浅谈tomcat的ThreadLocalLeakPreventionListener实现原理

为什么需要threadlocalLeakPerventionListener

    当context reload的时候，如果正在执行的worker线程引用了threadlocal中的变量，会造成整个webclassloader回收不了造成内存泄露，具体请移步tomcat的wiki http://wiki.apache.org/tomcat/MemoryLeakProtection，wiki上比我说得明白。那具体方案也很明确，当context reload的时候，renew线程中的所有的线程，呵呵这活儿得交给listener来做了，所以就有了ThreadLocalLeakPreventionListener，那如何使线程停下来呢，stop这是一个不靠谱的方法，在api已将该方法废弃，幸亏我们还有异常，线程池中的线程遇到异常就会自杀。说多都是废话，还是代码实际

 

private void stopIdleThreads(Context context) {
        if (serverStopping) return;

        if (context instanceof StandardContext &&
            !((StandardContext) context).getRenewThreadsWhenStoppingContext()) {
            log.debug("Not renewing threads when the context is stopping, "
                + "it is configured not to do it.");
            return;
        }

        Engine engine = (Engine) context.getParent().getParent();
        Service service = engine.getService();
        Connector[] connectors = service.findConnectors();
        if (connectors != null) {
            for (Connector connector : connectors) {
               //获取该context中的所有connector，并获取thread pool
                ProtocolHandler handler = connector.getProtocolHandler();
                Executor executor = null;
                if (handler != null) {
                    executor = handler.getExecutor();
                }

                if (executor instanceof ThreadPoolExecutor) {
                    ThreadPoolExecutor threadPoolExecutor =
                        (ThreadPoolExecutor) executor;
                    threadPoolExecutor.contextStopping();
                } else if (executor instanceof StandardThreadExecutor) {
                    StandardThreadExecutor stdThreadExecutor =
                        (StandardThreadExecutor) executor;
                    stdThreadExecutor.contextStopping();
                }

            }
        }
    }

 

public void contextStopping() {
       //这个时间很关键，当worker线程执行完毕，会根据这个时间判断是否自杀
        this.lastContextStoppedTime.set(System.currentTimeMillis());

        // save the current pool parameters to restore them later
        int savedCorePoolSize = this.getCorePoolSize();
        TaskQueue taskQueue =
                getQueue() instanceof TaskQueue ? (TaskQueue) getQueue() : null;
        if (taskQueue != null) {
            // note by slaurent : quite oddly threadPoolExecutor.setCorePoolSize
            // checks that queue.remainingCapacity()==0. I did not understand
            // why, but to get the intended effect of waking up idle threads, I
            // temporarily fake this condition.
            taskQueue.setForcedRemainingCapacity(Integer.valueOf(0));
        }

        // setCorePoolSize(0) wakes idle threads
        this.setCorePoolSize(0);

        // wait a little so that idle threads wake and poll the queue again,
        // this time always with a timeout (queue.poll() instead of
        // queue.take())
        // even if we did not wait enough, TaskQueue.take() takes care of timing
        // out, so that we are sure that all threads of the pool are renewed in
        // a limited time, something like 
        // (threadKeepAlive + longest request time)
        try {
            Thread.sleep(200L);
        } catch (InterruptedException e) {
            // yes, ignore
        }

        if (taskQueue != null) {
            // ok, restore the state of the queue and pool
            taskQueue.setForcedRemainingCapacity(null);
        }
        this.setCorePoolSize(savedCorePoolSize);
    }

 

protected void stopCurrentThreadIfNeeded() {
        if (currentThreadShouldBeStopped()) {
            long lastTime = lastTimeThreadKilledItself.longValue();
           判断这个线程是否需要自杀，就是根据context stop的时间+delay是否小于当前，如小于抛出异常自杀
            if (lastTime + threadRenewalDelay < System.currentTimeMillis()) {
                if (lastTimeThreadKilledItself.compareAndSet(lastTime,
                        System.currentTimeMillis() + 1)) {
                    // OK, it's really time to dispose of this thread

                    final String msg = sm.getString(
                                    "threadPoolExecutor.threadStoppedToAvoidPotentialLeak",
                                    Thread.currentThread().getName());

                    Thread.currentThread().setUncaughtExceptionHandler(
                            new UncaughtExceptionHandler() {
                                @Override
                                public void uncaughtException(Thread t,
                                        Throwable e) {
                                    // yes, swallow the exception
                                    log.debug(msg);
                                }
                            });
                    throw new RuntimeException(msg);
                }
            }
        }
    }

 

  粗略地marker一下thread renew的整个过程，如有想法，敬请拍砖