java.util.Collection的fail-fast機制
阿新 • • 發佈:2018-08-16
bool final cti main 一個 date bstr rac null fail-fast 機制是java集合(Collection)中的一種錯誤機制。
fail-fast如何產生:
1.多個線程同時操作同一個collection
fail-fast如何產生:
1.多個線程同時操作同一個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.通過增強循環for(Object obj:collection)遍歷collection 時同時修改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); } }
通過ArrayList 源碼
/** * 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(); } } //這段 final void checkForComodification() { if (modCount != expectedModCount) //modCount chushi throw new ConcurrentModificationException(); }
增強遍歷時 使用的是arraylist 的實現的 Iterator 每次遍歷的時候都會檢查 arraylist修改的記錄數是否一致 不一致就拋出ConcurrentModificationException異常
java.util.Collection的fail-fast機制