LinkedList.addAll在多线程应用程序中触发空指针异常

Question

I have a LinkedList that is being modified by a single thread. But there are many threads reading it.

protected volatile LinkedList<V> list = new LinkedList<V>();

I need to retrieve this list at some point. So, when I do,

List<V> retList = new LinkedList<V>();
retList.addAll(list);
return Collections.unmodifiableList(list);

I receive the following exception

java.lang.NullPointerException at java.util.LinkedList.toArray(LinkedList.java:866) at java.util.LinkedList.addAll(LinkedList.java:269) at java.util.LinkedList.addAll(LinkedList.java:247)

I have checked that the list or the values within it are not null. What is the correct approach when you have a single writer-multiple readers. The aim is not to have a synchronized list.

Answer 1

Your volatile tag doesn't mean anything -- this has to do with the thread safety of the writable object. If you do not want to implement proper memory barriers yourself, then you need to use a different class, like maybe:

Instead of:

protected volatile LinkedList<V> list = new LinkedList<V>();

Try using:

protected Deque<V> list = new [LinkedBlockingDeque][1]<V>();

I assume your using LinkedList instead of ArrayList because you want the Deque methods rather then the list methods.

If you really need list methods instead of dequeu methods, you can use CopyOnWriteArrayList

Anyway -- You need something that spits out a weakly consistent iterator rather then one of the standard java.util collections which all disclaim in their javadocs that they are not inherently thread-safe with any mix of readers and writers and are thus inappropriate for your use case without proper memory barriers.