本文简单解读Tomcat的Context和Wrapper组件。 本系列的源码工程为：HowTomcatWorksSourceCode。
本文目录为：
一、概述
Context表示一个Web应用,其默认实现类是StandardContext，其对应的阀是StandardContextValve
Wrapper表示一个单独的Servlet，比如普通的经典SpringMVC应用配置的DispatcherServlet和Tomcat自带的DefaultServlet，Wrapper对应的阀则是StandardWrapperValve。
二、源码解读
2，1 StandardContext
解读StandardContext可以从一个单独的应用的’web.xml’谈起。
比如：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd" >
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
version="3.1">
<display-name>SpringMVC Demo</display-name>
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath*:springmvc-context.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<servlet>
<servlet-name>springmvc-demo</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:springmvc-servlet.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>springmvc-demo</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
<filter>
<filter-name>encodingFilter</filter-name>
<filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>UTF-8</param-value>
</init-param>
<init-param>
<param-name>forceEncoding</param-name>
<param-value>true</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>encodingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
</web-app>
web的xml的配置基本和StandardContext的属性是保持一致的，比如这里是servlet-mapping：
/**
* The servlet mappings for this web application, keyed by matching pattern.
*/
private Map<String,String> servletMappings = new HashMap<>();
以及loadOnStartup的操作：
/**
* Load and initialize all servlets marked "load on startup" in the web application deployment descriptor.
*
* @param children Array of wrappers for all currently defined servlets (including those not declared load on
* startup)
*
* @return <code>true</code> if load on startup was considered successful
*/
public boolean loadOnStartup(Container children[]) {
// Collect "load on startup" servlets that need to be initialized
TreeMap<Integer, ArrayList<Wrapper>> map = new TreeMap<>();
for (Container child : children) {
Wrapper wrapper = (Wrapper) child;
int loadOnStartup = wrapper.getLoadOnStartup();
if (loadOnStartup < 0) {
continue;
}
Integer key = Integer.valueOf(loadOnStartup);
map.computeIfAbsent(key, k -> new ArrayList<>()).add(wrapper);
}
//.......
}
总结的说就是解析web.xml来生成Context的各项属性。
要了解的是，一个Context是可以有多个Servlet的。
下面再来解读一些StandardContext的源码,这里init、start、stop不作详细解读了。
在Context的启动方法中启动了很多组件，这里列举一个监听器的启动代码：
public boolean listenerStart() {
// 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];
//创建监听器
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;
}
List<Object> eventListeners = new ArrayList<>();
List<Object> lifecycleListeners = new ArrayList<>();
for (Object result : results) {
if ((result instanceof ServletContextAttributeListener) ||
(result instanceof ServletRequestAttributeListener) || (result instanceof ServletRequestListener) ||
(result instanceof HttpSessionIdListener) || (result instanceof HttpSessionAttributeListener)) {
eventListeners.add(result);
}
if ((result instanceof ServletContextListener) || (result instanceof HttpSessionListener)) {
lifecycleListeners.add(result);
}
}
eventListeners.addAll(Arrays.asList(getApplicationEventListeners()));
setApplicationEventListeners(eventListeners.toArray());
for (Object lifecycleListener : getApplicationLifecycleListeners()) {
lifecycleListeners.add(lifecycleListener);
if (lifecycleListener instanceof ServletContextListener) {
noPluggabilityListeners.add(lifecycleListener);
}
}
setApplicationLifecycleListeners(lifecycleListeners.toArray());
// 获得ServletContext
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 (Object instance : instances) {
if (!(instance instanceof ServletContextListener)) {
continue;
}
ServletContextListener listener = (ServletContextListener) instance;
try {
fireContainerEvent("beforeContextInitialized", listener);
//监听器处理对应的事件
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", instance.getClass().getName()), t);
ok = false;
}
}
return ok;
}
上面代码创建了监听器并执行了对应的监听事件。
然后我们关注一下Context的子容器Wrapper(Servlet)相关的源码：
@Override
public void addChild(Container child) {
if (log.isDebugEnabled()) {
log.debug("Add child " + child + " " + this);
}
//添加子容器Wrapper(Servlet)
synchronized (children) {
if (children.get(child.getName()) != null) {
throw new IllegalArgumentException(sm.getString("containerBase.child.notUnique", child.getName()));
}
child.setParent(this); // May throw IAE
children.put(child.getName(), child);
}
fireContainerEvent(ADD_CHILD_EVENT, child);
// Start child
// Don't do this inside sync block - start can be a slow process and
// locking the children object can cause problems elsewhere
try {
//子容器Wrapper(Servlet)启动
if ((getState().isAvailable() || LifecycleState.STARTING_PREP.equals(getState())) && startChildren) {
child.start();
}
} catch (LifecycleException e) {
throw new IllegalStateException(sm.getString("containerBase.child.start"), e);
}
}
public boolean loadOnStartup(Container children[]) {
// Collect "load on startup" servlets that need to be initialized
TreeMap<Integer,ArrayList<Wrapper>> map = new TreeMap<>();
for (Container child : children) {
Wrapper wrapper = (Wrapper) child;
int loadOnStartup = wrapper.getLoadOnStartup();
if (loadOnStartup < 0) {
continue;
}
Integer key = Integer.valueOf(loadOnStartup);
map.computeIfAbsent(key, k -> new ArrayList<>()).add(wrapper);
}
// Load the collected "load on startup" servlets
for (ArrayList<Wrapper> list : map.values()) {
for (Wrapper wrapper : list) {
try {
//执行子容器Wrapper(Servlet)的load操作
wrapper.load();
} catch (ServletException e) {
getLogger().error(
sm.getString("standardContext.loadOnStartup.loadException", getName(), wrapper.getName()),
StandardWrapper.getRootCause(e));
// NOTE: load errors (including a servlet that throws
// UnavailableException from the init() method) are NOT
// fatal to application startup
// unless failCtxIfServletStartFails="true" is specified
if (getComputedFailCtxIfServletStartFails()) {
return false;
}
}
}
}
return true;
}
上面的源码创建Wrapper，并进行子容器Wrapper(Servlet)的load操作。
我们继续来到StandardWrapper的源码。
2.2 StandardWrapper
Wrapper表示一个独立的Servlet,StandardWrapper是其默认实现类。
StandardWrapper最主要的功能就载入Servlet类，实例化并执行初始化(init)方法。但实际处理一个Http请求是由StandardWrapperValve的invoke处理的，而不是StandardWrapper。
StandardWrapperValve通过StandardWrapper的allocate方法获得构造好的Servlet实例，其源码如下：
/**
* Allocate an initialized instance of this Servlet that is ready to have its <code>service()</code> method called.
*
* @exception ServletException if the servlet init() method threw an exception
* @exception ServletException if a loading error occurs
*/
@Override
public Servlet allocate() throws ServletException {
// If we are currently unloading this servlet, throw an exception
if (unloading) {
throw new ServletException(sm.getString("standardWrapper.unloading", getName()));
}
boolean newInstance = false;
// Load and initialize our instance if necessary
if (instance == null || !instanceInitialized) {
synchronized (this) {
if (instance == null) {
try {
if (log.isDebugEnabled()) {
log.debug("Allocating instance");
}
instance = loadServlet();
newInstance = true;
// 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);
}
}
if (!instanceInitialized) {
initServlet(instance);
}
}
}
if (log.isTraceEnabled()) {
log.trace(" Returning instance");
}
// For new instances, count will have been incremented at the
// time of creation
if (!newInstance) {
countAllocated.incrementAndGet();
}
return instance;
}
加载(实例化)和初始化(init)Servlet的源码如下:
/**
* Load and initialize an instance of this servlet, if there is not already an initialized instance. This can be
* used, for example, to load servlets that are marked in the deployment descriptor to be loaded at server startup
* time.
*
* @return the loaded Servlet instance
*
* @throws ServletException for a Servlet load error
*/
public synchronized Servlet loadServlet() throws ServletException {
// Nothing to do if we already have an instance or an instance pool
if (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()));
}
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);
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;
}
private synchronized void initServlet(Servlet servlet) throws ServletException {
if (instanceInitialized) {
return;
}
// Call the initialization method of this servlet
try {
servlet.init(facade);
instanceInitialized = true;
} catch (UnavailableException f) {
unavailable(f);
throw f;
} catch (ServletException f) {
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw f;
} catch (Throwable f) {
ExceptionUtils.handleThrowable(f);
getServletContext().log(sm.getString("standardWrapper.initException", getName()), f);
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw new ServletException(sm.getString("standardWrapper.initException", getName()), f);
}
}
上面代码主要就是使用反射创建Servlet然后执行init方法。
我们回到DispatcherServlet的顶层接口Servlet的定义：
public interface Servlet {
public void init(ServletConfig config) throws ServletException;
public ServletConfig getServletConfig();
//......
}
从这里联系我们知道了Servlet的创建和初始化是由Servlet容器来执行的，以及ServletConfig也是容器创建的。而这里的ServletConfig类的对象facade最终指向了ApplicationContextFacade。 ApplicationContextFacade也实现了ServletContext。
三、简单测试
这里以原书第11章的源码坐下简单的测试：
public final class Bootstrap {
public static void main(String[] args) {
//invoke: http://localhost:8080/Modern or http://localhost:8080/Primitive
System.setProperty("catalina.base", System.getProperty("user.dir"));
Connector connector = new HttpConnector();
Wrapper wrapper1 = new StandardWrapper();
wrapper1.setName("Primitive");
wrapper1.setServletClass("PrimitiveServlet");
Wrapper wrapper2 = new StandardWrapper();
wrapper2.setName("Modern");
wrapper2.setServletClass("ModernServlet");
Context context = new StandardContext();
// StandardContext's start method adds a default mapper
context.setPath("/myApp");
context.setDocBase("myApp");
LifecycleListener listener = new SimpleContextConfig();
((Lifecycle) context).addLifecycleListener(listener);
context.addChild(wrapper1);
context.addChild(wrapper2);
// for simplicity, we don't add a valve, but you can add
// valves to context or wrapper just as you did in Chapter 6
Loader loader = new WebappLoader();
context.setLoader(loader);
// context.addServletMapping(pattern, name);
context.addServletMapping("/Primitive", "Primitive");
context.addServletMapping("/Modern", "Modern");
// add ContextConfig. This listener is important because it configures
// StandardContext (sets configured to true), otherwise StandardContext
// won't start
connector.setContainer(context);
try {
connector.initialize();
((Lifecycle) connector).start();
((Lifecycle) context).start();
// make the application wait until we press a key.
System.in.read();
((Lifecycle) context).stop();
}
catch (Exception e) {
e.printStackTrace();
}
}
}
上面的测试代码中Context内设置了两个Wrapper，输入http://localhost:8080/Primitive执行测试。
正常成功返回:
HTTP/1.1 200 ok
Content-Type: text/html
<html><h1></h1><body><p>Hello. Roses are red.</p></body></html>
四、参数资料
1.《深入剖析Tomcat》