热部署 

在Tomcat中可以通过Host标签设置热部署，当 autoDeploy为true时，在运行中的Tomcat中丢入一个war包，那么Tomcat不需要重启就可以自动加载该war包。

      <Host name="localhost"  appBase="webapps"
            unpackWARs="true" autoDeploy="true" >

Tomcat的容器中都包含有 backgroundProcessorDelay 属性和 backgroundProcess方法，默认的实现是，在每个容器启动时，当backgroundProcessorDelay大于1时（单位是秒），则会周期性的执行当前容器及所有子容器的backgroundProcess方法。

ContainerBase：
    protected synchronized void startInternal() throws LifecycleException {
        //...省略
        threadStart();
    }
 
 
    protected void threadStart() {
        if (thread != null)
            return;
        if (backgroundProcessorDelay <= 0)
            return;
        threadDone = false;
        String threadName = "ContainerBackgroundProcessor[" + toString() + "]";
        //开启线程周期性执行该后台任务
        thread = new Thread(new ContainerBackgroundProcessor(), threadName);
        thread.setDaemon(true);
        thread.start();
    }
 
ContainerBackgroundProcessor：
        public void run() {
            Throwable t = null;
            String unexpectedDeathMessage = sm.getString(
                    "containerBase.backgroundProcess.unexpectedThreadDeath",
                    Thread.currentThread().getName());
            try {
                while (!threadDone) {
                    try {
                        Thread.sleep(backgroundProcessorDelay * 1000L);
                    } catch (InterruptedException e) {
                        // Ignore
                    }
                    if (!threadDone) {
                        Container parent = (Container) getMappingObject();
                        ClassLoader cl = 
                            Thread.currentThread().getContextClassLoader();
                        if (parent.getLoader() != null) {
                            cl = parent.getLoader().getClassLoader();
                        }
                        //note 执行所有子容器的backgroundProcessorDelay方法
                        processChildren(parent, cl);
                    }
                }
            } catch (RuntimeException e) {
                t = e;
                throw e;
            } catch (Error e) {
                t = e;
                throw e;
            } finally {
                if (!threadDone) {
                    log.error(unexpectedDeathMessage, t);
                }
            }
        }  
 
        protected void processChildren(Container container, ClassLoader cl) {
            try {
                if (container.getLoader() != null) {
                    Thread.currentThread().setContextClassLoader
                        (container.getLoader().getClassLoader());
                }
                //执行当前容器的backgroundProcess
                container.backgroundProcess();
            } catch (Throwable t) {
                ExceptionUtils.handleThrowable(t);
            } finally {
                Thread.currentThread().setContextClassLoader(cl);
            }
            //在执行子容器的backgroundProcess
            Container[] children = container.findChildren();
            for (int i = 0; i < children.length; i++) {
                if (children[i].getBackgroundProcessorDelay() <= 0) {
                    processChildren(children[i], cl);
                }
            }
        }

 backgroundProcessorDelay的设置在Engine标签中，因为默认实现包含所有子容器，所有在最顶层的容器上配置一个即可。 

<Engine name="Catalina" defaultHost="localhost" backgroundProcessorDelay="20">

我们知道war包的管理应该是属于它的上一层容器Host，在Host中并没有重写backgroundProcess方法。 

StandardHost：
    public void backgroundProcess() {
      
        if (!getState().isAvailable())
            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);