并发容器-CopyOnWriteArrayList

并发容器一览

　　图源：https://time.geekbang.org/column/article/90201?utm_term=pc_interstitial_938

CopyOnWriteArrayList

       该容器支持多个线程同时读（读不加锁），但同一时刻只能一个线程写（写加锁）。

　　内部维护了一个数组

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

1.获取元素

可以看到没有做任何处理，直接返回数组中的元素。读取元素时是不加锁的，也就是可以任意线程同时读取。

/**
 * {@inheritDoc}
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E get(int index) {
    return get(getArray(), index);
}
/**
 * Gets the array.  Non-private so as to also be accessible
 * from CopyOnWriteArraySet class.
 */
final Object[] getArray() {
    return array;
}
@SuppressWarnings("unchecked")
private E get(Object[] a, int index) {
    return (E) a[index];
}

2.更新元素

　　可以看到，是直接深拷贝了原数组，然后修改新数组的值，最后将对象的数组指向新数组。更新元素是加了可重入锁的，同一时刻只有一个线程能够修改数组的值，其他线程要修改只能等待获得锁。

/**
 * Replaces the element at the specified position in this list with the
 * specified element.
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E set(int index, E element) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        E oldValue = get(elements, index);
        // 判断原有位置元素是否和插入元素相同，不相同才插入
        if (oldValue != element) {
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len);
            newElements[index] = element;
            setArray(newElements);
        } else {
            // Not quite a no-op; ensures volatile write semantics
            // 不是无用操作，是为了保证volatile写的意义
            setArray(elements);
        }
        return oldValue;
    } finally {
        lock.unlock();
    }
}
/**
 * Sets the array.
 */
final void setArray(Object[] a) {
    array = a;
}

22行代码处的setArray(elements)，这里操作看似没有意义。因为elements是volatile修饰的， 这里是为了保证happens-before原则：

　　1.volatile变量读操作happens-before写操作；

　　2.同一线程的代码顺序执行，先前的操作 happens-before 之后的操作

　　3.传递性

实际上这是为了保证非volatile变量的可见性。

像下面这个例子，线程1的set操作可以保证nonVolatileField 变量的修改对线程2的读取可见。

// initial conditions
int nonVolatileField = 0;
CopyOnWriteArrayList<String> list = /* a single String */

// Thread 1
nonVolatileField = 1;                 // (1)
list.set(0, "x");                     // (2)

// Thread 2
String s = list.get(0);               // (3)
if (s == "x") {
    int localVar = nonVolatileField;  // (4)
}

　　具体可以参考这个帖子：https://stackoverflow.com/questions/28772539/why-setarray-method-call-required-in-copyonwritearraylist

　　好像JDK11移除了这行代码。。。

 

3.插入元素

/**
 * Appends the specified element to the end of this list.
 * 末尾增加元素
 * @param e element to be appended to this list
 * @return {@code true} (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        Object[] newElements = Arrays.copyOf(elements, len + 1);
        newElements[len] = e;
        setArray(newElements);
        return true;
    } finally {
        lock.unlock();
    }
}
/**
 * Inserts the specified element at the specified position in this
 * list. Shifts the element currently at that position (if any) and
 * any subsequent elements to the right (adds one to their indices).
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public void add(int index, E element) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        if (index > len || index < 0)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+len);
        Object[] newElements;
        int numMoved = len - index;
        if (numMoved == 0)
            newElements = Arrays.copyOf(elements, len + 1);
        else {
            newElements = new Object[len + 1];
            System.arraycopy(elements, 0, newElements, 0, index);
            System.arraycopy(elements, index, newElements, index + 1,
                             numMoved);
        }
        newElements[index] = element;
        setArray(newElements);
    } finally {
        lock.unlock();
    }
}

　　可以看到，增加元素和更新元素一样，也是创建一个新数组复制原有元素，然后插入新增元素。同时也是加了可重入锁，同一时刻只有一个线程能进行修改。

4.删除元素

/**
 * Removes the element at the specified position in this list.
 * Shifts any subsequent elements to the left (subtracts one from their
 * indices).  Returns the element that was removed from the list.
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E remove(int index) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        E oldValue = get(elements, index);
        int numMoved = len - index - 1;
        if (numMoved == 0)
            setArray(Arrays.copyOf(elements, len - 1));
        else {
            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();
    }
}

　　可以看到也是创建了一个新数组，直接复制原数组元素。然后将数组引用指向新数组。