14. Java JUC源码分析系列笔记-CopyOnWriteArraySet
1. 是什么
写时复制的set，有序不重复，底层使用CopyOnWriteArrayList实现
2. 如何使用
public class CopyOnWriteArraySetTest
{
public static void main(String[] args)
{
CopyOnWriteArraySet<String> set = new CopyOnWriteArraySet<>();
set.add("a");
set.add("b");
set.add("c");
set.remove("a");
System.out.println(set.contains("a"));//false
System.out.println(Thread.currentThread().getName() + "读取set:" + set);//[b, c]
new Thread(()->{
System.out.println(Thread.currentThread().getName() + "读取set:" + set);//[c]
try
{
TimeUnit.SECONDS.sleep(2);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "再次读取set:" + set);//[c]
}).start();
new Thread(()->{
System.out.println(Thread.currentThread().getName() + "读取set:" + set);//[c]
try
{
TimeUnit.SECONDS.sleep(2);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "再次读取set:" + set);//[c]
}).start();
try
{
TimeUnit.SECONDS.sleep(1);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
set.remove("b");
System.out.println(Thread.currentThread().getName() + "读取修改后的set:" + set);//[c]
}
}
3. 原理分析
3.1. 构造方法
3.1.1. 底层使用CopyOnWriteArrayList实现
public class CopyOnWriteArraySet<E> extends AbstractSet<E>
implements java.io.Serializable {
//底层使用的是CopyOnWriteArrayList实现
private final CopyOnWriteArrayList<E> al;
public CopyOnWriteArraySet() {
//默认一个元素的数组
al = new CopyOnWriteArrayList<E>();
}
}
3.2. add方法
public boolean add(E e) {
//调用CopyOnWriteArrayList addIfAbsent方法
return al.addIfAbsent(e);
}
3.2.1. 转调CopyOnWriteArrayList addIfAbsent
- CopyOnWriteArrayList addIfAbsent
public boolean addIfAbsent(E e) {
//获取原数组
Object[] snapshot = getArray();
//如果在数组中已经存在，那么返回false
return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :
//元素还不在数组中，则加入
addIfAbsent(e, snapshot);
}
- 3行：获取原数组
- 5行：遍历数组是否存在该object
- 7行：不在的话插入list尾部
下面分别说明：
3.2.1.1. 遍历数组是否存在该object
private static int indexOf(Object o, Object[] elements,
int index, int fence) {
//object为null
if (o == null) {
for (int i = index; i < fence; i++)
if (elements[i] == null)
return i;
//object不为null
} else {
for (int i = index; i < fence; i++)
if (o.equals(elements[i]))
return i;
}
return -1;
}
3.2.1.2. 不在的话则加锁插入list尾部
- CopyOnWriteArrayList addIfAbsent
//把e加入到snapshot数组中
private boolean addIfAbsent(E e, Object[] snapshot) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] current = getArray();
int len = current.length;
//原数组跟现在的数组不一样了
if (snapshot != current) {
// Optimize for lost race to another addXXX operation
int common = Math.min(snapshot.length, len);
//遍历改变后的数组左边
for (int i = 0; i < common; i++)
//有一个位置不相同 且 改变后的数组这个位置xx的元素与要添加的元素相同，说明已经存在，返回false
if (current[i] != snapshot[i] && eq(e, current[i]))
return false;
//从改变后的数组右边查看是否存在要添加的元素
if (indexOf(e, current, common, len) >= 0)
return false;
}
//复制原数组并扩容，把e加入到数组的末尾，set回array
Object[] newElements = Arrays.copyOf(current, len + 1);
newElements[len] = e;
setArray(newElements);
return true;
} finally {
lock.unlock();
}
}
大体逻辑跟10.CopyOnWriteArrayList.md的remove方法差不多
3.3. contains方法
public boolean contains(Object o) {
//转调CopyOnWriteArrayList的contains方法
return al.contains(o);
}
3.3.1. 转调CopyOnWriteArrayList的contains
- CopyOnWriteArrayList的contains
//CopyOnWriteArrayList的contains方法
public boolean contains(Object o) {
Object[] elements = getArray();
//遍历数组查找
return indexOf(o, elements, 0, elements.length) >= 0;
}
3.3.1.1. 遍历数组判断是否存在该object
private static int indexOf(Object o, Object[] elements,
int index, int fence) {
//object为null
if (o == null) {
for (int i = index; i < fence; i++)
if (elements[i] == null)
return i;
//object不为null
} else {
for (int i = index; i < fence; i++)
if (o.equals(elements[i]))
return i;
}
return -1;
}
3.4. remove方法
- remove
public boolean remove(Object o) {
//转调CopyOnWriteArrayList的remove方法
return al.remove(o);
}
3.4.1. 转调CopyOnWriteArrayList的remove方法
- CopyOnWriteArrayList remove
//CopyOnWriteArrayList的remove方法
public boolean remove(Object o) {
Object[] snapshot = getArray();
int index = indexOf(o, snapshot, 0, snapshot.length);
return (index < 0) ? false : remove(o, snapshot, index);
}
- 3行：获取原数组
- 4行：遍历数组是否存在该object
- 5行：存在的话调用CopyOnWriteArrayList的remove方法删除
3.4.1.1. 遍历底层数组判断是否存在该object
private static int indexOf(Object o, Object[] elements,
int index, int fence) {
//object为null
if (o == null) {
for (int i = index; i < fence; i++)
if (elements[i] == null)
return i;
//object不为null
} else {
for (int i = index; i < fence; i++)
if (o.equals(elements[i]))
return i;
}
return -1;
}
3.4.1.2. 存在的话再去删除
参考10.CopyOnWriteArrayList.md remove方法
4. 总结
底层直接通过调用CopyOnWriteArrayList实现的，因此有序
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