remove() {        if (lastRet < 0)            throw new IllegalStateException();        checkForComodification();        try {            ArrayList.this.remove(lastRet);            cursor = lastRet;            lastRet = -1;            expectedModCount = modCount;        } catch (IndexOutOfBoundsException ex) {            throw new ConcurrentModificationException();        }    }    @Override    @SuppressWarnings("unchecked")    public void forEachRemaining(Consumer<? super E> consumer) {        Objects.requireNonNull(consumer);        final int size = ArrayList.this.size;        int i = cursor;        if (i >= size) {            return;        }        final Object[] elementData = ArrayList.this.elementData;        if (i >= elementData.length) {            throw new ConcurrentModificationException();        }        while (i != size && modCount == expectedModCount) {            consumer.accept((E) elementData[i++]);        }        // update once at end of iteration to reduce heap write traffic        cursor = i;        lastRet = i - 1;        checkForComodification();    }    final void checkForComodification() {        if (modCount != expectedModCount)            throw new ConcurrentModificationException();    }}    //这段     final void checkForComodification() {                        if (modCount != expectedModCount)  //modCount chushi                                 throw new ConcurrentModificationException();            }

增强遍历时 使用的是arraylist 的实现的 Iterator  每次遍历的时候都会检查 arraylist修改的记录数是否一致 不一致就抛出ConcurrentModificationException异常