tomcat容器实现类都继承了ContainerBase类,容器在启动的时候都会调用ContainerBase类的threadStart()方法,threadStart()方法如下:
protected void threadStart() { if (thread != null) return; if (backgroundProcessorDelay <= 0) return; threadDone = false; String threadName = "ContainerBackgroundProcessor[" + toString() + "]"; thread = new Thread(new ContainerBackgroundProcessor(), threadName); thread.setDaemon(true); thread.start(); }
StandardHost,StandardContext,StandardWrapper类的backgroundProcessorDelay的值都为-1,所以ContainerBackgroundProcessor线程都没有启动.而StandardEngine类在创建的时候 将backgroundProcessorDelay 的值设置为10.所以ContainerBackgroundProcessor线程在StandardEngine里启动,ContainerBackgroundProcessor线程代码如下:
protected class ContainerBackgroundProcessor implements Runnable { public void run() { while (!threadDone) { try { Thread.sleep(backgroundProcessorDelay * 1000L); } catch (InterruptedException e) { ; } if (!threadDone) { Container parent = (Container) getMappingObject();//得到StandardEngine ClassLoader cl = Thread.currentThread().getContextClassLoader(); if (parent.getLoader() != null) { cl = parent.getLoader().getClassLoader(); } processChildren(parent, cl); } } } protected void processChildren(Container container, ClassLoader cl) { try { if (container.getLoader() != null) { Thread.currentThread().setContextClassLoader (container.getLoader().getClassLoader()); } container.backgroundProcess(); } catch (Throwable t) { log.error("Exception invoking periodic operation: ", t); } finally { Thread.currentThread().setContextClassLoader(cl); } Container[] children = container.findChildren(); for (int i = 0; i < children.length; i++) { if (children[i].getBackgroundProcessorDelay() <= 0) { processChildren(children[i], cl); } } } }
ContainerBackgroundProcessor每隔10秒执行一下线程,线程递归调用StandardEngine,StandardHost,StandardContext,StandardWrapper类的backgroundProcess()方法(backgroundProcess()方法在ContainerBase类中)
backgroundProcess()方法如下:
public void backgroundProcess() { if (!started) return; if (cluster != null) { try { cluster.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString("containerBase.backgroundProcess.cluster", cluster), e); } } if (loader != null) { try { loader.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString("containerBase.backgroundProcess.loader", loader), e); } } if (manager != null) {//manager的实现类StandardManager try { manager.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString("containerBase.backgroundProcess.manager", manager), e); } } if (realm != null) { try { realm.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString("containerBase.backgroundProcess.realm", realm), e); } } Valve current = pipeline.getFirst(); while (current != null) { try { current.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString("containerBase.backgroundProcess.valve", current), e); } current = current.getNext(); } lifecycle.fireLifecycleEvent(Lifecycle.PERIODIC_EVENT, null); }
在调用StandContext类的backgroundProcess()方法时, 会调用StandardManager的backgroundProcess()方法,该方法如下:
public void backgroundProcess() { count = (count + 1) % processExpiresFrequency; if (count == 0) processExpires(); }
processExpiresFrequency的默认值为6,所以每6*10秒会调一次processExpires()方法,processExpires()方法如下:
public void processExpires() { long timeNow = System.currentTimeMillis(); Session sessions[] = findSessions(); int expireHere = 0 ; if(log.isDebugEnabled()) log.debug("Start expire sessions " + getName() + " at " + timeNow + " sessioncount " + sessions.length); for (int i = 0; i < sessions.length; i++) { if (sessions[i]!=null && !sessions[i].isValid()) {//判断session是否有效,无效则清除 expireHere++; } } long timeEnd = System.currentTimeMillis(); if(log.isDebugEnabled()) log.debug("End expire sessions " + getName() + " processingTime " + (timeEnd - timeNow) + " expired sessions: " + expireHere); processingTime += ( timeEnd - timeNow ); }
该方法主要判断session是否过期,过期则清除.
整个过程看起来有点复杂,简单地来说,就是tomcat服务器在启动的时候初始化了一个守护线程,定期6*10秒去检查有没有Session过期.过期则清除.
