目录
集合的线程安全
list集合线程不安全演示
Vector解决
Collections.synchronizedList
JUC 解决方案:CopyOnWriteArrayList
HashSet和HashMap线程不安全演示
JUC 解决方案:CopyOnWriteArrayList
JUC 解决方案:ConcurrentHashMap
集合的线程安全
list集合线程不安全演示
List中的add方法没有添加Synchronized,因此是线程不安全的:
public boolean add(E e) {
modCount++;
add(e, elementData, size);
return true;
}
public static void main(String[] args) {
var list = new ArrayList<String>();
for (int i = 0; i < 30; i++) {
int loop = i;
new Thread(() -> {
list.add(UUID.randomUUID().toString().substring(0, 8));
System.out.println(list);
}, String.valueOf(i)).start();
}
}
执行会出现并发修改异常:
Exception in thread "17" java.util.ConcurrentModificationException
Vector解决
public static void main(String[] args) {
var list = new Vector<String>();
for (int i = 0; i < 300; i++) {
new Thread(() -> {
list.add(UUID.randomUUID().toString().substring(0, 8));
System.out.println(list);
}, String.valueOf(i)).start();
}
}
Vector源码使用synchronized关键字,因此不存在线程安全问题,但是效率较低比较古老,不经常使用
Collections.synchronizedList
public static void main(String[] args) {
var list = Collections.synchronizedList(new ArrayList<>());
for (int i = 0; i < 300; i++) {
new Thread(() -> {
list.add(UUID.randomUUID().toString().substring(0, 8));
System.out.println(list);
}, String.valueOf(i)).start();
}
}
效率较低比较古老,不经常使用
JUC 解决方案:CopyOnWriteArrayList
public static void main(String[] args) {
var list = new CopyOnWriteArrayList<String>();
for (int i = 0; i < 300; i++) {
new Thread(() -> {
list.add(UUID.randomUUID().toString().substring(0, 8));
System.out.println(list);
}, String.valueOf(i)).start();
}
}
CopyOnWriteArrayList:写时复制技术,允许并发读取集合,但是只能单独写,即在写的时候首先复制一份集合,写完之后覆盖之前的内容
HashSet和HashMap线程不安全演示
public static void main(String[] args) {
var set = new HashSet<String>();
for (int i = 0; i < 300; i++) {
new Thread(() -> {
set.add(UUID.randomUUID().toString().substring(0, 8));
System.out.println(set);
}, String.valueOf(i)).start();
}
}
public static void main(String[] args) {
var map = new HashMap<String, String>();
for (int i = 0; i < 300; i++) {
int loop = i;
new Thread(() -> {
map.put(String.valueOf(loop), UUID.randomUUID().toString().substring(0, 8));
System.out.println(map);
}, String.valueOf(i)).start();
}
}
JUC 解决方案:CopyOnWriteArrayList
public static void main(String[] args) {
var set = new CopyOnWriteArraySet<String>();
for (int i = 0; i < 300; i++) {
new Thread(() -> {
set.add(UUID.randomUUID().toString().substring(0, 8));
System.out.println(set);
}, String.valueOf(i)).start();
}
}
JUC 解决方案:ConcurrentHashMap
public static void main(String[] args) {
var map = new ConcurrentHashMap<String, String>();
for (int i = 0; i < 300; i++) {
int loop = i;
new Thread(() -> {
map.put(String.valueOf(loop), UUID.randomUUID().toString().substring(0, 8));
System.out.println(map);
}, String.valueOf(i)).start();
}
}