一、单线程

1. 异常情况举例

           List<String> myList = new ArrayList<String>();

           myList.add( "1");
           myList.add( "2");
           myList.add( "3");
           myList.add( "4");
           myList.add( "5");

           Iterator<String> it = myList.iterator();
            while (it.hasNext()) {
                String value = it.next();
                 if (value.equals( "3")) {
                     myList.remove(value);  // error
                }
           }
      
            for (Iterator<String> it = myList.iterator(); it.hasNext();) {
                String value = it.next();
                 if (value.equals( "3")) {
                     myList.remove(value);  // error
                }
           }
      
      
            for (String value : myList) {
                System. out.println( "List Value:" + value);
                 if (value.equals( "3")) {
                     myList.remove(value);  // error
                }
           }   

2. 根本原因

public class ArrayList<E> extends AbstractList<E>
        implements Cloneable, Serializable, RandomAccess {
        
        
         @Override public boolean remove(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    System.arraycopy(a, i + 1, a, i, --s - i);
                    a[s] = null;  // Prevent memory leak
                    size = s;
                    modCount++;  // 只要删除成功都是累加
                    return true;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    System.arraycopy(a, i + 1, a, i, --s - i);
                    a[s] = null;  // Prevent memory leak
                    size = s;
                    modCount++;  // 只要删除成功都是累加
                    return true;
                }
            }
        }
        return false;
    }   


    @Override public Iterator<E> iterator() {
        return new ArrayListIterator();
    }   
        
    private class ArrayListIterator implements Iterator<E> {
          ......
   
          // 全局修改总数保存到当前类中
        /** The expected modCount value */
        private int expectedModCount = modCount;

        @SuppressWarnings("unchecked") public E next() {
            ArrayList<E> ourList = ArrayList.this;
            int rem = remaining;
               // 如果创建时的值不相同，抛出异常
            if (ourList.modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            if (rem == 0) {
                throw new NoSuchElementException();
            }
            remaining = rem - 1;
            return (E) ourList.array[removalIndex = ourList.size - rem];
        }   
        
          ......
     }
}   

3. 解决方案

            // 1 使用Iterator提供的remove方法，用于删除当前元素
            for (Iterator<String> it = myList.iterator(); it.hasNext();) {
                String value = it.next();
                 if (value.equals( "3")) {
                     it.remove();  // ok
                }
           }
           System. out.println( "List Value:" + myList.toString());
      
            // 2 建一个集合，记录需要删除的元素，之后统一删除             
           List<String> templist = new ArrayList<String>();
            for (String value : myList) {
                 if (value.equals( "3")) {
                     templist.remove(value);
                }
           }
            // 可以查看removeAll源码，其中使用Iterator进行遍历
           myList.removeAll(templist);
           System. out.println( "List Value:" + myList.toString());        
        
             // 3. 使用线程安全CopyOnWriteArrayList进行删除操作
           List<String> myList = new CopyOnWriteArrayList<String>();
           myList.add( "1");
           myList.add( "2");
           myList.add( "3");
           myList.add( "4");
           myList.add( "5");

           Iterator<String> it = myList.iterator();

            while (it.hasNext()) {
                String value = it.next();
                 if (value.equals( "3")) {
                     myList.remove( "4");
                     myList.add( "6");
                     myList.add( "7");
                }
           }
           System. out.println( "List Value:" + myList.toString());
          
            // 4. 不使用Iterator进行遍历，需要注意的是自己保证索引正常
            for ( int i = 0; i < myList.size(); i++) {
                String value = myList.get(i);
                System. out.println( "List Value:" + value);
                 if (value.equals( "3")) {
                     myList.remove(value);  // ok
                     i--; // 因为位置发生改变，所以必须修改i的位置
                }
           }
           System. out.println( "List Value:" + myList.toString());

二、多线程

1. 同步异常情况举例

          final List<String> myList = createTestData();
      
          new Thread(new Runnable() {
           
               @Override
               public void run() {
                    for (String string : myList) {
                         System.out.println("遍历集合 value = " + string);
                     
                         try {
                              Thread.sleep(100);
                         } catch (InterruptedException e) {
                              e.printStackTrace();
                         }
                    }
               }
          }).start();
      
          new Thread(new Runnable() {
           
               @Override
               public void run() {
                
                 for (Iterator<String> it = myList.iterator(); it.hasNext();) {
                     String value = it.next();
                 
                     System.out.println("删除元素 value = " + value);
                 
                     if (value.equals( "3")) {
                          it.remove();
                     }
                 
                     try {
                              Thread.sleep(100);
                         } catch (InterruptedException e) {
                              e.printStackTrace();
                         }
                }
               }
          }).start();

2. 尝试方案

3. CopyOnWriteArrayList注意事项

4. 解决方案

           List<String> myList = new CopyOnWriteArrayList<String>();
           myList.add( "1");
           myList.add( "2");
           myList.add( "3");
           myList.add( "4");
           myList.add( "5");

          new Thread(new Runnable() {
            
               @Override
               public void run() {
                    for (String string : myList) {
                         System.out.println("遍历集合 value = " + string);
                      
                         try {
                              Thread.sleep(100);
                         } catch (InterruptedException e) {
                              e.printStackTrace();
                         }
                    }
               }
          }).start();
       
          new Thread(new Runnable() {
            
               @Override
               public void run() {
                 
                    for (int i = 0; i < myList.size(); i++) {
                         String value = myList.get(i);
                      
                         System.out.println("删除元素 value = " + value);
                  
                     if (value.equals( "3")) {
                          myList.remove(value);
                          i--; // 注意                           
                     }
                  
                     try {
                              Thread.sleep(100);
                         } catch (InterruptedException e) {
                              e.printStackTrace();
                         }
                    }
               }
          }).start();

三、参考资料