Executor.execute() JMM保证
考虑以下代码片段：
public class A {
private final Executor executor = Executors.newCachedThreadPool();
private final Queue<Object> messageQueue = new ConcurrentLinkedQueue<M>();
public void sendMessage(Object message) {
messageQueue.offer(message);
executor.execute(new Runnable() {
@Override
public void run() {
final Object message = messageQueue.poll();
// Can message == null?
}
});
}
}
当 Runnable 实例尝试检索消息时，是否可以保证 messageQueue 包含该消息？或者更笼统地说：JIT/JVM 可以根据 JMM 对两个函数调用重新排序吗？
Consider the following code snipet:
public class A {
private final Executor executor = Executors.newCachedThreadPool();
private final Queue<Object> messageQueue = new ConcurrentLinkedQueue<M>();
public void sendMessage(Object message) {
messageQueue.offer(message);
executor.execute(new Runnable() {
@Override
public void run() {
final Object message = messageQueue.poll();
// Can message == null?
}
});
}
}
Is it guaranteed that messageQueue contains the message by the time when the Runnable instance will try to retrieve it? Or to phraise it a little bit more general: can two function calls be reordered by JIT/JVM according to JMM?
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是的，如果没有其他生产者/消费者。
Executor.execute()建立一个happens-before关系。因此，offer() 中的所有内容都发生在 poll() 之前。poll()可以看到offer()的效果。尽管没有正式指定，但根据任何常识，poll()应该返回刚刚添加到队列中的对象。Yes, if there are not other producers/consumers.
Executor.execute()establish a happens-before relationship. Therefore everything inoffer()happens-beforepoll().poll()sees the effect ofoffer(). Although not formally specified, by any common sense,poll()should then return the object just added to the queue.