为什么此代码会导致 "this reference" 隐式转义？

Question

“当内部 EventListener 实例发布时，封闭的 ThisEscape 实例也发布”是什么意思。意思是？

此引自《java 并发实践》

public class ThisEscape {
public ThisEscape(EventSource source)
{
    source.registerListener(
            new EventListener(){
                public void onEvent(Event e)
                {
                    doSomething(e);
                }
            }
    );
}

}

Answer 1

ThisEscape 实例是在构建时发布的，因为 doSomething 是 ThisEscape 的方法，可能会导致无法预料的问题。

一个简单的代码示例，当一个事件发生时，ThisEscape对象还没有被构建，但是由于这个ThisEscape注册到了EventSource，那么onSomeThing方法被触发，由于ThisEscape#data还没有初始化，所以生成了一个NPE：

public class ThisEscape {
    static class EventSource {
        private List<EventListener> eventListeners = new ArrayList<>();
        public void registerListener(EventListener eventListener) {
            eventListeners.add(eventListener);
        }
        public void eventHappen(Event event) {
            eventListeners.forEach(eventListener -> {
                eventListener.onEvent(event);
            });
        }
    }
    static class Event {
        public String eventString;
        public Event(String eventString) {
            this.eventString = eventString;
        }
    }
    interface EventListener {
        void onEvent(Event e);
    }
    private List<String> data;
    public void doSomething(Event e) {
        data.add(e.eventString);
    }
    public ThisEscape(EventSource source) {
        source.registerListener(
                new EventListener() {
                    public void onEvent(Event e) {
                        doSomething(e);
                    }
                }
        );
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.data = new ArrayList<>();
    }
    public static void main(String[] args) {
        EventSource eventSource = new EventSource();
        new Thread(() -> {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            eventSource.eventHappen(new Event("hello"));
        }).start();
        ThisEscape thisEscape = new ThisEscape(eventSource);
    }
}

为什么此代码会导致 "this reference" 隐式转义？

Why does this code cause "this reference" to escape implicitly?

java

escaping

reference

this

java.util.concurrent