Java集合-05fail-fast(快速失败)机制原理及解决方法

fail-fast简介

fail-fast出现场景

• 单线程场景

  •     public class FailFastSingleThreadTest {
            public static void main(String[] args) {
                List<String> lists = new ArrayList<>(10);
                for (int i = 0; i < 4; i++){
                    lists.add(String.valueOf(i));
                }
    
                //fail-fast
                for (String list : lists) {
                    lists.remove(3);
                }
            }
        }
    //output:Exception in thread "main" java.util.ConcurrentModificationException

• 多线程场景

  •   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

      //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();
          }
      }

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();
                }
            }
  
        }
    }

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