tomcat(13)Host和Engine容器

【0】README
0.1）本文部分文字描述转自 “how tomcat works”，旨在学习 “tomcat(13)Host和Engine容器” 的基础知识；
0.2）Host容器：若你想在同一个tomcat部署 上运行多个Context 容器的话，就需要使用 Host 容器；它是Context容器的父容器；（干货——若你想在同一个tomcat部署
上运行多个Context 容器的话，就需要使用 Host 容器，Host容器表示包含有一个或多个 Context容器的虚拟主机）
0.3）Engine容器：表示Catalina的整个servlet 引擎。如果使用了Engine容器，那么它总是处于容器层级的最顶层；添加到Engine 容器中的子容器通常是 org.apache.catalina.Host的实现 。默认情况下，tomcat 会使用 Engine容器的，并且有一个Host容器作为其子容器；（干货——Engine表示Catalina的整个servlet
引擎，并且有一个Host容器作为其子容器）




0.4）本文内容概览

step1）首先介绍Host接口及其 相关实现类 StandardHost类， StandardHostMapper 类 和 StandardHostValve类；
step2）接下来使用一个应用程序来说明如何使用Host实例作为一个顶层servlet容器；
step3）介绍Engine接口 及其 相关实现类 StandardEngine 和 StandardEngineValve类，并使用一个app 说明如何使用Engine实例作为顶层 servlet容器；

 0.5）for complete source code, please visit  https://github.com/pacosonTang/HowTomcatWorks/tree/master/chapter13

【1】Host接口（org.apache.catalina.Host）
1）源码如下  

public interface Host extends Container { // org.apache.catalina.Host
public static final String ADD_ALIAS_EVENT = "addAlias";
public static final String REMOVE_ALIAS_EVENT = "removeAlias";
public String getAppBase();
public void setAppBase(String appBase);
public boolean getAutoDeploy();
public void setAutoDeploy(boolean autoDeploy);
public void addDefaultContext(DefaultContext defaultContext);
public DefaultContext getDefaultContext();
public String getName();
public void setName(String name);
public void importDefaultContext(Context context);
public void addAlias(String alias);
public String[] findAliases();
public Context map(String uri);  // highlight line.
public void removeAlias(String alias);
}

对以上代码的分析（Analysis）：Host接口中比较重要的方法是 map()方法，该方法返回一个 用来处理引入的http 请求的Context容器的实例；具体实现在
StandardHost类中；

【2】StandardHost类
1）StandardHost的构造函数

public class StandardHost  extends ContainerBase implements Deployer, Host { //org.apache.catalina.core.StandardHost
public StandardHost() {
super();
pipeline.setBasic(new StandardHostValve());
}

2）调用其start()方法时，会添加两个阀，分别是ErrorReportValve类和ErrorDispatchValve类的实例。start()方法源代码如下：

public synchronized void start() throws LifecycleException {  //org.apache.catalina.core.StandardHost.start().
// Set error report valve
if ((errorReportValveClass != null)
&& (!errorReportValveClass.equals(""))) {
try {
Valve valve = (Valve) Class.forName(errorReportValveClass)
.newInstance();
addValve(valve); // highlight line.
} catch (Throwable t) {
log(sm.getString
("standardHost.invalidErrorReportValveClass",
errorReportValveClass));
}
}
// Set dispatcher valve
addValve(new ErrorDispatcherValve()); // highlight line.
super.start(); // highlight line.
}
// private String errorReportValveClass =  "org.apache.catalina.valves.ErrorReportValve"; // defined in StandardHost

public synchronized void addValve(Valve valve) { //org.apache.catalina.core.ContainerBase.addValve().
pipeline.addValve(valve);
fireContainerEvent(ADD_VALVE_EVENT, valve);
}

3）每当引入一个http请求时都会调用Host.invoke()方法：由于StandardHost类并没有invoke方法，故调用ContainerBase.invoke()方法，接着调用StandardHost实例的基础阀StandardHostValve.invoke()方法；而StandardHostValve.invoke()方法会调用 StandardHost.map()方法来获取相应的Context实例来处理http
请求。
4）StandardHost.map()方法源代码如下：

public Context map(String uri) { //org.apache.catalina.core.StandardHost.map().
if (debug > 0)
log("Mapping request URI '" + uri + "'");
if (uri == null)
return (null);
// Match on the longest possible context path prefix
if (debug > 1)
log("  Trying the longest context path prefix");
Context context = null;
String mapuri = uri;
while (true) {
context = (Context) findChild(mapuri);
if (context != null)
break;
int slash = mapuri.lastIndexOf('/');
if (slash < 0)
break;
mapuri = mapuri.substring(0, slash);
}
// If no Context matches, select the default Context
if (context == null) {
if (debug > 1)
log("  Trying the default context");
context = (Context) findChild("");
}
// Complain if no Context has been selected
if (context == null) {
log(sm.getString("standardHost.mappingError", uri));
return (null);
}
// Return the mapped Context (if any)
if (debug > 0)
log(" Mapped to context '" + context.getPath() + "'");
return (context);
}

Attention）在tomcat4中，
ContainerBase类也有map()方法，而 ContainerBase.map()方法 会调用 StandardHost.map()方法；（而在tomcat5中，映射器组件已经移除，Context实例是通过request对象来获取的）

【3】StandardHostMapper类（org.apache.catalina.core.StandardHostMapper）
1）在tomcat4中， ContainerBase类（即StandardHost的父类）会调用其 addDefaultMapper() 方法创建一个默认映射器。默认映射器的类型由 mapperClass属性的值决定，ContainerBase.addDefaultMapper()方法的源代码如下： 

protected void addDefaultMapper(String mapperClass) { //org.apache.catalina.core.ContainerBase.addDefaultMapper().
if (mapperClass == null)
return;
if (mappers.size() >= 1)
return;
// Instantiate and add a default Mapper
try {
Class clazz = Class.forName(mapperClass);
Mapper mapper = (Mapper) clazz.newInstance();
mapper.setProtocol("http");
addMapper(mapper);
} catch (Exception e) {
log(sm.getString("containerBase.addDefaultMapper", mapperClass),
e);
}
}

对上述代码的分析（Analysis）：

A1）mapperClass的值定义在 StandardHost 类中：

private String mapperClass =  "org.apache.catalina.core.StandardHostMapper"; //defined in org.apache.catalina.core.StandardHost

A2）StandardHost.start()方法：会在方法末尾调用父类的 start()方法（ContainerBase.start()方法），确保默认映射器创建完成；

public synchronized void start() throws LifecycleException { //org.apache.catalina.core.ContainerBase.start().
// Validate and update our current component state
if (started)
throw new LifecycleException
(sm.getString("containerBase.alreadyStarted", logName()));

// Notify our interested LifecycleListeners
lifecycle.fireLifecycleEvent(BEFORE_START_EVENT, null);

addDefaultMapper(this.mapperClass);
started = true;

// Start our subordinate components, if any
if ((loader != null) && (loader instanceof Lifecycle))
((Lifecycle) loader).start();
if ((logger != null) && (logger instanceof Lifecycle))
((Lifecycle) logger).start();
if ((manager != null) && (manager instanceof Lifecycle))
((Lifecycle) manager).start();
if ((cluster != null) && (cluster instanceof Lifecycle))
((Lifecycle) cluster).start();
if ((realm != null) && (realm instanceof Lifecycle))
((Lifecycle) realm).start();
if ((resources != null) && (resources instanceof Lifecycle))
((Lifecycle) resources).start();

// Start our Mappers, if any
Mapper mappers[] = findMappers();
for (int i = 0; i < mappers.length; i++) {
if (mappers[i] instanceof Lifecycle)
((Lifecycle) mappers[i]).start();
}

// Start our child containers, if any
Container children[] = findChildren();
for (int i = 0; i < children.length; i++) {
if (children[i] instanceof Lifecycle)
((Lifecycle) children[i]).start();
}

// Start the Valves in our pipeline (including the basic), if any
if (pipeline instanceof Lifecycle)
((Lifecycle) pipeline).start();

// Notify our interested LifecycleListeners
lifecycle.fireLifecycleEvent(START_EVENT, null);

// Notify our interested LifecycleListeners
lifecycle.fireLifecycleEvent(AFTER_START_EVENT, null);

}

2）StandardHostMapper类中最重要的方法是 map()方法，源代码如下：

public Container map(Request request, boolean update) { //org.apache.catalina.core.StandardHostMapper.map()
// Has this request already been mapped?
if (update && (request.getContext() != null))
return (request.getContext());
// Perform mapping on our request URI
String uri = ((HttpRequest) request).getDecodedRequestURI();
Context context = host.map(uri); // highlight line.
// Update the request (if requested) and return the selected Context
if (update) {
request.setContext(context);
if (context != null)
((HttpRequest) request).setContextPath(context.getPath());
else
((HttpRequest) request).setContextPath(null);
}
return (context);
}

Attention）StandardHostMapper.map()方法仅仅调用了 Host.map()方法；

【4】StandardHostValve类（org.apache.catalina.core.StandardHostValve）
1）StandardHostValve是StandardHost 的基础阀。当 有引入的http 请求时，会调用 StandardHostValve.invoke()方法对其进行处理； 

public void invoke(Request request, Response response,
ValveContext valveContext)  //org.apache.catalina.core.StandardHostValve.invoke().
throws IOException, ServletException {

// Validate the request and response object types
if (!(request.getRequest() instanceof HttpServletRequest) ||
!(response.getResponse() instanceof HttpServletResponse)) {
return;     // NOTE - Not much else we can do generically
}

// Select the Context to be used for this Request
StandardHost host = (StandardHost) getContainer();
Context context = (Context) host.map(request, true); // highlight line.
if (context == null) {
((HttpServletResponse) response.getResponse()).sendError
(HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
sm.getString("standardHost.noContext"));
return;
}

// Bind the context CL to the current thread
Thread.currentThread().setContextClassLoader
(context.getLoader().getClassLoader());

// Update the session last access time for our session (if any)
HttpServletRequest hreq = (HttpServletRequest) request.getRequest();
String sessionId = hreq.getRequestedSessionId();
if (sessionId != null) {
Manager manager = context.getManager();
if (manager != null) {
Session session = manager.findSession(sessionId);
if ((session != null) && session.isValid())
session.access(); //highlight line.
}
}
// Ask this Context to process this request
context.invoke(request, response);
Thread.currentThread().setContextClassLoader
(StandardHostValve.class.getClassLoader());
}

对以上代码的分析（Analysis）：

step1）在tomcat4中， 首先invoke()方法会调用 StandardHost.map() 方法来获取一个相应的 Context实例；

// Select the Context to be used for this Request
StandardHost host = (StandardHost) getContainer();
Context context = (Context) host.map(request, true); // highlight line.

Attention）在获取Context实例的代码中有一个往复的过程。上面的map()方法需要两个参数，该方法定义在 ContainerBase类中。ContainerBase.map()方法会找到其子容器的映射器，并调用其map()方法；

step2）然后，invoke()方法会获取与该request对象相关联的session 对象，并调用其 access()方法。access()方法会修改 session对象的最后访问时间。
step3）org.apache.catalina.session.StandardSession.access()方法的源代码如下：

public void access() {
this.isNew = false;
this.lastAccessedTime = this.lastUsedTime;
this.lastUsedTime = System.currentTimeMillis();
}

step4）最后StandardHostValve.invoke()方法调用Context实例的 invoke()来处理http 请求；

【5】为什么必须要有一个Host容器
1）在tomcat4 和 tomcat5的部署中，若一个Context实例使用ContextConfig对象进行设置，就必须使用一个 Host对象，原因如下（reasons）：

r1）使用ContextConfig对象 需要知道应用程序 web.xml 文件的位置。在其 applicationConfig() 方法中，它会试图打开web.xml ；applicationConfig()方法的源码片段如下：

private void applicationConfig() { // org.apache.catalina.startup.ContextConfig.applicationConfig()
// Open the application web.xml file, if it exists
InputStream stream = null;
ServletContext servletContext = context.getServletContext();
if (servletContext != null)
stream = servletContext.getResourceAsStream
(Constants.ApplicationWebXml); //  public static final String ApplicationWebXml = "/WEB-INF/web.xml";
if (stream == null) {
log(sm.getString("contextConfig.applicationMissing"));
return;
}
// Process the application web.xml file
synchronized (webDigester) { // highlight code begins.
try {
URL url =
servletContext.getResource(Constants.ApplicationWebXml);
InputSource is = new InputSource(url.toExternalForm());
is.setByteStream(stream);
webDigester.setDebug(getDebug());
if (context instanceof StandardContext) {
((StandardContext) context).setReplaceWelcomeFiles(true);
}
webDigester.clear();
webDigester.push(context);
webDigester.parse(is);
webDigester.push(null);
} catch (SAXParseException e) {
log(sm.getString("contextConfig.applicationParse"), e);
log(sm.getString("contextConfig.applicationPosition",
"" + e.getLineNumber(),
"" + e.getColumnNumber()));
ok = false;
} catch (Exception e) {
log(sm.getString("contextConfig.applicationParse"), e);
ok = false;
} finally {
try {
if (stream != null) {
stream.close();
}
} catch (IOException e) {
log(sm.getString("contextConfig.applicationClose"), e);
}
}        }    }<strong style="font-family: SimSun; background-color: rgb(255, 255, 255);"> </strong>

对以上代码的分析（Analysis）：

A1）其中public static final String ApplicationWebXml = "/WEB-INF/web.xml"；web.xml 文件的相对路径，servletContext是一个 org.apache.catalina.core.ApplicationContext类型（实现了
javax.servlet.servletContext接口）的对象；
A2）下面是 ApplicationContext.getResource()方法的部分实现代码：

public URL getResource(String path)  throws MalformedURLException { // org.apache.catalina.core.ApplicationContext.getResource().
path = normalize(path);
if (path == null)
return (null);
DirContext resources = context.getResources();
if (resources != null) {
String fullPath = context.getName() + path;
String hostName = context.getParent().getName(); //highlight line.
try {
resources.lookup(path);
if( System.getSecurityManager() != null ) {
try {
PrivilegedGetResource dp =
new PrivilegedGetResource
(hostName, fullPath, resources);
return (URL)AccessController.doPrivileged(dp);
} catch( PrivilegedActionException pe) {
throw pe.getException();
}
} else {
return new URL
("jndi", null, 0, getJNDIUri(hostName, fullPath),
new DirContextURLStreamHandler(resources));
}
} catch (Exception e) {
//e.printStackTrace();
}
}
return (null);
}

Attention）上述“highlight line”
所标识的行表明了，如果要使用 ContextConfig实例来进行配置的话，Context实例必须有一个 Host实例作为其父容器，否则context.getParent().getName()会抛出异常导致获取资源文件（web.xml）的URL不成功；

【6】应用程序1（StandardHost测试用例）
6.1）源代码

public final class Bootstrap1 {
public static void main(String[] args) {
//invoke: http://localhost:8080/app1/Primitive or http://localhost:8080/app1/Modern System.setProperty("catalina.base", System.getProperty("user.dir"));
Connector connector = new HttpConnector();

 Wrapper wrapper1 = new StandardWrapper();
    wrapper1.setName("Primitive");
    //wrapper1.setServletClass("servlet.PrimitiveServlet");
    wrapper1.setServletClass("PrimitiveServlet");
    Wrapper wrapper2 = new StandardWrapper();
    wrapper2.setName("Modern");
    //wrapper2.setServletClass("servlet.ModernServlet");
    wrapper2.setServletClass("ModernServlet");
Context context = new StandardContext();
// StandardContext's start method adds a default mapper
context.setPath("/app1");
context.setDocBase("app1");

context.addChild(wrapper1);
context.addChild(wrapper2);

LifecycleListener listener = new SimpleContextConfig();
((Lifecycle) context).addLifecycleListener(listener);

Host host = new StandardHost();
host.addChild(context);
host.setName("localhost");
host.setAppBase("webapps");

Loader loader = new WebappLoader();
context.setLoader(loader);
// context.addServletMapping(pattern, name);
context.addServletMapping("/Primitive", "Primitive");
context.addServletMapping("/Modern", "Modern");

connector.setContainer(host);
try {
connector.initialize();
((Lifecycle) connector).start();
((Lifecycle) host).start(); // 与以往的Bootstrap.java不同的是，这里是host.start() 而不是 context.start()

// make the application wait until we press a key.
System.in.read();
((Lifecycle) host).stop();
}
catch (Exception e) {
e.printStackTrace();
}
}
}

对以上代码的分析（Analysis）：
A1）本文结合 “tomcat(11)的Tomcat 处理http请求的方法调用协作图（以Context容器为起点）”总结了“《Tomcat 处理http请求的方法调用协作图（以Host容器为起点）》”，如下：



6.2）console info

E:\bench-cluster\cloud-data-preprocess\HowTomcatWorks\src>java -cp .;lib/servlet.jar;lib/catalina_4_1_24.jar;lib/catalina-5.5.4.jar;lib/naming-common
jar;lib/commons-collections.jar;lib/naming-resources.jar;lib/commons-digester.jar;lib/catalina.jar;E:\bench-cluster\cloud-data-preprocess\HowTomcatWo
ks\webroot com.tomcat.chapter13.startup.Bootstrap1
HttpConnector Opening server socket on all host IP addresses
HttpConnector[8080] Starting background thread
WebappLoader[/app1]: Deploying class repositories to work directory E:\bench-cluster\cloud-data-preprocess\HowTomcatWorks\src\work\_\localhost\app1
StandardManager[/app1]: Seeding random number generator class java.security.SecureRandom
StandardManager[/app1]: Seeding of random number generator has been completed
StandardManager[/app1]: IOException while loading persisted sessions: java.io.EOFException
java.io.EOFException // <span class="comment" style="margin: 0px; padding: 0px; border: none; color: rgb(0, 130, 0); font-family: Consolas, 'Courier New', Courier, mono, serif; line-height: 18px;">这是从文件中加载 session对象到内存，由于没有相关文件，所以加载失败，抛出异常，但这不会影响我们访问servlet，大家不要惊慌；  </span><span style="margin: 0px; padding: 0px; border: none; font-family: Consolas, 'Courier New', Courier, mono, serif; line-height: 18px;">  </span>
at java.io.ObjectInputStream$PeekInputStream.readFully(Unknown Source)
at java.io.ObjectInputStream$BlockDataInputStream.readShort(Unknown Source)
at java.io.ObjectInputStream.readStreamHeader(Unknown Source)
at java.io.ObjectInputStream.<init>(Unknown Source)
at org.apache.catalina.util.CustomObjectInputStream.<init>(CustomObjectInputStream.java:103)
at org.apache.catalina.session.StandardManager.load(StandardManager.java:408)
at org.apache.catalina.session.StandardManager.start(StandardManager.java:655)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:3570)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1188)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:738)
at com.tomcat.chapter13.startup.Bootstrap1.main(Bootstrap1.java:59)
StandardManager[/app1]: Exception loading sessions from persistent storage
java.io.EOFException
at java.io.ObjectInputStream$PeekInputStream.readFully(Unknown Source)
at java.io.ObjectInputStream$BlockDataInputStream.readShort(Unknown Source)
at java.io.ObjectInputStream.readStreamHeader(Unknown Source)
at java.io.ObjectInputStream.<init>(Unknown Source)
at org.apache.catalina.util.CustomObjectInputStream.<init>(CustomObjectInputStream.java:103)
at org.apache.catalina.session.StandardManager.load(StandardManager.java:408)
at org.apache.catalina.session.StandardManager.start(StandardManager.java:655)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:3570)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1188)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:738)
at com.tomcat.chapter13.startup.Bootstrap1.main(Bootstrap1.java:59)
ModernServlet -- init
StandardHost[localhost]: MAPPING configuration error for request URI /favicon.ico

6.3）访问效果





【7】Engine接口（org.apche.catalina.Engine）
1）intro to Engine：Engine容器是org.apche.catalina.Engine接口的实例。Engine容器也就是tomcat的servlet 引擎。当部署tomcat时 要支持多个虚拟机（Host）的话，就需要使用 Engine容器。一般
case下，部署的tomcat 都会使用一个 Engine容器；
2）org.apche.catalina.Engine定义如下：

public interface Engine extends Container {
public String getDefaultHost();
public void setDefaultHost(String defaultHost);
public String getJvmRoute();
public void setJvmRoute(String jvmRouteId);
public Service getService();
public void setService(Service service);
public void addDefaultContext(DefaultContext defaultContext);
public DefaultContext getDefaultContext();
public void importDefaultContext(Context context);
}

3）在Engine容器中，可以设置一个默认的Host容器或Context容器。注意，Engine容器可以与一个服务实例相关联；

【8】StandardEngine类（org.apache.catalina.core.StandardEngine）
1）其构造函数如下：

public class StandardEngine  extends ContainerBase implements Engine {
public StandardEngine() {
super();
pipeline.setBasic(new StandardEngineValve());
}

2）作为一个顶层容器，Engine容器可以有子容器，而它的子容器只能是Host容器；将一个非Host容器设置为其子容器，则抛出异常，参见StandardEngine.addChild()： 

public void addChild(Container child) { // org.apache.catalina.core.StandardEngine.addChild().
if (!(child instanceof Host))
throw new IllegalArgumentException
(sm.getString("standardEngine.notHost"));
super.addChild(child);
}

3）当然了，由于Engine是顶层容器，它就不能再有父容器了，若调用 StandardEngine.setParent方法为其设置父容器时，抛出异常：

public void setParent(Container container) {   // org.apache.catalina.core.StandardEngine.setParent().
throw new IllegalArgumentException
(sm.getString("standardEngine.notParent"));
}

【9】StandardEngineValve类（org.apache.catalina.core.StandardEngineValve）
1）StandardEngineValve：是 StandardEngine容器的基础阀；
2）StandardEngineValve.invoke()方法如下：

public void invoke(Request request, Response response,
ValveContext valveContext) // org.apache.catalina.core.StandardEngineValve.invoke().
throws IOException, ServletException {
// Validate the request and response object types
if (!(request.getRequest() instanceof HttpServletRequest) ||
!(response.getResponse() instanceof HttpServletResponse)) {
return;     // NOTE - Not much else we can do generically
}
// Validate that any HTTP/1.1 request included a host header
HttpServletRequest hrequest = (HttpServletRequest) request;
if ("HTTP/1.1".equals(hrequest.getProtocol()) &&
(hrequest.getServerName() == null)) {
((HttpServletResponse) response.getResponse()).sendError
(HttpServletResponse.SC_BAD_REQUEST,
sm.getString("standardEngine.noHostHeader",
request.getRequest().getServerName()));
return;
}
// Select the Host to be used for this Request
StandardEngine engine = (StandardEngine) getContainer();
Host host = (Host) engine.map(request, true);
if (host == null) {
((HttpServletResponse) response.getResponse()).sendError
(HttpServletResponse.SC_BAD_REQUEST,
sm.getString("standardEngine.noHost",
request.getRequest().getServerName()));
return;
}
// Ask this Host to process this request
host.invoke(request, response);
}

对以上代码的分析（Analysis）：

step1）在验证了request和response对象的类型后，invoke()方法得到Host实例，用于处理该请求；
step2）invoke()方法会通过调用Engine实例的map()方法获取Host对象；
step3）得到Host对象后，调用其invoke() 方法处理请求；

【10】应用程序2[b]（StandardEngine测试用例）[/b]
1）源代码：

public final class Bootstrap2 {
public static void main(String[] args) {
//invoke: http://localhost:8080/app1/Primitive or http://localhost:8080/app1/Modern System.setProperty("catalina.base", System.getProperty("user.dir"));
Connector connector = new HttpConnector();

 Wrapper wrapper1 = new StandardWrapper();
    wrapper1.setName("Primitive");
    //wrapper1.setServletClass("servlet.PrimitiveServlet");
    wrapper1.setServletClass("PrimitiveServlet");
    Wrapper wrapper2 = new StandardWrapper();
    wrapper2.setName("Modern");
    //wrapper2.setServletClass("servlet.ModernServlet");
    wrapper2.setServletClass("ModernServlet");

Context context = new StandardContext();
// StandardContext's start method adds a default mapper
context.setPath("/app1");
context.setDocBase("app1");

context.addChild(wrapper1);
context.addChild(wrapper2);

LifecycleListener listener = new SimpleContextConfig();
((Lifecycle) context).addLifecycleListener(listener);

Host host = new StandardHost();
host.addChild(context);
host.setName("localhost");
host.setAppBase("webapps");

Loader loader = new WebappLoader();
context.setLoader(loader);
// context.addServletMapping(pattern, name);
context.addServletMapping("/Primitive", "Primitive");
context.addServletMapping("/Modern", "Modern");

Engine engine = new StandardEngine();
engine.addChild(host);
engine.setDefaultHost("localhost");

connector.setContainer(engine);
try {
connector.initialize();
((Lifecycle) connector).start();
((Lifecycle) engine).start();

// make the application wait until we press a key.
System.in.read();
((Lifecycle) engine).stop(); // 与以往不同的是，这里是engine.start()而不是 context.start()，因为最上层容易已经改变了，而enginge包含host，host包含context，context包含wrapper.
}
catch (Exception e) {
e.printStackTrace();
}
}
}

对以上代码的分析（Analysis）：
A1）同理，本小节也总结了 “Tomcat 处理http请求的方法调用协作图（以Engine容器为起点）”，如下：




A2）对上面的协作图的调用steps做分析：

step1）当http 请求到达 tomcat server时，HttpConnector得到 ServerSocket(（accept()方法）），然后调用最上层容器的invoke（）方法，而所有容器都继承自ContainerBase，所以实际上调用的是 ContainerBase.invoke（）方法；
step2）之后调用管道StandardPipeline.invoke()方法，进而调用管道的非基础阀和基础阀的invoke方法（而基础阀在容器的构造器中设置了）；（干货——所有容器都继承自ContainerBase，而管道StandardPipeline是在
ContainerBase中创建的，所以所有容器都共用同一个管道对象，而当调用到某容器的时候，设置其对应的基础阀即可）
step3）基础阀的invoke()方法会调用下一层级容器的invoke方法，接着继续调用管道StandardPipeline.invoke()方法，一直进行下去（回到step2的过程）.......直到到达Wrapper容器（因为它是最小的容器，是一个具体servlet的封装）；
step4）Wrapper.invoke（）方法，同样也要调用其管道StandardPipeline.invoke()方法，接着调用基础阀StandardWrapperValve，与其他基础阀不同的是，StandardWrapperValve.invoke()方法会调用
ApplicationFilterChain.doFilter()方法，接着调用具体的HttpServlet.allocate()方法和service()方法，写出响应info（html）到client，ending（Bingo）；

2）console info

E:\bench-cluster\cloud-data-preprocess\HowTomcatWorks\src>java -cp .;lib/servlet.jar;lib/catalina_4_1_24.jar;lib/catalina-5.5.4.jar;lib/naming-common.
jar;lib/commons-collections.jar;lib/naming-resources.jar;lib/commons-digester.jar;lib/catalina.jar;E:\bench-cluster\cloud-data-preprocess\HowTomcatWok
s\webroot com.tomcat.chapter13.startup.Bootstrap2
HttpConnector Opening server socket on all host IP addresses
HttpConnector[8080] Starting background thread
Apache Tomcat/4.1.24
WebappLoader[/app1]: Deploying class repositories to work directory E:\bench-cluster\cloud-data-preprocess\HowTomcatWorks\src\work\_\localhost\app1
WebappLoader[/app1]: Deploy class files /WEB-INF/classes to E:\bench-cluster\cloud-data-preprocess\HowTomcatWorks\src\webapps\app1\WEB-INF\classes
StandardManager[/app1]: Seeding random number generator class java.security.SecureRandom
StandardManager[/app1]: Seeding of random number generator has been completed
StandardManager[/app1]: IOException while loading persisted sessions: java.io.EOFException
java.io.EOFException
at java.io.ObjectInputStream$PeekInputStream.readFully(Unknown Source)
at java.io.ObjectInputStream$BlockDataInputStream.readShort(Unknown Source)
at java.io.ObjectInputStream.readStreamHeader(Unknown Source)
at java.io.ObjectInputStream.<init>(Unknown Source)
at org.apache.catalina.util.CustomObjectInputStream.<init>(CustomObjectInputStream.java:103)
at org.apache.catalina.session.StandardManager.load(StandardManager.java:408)
at org.apache.catalina.session.StandardManager.start(StandardManager.java:655)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:3570)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1188)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:738)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1188)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:347)
at com.tomcat.chapter13.startup.Bootstrap2.main(Bootstrap2.java:67)
StandardManager[/app1]: Exception loading sessions from persistent storage
java.io.EOFException
at java.io.ObjectInputStream$PeekInputStream.readFully(Unknown Source)
at java.io.ObjectInputStream$BlockDataInputStream.readShort(Unknown Source)
at java.io.ObjectInputStream.readStreamHeader(Unknown Source)
at java.io.ObjectInputStream.<init>(Unknown Source)
at org.apache.catalina.util.CustomObjectInputStream.<init>(CustomObjectInputStream.java:103)
at org.apache.catalina.session.StandardManager.load(StandardManager.java:408)
at org.apache.catalina.session.StandardManager.start(StandardManager.java:655)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:3570)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1188)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:738)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1188)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:347)
at com.tomcat.chapter13.startup.Bootstrap2.main(Bootstrap2.java:67)
ModernServlet -- init
StandardHost[localhost]: MAPPING configuration error for request URI /favicon.ico
StandardHost[localhost]: MAPPING configuration error for request URI /favicon.ico

3）访问效果