本文接下来会介绍Host容器和Engine容器，在tomcat的实际部署中，总是会使用一个Host容器；本文介绍Host接口和Engine接口及其相关类
Host容器是org.apache.catalina.Host接口的实例，Host接口继承自Container接口， 其定义如下
public
interface
Host
extends
Container {
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);
public
void
removeAlias(String alias);
}
该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，该方法的具体实现在StandardHost类中
在tomcat中org.apache.catalina.core.StandardHost类是Host接口的标准实现，该类继承自org.apache.catalina.core.ContainerBase类，实现了Host和Deployer接口
与StandardContext类和StandardWrapper类相似，StandardHost类的构造函数会将一个基础阀的实例添加到其管道对象中：
public
StandardHost() {
super
();
pipeline.setBasic(
new
StandardHostValve());
}
基础阀是org.apache.catalina.core.StandardHostValve类的实例
当调用其start()方法时，StandardHost实例会添加两个阀，分别为ErrorReportValve类和ErrorDispatcherValve类的实例，这两个阀均位于org.apache.catalina.valves包下
public
synchronized
void
start()
throws
LifecycleException {
//
Set error report valve
if
((errorReportValveClass !=
null
)
&& (!errorReportValveClass.equals(""
))) {
try
{
Valve valve
=
(Valve) Class.forName(errorReportValveClass)
.newInstance();
addValve(valve);
}
catch
(Throwable t) {
log(sm.getString
(
"standardHost.invalidErrorReportValveClass"
,
errorReportValveClass));
}
}
//
Set dispatcher valve
addValve(
new
ErrorDispatcherValve());
super
.start();
}
每当引入一个HTTP请求，都会调用Host实例的invoke()方法，这里是StandardHost的父类ContainerBase类的invoke()方法，而ContainerBase类的invoke()方法会调用StandardHost实例的基础阀StandardHostValve实例的invoke()方法；StandardHostValve类的invoke()方法会调用StandardHost类的map()方法来获取相应的Context实例来处理HTTP请求
public
Context map(String uri) {
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);
}
在tomcat4中， StandardHost的父类ContainerBase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
protected
void
addDefaultMapper(String mapperClass) {
//
Do we need a default Mapper?
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);
}
}
变量mapperClass的值定义在StandardHst类中
private String mapperClass =
"org.apache.catalina.core.StandardHostMapper";
在tomcat4中，StandardHost类的start()方法会在方法末尾调用addDefaultMapper()方法，确保默认映射器的创建完成
当然，StandardHostMapper类中最重要的方法是map()方法，下面是map()方法的实现
public
Container map(Request request,
boolean
update) {
//
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);
//
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);
}
注意，这里map()方法仅仅是简单地调用了Host实例的map()方法
org.apache.catalina.core.StandardHostValve类是StandardHost实例的基础阀，当有引入的HTTP请求时，会调用StandardHostValve类的invoke()方法对其进行处理
public
void
invoke(Request request, Response response,
ValveContext valveContext)
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
);
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();
}
}
//
Ask this Context to process this request
context.invoke(request, response);
}
在tomcat4中，invoke()方法会调用StandardHost实例的map()方法获取一个相应的Context实例；然后获取与该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；最后调用Context实例的invoke()来处理HTTP请求
接下来描述Engine容器，Engine容器是org.apache.catalina.Engine接口的实例，Engine容器也就是Tomcat的servlet引擎
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);
}
在Engine容器中，可以设置一个默认的Host容器或一个默认的Context容器，注意，Engine容器可以与一个服务实例相关联
org.apache.catalina.core.StandardEngine类是Engine接口的标准实现，在实例化的时候，StandardEngine类会添加一个基础阀
public
StandardEngine() {
super
();
pipeline.setBasic(
new
StandardEngineValve());
}
org.apache.catalina.core.StandardEngineValve类是StandardEngine容器的基础阀，下面是它的invoke()方法的实现代码
public
void
invoke(Request request, Response response,
ValveContext valveContext)
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);
}
在验证了request和response对象的类型后，invoke()方法会通过调用Engine实例的map()方法获取Host对象；得到Host对象以后，调用其invoke()方法处理请求
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