前言
在Tomcat中，容器（Container）主要包括四种，Engine、Host、Context和Wrapper。在详细分析tomcat源码之前，我们先来看看容器的类继承层级是怎样的。
容器类继承层次图
这儿，我们先说明一下容器的包含关系。Engine包含多个Host，Host包含多个Context，Context包含多个Wrapper，每个Wrapper对应一个Servlet。我们如何来理解各个容器呢？Tomcat又为什么要把容器分成这4种呢？
- Engine，我们可以看成是容器对外提供功能的入口，每个Engine是Host的集合，用于管理各个Host。
- Host，我们可以看成
虚拟主机，一个tomcat可以支持多个虚拟主机。 - Context，又叫做上下文容器，我们可以看成
应用服务，每个Host里面可以运行多个应用服务。同一个Host里面不同的Context，其contextPath必须不同，默认Context的contextPath为空格("")或斜杠(/)。 - Wrapper，是Servlet的抽象和包装，每个Context可以有多个Wrapper，用于支持不同的Servlet。另外，每个JSP其实也是一个个的Servlet。
什么是虚拟主机
http协议从1.1开始，支持在请求头里面添加Host字段用来表示请求的域名。DNS域名解析的时候，可以将不同的域名解析到同一个ip或者主机。
假如我们需要在一个tomcat里面同时支持三个域名：
其轮廓图如下所示：
一个tomcat多个域名
要想在tomcat里面支持多域名，我们需要在server.xml文件里面的Engine标签下面添加多个Host标签，如下所示：
<Host name="www.ramki.com" appbase="ramki_webapps" />
<Host name="www.krishnan.com" appbase="krishnan_webapps" />
<Host name="www.blog.ramki.com" appbase="blog_webapps" />
其中name表示域名，appbase表示虚拟主机的目录。
当我们在浏览器输入http://www.ramki.com之后，相应域名将请求到tomcat。tomcat通过读取并搜索server.xml，找到www.ramki.com对应的虚拟主机Host，然后就使用查找到的Host来处理请求。
请求流程
在浏览器请求的时候，请求头信息如下，这儿我们重点关注Host header。
GET / HTTP/1.1
Host: www.ramki.com
Proxy-Connection: keep-alive
User-Agent: Mozilla/5.0 (Windows NT 6.2) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11
Accept: text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
什么是contextPath
我们在上面提到：同一个Host里面不同的Context，其contextPath必须不同，默认Context的contextPath为空格("")或斜杠(/)，那么我们如何理解这句话呢？
还是以例子来说明，假如我们在www.ramki.com映射的Host下配置3个Context子容器，其contextPath分别如下：
- AContext为：/
- BContext为：user
- CContext为：manager
如果我们请求www.ramki.com/simple（域名后面不是user，也不是manager），Host将请求转发到AContext；
请求www.ramki.com/user/add，Host将请求转发到BContext；
请求www.ramki.com/manager/view，Host将请求转发到CContext。
接下来做什么
通过上面的讲述，我们知道了每个容器的作用，接下来我们要做的就是分析每个容器的源码实现。
Engine
Engine的标准实现为org.apache.catalina.core.StandardEngine。我们先来看看构造函数。其主要职责为：使用默认的基础阀门创建标准Engine组件。
/**
* Create a new StandardEngine component with the default basic Valve.
*/
public StandardEngine() {
super();
pipeline.setBasic(new StandardEngineValve());
/* Set the jmvRoute using the system property jvmRoute */
try {
setJvmRoute(System.getProperty("jvmRoute"));
} catch(Exception ex) {
log.warn(sm.getString("standardEngine.jvmRouteFail"));
}
// By default, the engine will hold the reloading thread
backgroundProcessorDelay = 10;
}
接下来我们看看StandardEngineValve的invoke()方法。该方法主要是选择合适的Host，然后调用Host中pipeline的第一个Valve的invoke()方法。
public final void invoke(Request request, Response response)
throws IOException, ServletException {
// Select the Host to be used for this Request
Host host = request.getHost();
if (host == null) {
response.sendError
(HttpServletResponse.SC_BAD_REQUEST,
sm.getString("standardEngine.noHost",
request.getServerName()));
return;
}
if (request.isAsyncSupported()) {
request.setAsyncSupported(host.getPipeline().isAsyncSupported());
}
// Ask this Host to process this request
host.getPipeline().getFirst().invoke(request, response);
}
其实Engine主要的功能就上面的两个方法。额外还有一些细小的功能，我们放在Engine拾遗里面分析。
Engine拾遗
addChild只会让Host作为其子容器添加到子容器列表中，非Host在添加子容器的时候会抛异常！
@Override
public void addChild(Container child) {
if (!(child instanceof Host))
throw new IllegalArgumentException
(sm.getString("standardEngine.notHost"));
super.addChild(child);
}
因为Engine为tomcat中的顶级容器，所以没有父容器了，当调用setParent时将抛出异常。
@Override
public void setParent(Container container) {
throw new IllegalArgumentException
(sm.getString("standardEngine.notParent"));
}
getParentClassLoader和Service中同名的该方法一样，先判断父类加载器是否存在，存在的话直接返回。因为Engine肯定属于某一个Service，所以service不会为null，在父类加载器不存在的情况下，会获取Service的parentClassLoader。
@Override
public ClassLoader getParentClassLoader() {
if (parentClassLoader != null)
return (parentClassLoader);
if (service != null) {
return (service.getParentClassLoader());
}
return (ClassLoader.getSystemClassLoader());
}
Host
分析Host的时候，我们从Host的构造函数入手，该方法主要是设置基础阀门。
public StandardHost() {
super();
pipeline.setBasic(new StandardHostValve());
}
接下来我们看看基础阀门StandardHostValve的invoke()方法，该方法比较长。
@Override
public final void invoke(Request request, Response response)
throws IOException, ServletException {
// Select the Context to be used for this Request
Context context = request.getContext();
if (context == null) {
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
sm.getString("standardHost.noContext"));
return;
}
if (request.isAsyncSupported()) {
request.setAsyncSupported(context.getPipeline().isAsyncSupported());
}
boolean asyncAtStart = request.isAsync();
boolean asyncDispatching = request.isAsyncDispatching();
try {
context.bind(Globals.IS_SECURITY_ENABLED, MY_CLASSLOADER);
if (!asyncAtStart && !context.fireRequestInitEvent(request.getRequest())) {
// Don't fire listeners during async processing (the listener
// fired for the request that called startAsync()).
// If a request init listener throws an exception, the request
// is aborted.
return;
}
// Ask this Context to process this request. Requests that are in
// async mode and are not being dispatched to this resource must be
// in error and have been routed here to check for application
// defined error pages.
try {
if (!asyncAtStart || asyncDispatching) {
context.getPipeline().getFirst().invoke(request, response);
} else {
// Make sure this request/response is here because an error
// report is required.
if (!response.isErrorReportRequired()) {
throw new IllegalStateException(sm.getString("standardHost.asyncStateError"));
}
}
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
container.getLogger().error("Exception Processing " + request.getRequestURI(), t);
// If a new error occurred while trying to report a previous
// error allow the original error to be reported.
if (!response.isErrorReportRequired()) {
request.setAttribute(RequestDispatcher.ERROR_EXCEPTION, t);
throwable(request, response, t);
}
}
// Now that the request/response pair is back under container
// control lift the suspension so that the error handling can
// complete and/or the container can flush any remaining data
response.setSuspended(false);
Throwable t = (Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION);
// Protect against NPEs if the context was destroyed during a
// long running request.
if (!context.getState().isAvailable()) {
return;
}
// Look for (and render if found) an application level error page
if (response.isErrorReportRequired()) {
if (t != null) {
throwable(request, response, t);
} else {
status(request, response);
}
}
if (!request.isAsync() && !asyncAtStart) {
context.fireRequestDestroyEvent(request.getRequest());
}
} finally {
// Access a session (if present) to update last accessed time, based
// on a strict interpretation of the specification
if (ACCESS_SESSION) {
request.getSession(false);
}
context.unbind(Globals.IS_SECURITY_ENABLED, MY_CLASSLOADER);
}
}
invoke()方法关键的地方就一行-context.getPipeline().getFirst().invoke(request, response);，正是调用Context 中pipeline的第一个Valve的invoke()方法。作用和Engine下基础阀门的invoke()调用类似。
Context
接下来我们分析一下Context的实现org.apache.catalina.core.StandardContext。
先来看看构造方法，该方法用于设置Context.pipeline的基础阀门。
public StandardContext() {
super();
pipeline.setBasic(new StandardContextValve());
broadcaster = new NotificationBroadcasterSupport();
// Set defaults
if (!Globals.STRICT_SERVLET_COMPLIANCE) {
// Strict servlet compliance requires all extension mapped servlets
// to be checked against welcome files
resourceOnlyServlets.add("jsp");
}
}
接下来我们分析一下StandardContextValve的invoke()方法。在Wrapper不为null的时候，调用下面的代码：wrapper.getPipeline().getFirst().invoke(request, response);。
@Override
public final void invoke(Request request, Response response)
throws IOException, ServletException {
// Disallow any direct access to resources under WEB-INF or META-INF
MessageBytes requestPathMB = request.getRequestPathMB();
if ((requestPathMB.startsWithIgnoreCase("/META-INF/", 0))
|| (requestPathMB.equalsIgnoreCase("/META-INF"))
|| (requestPathMB.startsWithIgnoreCase("/WEB-INF/", 0))
|| (requestPathMB.equalsIgnoreCase("/WEB-INF"))) {
response.sendError(HttpServletResponse.SC_NOT_FOUND);
return;
}
// Select the Wrapper to be used for this Request
Wrapper wrapper = request.getWrapper();
if (wrapper == null || wrapper.isUnavailable()) {
response.sendError(HttpServletResponse.SC_NOT_FOUND);
return;
}
// Acknowledge the request
try {
response.sendAcknowledgement();
} catch (IOException ioe) {
container.getLogger().error(sm.getString(
"standardContextValve.acknowledgeException"), ioe);
request.setAttribute(RequestDispatcher.ERROR_EXCEPTION, ioe);
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
return;
}
if (request.isAsyncSupported()) {
request.setAsyncSupported(wrapper.getPipeline().isAsyncSupported());
}
wrapper.getPipeline().getFirst().invoke(request, response);
}
Wrapper
Wrapper是一个Servlet的包装，我们先来看看构造方法。主要作用就是设置基础阀门StandardWrapperValve。
public StandardWrapper() {
super();
swValve=new StandardWrapperValve();
pipeline.setBasic(swValve);
broadcaster = new NotificationBroadcasterSupport();
}
接下来我们看看StandardWrapperValve的invoke()方法。该方法非常长，已经超过了200行。
前方高能！
@Override
public final void invoke(Request request, Response response)
throws IOException, ServletException {
// Initialize local variables we may need
boolean unavailable = false;
Throwable throwable = null;
// This should be a Request attribute...
long t1=System.currentTimeMillis();
requestCount.incrementAndGet();
StandardWrapper wrapper = (StandardWrapper) getContainer();
Servlet servlet = null;
Context context = (Context) wrapper.getParent();
// Check for the application being marked unavailable
if (!context.getState().isAvailable()) {
response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardContext.isUnavailable"));
unavailable = true;
}
// Check for the servlet being marked unavailable
if (!unavailable && wrapper.isUnavailable()) {
container.getLogger().info(sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
long available = wrapper.getAvailable();
if ((available > 0L) && (available < Long.MAX_VALUE)) {
response.setDateHeader("Retry-After", available);
response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
} else if (available == Long.MAX_VALUE) {
response.sendError(HttpServletResponse.SC_NOT_FOUND,
sm.getString("standardWrapper.notFound",
wrapper.getName()));
}
unavailable = true;
}
// Allocate a servlet instance to process this request
try {
// 关键点1：这儿调用Wrapper的allocate()方法分配一个Servlet实例
if (!unavailable) {
servlet = wrapper.allocate();
}
} catch (UnavailableException e) {
container.getLogger().error(
sm.getString("standardWrapper.allocateException",
wrapper.getName()), e);
long available = wrapper.getAvailable();
if ((available > 0L) && (available < Long.MAX_VALUE)) {
response.setDateHeader("Retry-After", available);
response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
} else if (available == Long.MAX_VALUE) {
response.sendError(HttpServletResponse.SC_NOT_FOUND,
sm.getString("standardWrapper.notFound",
wrapper.getName()));
}
} catch (ServletException e) {
container.getLogger().error(sm.getString("standardWrapper.allocateException",
wrapper.getName()), StandardWrapper.getRootCause(e));
throwable = e;
exception(request, response, e);
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
container.getLogger().error(sm.getString("standardWrapper.allocateException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
servlet = null;
}
MessageBytes requestPathMB = request.getRequestPathMB();
DispatcherType dispatcherType = DispatcherType.REQUEST;
if (request.getDispatcherType()==DispatcherType.ASYNC) dispatcherType = DispatcherType.ASYNC;
request.setAttribute(Globals.DISPATCHER_TYPE_ATTR,dispatcherType);
request.setAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR,
requestPathMB);
// Create the filter chain for this request
// 关键点2，创建过滤器链，类似于Pipeline的功能
ApplicationFilterChain filterChain =
ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);
// Call the filter chain for this request
// NOTE: This also calls the servlet's service() method
try {
if ((servlet != null) && (filterChain != null)) {
// Swallow output if needed
if (context.getSwallowOutput()) {
try {
SystemLogHandler.startCapture();
if (request.isAsyncDispatching()) {
request.getAsyncContextInternal().doInternalDispatch();
} else {
// 关键点3，调用过滤器链的doFilter，最终会调用到Servlet的service方法
filterChain.doFilter(request.getRequest(),
response.getResponse());
}
} finally {
String log = SystemLogHandler.stopCapture();
if (log != null && log.length() > 0) {
context.getLogger().info(log);
}
}
} else {
if (request.isAsyncDispatching()) {
request.getAsyncContextInternal().doInternalDispatch();
} else {
// 关键点3，调用过滤器链的doFilter，最终会调用到Servlet的service方法
filterChain.doFilter
(request.getRequest(), response.getResponse());
}
}
}
} catch (ClientAbortException e) {
throwable = e;
exception(request, response, e);
} catch (IOException e) {
container.getLogger().error(sm.getString(
"standardWrapper.serviceException", wrapper.getName(),
context.getName()), e);
throwable = e;
exception(request, response, e);
} catch (UnavailableException e) {
container.getLogger().error(sm.getString(
"standardWrapper.serviceException", wrapper.getName(),
context.getName()), e);
// throwable = e;
// exception(request, response, e);
wrapper.unavailable(e);
long available = wrapper.getAvailable();
if ((available > 0L) && (available < Long.MAX_VALUE)) {
response.setDateHeader("Retry-After", available);
response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
} else if (available == Long.MAX_VALUE) {
response.sendError(HttpServletResponse.SC_NOT_FOUND,
sm.getString("standardWrapper.notFound",
wrapper.getName()));
}
// Do not save exception in 'throwable', because we
// do not want to do exception(request, response, e) processing
} catch (ServletException e) {
Throwable rootCause = StandardWrapper.getRootCause(e);
if (!(rootCause instanceof ClientAbortException)) {
container.getLogger().error(sm.getString(
"standardWrapper.serviceExceptionRoot",
wrapper.getName(), context.getName(), e.getMessage()),
rootCause);
}
throwable = e;
exception(request, response, e);
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
container.getLogger().error(sm.getString(
"standardWrapper.serviceException", wrapper.getName(),
context.getName()), e);
throwable = e;
exception(request, response, e);
}
// Release the filter chain (if any) for this request
// 关键点4，释放掉过滤器链及其相关资源
if (filterChain != null) {
filterChain.release();
}
// 关键点5，释放掉Servlet及相关资源
// Deallocate the allocated servlet instance
try {
if (servlet != null) {
wrapper.deallocate(servlet);
}
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
container.getLogger().error(sm.getString("standardWrapper.deallocateException",
wrapper.getName()), e);
if (throwable == null) {
throwable = e;
exception(request, response, e);
}
}
// If this servlet has been marked permanently unavailable,
// unload it and release this instance
// 关键点6，如果servlet被标记为永远不可达，则需要卸载掉它，并释放这个servlet实例
try {
if ((servlet != null) &&
(wrapper.getAvailable() == Long.MAX_VALUE)) {
wrapper.unload();
}
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
container.getLogger().error(sm.getString("standardWrapper.unloadException",
wrapper.getName()), e);
if (throwable == null) {
throwable = e;
exception(request, response, e);
}
}
long t2=System.currentTimeMillis();
long time=t2-t1;
processingTime += time;
if( time > maxTime) maxTime=time;
if( time < minTime) minTime=time;
}
通过阅读源码，我们发现了几个关键点。现罗列如下，后面我们会逐一分析这些关键点相关的源码。
- 关键点1：这儿调用Wrapper的allocate()方法分配一个Servlet实例
- 关键点2，创建过滤器链，类似于Pipeline的功能
- 关键点3，调用过滤器链的doFilter，最终会调用到Servlet的service方法
- 关键点4，释放掉过滤器链及其相关资源
- 关键点5，释放掉Servlet及相关资源
- 关键点6，如果servlet被标记为永远不可达，则需要卸载掉它，并释放这个servlet实例
关键点1 - Wrapper分配Servlet实例
我们来分析一下Wrapper.allocate()方法
@Override
public Servlet allocate() throws ServletException {
// If we are currently unloading this servlet, throw an exception
// 卸载过程中，不能分配Servlet
if (unloading) {
throw new ServletException(sm.getString("standardWrapper.unloading", getName()));
}
boolean newInstance = false;
// If not SingleThreadedModel, return the same instance every time
// 如果Wrapper没有实现SingleThreadedModel，则每次都会返回同一个Servlet
if (!singleThreadModel) {
// Load and initialize our instance if necessary
// 实例为null或者实例还未初始化，使用synchronized来保证并发时的原子性
if (instance == null || !instanceInitialized) {
synchronized (this) {
if (instance == null) {
try {
if (log.isDebugEnabled()) {
log.debug("Allocating non-STM instance");
}
// Note: We don't know if the Servlet implements
// SingleThreadModel until we have loaded it.
// 加载Servlet
instance = loadServlet();
newInstance = true;
if (!singleThreadModel) {
// For non-STM, increment here to prevent a race
// condition with unload. Bug 43683, test case
// #3
countAllocated.incrementAndGet();
}
} catch (ServletException e) {
throw e;
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("standardWrapper.allocate"), e);
}
}
// 初始化Servlet
if (!instanceInitialized) {
initServlet(instance);
}
}
}
if (singleThreadModel) {
if (newInstance) {
// Have to do this outside of the sync above to prevent a
// possible deadlock
synchronized (instancePool) {
instancePool.push(instance);
nInstances++;
}
}
}
// 非单线程模型，直接返回已经创建的Servlet，也就是说，这种情况下只会创建一个Servlet
else {
if (log.isTraceEnabled()) {
log.trace(" Returning non-STM instance");
}
// For new instances, count will have been incremented at the
// time of creation
if (!newInstance) {
countAllocated.incrementAndGet();
}
return instance;
}
}
// 如果是单线程模式，则使用servlet对象池技术来加载多个Servlet
synchronized (instancePool) {
while (countAllocated.get() >= nInstances) {
// Allocate a new instance if possible, or else wait
if (nInstances < maxInstances) {
try {
instancePool.push(loadServlet());
nInstances++;
} catch (ServletException e) {
throw e;
} catch (Throwable e) {
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("standardWrapper.allocate"), e);
}
} else {
try {
instancePool.wait();
} catch (InterruptedException e) {
// Ignore
}
}
}
if (log.isTraceEnabled()) {
log.trace(" Returning allocated STM instance");
}
countAllocated.incrementAndGet();
return instancePool.pop();
}
}
总结下来，注意以下几点即可：
- 卸载过程中，不能分配Servlet
- 如果不是单线程模式，则每次都会返回同一个Servlet（默认Servlet实现方式）
-
Servlet实例为null或者Servlet实例还未初始化，使用synchronized来保证并发时的原子性 - 如果是单线程模式，则使用servlet对象池技术来加载多个Servlet
接下来我们看看loadServlet()方法
public synchronized Servlet loadServlet() throws ServletException {
// Nothing to do if we already have an instance or an instance pool
if (!singleThreadModel && (instance != null))
return instance;
PrintStream out = System.out;
if (swallowOutput) {
SystemLogHandler.startCapture();
}
Servlet servlet;
try {
long t1=System.currentTimeMillis();
// Complain if no servlet class has been specified
if (servletClass == null) {
unavailable(null);
throw new ServletException
(sm.getString("standardWrapper.notClass", getName()));
}
// 关键的地方，就是通过实例管理器，创建Servlet实例，而实例管理器是通过特殊的类加载器来加载给定的类
InstanceManager instanceManager = ((StandardContext)getParent()).getInstanceManager();
try {
servlet = (Servlet) instanceManager.newInstance(servletClass);
} catch (ClassCastException e) {
unavailable(null);
// Restore the context ClassLoader
throw new ServletException
(sm.getString("standardWrapper.notServlet", servletClass), e);
} catch (Throwable e) {
e = ExceptionUtils.unwrapInvocationTargetException(e);
ExceptionUtils.handleThrowable(e);
unavailable(null);
// Added extra log statement for Bugzilla 36630:
// https://bz.apache.org/bugzilla/show_bug.cgi?id=36630
if(log.isDebugEnabled()) {
log.debug(sm.getString("standardWrapper.instantiate", servletClass), e);
}
// Restore the context ClassLoader
throw new ServletException
(sm.getString("standardWrapper.instantiate", servletClass), e);
}
if (multipartConfigElement == null) {
MultipartConfig annotation =
servlet.getClass().getAnnotation(MultipartConfig.class);
if (annotation != null) {
multipartConfigElement =
new MultipartConfigElement(annotation);
}
}
// Special handling for ContainerServlet instances
// Note: The InstanceManager checks if the application is permitted
// to load ContainerServlets
if (servlet instanceof ContainerServlet) {
((ContainerServlet) servlet).setWrapper(this);
}
classLoadTime=(int) (System.currentTimeMillis() -t1);
if (servlet instanceof SingleThreadModel) {
if (instancePool == null) {
instancePool = new Stack<>();
}
singleThreadModel = true;
}
// 调用Servlet的init方法
initServlet(servlet);
fireContainerEvent("load", this);
loadTime=System.currentTimeMillis() -t1;
} finally {
if (swallowOutput) {
String log = SystemLogHandler.stopCapture();
if (log != null && log.length() > 0) {
if (getServletContext() != null) {
getServletContext().log(log);
} else {
out.println(log);
}
}
}
}
return servlet;
}
关键的地方有两个：
- 通过实例管理器，创建Servlet实例，而实例管理器是通过特殊的类加载器来加载给定的类
- 调用Servlet的init方法
关键点2 - 创建过滤器链
创建过滤器链是调用的org.apache.catalina.core.ApplicationFilterFactory的createFilterChain()方法。我们来分析一下这个方法。该方法需要注意的地方已经在代码的comments里面说明了。
public static ApplicationFilterChain createFilterChain(ServletRequest request,
Wrapper wrapper, Servlet servlet) {
// If there is no servlet to execute, return null
if (servlet == null)
return null;
// Create and initialize a filter chain object
// 1. 如果加密打开了，则可能会多次调用这个方法
// 2. 为了避免重复生成filterChain对象，所以会将filterChain对象放在Request里面进行缓存
ApplicationFilterChain filterChain = null;
if (request instanceof Request) {
Request req = (Request) request;
if (Globals.IS_SECURITY_ENABLED) {
// Security: Do not recycle
filterChain = new ApplicationFilterChain();
} else {
filterChain = (ApplicationFilterChain) req.getFilterChain();
if (filterChain == null) {
filterChain = new ApplicationFilterChain();
req.setFilterChain(filterChain);
}
}
} else {
// Request dispatcher in use
filterChain = new ApplicationFilterChain();
}
filterChain.setServlet(servlet);
filterChain.setServletSupportsAsync(wrapper.isAsyncSupported());
// Acquire the filter mappings for this Context
StandardContext context = (StandardContext) wrapper.getParent();
// 从这儿看出过滤器链对象里面的元素是根据Context里面的filterMaps来生成的
FilterMap filterMaps[] = context.findFilterMaps();
// If there are no filter mappings, we are done
if ((filterMaps == null) || (filterMaps.length == 0))
return (filterChain);
// Acquire the information we will need to match filter mappings
DispatcherType dispatcher =
(DispatcherType) request.getAttribute(Globals.DISPATCHER_TYPE_ATTR);
String requestPath = null;
Object attribute = request.getAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR);
if (attribute != null){
requestPath = attribute.toString();
}
String servletName = wrapper.getName();
// Add the relevant path-mapped filters to this filter chain
// 类型和路径都匹配的情况下，将context.filterConfig放到过滤器链里面
for (int i = 0; i < filterMaps.length; i++) {
if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
continue;
}
if (!matchFiltersURL(filterMaps[i], requestPath))
continue;
ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
context.findFilterConfig(filterMaps[i].getFilterName());
if (filterConfig == null) {
// FIXME - log configuration problem
continue;
}
filterChain.addFilter(filterConfig);
}
// Add filters that match on servlet name second
// 类型和servlet名称都匹配的情况下，将context.filterConfig放到过滤器链里面
for (int i = 0; i < filterMaps.length; i++) {
if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
continue;
}
if (!matchFiltersServlet(filterMaps[i], servletName))
continue;
ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
context.findFilterConfig(filterMaps[i].getFilterName());
if (filterConfig == null) {
// FIXME - log configuration problem
continue;
}
filterChain.addFilter(filterConfig);
}
// Return the completed filter chain
return filterChain;
}
关键点3 - 调用过滤器链的doFilter
// 1. `doFilter`方法会对异常进行重新抛出，也就是将检查性异常变更为运行时异常
// 2. `doFilter`如果访问授权开关打开，则会有访问鉴权
// 3. `doFilter`最终会调用internalDoFilter方法
@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<Void>() {
@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);
}
}
// 1. `internalDoFilter`方法通过pos和n来调用过滤器链里面的每个过滤器。pos表示当前的过滤器下标，n表示总的过滤器数量
// 2. `internalDoFilter`方法最终会调用servlet.service()方法
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);
}
}
}
-
doFilter方法会对异常进行重新抛出，也就是将检查性异常变更为运行时异常 -
doFilter如果访问授权开关打开，则会有访问鉴权 -
doFilter最终会调用internalDoFilter方法 -
internalDoFilter方法通过pos和n来调用过滤器链里面的每个过滤器。pos表示当前的过滤器下标，n表示总的过滤器数量 -
internalDoFilter方法最终会调用servlet.service()方法
关键点4 - 释放掉过滤器链及其相关资源
调用的FilterChain.release()方法，我们来分析一下，逻辑较之前的代码简单很多。
- 过滤器置为null，help gc
- n和pos置为0
- servlet置为null
void release() {
for (int i = 0; i < n; i++) {
filters[i] = null;
}
n = 0;
pos = 0;
servlet = null;
servletSupportsAsync = false;
}
关键点5 - 释放掉Servlet及相关资源
调用的是StandardWrapper的deallocate()方法，我们来分析一下。
- 如果是非单线程模型，直接返回。
- 如果是单线程模型，则将Servlet对象放回对象池。
@Override
public void deallocate(Servlet servlet) throws ServletException {
// If not SingleThreadModel, no action is required
if (!singleThreadModel) {
countAllocated.decrementAndGet();
return;
}
// Unlock and free this instance
synchronized (instancePool) {
countAllocated.decrementAndGet();
instancePool.push(servlet);
instancePool.notify();
}
}
关键点6 - 卸载servlet实例
该逻辑调用的是StandardWrapper的unload()方法，该方法非常得长！
前方高能！
@Override
public synchronized void unload() throws ServletException {
// Nothing to do if we have never loaded the instance
// 如果实例未加载，那么什么事情都不做
if (!singleThreadModel && (instance == null))
return;
// 设置正在卸载标记为true
unloading = true;
// Loaf a while if the current instance is allocated
// (possibly more than once if non-STM)
// 当前有实例已经分配了，则需要等待分配的实例完成处理。所以这儿会有线程休眠操作
if (countAllocated.get() > 0) {
int nRetries = 0;
long delay = unloadDelay / 20;
while ((nRetries < 21) && (countAllocated.get() > 0)) {
if ((nRetries % 10) == 0) {
log.info(sm.getString("standardWrapper.waiting",
countAllocated.toString(),
getName()));
}
try {
Thread.sleep(delay);
} catch (InterruptedException e) {
// Ignore
}
nRetries++;
}
}
if (instanceInitialized) {
PrintStream out = System.out;
if (swallowOutput) {
SystemLogHandler.startCapture();
}
// Call the servlet destroy() method
// 如果为非单线程模型，则调用servlet的destroy方法
try {
if( Globals.IS_SECURITY_ENABLED) {
try {
SecurityUtil.doAsPrivilege("destroy", instance);
} finally {
SecurityUtil.remove(instance);
}
} else {
instance.destroy();
}
} catch (Throwable t) {
t = ExceptionUtils.unwrapInvocationTargetException(t);
ExceptionUtils.handleThrowable(t);
instance = null;
instancePool = null;
nInstances = 0;
fireContainerEvent("unload", this);
unloading = false;
throw new ServletException
(sm.getString("standardWrapper.destroyException", getName()),
t);
} finally {
// Annotation processing
if (!((Context) getParent()).getIgnoreAnnotations()) {
try {
((Context)getParent()).getInstanceManager().destroyInstance(instance);
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
log.error(sm.getString("standardWrapper.destroyInstance", getName()), t);
}
}
// Write captured output
if (swallowOutput) {
String log = SystemLogHandler.stopCapture();
if (log != null && log.length() > 0) {
if (getServletContext() != null) {
getServletContext().log(log);
} else {
out.println(log);
}
}
}
}
}
// Deregister the destroyed instance
// 将销毁的servlet从jmx中unregister操作
instance = null;
instanceInitialized = false;
if (isJspServlet && jspMonitorON != null ) {
Registry.getRegistry(null, null).unregisterComponent(jspMonitorON);
}
// 如果是单线程模型，需要将servlet进行出栈(pop）操作，并调用servlet的destory方法
if (singleThreadModel && (instancePool != null)) {
try {
while (!instancePool.isEmpty()) {
Servlet s = instancePool.pop();
if (Globals.IS_SECURITY_ENABLED) {
try {
SecurityUtil.doAsPrivilege("destroy", s);
} finally {
SecurityUtil.remove(s);
}
} else {
s.destroy();
}
// Annotation processing
if (!((Context) getParent()).getIgnoreAnnotations()) {
((StandardContext)getParent()).getInstanceManager().destroyInstance(s);
}
}
} catch (Throwable t) {
t = ExceptionUtils.unwrapInvocationTargetException(t);
ExceptionUtils.handleThrowable(t);
instancePool = null;
nInstances = 0;
unloading = false;
fireContainerEvent("unload", this);
throw new ServletException
(sm.getString("standardWrapper.destroyException",
getName()), t);
}
instancePool = null;
nInstances = 0;
}
singleThreadModel = false;
// 卸载完成，卸载标记置为false
unloading = false;
// 发送容器unload事件
fireContainerEvent("unload", this);
}
总结一下该方法的逻辑：
- 如果实例未加载，那么什么事情都不做
- 设置正在卸载标记为true
- 当前有实例已经分配了，则需要等待分配的实例完成处理。所以这儿会有线程休眠操作
- 如果为非单线程模型，则调用servlet的destroy方法
- 将销毁的servlet从jmx中unregister操作
- 如果是单线程模型，需要将servlet进行出栈(pop）操作，并调用servlet的destory方法
- 卸载完成，卸载标记置为false
- 发送容器unload事件
总结
在tomcat中，容器(Container)分4种，Engine、Host、Context和Wrapper。Engine是容器的入口，Host是虚拟主机，Context是应用服务，Wrapper为具体的Servlet。
从外层的Engine到内层的Wrapper，我们逐层进行了详细分析。尤其是对Wrapper进行了详细分析，包括Servlet的加载、过滤器链的生成、过滤器链的调用等等各项复杂的设计和代码。
本文主要对Container进行了分析，关于其他的组件我们只是一笔带过。
至此，楼主已经完全清楚了容器的初始化、加载和运行的机制。那么，你呢？是否也完全明白了呢？
参考链接
https://www.ramkitech.com/2012/02/understanding-virtual-host-concept-in.html
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