foreach + remove = ConcurrentModificationException

foreach + remove = ConcurrentModificationException

---

问题：

List<String> list = new ArrayList<>();
list.add("0");
list.add("1");
list.add("2");
list.add("3");
for (String s : list) {
    if (条件语句) {
        list.remove(s);
    }
}

在上述代码的 if 语句中依次使用下列条件语句并执行代码：

条件语句	执行结果
"0".equals(s)	抛出 java.util.ConcurrentModificationException 异常
"1".equals(s)	抛出 java.util.ConcurrentModificationException 异常
"2".equals(s)	正常执行
"3".equals(s)	抛出 java.util.ConcurrentModificationException 异常

分析：

对代码进行反编译，可以发现 foreach 只是一个语法糖，最终需要转换为 iterator 进行实现：

ArrayList<String> list = new ArrayList<String>();
list.add("0");
list.add("1");
list.add("2");
list.add("3");
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
    String s = (String)iterator.next();
    if (!"2".equals(s)) continue;
    list.remove(s);
}

ArrayList 存在一个名为 Itr 的实现了 Iterator 接口的内部类，通过 iterator() 方法可以获取该内部类的实例：

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {

    transient Object[] elementData;
    
    private int size;
    
    protected transient int modCount = 0;
    
    public Iterator<E> iterator() { return new Itr(); }
    
    private class Itr implements Iterator<E> {

        int cursor = 0;

        int lastRet = -1;

        int expectedModCount = modCount;

        public boolean hasNext() {...}

        public E next() {...}

        public void remove() {...}

        final void checkForComodification() {...}
    }
}

• elementData

  集合中存储的所有元素，elementDate 的数组长度代表了 集合的容量。

• size

  集合包含元素的个数。

• modCount

  集合发生“结构性修改”的次数。对集合进行结构性修改的常见方式为调用add()、remove()等方法改变集合包含的元素数量。

  ArrayList<String> list = new ArrayList<String>(); // modCount == 0
  list.add("0"); // modCount == 1
  list.add("1"); // modCount == 2
  list.add("2"); // modCount == 3
  list.add("3"); // modCount == 4
  Iterator iterator = list.iterator();
  while (iterator.hasNext()) {
      String s = (String)iterator.next();
      if (!"2".equals(s)) continue;
      list.remove(s); // modCount == 5
  }
  

  就本文分析的代码而言，创建 ArrayList 实例时，modCount 的初值为0，即尚未对集合进行任何修改，在调用了4次 add() 方法后，modCount的值就累加到了4。

• cursor

  当前遍历到的集合元素的下标。

• lastRet

  上一次遍历到的集合元素的下标。

• expectedModCount

  在只有当前迭代器对集合进行“结构性修改”的情况下，集合应该具有的结构性修改次数。

• checkForComodification()

  检查集合是否发生“并发修改”。所有并发修改，指的是除了当前迭代器，外界也对集合进行了修改。由于 ArrayList 没有对并发访问进行控制，因此并发修改将使得当前迭代器后续的行为变得不可控。

  就 Itr 而言，它通过比较 modCount 和 expectedModCount 来判断是否还有元素尚未遍历。

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

• hasNext()1111

  判断是否还有元素尚未遍历。就 Itr 而言，它通过比较 cusor 和 size 是否相等来判断是否还有元素尚未遍历。

  public boolean hasNext() {
      return cursor != size;
  }
  

• next()

  向后遍历一个集合元素。

  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];
  }
  

• remove()

  从集合中移除上一次调用 next() 方法遍历到的元素。

  public void remove() {
      // 每调用一次 next() 方法 最多只能调用一次 remove() 方法。
      if (lastRet < 0)
          throw new IllegalStateException();    
      
      checkForComodification();
  
      try {
          ArrayList.this.remove(lastRet);  // 从集合中移除元素，remove方法会更新modCount的值。
          cursor = lastRet;
          lastRet = -1;                    // 每调用一次 next() 方法 最多只能调用一次 remove() 方法。
          expectedModCount = modCount;     // 记录新的modCount。
      } catch (IndexOutOfBoundsException ex) {
          throw new ConcurrentModificationException();
      }
  }
  

条件语句	代码序列	执行结果
"0".equals(s)	hashNext(): true cusor = 0,size = 4 next(): “0” checkForComodification(): OK modCount = 4,expectedModCount = 4 cusor = 1 ”0”.equals(“0”): true remove(): OK modCount = 5,size = 3 hashNext(): true cusor = 1,size = 3 next(): Exception checkForComodification(): Exception modCount = 5,expectedModCount = 4	java.util.ConcurrentModificationException
"1".equals(s)	略	java.util.ConcurrentModificationException
"2".equals(s)	hashNext(): true cusor = 2,size = 4 next(): “2” checkForComodification(): OK modCount = 4,expectedModCount = 4 cusor = 3 ”2”.equals(“2”): true remove(): OK modCount = 5,size = 3 hashNext(): false cusor = 3,size = 3	正常执行
"3".equals(s)	hashNext(): true cusor = 3,size = 4 next(): “3” checkForComodification(): OK modCount = 4,expectedModCount = 4 cusor = 4 ”3”.equals(“3”): true remove(): OK modCount = 5,size = 3 hashNext(): true cusor = 4,size = 3 next(): Exception checkForComodification(): Exception modCount = 5,expectedModCount = 4	java.util.ConcurrentModificationException

上面4个条件语句都在迭代器外部对集合进行了结构性修改，不同之处在于，"2".equals(s) 条件语句对集合进行结构行修改以后，就停止了在迭代器中修改集合，这就避免了冲突的发生，所以没有抛出异常。

附件：

完整源码：

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access
    
    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
     private int size;
    
    /**
     * The number of times this list has been <i>structurally modified</i>.
     * Structural modifications are those that change the size of the
     * list, or otherwise perturb it in such a fashion that iterations in
     * progress may yield incorrect results.
     *
     * <p>This field is used by the iterator and list iterator implementation
     * returned by the {@code iterator} and {@code listIterator} methods.
     * If the value of this field changes unexpectedly, the iterator (or list
     * iterator) will throw a {@code ConcurrentModificationException} in
     * response to the {@code next}, {@code remove}, {@code previous},
     * {@code set} or {@code add} operations.  This provides
     * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *
     * <p><b>Use of this field by subclasses is optional.</b> If a subclass
     * wishes to provide fail-fast iterators (and list iterators), then it
     * merely has to increment this field in its {@code add(int, E)} and
     * {@code remove(int)} methods (and any other methods that it overrides
     * that result in structural modifications to the list).  A single call to
     * {@code add(int, E)} or {@code remove(int)} must add no more than
     * one to this field, or the iterators (and list iterators) will throw
     * bogus {@code ConcurrentModificationExceptions}.  If an implementation
     * does not wish to provide fail-fast iterators, this field may be
     * ignored.
     */
    protected transient int modCount = 0;
    
    public Iterator<E> iterator() {
        return new Itr();
    }
    
    /**
     * 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();
        }
    }
    
}