浅谈tomcat的ThreadLocalLeakPreventionListener实现原理

bingtuag

　　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