并发编程--this escape 问题

[wyc_]

public class SafeListener {
private final EventListener listener;
private SafeListener() {
listener = new EventListener() {
public void onEvent(Event e) {
doSomething(e);
}
};
}
public static SafeListener newInstance(EventSource source) {
SafeListener safe = new SafeListener();
source.registerListener(safe.listener);
return safe;
}
}

谁能解释一下为什么使用工厂方法可以避免this escape？