如题，最近项目里有个模块我做了异步处理方面的事情，在code过程中发现一个颠覆我对synchronized这个关键字和用法的地方，请问各位java开发者们是否对此有一个合理的解释，不多说，我直接贴出问题代码：
（事实证明这是一个坑，各位读者，如果有兴趣，可以先不看答案，自己看看能不能发现这个坑）
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
public class ConcurrentList {
//private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());
public static void main(String[] args) {
new Thread(new Runnable() {
@Override
public void run() {
while (true) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (TEST_LIST) {
TEST_LIST.add("11");
}
System.out.println("Thread1 running");
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
while (true) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (TEST_LIST) {
for (String at : TEST_LIST) {
TEST_LIST.add("22");
}
}
System.out.println("Thread2 running");
}
}
}).start();
}
}
输出结果是：
Thread1 running
Exception in thread "Thread-1" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at com.free4lab.lol.ConcurrentList$2.run(ConcurrentList.java:40)
at java.lang.Thread.run(Thread.java:619)
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running
-----------------------------------分隔线，以下是解释--------------------------------
问题明了了：
以上问题不是并发的问题，是ArrayList的问题，是个坑！且看如下代码，以及运行结果：
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
public class ConcurrentList {
//private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());
public static void main(String[] args) {
TEST_LIST.add("111");
TEST_LIST.add("222");
for (String at : TEST_LIST) {
System.out.println(at);
TEST_LIST.add("333");
System.out.println("add over");
}
}
}
结果是：
111
add over
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at com.free4lab.lol.ConcurrentList.main(ConcurrentList.java:15)
分析：我们发现迭代了一次之后就抛出所谓的并发修改异常，不过这里没有多线程，看下源代码就知道了
list.add的时候执行了，修改了modCount，循环外面一次add到第一次迭代不会有问题，因为初始化的时候在AbstractList中int expectedModCount = modCount;，
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacity(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
public void ensureCapacity(int minCapacity) {
modCount++;
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) {
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
}
public E next() {
checkForComodification();
try {
E next = get(cursor);
lastRet = cursor++;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
这样迭代器next()第一次 checkForComodification() 是不会抛出异常的，第二次才会抛出异常，因为在checkForComodification()里检查了
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
这样，在循环迭代中，进行了一次add操作，修改了modcount变量，再次迭代的时候，异常就throw出来了!
如果非要进行这样的操作，那么声明list为CopyOnWriteArrayList，就ok！因为用了copyonwrite技术
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
public class ConcurrentList {
private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
//private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());
public static void main(String[] args) {
TEST_LIST.add("111");
TEST_LIST.add("222");
for (String at : TEST_LIST) {
System.out.println(at);
TEST_LIST.add("333");
System.out.println("add over");
}
}
}
输出是正确的：
111 add over 222 add over
额外再说一点，也可以用iterator迭代，不过同样也无法调用next()方法（我注释掉了），这样程序就是死循环了，不断的加，不断的迭代。所以我感觉如果需要在迭代中增加元素，真正有用的还是CopyOnWriteArrayList，不过实际中，如果CopyOnWriteArrayList代价太高，可能我们可以申请一个临时list存放，在迭代后合并到主list中！
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
public class ConcurrentList {
//private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());
public static void main(String[] args) {
TEST_LIST.add("111");
TEST_LIST.add("222");
Iterator iterator = TEST_LIST.iterator();
while(iterator.hasNext()){
//System.out.println(iterator.next());
TEST_LIST.add("333");
System.out.println("add over");
}
}
}
E-mail: huahuiyang@gmail.com https://cn.linkedin.com/pub/huahui-yang/91/13a/105