本文接下来介绍并分析servlet容器，servlet容器是用来处理请求servlet资源，并为web客户端填充response对象的模块。
servlet容器是org.apache.catalina.Container接口的实例，在tomcat中，有四种类型的容器，分别为Engine、Host 、Context和Wrapper。
Engine. 代表整个容器的servlet引擎。
Host.代表一个拥有一定数量Context的虚拟主机。
Context.代表一个Web项目.一个Context包含一个或多个Wrapper。
Wrapper.代表单独的一个servlet。
这些容器构成一个自顶向下的等级结构，高等级的容器可以具有多个直接下属等级的容器实例（子容器），这有点类似于composite模式，不过还是有差别的
org.apache.catalina.Container接口声明如下
//
添加
public
void
addChild(Container child);
//
删除
public
void
removeChild(Container child);
//
查找
public
Container findChild(String name);
//
查找全部
public
Container[] findChildren();
上面方法均为操作子容器的相关方法
容器可以包含一些支持的组件，诸如载入器、记录器、管理器、领域和资源等，我们可以通过编辑server.xml文件来决定使用哪种容器。
下面我们来分析servlet容器是怎么执行任务的，这里就要提到servlet容器的管道模型，管道包含了该servlet容器将要调用的任务，而阀则表示一个具体的执行任务；在servlet容器的管道中，有一个基础阀，也可以添加任意数量的额外阀，阀的数量通常是指额外添加的阀的数量，不包括基础阀
这里就好像servlet编程中的过滤器模型，管道好比过滤器链，而阀则是具体的过滤器；基础阀总是最后一个执行的。
这里涉及几个相关的接口，包括Pipeline、Valve、ValveContext 和Contained
Pipeline接口声明如下
public
interface
Pipeline {
public
Valve getBasic();
public
void
setBasic(Valve valve);
public
void
addValve(Valve valve);
public
Valve[] getValves();
public
void
invoke(Request request, Response response)
throws
IOException, ServletException;
public
void
removeValve(Valve valve);
}
Valve接口声明如下
public
interface
Valve {
public
String getInfo();
public
void
invoke(Request request, Response response,
ValveContext context)
throws
IOException, ServletException;
}
ValveContext接口声明
public
interface
ValveContext {
public
String getInfo();
public
void
invokeNext(Request request, Response response)
throws
IOException, ServletException;
}
Contained接口声明
public
interface
Contained {
public
Container getContainer();
public
void
setContainer(Container container);
}
阀可以选择是否实现该接口，设置阀与一个servlet容器相关连
下面我们来学习Wrapper容器，Wrapper容器表示一个独立的servlet定义，负责管理其基础servlet类的生命周期，它继承了Container接口，另外添加了额外方法声明。其中比较重要的方法声明是load()方法和allocate()方法，均与载入及初始化servlet类相关（供基础阀调用，基础阀持有Wrapper容器实例引用）
下面来分析一个简单的Wrapper类，该类实现了org.apache.catalina.Wrapper接口和org.apache.catalina.Pipeline接口
public
class
SimpleWrapper
implements
Wrapper, Pipeline {
//
the servlet instance
private
Servlet instance =
null
;
private
String servletClass;
private
Loader loader;
private
SimplePipeline pipeline =
new
SimplePipeline(
this
);
protected
Container parent =
null
;
public
SimpleWrapper() {
pipeline.setBasic(
new
SimpleWrapperValve());
}
public
synchronized
void
addValve(Valve valve) {
pipeline.addValve(valve);
}
public
Servlet allocate()
throws
ServletException {
//
Load and initialize our instance if necessary
if
(instance==
null
) {
try
{
instance
=
loadServlet();
}
catch
(ServletException e) {
throw
e;
}
catch
(Throwable e) {
throw
new
ServletException("Cannot allocate a servlet instance"
, e);
}
}
return
instance;
}
private
Servlet loadServlet()
throws
ServletException {
if
(instance!=
null
)
return
instance;
Servlet servlet
=
null
;
String actualClass
=
servletClass;
if
(actualClass ==
null
) {
throw
new
ServletException("servlet class has not been specified"
);
}
Loader loader
=
getLoader();
//
Acquire an instance of the class loader to be used
if
(loader==
null
) {
throw
new
ServletException("No loader."
);
}
ClassLoader classLoader
=
loader.getClassLoader();
//
Load the specified servlet class from the appropriate class loader
Class classClass =
null
;
try
{
if
(classLoader!=
null
) {
classClass
=
classLoader.loadClass(actualClass);
}
}
catch
(ClassNotFoundException e) {
throw
new
ServletException("Servlet class not found"
);
}
//
Instantiate and initialize an instance of the servlet class itself
try
{
servlet
=
(Servlet) classClass.newInstance();
}
catch
(Throwable e) {
throw
new
ServletException("Failed to instantiate servlet"
);
}
//
Call the initialization method of this servlet
try
{
servlet.init(
null
);
}
catch
(Throwable f) {
throw
new
ServletException("Failed initialize servlet."
);
}
return
servlet;
}
public
Loader getLoader() {
if
(loader !=
null
)
return
(loader);
if
(parent !=
null
)
return
(parent.getLoader());
return
(
null
);
}
public
void
invoke(Request request, Response response)
throws
IOException, ServletException {
pipeline.invoke(request, response);
}
public
void
load()
throws
ServletException {
instance
=
loadServlet();
}
//
method implementations of Pipeline
public
Valve getBasic() {
return
pipeline.getBasic();
}
public
void
setBasic(Valve valve) {
pipeline.setBasic(valve);
}
public
Valve[] getValves() {
return
pipeline.getValves();
}
public
void
removeValve(Valve valve) {
pipeline.removeValve(valve);
}
}
上面的SimpleWrapper类由于实现了org.apache.catalina.Pipeline接口接口，同时与该接口相关的实现方法都是调用引用的成员变量SimplePipeline pipeline = new SimplePipeline(this)的对应方法，因此我们可以理解为SimpleWrapper类为SimplePipeline的包装类
在它的invoke()方法里面调用了成员变量的SimplePipeline pipeline = new SimplePipeline(this)的invoke()方法，这里构造函数传入SimpleWrapper实例本身，可以猜想是为了获取其载入器及具体的servlet实现类（注：该方法为Container接口与Pipeline接口都具有的方法声明，因此SimpleWrapper类只要一个实现），下面我们继续分析SimplePipeline相关实现
public
class
SimplePipeline
implements
Pipeline {
public
SimplePipeline(Container container) {
setContainer(container);
}
//
The basic Valve (if any) associated with this Pipeline.
protected
Valve basic =
null
;
//
The Container with which this Pipeline is associated.
protected
Container container =
null
;
//
the array of Valves
protected
Valve valves[] =
new
Valve[0
];
public
void
setContainer(Container container) {
this
.container =
container;
}
public
Valve getBasic() {
return
basic;
}
public
void
setBasic(Valve valve) {
this
.basic =
valve;
((Contained) valve).setContainer(container);
}
public
void
addValve(Valve valve) {
if
(valve
instanceof
Contained)
((Contained) valve).setContainer(
this
.container);
synchronized
(valves) {
Valve results[]
=
new
Valve[valves.length +1
];
System.arraycopy(valves,
0, results, 0
, valves.length);
results[valves.length]
=
valve;
valves
=
results;
}
}
public
Valve[] getValves() {
return
valves;
}
public
void
invoke(Request request, Response response)
throws
IOException, ServletException {
//
Invoke the first Valve in this pipeline for this request
(
new
SimplePipelineValveContext()).invokeNext(request, response);
}
public
void
removeValve(Valve valve) {
}
//
this class is copied from org.apache.catalina.core.StandardPipeline class's
//
StandardPipelineValveContext inner class.
protected
class
SimplePipelineValveContext
implements
ValveContext {
protected
int
stage = 0
;
public
String getInfo() {
return
null
;
}
public
void
invokeNext(Request request, Response response)
throws
IOException, ServletException {
int
subscript =
stage;
stage
= stage + 1
;
//
Invoke the requested Valve for the current request thread
if
(subscript <
valves.length) {
valves[subscript].invoke(request, response,
this
);
}
else
if
((subscript == valves.length) && (basic !=
null
)) {
basic.invoke(request, response,
this
);
}
else
{
throw
new
ServletException("No valve"
);
}
}
}
//
end of inner class
}
invoke()方法里面调用内部类SimplePipelineValveContext（实现了ValveContext接口），遍历执行各个阀的invoke()方法
wrapper容器执行的基本流程如上所述，下面我们来进一步分析相关辅助类及实现类等
在应用初始化servlet容器时，我们需要为其指定一个载入器，下面是一个简单的载入器，实现了Loader接口
public
class
SimpleLoader
implements
Loader {
public
static
final
String WEB_ROOT =
System.getProperty(
"user.dir") + File.separator + "webroot"
;
ClassLoader classLoader
=
null
;
Container container
=
null
;
public
SimpleLoader() {
try
{
URL[] urls
=
new
URL[1
];
URLStreamHandler streamHandler
=
null
;
File classPath
=
new
File(WEB_ROOT);
String repository
= (
new
URL("file",
null
, classPath.getCanonicalPath() +
File.separator)).toString() ;
urls[
0] =
new
URL(
null
, repository, streamHandler);
classLoader
=
new
URLClassLoader(urls);
}
catch
(IOException e) {
System.out.println(e.toString() );
}
}
public
ClassLoader getClassLoader() {
return
classLoader;
}
public
Container getContainer() {
return
container;
}
//
这里省略其余代码
}
基础阀是干嘛的呢，具体来说是调用具体servlet的service()方法（管道持有对基础阀的引用）
public
class
SimpleWrapperValve
implements
Valve, Contained {
protected
Container container;
public
void
invoke(Request request, Response response, ValveContext valveContext)
throws
IOException, ServletException {
SimpleWrapper wrapper
=
(SimpleWrapper) getContainer();
ServletRequest sreq
=
request.getRequest();
ServletResponse sres
=
response.getResponse();
Servlet servlet
=
null
;
HttpServletRequest hreq
=
null
;
if
(sreq
instanceof
HttpServletRequest)
hreq
=
(HttpServletRequest) sreq;
HttpServletResponse hres
=
null
;
if
(sres
instanceof
HttpServletResponse)
hres
=
(HttpServletResponse) sres;
//
Allocate a servlet instance to process this request
try
{
servlet
=
wrapper.allocate();
if
(hres!=
null
&& hreq!=
null
) {
servlet.service(hreq, hres);
}
else
{
servlet.service(sreq, sres);
}
}
catch
(ServletException e) {
}
}
public
String getInfo() {
return
null
;
}
public
Container getContainer() {
return
container;
}
public
void
setContainer(Container container) {
this
.container =
container;
}
}
其他额外添加的阀本人就不在具体描述了，至此SimpleWrapper容器分析完毕！
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