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你写程序有写诗一样的感觉吗?

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CopyOnWriteArrayList和CopyOnWriteArraySet

CopyOnWriteArrayList和CopyOnWriteArraySet应用场合一般 是在读多写少的情况,比如黑名单,他们之间的区别就是list和set的区别,但是在实现上CopyOnWriteArraySet使用CopyOnWriteArrayList来实现的,就像set使用了hashmap,treeset使用treemap,下面先看CopyOnWriteArrayList的添加操作.

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/**
* 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;
//加锁,CopyOnWriteArrayList是线程安全的
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;
//如果位移实在列表的尾部,那么直接复制数组,,使用Arrays.copyOf()
if (numMoved == 0)
newElements = Arrays.copyOf(elements, len + 1);
else {
//否则,以index为中心点,把原始数组分成2段,index左边一段,右边一段,index放要插入的数据元素,放进新的数组newElements。
newElements = new Object[len + 1];
System.arraycopy(elements, 0, newElements, 0, index);
System.arraycopy(elements, index, newElements, index + 1,
numMoved);
}
newElements[index] = element;
//覆盖原来的数组,原来的数组失去引用,被jvm回收
setArray(newElements);
} finally {
//解锁
lock.unlock();
}
}

再看remove方法:

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/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If this list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
* (if such an element exists). Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
public boolean remove(Object o) {
final ReentrantLock lock = this.lock;
//加锁,
lock.lock();
try {
Object[] elements = getArray();
int len = elements.length;
if (len != 0) {
// Copy while searching for element to remove
// This wins in the normal case of element being present
int newlen = len - 1;
Object[] newElements = new Object[newlen];

for (int i = 0; i < newlen; ++i) {
if (eq(o, elements[i])) {
// found one; copy remaining and exit
//找到要删除的元素,然后将元素后面的数据复制到新的数组里边
for (int k = i + 1; k < len; ++k)
newElements[k-1] = elements[k];
setArray(newElements);
return true;
} else
//不是删除数据,直接进入新的数组
newElements[i] = elements[i];
}

// special handling for last cell
//最后一个元素的单独处理
if (eq(o, elements[newlen])) {
setArray(newElements);
return true;
}
}
return false;
} finally {
//解锁
lock.unlock();
}
}

再看CopyOnWriteArraySet
内部维护了一个

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private final CopyOnWriteArrayList<E> al;

因为CopyOnWriteArraySet元素不能重复,所以CopyOnWriteArraySet的add用的是CopyOnWriteArrayList的addIfAbsent,

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public boolean add(E e) {
return al.addIfAbsent(e);
}

删除操作用的是CopyOnWriteArrayList的remove

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public boolean remove(Object o) {
return al.remove(o);
}