java - 自定义 ThreadLocal 类的 ThreadLocal 泄漏说明

Question

我一直在读这个article关于 Tomcat 中的 ThreadLocal 泄漏。第一个示例包含以下代码:

public class MyCounter {
        private int count = 0;

        public void increment() {
                count++;
        }

        public int getCount() {
                return count;
        }
}

public class MyThreadLocal extends ThreadLocal<MyCounter> {
}

public class LeakingServlet extends HttpServlet {
        private static MyThreadLocal myThreadLocal = new MyThreadLocal();

        protected void doGet(HttpServletRequest request,
                        HttpServletResponse response) throws ServletException, IOException {

                MyCounter counter = myThreadLocal.get();
                if (counter == null) {
                        counter = new MyCounter();
                        myThreadLocal.set(counter);
                }

                response.getWriter().println(
                                "The current thread served this servlet " + counter.getCount()
                                                + " times");
                counter.increment();
        }
}

它说所有的类都驻留在 webapp 中。然后它概述了以下对类加载器(不是 ThreadLocal)泄漏的解释，我不明白:

If the LeakingServlet is invoked at least once and the Thread that served it is not stopped, then we created a classloader leak !

The leak is caused because we have a custom class for the ThreadLocal instance, and also a custom class for the value bound to the Thread. Actually the important thing is that both classes were loaded by the webapp classloader.

ThreadLocal 泄漏的解释很糟糕。如果为请求提供服务的线程没有停止，我们怎么会有 ThreadLocal 泄漏(实际上文章说类加载器泄漏)？这是否意味着，如果 Thread 是 Tomcat 线程池的一部分，并且由于 MyThreadLocal.remove() 尚未关闭，则 MyCounter/MyThreadLocal 的 webapp 类加载器不能被垃圾回收，因为 Thread 已返回到游泳池？调用 MyThreadLocal.remove() 会解决这个问题吗？我不明白。

Answer 1

解释不正确:

The leak is caused because we have a custom class for the ThreadLocal instance

那部分是错误的。 Thread 类中的 ThreadLocalMap 使用 weakReferences 作为键，因此 threadLocal 实例是否为自定义类并不重要。我用我的自定义 ThreadLocal 类对此进行了测试，没有内存泄漏。

, and also a custom class for the value bound to the Thread.

这是正确的。对于值线程不使用弱引用，因此如果值是自定义类，将阻止它的类加载器被垃圾收集，这将导致该类加载器加载的所有类保留在内存中，直到线程从线程池中删除。