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Shiro在Tomcat下内存泄漏问题
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警告 [main] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [ROOT] appears to have started a thread named [SessionValidationThread-1] but has failed to stop it. This is very likely to create a memory leak.

关闭Tomcat后报错：

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org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [ROOT] appears to have started a thread named [SessionValidationThread-1] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 sun.misc.Unsafe.park(Native Method)
 java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
 java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078)
 java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1093)
 java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
 java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)
 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)
 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
 java.lang.Thread.run(Thread.java:748)

1.首先看看org.apache.catalina这个包是谁的？用来干嘛的？

catalina就是Tomcat服务器使用的Apache实现的servlet容器的名字。

2.clearReferencesThreads是用来干嘛的？

这是tomcat关闭应用时检测到了应用启动的线程未被终止，tomcat为防止造成内存泄露，给出警告，并根据配置

来决定是否强制停止该线程（默认不会强制停止）。

这样一来问题的原因就清楚了，不就是关闭Tomcat时有个线程没有终止导致的内存泄漏嘛。

接下来就是找出未关闭线程所属的Class并想办法让他在Tomcat停止前结束就解决了。

1.用jps找出应用进程的pid

jps -l

jps

定位到pid为6168的应用

2.用jstack导出线程dump（Windows可以直接用jvisualvm图形化界面）

jstack 6168 > D:/dump.txt

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"SessionValidationThread-1" #63 daemon prio=5 os_prio=0 tid=0x000000001fe10800 nid=0x974 waiting on condition [0x000000000107e000]
   java.lang.Thread.State: TIMED_WAITING (parking)
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x0000000774472b78> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
	at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078)
	at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1093)
	at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
	at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:748)

掌握到以下信息：是个守护线程，是个定时任务。下面找出是谁创建的SessionValidationThread为前缀的线程就行了，直接在IDEA中全局搜索关键字SessionValidationThread（我这里没下载Maven依赖的Source，所以搜的类名）

class

ExecutorServiceSessionValidationScheduler这个一看就像是了

进去后看到，只截取关键部分：

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public class ExecutorServiceSessionValidationScheduler implements SessionValidationScheduler, Runnable {

    ......

    public void enableSessionValidation() {
        if (this.interval > 0L) {
            this.service = Executors.newSingleThreadScheduledExecutor(new ThreadFactory() {
                private final AtomicInteger count = new AtomicInteger(1);

                public Thread newThread(Runnable r) {
                    Thread thread = new Thread(r);
                    thread.setDaemon(true);
                    thread.setName(ExecutorServiceSessionValidationScheduler.this.threadNamePrefix + this.count.getAndIncrement());
                    return thread;
                }
            });
            this.service.scheduleAtFixedRate(this, this.interval, this.interval, TimeUnit.MILLISECONDS);
        }

        this.enabled = true;
    }
    
    public void run() {
        if (log.isDebugEnabled()) {
            log.debug("Executing session validation...");
        }

        long startTime = System.currentTimeMillis();
        this.sessionManager.validateSessions();
        long stopTime = System.currentTimeMillis();
        if (log.isDebugEnabled()) {
            log.debug("Session validation completed successfully in " + (stopTime - startTime) + " milliseconds.");
        }

    }
    
    public void disableSessionValidation() {
        if (this.service != null) {
            this.service.shutdownNow();
        }

        this.enabled = false;
    }

    ......

}

现在看怎么关闭这个线程，ExecutorServiceSessionValidationScheduler自带销毁的方法，但是怎么调用到他呢？

再来找找ExecutorServiceSessionValidationScheduler是从哪被调用的，这个类是Shiro用作session的定时校验的，我们去Shiri的xml配置中看看session相关配置。

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<!-- 会话管理器 -->
<bean id="sessionManager" class="org.apache.shiro.web.session.mgt.DefaultWebSessionManager">
    <property name="sessionDAO" ref="redisSessionDAO" />
    <!--去掉URL中的JSESSIONID-->
    <property name="sessionIdUrlRewritingEnabled" value="false"/>
    <!-- 设置超时时间，这里是毫秒 -->
    <property name="globalSessionTimeout" value="1800000"/>
    <property name="deleteInvalidSessions" value="true"/>
    <property name="sessionValidationSchedulerEnabled" value="true"/>
    <property name="sessionIdCookieEnabled" value="true"/>
    <property name="sessionIdCookie" ref="simpleCookie"/>
</bean>

很清楚了吧，sessionValidationSchedulerEnabled设置为true了，因为DefaultWebSessionManager是在Spring的IOC里的，所以找到里边的销毁方法就好了。但是DefaultWebSessionManager的类继承的有点多，为了方便就Debug吧。

直接在ExecutorServiceSessionValidationScheduler的enableSessionValidation方法上下断点，追踪下链路。

找到了，AbstractValidatingSessionManager类中的enableSessionValidation方法

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protected synchronized void enableSessionValidation() {
    SessionValidationScheduler scheduler = this.getSessionValidationScheduler();
        if (scheduler == null) {
            scheduler = this.createSessionValidationScheduler();
            this.setSessionValidationScheduler(scheduler);
        }

        if (!scheduler.isEnabled()) {
            if (log.isInfoEnabled()) {
                log.info("Enabling session validation scheduler...");
            }

        scheduler.enableSessionValidation();
        this.afterSessionValidationEnabled();
    }

}

同时也找到了提供的销毁方法

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public void destroy() {
    this.disableSessionValidation();
}

他会最终调用ExecutorServiceSessionValidationScheduler的disableSessionValidation方法，対线程执行shutdownNow()操作

最终解决方法：

新建类MyApplicationListener，监听Spring容器关闭（其他的钩子也行，只要能调用到就OK）

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package cn.qinshuang;

import org.apache.shiro.session.mgt.AbstractValidatingSessionManager;
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextClosedEvent;

public class MyApplicationListener implements ApplicationListener<ApplicationEvent>, ApplicationContextAware {
    private ApplicationContext applicationContext;

    @Override
    public void onApplicationEvent(ApplicationEvent applicationEvent) {
        if (applicationEvent instanceof ContextClosedEvent) {
            AbstractValidatingSessionManager s = (AbstractValidatingSessionManager) applicationContext.getBean("sessionManager");
            if (s != null) {
                s.destroy();
            }
        }
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}

至此，Tomcat正常关闭，控制台也输出了session validation停止的log

[main] INFO o.a.s.s.m.AbstractValidatingSessionManager - Disabled session validation scheduler.

文章作者: Shawn Qin
文章链接: https://qinshuang1998.github.io/2019/09/23/shiro-tomcat-memory-leak/
版权声明: 本博客所有文章除特别声明外，均采用 CC BY-NC-SA 4.0 许可协议。转载请注明来自 Shawn's Blog！
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