Tomcat容器核心逻辑及源码分析 置顶！ 有更新！

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Tomcat容器核心逻辑及源码分析

Tomcat架构图

整体架构图

架构图

连接器

连接器

容器及处理流程

容器

容器

Tomcat的热加载和热部署

热加载和热部署核心原理

在容器的基类ContainerBase类中的模板方法startInternal中会提交一个定时的后台任务ContainerBackgroundProcessorMonitor去监控容器，源码如下:

protected synchronized void startInternal() throws LifecycleException { // Start our subordinate components, if any logger = null; getLogger(); Cluster cluster = getClusterInternal(); if (cluster instanceof Lifecycle) { ((Lifecycle) cluster).start(); } Realm realm = getRealmInternal(); if (realm instanceof Lifecycle) { ((Lifecycle) realm).start(); } // Start our child containers, if any Container children[] = findChildren(); List
      
       > results = new ArrayList<>(); for (int i = 0; i < children.length; i++) { results.add(startStopExecutor.submit(new StartChild(children[i]))); } MultiThrowable multiThrowable = null; for (Future
       
         result : results) { try { result.get(); } catch (Throwable e) { log.error(sm.getString("containerBase.threadedStartFailed"), e); if (multiThrowable == null) { multiThrowable = new MultiThrowable(); } multiThrowable.add(e); } } if (multiThrowable != null) { throw new LifecycleException(sm.getString("containerBase.threadedStartFailed"), multiThrowable.getThrowable()); } // Start the Valves in our pipeline (including the basic), if any if (pipeline instanceof Lifecycle) { ((Lifecycle) pipeline).start(); } setState(LifecycleState.STARTING); // Start our thread if (backgroundProcessorDelay > 0) { monitorFuture = Container.getService(ContainerBase.this).getServer() .getUtilityExecutor().scheduleWithFixedDelay( new ContainerBackgroundProcessorMonitor(), 0, 60, TimeUnit.SECONDS); } }

在ContainerBackgroundProcessorMonitor任务中对当前容器进行了判断，如果满足backgroundProcessorDelay>0和容器当前状态属于正常，则启动了一个定时任务ContainerBackgroundProcessor,ContainerBackgroundProcessor任务核心处理如下:

 protected void processChildren(Container container) { ClassLoader originalClassLoader = null; try { if (container instanceof Context) { Loader loader = ((Context) container).getLoader(); // Loader will be null for FailedContext instances if (loader == null) { return; } // Ensure background processing for Contexts and Wrappers // is performed under the web app's class loader originalClassLoader = ((Context) container).bind(false, null); } container.backgroundProcess(); Container[] children = container.findChildren(); for (int i = 0; i < children.length; i++) { if (children[i].getBackgroundProcessorDelay() <= 0) { processChildren(children[i]); } } } catch (Throwable t) { ExceptionUtils.handleThrowable(t); log.error(sm.getString("containerBase.backgroundProcess.error"), t); } finally { if (container instanceof Context) { ((Context) container).unbind(false, originalClassLoader); } } }

在这个方法中调用当前容器的 backgroundProcess 方法，以及递归调用子孙的 backgroundProcess 方法。backgroundProcess 是 Container 接口中的方法，也就是说所有类型的容器都可以实现这个方法，在这个方法里完成需要周期性执行的任务,在递归调用子容器的backgroundProcess时同样也对backgroundProcessorDelay参数做了判断，在这个参数大于0时，当前容器会开始自己的周期任务来自己执行backgroundProcess方法，不需要父容器代为执行。
backgroundProcess在容器基类ContainerBase中提供了默认实现:

 public void backgroundProcess() { if (!getState().isAvailable()) return; Cluster cluster = getClusterInternal(); if (cluster != null) { try { cluster.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString("containerBase.backgroundProcess.cluster", cluster), e); } } Realm realm = getRealmInternal(); 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(); } fireLifecycleEvent(Lifecycle.PERIODIC_EVENT, null); }

基类中处理流程如下：
1. 执行容器中Cluster组件的周期性任务。
2. 执行容器中Realm组件的周期性任务。
3. 执行容器中valve的周期性任务。
4. 触发容器生命周期事件。

容器如果要自己去实现周期性任务只需重写此方法即可，如果不想受父容器管理则需同时修改backgroundProcessorDelay参数值，tomcat的热加载就是由StandardContext重写了此方法和修改了backgroundProcessorDelay>0实现，而热部署则是由HostConfig监听了容器的Lifecycle.PERIODIC_EVENT事件实现。

热加载

tomcat的热加载是由Context容器重写了周期性任务处理方法backgroundProcess实现的，其源码如下:

 public void backgroundProcess() { if (!getState().isAvailable()) return; Loader loader = getLoader(); if (loader != null) { try { loader.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString( "standardContext.backgroundProcess.loader", loader), e); } } Manager manager = getManager(); if (manager != null) { try { manager.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString( "standardContext.backgroundProcess.manager", manager), e); } } WebResourceRoot resources = getResources(); if (resources != null) { try { resources.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString( "standardContext.backgroundProcess.resources", resources), e); } } InstanceManager instanceManager = getInstanceManager(); if (instanceManager != null) { try { instanceManager.backgroundProcess(); } catch (Exception e) { log.warn(sm.getString( "standardContext.backgroundProcess.instanceManager", resources), e); } } super.backgroundProcess(); }

其处理流程为:
1. 获取WebappLoader，然后执行WebappLoader的周期性处理方法，检查WEB-INF/classes和WEB-INF/lib目录下的类文件，如果有变化，则会调用Context容器的reload方法。
2. 获取Manager，然后周期性检查是否有过期的session.
3. 获取WebResourceRoot，然后周期性检查静态资源是否有变化。
4. 调用父容器的backgroundProcess方法。

其中热加载处理的核心为Context的reload方法，在这个方法中会停止和销毁Context及其子容器、Context关联的listener和filter、Context的Pipline和各种valve、Context的类加载器以及加载器加载的类文件资源，然后重新启动Context重建被销毁的资源。在reload方法中并没有调用Manager的destroy方法，所以不会销毁Session，其源码如下:

 /** * Reload this web application, if reloading is supported. * 
 * IMPLEMENTATION NOTE: This method is designed to deal with * reloads required by changes to classes in the underlying repositories * of our class loader and changes to the web.xml file. It does not handle * changes to any context.xml file. If the context.xml has changed, you * should stop this Context and create (and start) a new Context instance * instead. Note that there is additional code in * CoyoteAdapter#postParseRequest() to handle mapping requests * to paused Contexts. * * @exception IllegalStateException if the reloadable * property is set to false. */ @Override public synchronized void reload() { // Validate our current component state if (!getState().isAvailable()) throw new IllegalStateException (sm.getString("standardContext.notStarted", getName())); if(log.isInfoEnabled()) www.jmhat.com(sm.getString("standardContext.reloadingStarted", getName())); // Stop accepting requests temporarily. setPaused(true); try { stop(); } catch (LifecycleException e) { log.error( sm.getString("standardContext.stoppingContext", getName()), e); } try { start(); } catch (LifecycleException e) { log.error( sm.getString("standardContext.startingContext", getName()), e); } setPaused(false); if(log.isInfoEnabled()) www.jmhat.com(sm.getString("standardContext.reloadingCompleted", getName())); }

tomncat的热加载是默认关闭的，如果需要开启，需要在conf目录下的context.xml文件中设置reloadable参数来开启这个功能。

热部署

tomcat的热部署就是重新部署我们的Web应用，对应到代码中也是就是我们的Context容器，上面我们提到了热加载，在热加载中我们只是停止和销毁了容器的部分组件，但是在热部署中会整个销毁或者新建Context对象，由于Context容器是Host容器的子容器，所以这个任务交给了Host容器来处理,但不是直接通过backgroundProcess方法来处理的，但是通过监听backgroundProcess方法触发的生命周期事件fireLifecycleEvent(Lifecycle.PERIODIC_EVENT, null)来实现Context的热部署，具体是由HostConfig来监听这个事件,然后它在接到通知后执行了Check方法，在Check方法中处理了检测是否Web应用目录由变化，如果有变化则进行部署，其源码如下:

 /** * Check status of all webapps. */ protected void check() { if (host.getAutoDeploy()) { // Check for resources modification to trigger redeployment DeployedApplication[] apps = deployed.values().toArray(new DeployedApplication[0]); for (int i = 0; i < apps.length; i++) { if (!isServiced(apps[i].name)) checkResources(apps[i], false); } // Check for old versions of applications that can now be undeployed if (host.getUndeployOldVersions()) { checkUndeploy(); } // Hotdeploy applications deployApps(); } }

在deployApps()方法中实现了对Web应用的部署，源码如下:

 protected void deployApps() { File appBase = host.getAppBaseFile(); File configBase = host.getConfigBaseFile(); String[] filteredAppPaths = filterAppPaths(appBase.list()); // Deploy XML descriptors from configBase deployDescriptors(configBase, configBase.list()); // Deploy WARs deployWARs(appBase, filteredAppPaths); // Deploy expanded folders deployDirectories(appBase, filteredAppPaths); }

类加载

Tomcat 类加载器的层次结构

类加载器层次结构

WebAppClassLoader

AppClassLoader 只能加载一个 Servlet 类，在加载第二个同名 Servlet 类时，AppClassLoader 会返回第一个 Servlet 类的 Class 实例，这是因为在 AppClassLoader 看来，同名的 Servlet 类只被加载一次。

因此 Tomcat 的解决方案是自定义一个类加载器 WebAppClassLoader， 并且给每个 Web 应用创建一个类加载器实例。我们知道，Context 容器组件对应一个 Web 应用，因此，每个 Context 容器负责创建和维护一个 WebAppClassLoader 加载器实例。这背后的原理是，不同的加载器实例加载的类被认为是不同的类，即使它们的类名相同。这就相当于在 Java 虚拟机内部创建了一个个相互隔离。

SharedClassLoader

两个 Web 应用之间怎么共享库类，并且不能重复加载相同的类。我们知道，在双亲委托机制里，各个子加载器都可以通过父加载器去加载类，那么把需要共享的类放到父加载器的加载路径下不就行了吗，应用程序也正是通过这种方式共享 JRE 的核心类。

因此 Tomcat 的设计者又加了一个类加载器 SharedClassLoader，作为 WebAppClassLoader 的父加载器，专门来加载 Web 应用之间共享的类。如果 WebAppClassLoader 自己没有加载到某个类，就会委托父加载器 SharedClassLoader 去加载这个类，SharedClassLoader 会在指定目录下加载共享类，之后返回给 WebAppClassLoader，这样共享的问题就解决了。

CatalinaClassLoader

如何隔离 Tomcat 本身的类和 Web 应用的类？我们知道，要共享可以通过父子关系，要隔离那就需要兄弟关系了。兄弟关系就是指两个类加载器是平行的，它们可能拥有同一个父加载器，但是两个兄弟类加载器加载的类是隔离的。基于此 Tomcat 又设计一个类加载器 CatalinaClassLoader，专门来加载 Tomcat 自身的类。

CommonClassLoader

Tomcat 和各 Web 应用之间需要共享一些类时该怎么办呢？还是再增加一个 CommonClassLoader，作为 CatalinaClassLoader 和 SharedClassLoader 的父加载器。CommonClassLoader 能加载的类都可以被 CatalinaClassLoader 和 SharedClassLoader 使用，而 CatalinaClassLoader 和 SharedClassLoader 能加载的类则与对方相互隔离。WebAppClassLoader 可以使用 SharedClassLoader 加载到的类，但各个 WebAppClassLoader 实例之间相互隔离。

WebAppClassLoader

Tomcat的自定义类加载器WebAppClassLoader 打破了双亲委托机制，它首先自己尝试去加载某个类，如果找不到再代理给父类加载器，其目的是优先加载 Web 应用自己定义的类。具体实现就是重写ClassLoader 的两个方法：findClass 和 loadClass。
其中findClass源码如下:

 /** * Find the specified class in our local repositories, if possible. If * not found, throw ClassNotFoundException. * * @param name The binary name of the class to be loaded * * @exception ClassNotFoundException if the class was not found */ @Override public Class
       findClass(String name) throws ClassNotFoundException { if (log.isDebugEnabled()) log.debug(" findClass(" + name + ")"); checkStateForClassLoading(name); // (1) Permission to define this class when using a SecurityManager if (securityManager != null) { int i = name.lastIndexOf('.'); if (i >= 0) { try { if (log.isTraceEnabled()) log.trace(" securityManager.checkPackageDefinition"); securityManager.checkPackageDefinition(name.substring(0,i)); } catch (Exception se) { if (log.isTraceEnabled()) log.trace(" -->Exception-->ClassNotFoundException", se); throw new ClassNotFoundException(name, se); } } } // Ask our superclass to locate this class, if possible // (throws ClassNotFoundException if it is not found) Class
       clazz = null; try { if (log.isTraceEnabled()) log.trace(" findClassInternal(" + name + ")"); try { if (securityManager != null) { PrivilegedAction
      
       > dp = new PrivilegedFindClassByName(name); clazz = AccessController.doPrivileged(dp); } else { clazz = findClassInternal(name); } } catch(AccessControlException ace) { log.warn(sm.getString("webappClassLoader.securityException", name, ace.getMessage()), ace); throw new ClassNotFoundException(name, ace); } catch (RuntimeException e) { if (log.isTraceEnabled()) log.trace(" -->RuntimeException Rethrown", e); throw e; } if ((clazz == null) && hasExternalRepositories) { try { clazz = super.findClass(name); } catch(AccessControlException ace) { log.warn(sm.getString("webappClassLoader.securityException", name, ace.getMessage()), ace); throw new ClassNotFoundException(name, ace); } catch (RuntimeException e) { if (log.isTraceEnabled()) log.trace(" -->RuntimeException Rethrown", e); throw e; } } if (clazz == null) { if (log.isDebugEnabled()) log.debug(" --> Returning ClassNotFoundException"); throw new ClassNotFoundException(name); } } catch (ClassNotFoundException e) { if (log.isTraceEnabled()) log.trace(" --> Passing on ClassNotFoundException"); throw e; } // Return the class we have located if (log.isTraceEnabled()) log.debug(" Returning class " + clazz); if (log.isTraceEnabled()) { ClassLoader cl; if (Globals.IS_SECURITY_ENABLED){ cl = AccessController.doPrivileged( new PrivilegedGetClassLoader(clazz)); } else { cl = clazz.getClassLoader(); } log.debug(" Loaded by " + cl.toString()); } return clazz; }

在 findClass 方法里，主要有四个步骤：
1. 如果使用了SecurityManager，则会先检查权限。
2. 先在 Web 应用本地目录下查找要加载的类。
3. 如果没有找到，交给父加载器去查找，它的父加载器就是上面提到的系统类加载器 AppClassLoader。
4. 如何父加载器也没找到这个类，抛出 ClassNotFound 异常。
loadClass源码如下:

/** * Load the class with the specified name, searching using the following * algorithm until it finds and returns the class. If the class cannot * be found, returns ClassNotFoundException. * 
      
    
        * 
       
  • Call findLoadedClass(String) to check if the * class has already been loaded. If it has, the same * Class object is returned.
    •  * 
       
  • If the delegate property is set to true, * call the loadClass() method of the parent class * loader, if any.
    •  * 
       
  • Call findClass() to find this class in our locally * defined repositories.
    •  * 
       
  • Call the loadClass() method of our parent * class loader, if any.
    •  * 
      
 * If the class was found using the above steps, and the * resolve flag is true, this method will then * call resolveClass(Class) on the resulting Class object. * * @param name The binary name of the class to be loaded * @param resolve If true then resolve the class * * @exception ClassNotFoundException if the class was not found */ @Override public Class
       loadClass(String name, boolean resolve) throws ClassNotFoundException { synchronized (getClassLoadingLock(name)) { if (log.isDebugEnabled()) log.debug("loadClass(" + name + ", " + resolve + ")"); Class
       clazz = null; // Log access to stopped class loader checkStateForClassLoading(name); // (0) Check our previously loaded local class cache clazz = findLoadedClass0(name); if (clazz != null) { if (log.isDebugEnabled()) log.debug(" Returning class from cache"); if (resolve) resolveClass(clazz); return clazz; } // (0.1) Check our previously loaded class cache clazz = findLoadedClass(name); if (clazz != null) { if (log.isDebugEnabled()) log.debug(" Returning class from cache"); if (resolve) resolveClass(clazz); return clazz; } // (0.2) Try loading the class with the system class loader, to prevent // the webapp from overriding Java SE classes. This implements // SRV.10.7.2 String resourceName = binaryNameToPath(name, false); ClassLoader javaseLoader = getJavaseClassLoader(); boolean tryLoadingFromJavaseLoader; try { // Use getResource as it won't trigger an expensive // ClassNotFoundException if the resource is not available from // the Java SE class loader. However (see // https://www.jmhat.com/bugzilla/show_bug.cgi?id=58125 for // details) when running under a security manager in rare cases // this call may trigger a ClassCircularityError. // See https://www.jmhat.com/bugzilla/show_bug.cgi?id=61424 for // details of how this may trigger a StackOverflowError // Given these reported errors, catch Throwable to ensure any // other edge cases are also caught URL url; if (securityManager != null) { PrivilegedAction
      
        dp = new PrivilegedJavaseGetResource(resourceName); url = AccessController.doPrivileged(dp); } else { url = javaseLoader.getResource(resourceName); } tryLoadingFromJavaseLoader = (url != null); } catch (Throwable t) { // Swallow all exceptions apart from those that must be re-thrown ExceptionUtils.handleThrowable(t); // The getResource() trick won't work for this class. We have to // try loading it directly and accept that we might get a // ClassNotFoundException. tryLoadingFromJavaseLoader = true; } if (tryLoadingFromJavaseLoader) { try { clazz = javaseLoader.loadClass(name); if (clazz != null) { if (resolve) resolveClass(clazz); return clazz; } } catch (ClassNotFoundException e) { // Ignore } } // (0.5) Permission to access this class when using a SecurityManager if (securityManager != null) { int i = name.lastIndexOf('.'); if (i >= 0) { try { securityManager.checkPackageAccess(name.substring(0,i)); } catch (SecurityException se) { String error = sm.getString("webappClassLoader.restrictedPackage", name); www.jmhat.com(error, se); throw new ClassNotFoundException(error, se); } } } boolean delegateLoad = delegate || filter(name, true); // (1) Delegate to our parent if requested if (delegateLoad) { if (log.isDebugEnabled()) log.debug(" Delegating to parent classloader1 " + parent); try { clazz = Class.forName(name, false, parent); if (clazz != null) { if (log.isDebugEnabled()) log.debug(" Loading class from parent"); if (resolve) resolveClass(clazz); return clazz; } } catch (ClassNotFoundException e) { // Ignore } } // (2) Search local repositories if (log.isDebugEnabled()) log.debug(" Searching local repositories"); try { clazz = findClass(name); if (clazz != null) { if (log.isDebugEnabled()) log.debug(" Loading class from local repository"); if (resolve) resolveClass(clazz); return clazz; } } catch (ClassNotFoundException e) { // Ignore } // (3) Delegate to parent unconditionally if (!delegateLoad) { if (log.isDebugEnabled()) log.debug(" Delegating to parent classloader at end: " + parent); try { clazz = Class.forName(name, false, parent); if (clazz != null) { if (log.isDebugEnabled()) log.debug(" Loading class from parent"); if (resolve) resolveClass(clazz); return clazz; } } catch (ClassNotFoundException e) { // Ignore } } } throw new ClassNotFoundException(name); }

loadClass 主要有六个步骤：

  1. 先在本地 Cache 查找该类是否已经加载过，也就是说 Tomcat 的类加载器是否已经加载过这个类。
  2. 如果 Tomcat 类加载器没有加载过这个类，再看看系统类加载器是否加载过。
  3. 如果都没有，就让ExtClassLoader去加载，这一步比较关键，目的防止 Web 应用自己的类覆盖 JRE 的核心类。因为 Tomcat 需要打破双亲委托机制，假如 Web 应用里自定义了一个叫 Object 的类，如果先加载这个 Object 类，就会覆盖 JRE 里面的那个 Object 类，这就是为什么 Tomcat 的类加载器会优先尝试用 ExtClassLoader 去加载，因为 ExtClassLoader 会委托给 BootstrapClassLoader 去加载，BootstrapClassLoader 发现自己已经加载了 Object 类，直接返回给 Tomcat 的类加载器，这样 Tomcat 的类加载器就不会去加载 Web 应用下的 Object 类了，也就避免了覆盖 JRE 核心类的问题。
  4. 如果 ExtClassLoader 加载器加载失败，也就是说 JRE 核心类中没有这类，那么就在本地 Web 应用目录下查找并加载。
  5. 如果本地目录下没有这个类，说明不是 Web 应用自己定义的类，那么由系统类加载器去加载。这里请你注意，Web 应用是通过Class.forName调用交给系统类加载器的，因为Class.forName的默认加载器就是系统类加载器。
  6. 如果上述加载过程全部失败，抛出 ClassNotFound 异常。

Tomcat如何实现Servlet规范

Servlet

Tomcat是用Wrapper容器来管理Servlet的,并且StandardWrapper通过loadServlet方法来实例化 Servlet，其流程图如下:
image

Filter

Filter的实例是在Context容器中进行管理的，Context容器用Map集合来保存Filter。其核心方法源码如下:

 /** * Invoke the next filter in this chain, passing the specified request * and response. If there are no more filters in this chain, invoke * the service() method of the servlet itself. * * @param request The servlet request we are processing * @param response The servlet response we are creating * * @exception IOException if an input/output error occurs * @exception ServletException if a servlet exception occurs */ @Override public void doFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException { if( Globals.IS_SECURITY_ENABLED ) { final ServletRequest req = request; final ServletResponse res = response; try { java.security.AccessController.doPrivileged( new java.security.PrivilegedExceptionAction
      
       () { @Override public Void run() throws ServletException, IOException { internalDoFilter(req,res); return null; } } ); } catch( PrivilegedActionException pe) { Exception e = pe.getException(); if (e instanceof ServletException) throw (ServletException) e; else if (e instanceof IOException) throw (IOException) e; else if (e instanceof RuntimeException) throw (RuntimeException) e; else throw new ServletException(e.getMessage(), e); } } else { internalDoFilter(request,response); } } private void internalDoFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException { // Call the next filter if there is one if (pos < n) { ApplicationFilterConfig filterConfig = filters[pos++]; try { Filter filter = filterConfig.getFilter(); if (request.isAsyncSupported() && "false".equalsIgnoreCase( filterConfig.getFilterDef().getAsyncSupported())) { request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE); } if( Globals.IS_SECURITY_ENABLED ) { final ServletRequest req = request; final ServletResponse res = response; Principal principal = ((HttpServletRequest) req).getUserPrincipal(); Object[] args = new Object[]{req, res, this}; SecurityUtil.doAsPrivilege ("doFilter", filter, classType, args, principal); } else { filter.doFilter(request, response, this); } } catch (IOException | ServletException | RuntimeException e) { throw e; } catch (Throwable e) { e = ExceptionUtils.unwrapInvocationTargetException(e); ExceptionUtils.handleThrowable(e); throw new ServletException(sm.getString("filterChain.filter"), e); } return; } // We fell off the end of the chain -- call the servlet instance try { if (ApplicationDispatcher.WRAP_SAME_OBJECT) { lastServicedRequest.set(request); lastServicedResponse.set(response); } if (request.isAsyncSupported() && !servletSupportsAsync) { request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE); } // Use potentially wrapped request from this point if ((request instanceof HttpServletRequest) && (response instanceof HttpServletResponse) && Globals.IS_SECURITY_ENABLED ) { final ServletRequest req = request; final ServletResponse res = response; Principal principal = ((HttpServletRequest) req).getUserPrincipal(); Object[] args = new Object[]{req, res}; SecurityUtil.doAsPrivilege("service", servlet, classTypeUsedInService, args, principal); } else { servlet.service(request, response); } } catch (IOException | ServletException | RuntimeException e) { throw e; } catch (Throwable e) { e = ExceptionUtils.unwrapInvocationTargetException(e); ExceptionUtils.handleThrowable(e); throw new ServletException(sm.getString("filterChain.servlet"), e); } finally { if (ApplicationDispatcher.WRAP_SAME_OBJECT) { lastServicedRequest.set(null); lastServicedResponse.set(null); } } }

从 ApplicationFilterChain 的源码我们可以看到几个关键信息：

  1. Filter 链中除了有 Filter 对象的数组，还有一个整数变量 pos，这个变量用来记录当前被调用的 Filter 在数组中的位置。
  2. Filter 链中有个 Servlet 实例，这个好理解，因为上面提到了，每个 Filter 链最后都会调到一个 Servlet。
  3. Filter 链本身也实现了 doFilter 方法，直接调用了一个内部方法 internalDoFilter。
  4. internalDoFilter 方法的实现比较有意思，它做了一个判断，如果当前 Filter 的位置小于 Filter 数组的长度，也就是说 Filter 还没调完，就从 Filter 数组拿下一个 Filter，调用它的 doFilter 方法。否则，意味着所有 Filter 都调到了，就调用 Servlet 的 service 方法。

Listener

Tomcat 是通过 Context 容器来管理这些监听器的。Context 容器将两类事件分开来管理，分别用不同的集合来存放不同类型事件的监听器：

// 监听属性值变化的监听器 private List applicationEventListenersList = new CopyOnWriteArrayList<>(); // 监听生命事件的监听器 private Object applicationLifecycleListenersObjects[] = new Object[0];  
在 Context 容器的启动方法里，通过调用listenerStart方法添加Listener和触发了所有的ServletContextListener，其源码如下:
 
/** * Configure the set of instantiated application event listeners * for this Context. * @return true if all listeners wre * initialized successfully, or false otherwise. */ public boolean listenerStart() { if (log.isDebugEnabled()) log.debug("Configuring application event listeners"); // Instantiate the required listeners String listeners[] = findApplicationListeners(); Object results[] = new Object[listeners.length]; boolean ok = true; for (int i = 0; i < results.length; i++) { if (getLogger().isDebugEnabled()) getLogger().debug(" Configuring event listener class '" + listeners[i] + "'"); try { String listener = listeners[i]; results[i] = getInstanceManager().newInstance(listener); } catch (Throwable t) { t = ExceptionUtils.unwrapInvocationTargetException(t); ExceptionUtils.handleThrowable(t); getLogger().error(sm.getString( "standardContext.applicationListener", listeners[i]), t); ok = false; } } if (!ok) { getLogger().error(sm.getString("standardContext.applicationSkipped")); return false; } // Sort listeners in two arrays List eventListeners = new ArrayList<>(); List lifecycleListeners = new ArrayList<>(); for (int i = 0; i < results.length; i++) { if ((results[i] instanceof ServletContextAttributeListener) || (results[i] instanceof ServletRequestAttributeListener) || (results[i] instanceof ServletRequestListener) || (results[i] instanceof HttpSessionIdListener) || (results[i] instanceof HttpSessionAttributeListener)) { eventListeners.add(results[i]); } if ((results[i] instanceof ServletContextListener) || (results[i] instanceof HttpSessionListener)) { lifecycleListeners.add(results[i]); } } // Listener instances may have been added directly to this Context by // ServletContextInitializers and other code via the pluggability APIs. // Put them these listeners after the ones defined in web.xml and/or // annotations then overwrite the list of instances with the new, full // list. for (Object eventListener: getApplicationEventListeners()) { eventListeners.add(eventListener); } setApplicationEventListeners(eventListeners.toArray()); for (Object lifecycleListener: getApplicationLifecycleListeners()) { lifecycleListeners.add(lifecycleListener); if (lifecycleListener instanceof ServletContextListener) { noPluggabilityListeners.add(lifecycleListener); } } setApplicationLifecycleListeners(lifecycleListeners.toArray()); // Send application start events if (getLogger().isDebugEnabled()) getLogger().debug("Sending application start events"); // Ensure context is not null getServletContext(); context.setNewServletContextListenerAllowed(false); Object instances[] = getApplicationLifecycleListeners(); if (instances == null || instances.length == 0) { return ok; } ServletContextEvent event = new ServletContextEvent(getServletContext()); ServletContextEvent tldEvent = null; if (noPluggabilityListeners.size() > 0) { noPluggabilityServletContext = new NoPluggabilityServletContext(getServletContext()); tldEvent = new ServletContextEvent(noPluggabilityServletContext); } for (int i = 0; i < instances.length; i++) { if (!(instances[i] instanceof ServletContextListener)) continue; ServletContextListener listener = (ServletContextListener) instances[i]; try { fireContainerEvent("beforeContextInitialized", listener); if (noPluggabilityListeners.contains(listener)) { listener.contextInitialized(tldEvent); } else { listener.contextInitialized(event); } fireContainerEvent("afterContextInitialized", listener); } catch (Throwable t) { ExceptionUtils.handleThrowable(t); fireContainerEvent("afterContextInitialized", listener); getLogger().error (sm.getString("standardContext.listenerStart", instances[i].getClass().getName()), t); ok = false; } } return ok; }  
Spring boot内嵌Tomcat的启动
 
核心启动流程
 
在Spring Boot应用的启动类上都会有@SpringBootApplication注解，其中核心为@EnableAutoConfiguration注解中包含的AutoConfigurationImportSelector，该类做了什么事情呢？进入源码，首先调用selectImport()方法，在该方法中调用了getAutoConfigurationEntry（）方法，在之中又调用了getCandidateConfigurations()方法，getCandidateConfigurations()方法就去META-INF/spring.factory配置文件中加载相关配置类，其中就加载了www.jmhat.com配置，该配置类用@Import引入了EmbeddedTomcat配置类，在这个类中中注册一个Bean为TomcatServletWebServerFactory,其中实现了Spring的ServletWebServerFactory方法，其中核心方法getWebServer源码如下:
 
 @Override public WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat"); tomcat.setBaseDir(baseDir.getAbsolutePath()); Connector connector = new Connector(www.jmhat.com); tomcat.getService().addConnector(connector); customizeConnector(connector); tomcat.setConnector(connector); tomcat.getHost().setAutoDeploy(false); configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } prepareContext(tomcat.getHost(), initializers); return getTomcatWebServer(tomcat); } 
 
在其中实现了tomcat的启动，那么这个方法在哪被调用的呢？在我们的Spring Boot应用启动的时候，核心方法SpringApplication.run()中，其中核心会调用到AbstractApplicationContext的refresh方法，源码如下:
 
public void refresh() throws BeansException, IllegalStateException { synchronized(this.startupShutdownMonitor) { this.prepareRefresh(); ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory(); this.prepareBeanFactory(beanFactory); try { this.postProcessBeanFactory(beanFactory); this.invokeBeanFactoryPostProcessors(beanFactory); this.registerBeanPostProcessors(beanFactory); this.initMessageSource(); this.initApplicationEventMulticaster(); this.onRefresh(); this.registerListeners(); this.finishBeanFactoryInitialization(beanFactory); this.finishRefresh(); } catch (BeansException var9) { if (this.logger.isWarnEnabled()) { this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9); } this.destroyBeans(); this.cancelRefresh(var9); throw var9; } finally { this.resetCommonCaches(); } } } 
 
其中核心处调用了模板方法onRefresh(),而ServletWebServerApplicationContext的实现源码如下:
 
 @Override protected void onRefresh() { super.onRefresh(); try { createWebServer(); } catch (Throwable ex) { throw new ApplicationContextException("Unable to start web server", ex); } } private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); if (webServer == null && servletContext == null) { ServletWebServerFactory factory = getWebServerFactory(); this.webServer = factory.getWebServer(getSelfInitializer()); } else if (servletContext != null) { try { getSelfInitializer().onStartup(servletContext); } catch (ServletException ex) { throw new ApplicationContextException("Cannot initialize servlet context", ex); } } initPropertySources(); } 
 
其中调用了getWebServer来启动tomcat。
 
请求在容器中的执行流程图
 
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