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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