JDK容器学习之CopyOnWriteArrayList：线程安全保障机制

/** The lock protecting all mutators */
final transient ReentrantLock lock = new ReentrantLock();

/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;

public int size() {
    return getArray().length;
}

final Object[] getArray() {
    return array;
}

private E get(Object[] a, int index) {
    return (E) a[index];
}

/**
 * {@inheritDoc}
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E get(int index) {
    return get(getArray(), index);
}

public E remove(int index) {
  final ReentrantLock lock = this.lock;
  // 加锁，保证同一时刻只能有一个线程对链表进行写操作
  lock.lock();
  try {
      Object[] elements = getArray();
      int len = elements.length;
      E oldValue = get(elements, index);
      int numMoved = len - index - 1;
      if (numMoved == 0) { // 删除最后一个元素时
          //直接进行数组拷贝，然后将tables数组引用指向拷贝后的数组
          setArray(Arrays.copyOf(elements, len - 1));
      } else {
          // 创建一个新的数组，将旧数组内容拷贝到新数组中
          // 然后将tables数组引用指向新的数组
          Object[] newElements = new Object[len - 1];
          System.arraycopy(elements, 0, newElements, 0, index);
          System.arraycopy(elements, index + 1, newElements, index,
                           numMoved);
          setArray(newElements);
      }
      return oldValue;
  } finally {
      lock.unlock();
  }
}

public void add(int index, E element) {
    final ReentrantLock lock = this.lock;
    // 新增，先加锁
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        if (index > len || index < 0) {
            // 数组越界判断
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+len);
        }
        Object[] newElements;
        int numMoved = len - index;
        // 将原数组拷贝到新的数组中
        if (numMoved == 0) { // 添加在最后一个
            newElements = Arrays.copyOf(elements, len + 1);
        } else { // 添加到中间，需要两次拷贝
            newElements = new Object[len + 1];
            System.arraycopy(elements, 0, newElements, 0, index);
            System.arraycopy(elements, index, newElements, index + 1,
                             numMoved);
        }
        
        // 直接将数据添加到新的数组
        newElements[index] = element;
        // 将tables引用指向新的数组
        setArray(newElements);
    } finally {
        lock.unlock();
    }
}

public class CopyOnWriteTest {
    List<String> list = new CopyOnWriteArrayList<>(
            new String[]{
                    "1", "2", "3", "4", "5", "6", "7", "8", "9"
            }
    );

    private void modify() {
        new Thread(() -> {
            list.add(8, "a8");
            list.remove(9);
            list.set(6, "6666");
            System.out.println("----修改完成----");
        }).start();
    }

    @Test
    public void testModify() throws InterruptedException {
        Iterator<String> iterable = list.iterator();
        int i = 0;
        while (iterable.hasNext()) {
            if (i++ == 1) {
                modify();
            } else if (i == 4) {
                Thread.sleep(1000);
            }
            System.out.println("index: " + i + " value: " + iterable.next());
        }
        
        Thread.sleep(1000);
        System.out.println(list);
    }
}

index: 1 value: 1
index: 2 value: 2
index: 3 value: 3
----修改完成----
index: 4 value: 4
index: 5 value: 5
index: 6 value: 6
index: 7 value: 7
index: 8 value: 8
index: 9 value: 9
[1, 2, 3, 4, 5, 6, 6666, 8, a8]

static final class COWIterator<E> implements ListIterator<E> {
    /** Snapshot of the array */
    private final Object[] snapshot;
    /** Index of element to be returned by subsequent call to next.  */
    private int cursor;

    private COWIterator(Object[] elements, int initialCursor) {
        cursor = initialCursor;
        snapshot = elements;
    }

    public boolean hasNext() {
        return cursor < snapshot.length;
    }

    public boolean hasPrevious() {
        return cursor > 0;
    }

    @SuppressWarnings("unchecked")
    public E next() {
        if (! hasNext())
            throw new NoSuchElementException();
        return (E) snapshot[cursor++];
    }

    @SuppressWarnings("unchecked")
    public E previous() {
        if (! hasPrevious())
            throw new NoSuchElementException();
        return (E) snapshot[--cursor];
    }

    public int nextIndex() {
        return cursor;
    }

    public int previousIndex() {
        return cursor-1;
    }

    /**
     * Not supported. Always throws UnsupportedOperationException.
     * @throws UnsupportedOperationException always; {@code remove}
     *         is not supported by this iterator.
     */
    public void remove() {
        throw new UnsupportedOperationException();
    }

    /**
     * Not supported. Always throws UnsupportedOperationException.
     * @throws UnsupportedOperationException always; {@code set}
     *         is not supported by this iterator.
     */
    public void set(E e) {
        throw new UnsupportedOperationException();
    }

    /**
     * Not supported. Always throws UnsupportedOperationException.
     * @throws UnsupportedOperationException always; {@code add}
     *         is not supported by this iterator.
     */
    public void add(E e) {
        throw new UnsupportedOperationException();
    }
}