在多線程環境下,要使用線程安全的集合,比如,ConcurrentHashMap是線程安全的HashMap,CopyOnWriteArrayList是線程安全的ArrayList。那麼HashSet對應的線程安全集合,是什麼呢?java有沒有提供預設實作呢?
在java的concurrent包中,我找到了CopyOnWriteArraySet,那麼它是線程安全的嗎?下面是測試代碼。
public static void main(String[] args) {
Set<String> set = new CopyOnWriteArraySet<>();
ExecutorService service = Executors.newFixedThreadPool(12);
int times = 10000;
AtomicInteger flag = new AtomicInteger(0);
for(int i = 0; i < times; i ++){
service.execute(()->{
set.add("a" + flag.getAndAdd(1));
});
}
service.shutdown();
try {
service.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
}catch (Exception e){
e.printStackTrace();
}
System.out.println(set.size());
}
經過多次執行,結果都是10000。可以說明,CopyOnWriteArraySet是線程安全的Set。另外,關注公衆号Java技術棧,在背景回複:面試,可以擷取我整理的 Java 系列面試題和答案,非常齊全。
那麼CopyOnWriteArraySet是如何保證寫入時的線程安全呢?以下是CopyOnWriteArraySet的add源碼。
public boolean add(E e) {
return al.addIfAbsent(e);
}
public boolean addIfAbsent(E e) {
Object[] snapshot = getArray();
return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :
addIfAbsent(e, snapshot);
}
private static int indexOf(Object o, Object[] elements,
int index, int fence) {
if (o == null) {
for (int i = index; i < fence; i++)
if (elements[i] == null)
return i;
} else {
for (int i = index; i < fence; i++)
if (o.equals(elements[i]))
return i;
}
return -1;
}
private boolean addIfAbsent(E e, Object[] snapshot) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] current = getArray();
int len = current.length;
if (snapshot != current) {
// Optimize for lost race to another addXXX operation
int common = Math.min(snapshot.length, len);
for (int i = 0; i < common; i++)
if (current[i] != snapshot[i] && eq(e, current[i]))
return false;
if (indexOf(e, current, common, len) >= 0)
return false;
}
Object[] newElements = Arrays.copyOf(current, len + 1);
newElements[len] = e;
setArray(newElements);
return true;
} finally {
lock.unlock();
}
}
從源碼可以看出,CopyOnWriteArraySet底層采用了CopyOnWriteArrayList資料結構來實作。在add元素時,采用的是可重入鎖來實作線程安全。