Java集合-05fail-fast(快速失败)机制原理及解决方法

public class FailFastMultiThread  {
private static List<String> lists = new ArrayList<>(10);
static {
for (int i = 0; i < 4; i++){
lists.add(String.valueOf(i));
}
}
public static void main(String[] args) {
new Thread(new ForEachThread()).start();
new Thread(new EditThread()).start();
}

//用于遍历
static class ForEachThread implements Runnable{
@Override
public void run() {
Iterator<String> iterator = lists.iterator();
while (iterator.hasNext()){
System.out.println(iterator.next());
try {
Thread.sleep(100);//为了另外的线程加入，也是为了结合在遍历时候修改结构
} catch (InterruptedException e) {
e.printStackTrace();
}
}

}
}

//用于修改结构
static class EditThread implements Runnable{
@Override
public void run() {
lists.add("8");
}
}

}
//output:Exception in thread "Thread-0" java.util.ConcurrentModificationException

产生原因

集合能够遍历是因为迭代器的原因，而Iterator接口只是定义了具体的方法，集合需要实现该接口方法，
查看ArrayList中具体的实现方法

//省略部分方法
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;

@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();
}
}

final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}

可以看出在Itr是ArrayList的一个内部类，迭代器操作通过这个内部类，Itr有个expectedModCount属性，这个属性判断是否与modCount相等，如果不相等抛出异常，modCount记录list结构上发生变化的次数，可以看出在迭代时候checkForComodification()方法检测两个的值不相等就抛出异常

解决方法

• 单线程 使用迭代器的remove方法

public class NoFailFastSingleThread {
public static void main(String[] args) {
List<String> lists = new ArrayList<>(10);
for (int i = 0; i < 4; i++){
lists.add(String.valueOf(i));
}

Iterator<String> iterator = lists.iterator();
while (iterator.hasNext()){
String next = iterator.next();
if (next != null){
iterator.remove();
}
}

}
}

使用java并发包下的类来代替对应的集合,如CopyOnWriteArrayList代替ArrayList，