一. GenericServlet
GenericServlet本身是一个抽象类，并且实现了Servlet和ServletConfig接口
其在内部定义了一个私有的ServletConfig类型的变量config，并在init(ServletConfig config)为其赋值，然后通过config实现其他方法。
service(ServletRequest servletrequest, ServletResponse servletresponse)为抽象方法。具体代码如下：
public abstract class GenericServlet implements Servlet, ServletConfig, Serializable
{
private transient ServletConfig config; public GenericServlet() { }
@override
public void init(ServletConfig config) throws ServletException
{
this.config = config;
init();
} // 此方法供我们实现的Servlet初始化时使用
public void init() throws ServletException {}
@override
public ServletConfig getServletConfig()
{
return config;
}
@override
public String getInitParameter(String name)
{
return getServletConfig().getInitParameter(name);
} public abstract void service(ServletRequest servletrequest, ServletResponse servletresponse)
throws ServletException, IOException; // 其他方法略
... }
二. HttpServlet
为了简化开发，Servlet容器提供了HttpServlet的实现，其继承了GenericServlet.
1. Servlet容器为我们封装了HttpServletRequest, HttpServletResponse类型的参数，传入service方法，在service方法内强转后传入对应的doXXX方法
2. 开发时继承HttpServlet，只需要重写doXXX方法即可，处理相应方式的HTTP请求。
public abstract class HttpServlet extends GenericServlet
{
// doXXX方法被子类覆盖
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_get_not_supported");
if(protocol.endsWith("1.1"))
resp.sendError(405, msg);
else
resp.sendError(400, msg);
} //doPost, doPut, doDelete类似实现， doHead, doTrace, doOption略复杂
...... //通过此方法调用不同HTTP请求方式的处理方法doXXX
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
if(method.equals("GET"))
{
long lastModified = getLastModified(req);
if(lastModified == -1L)
{
doGet(req, resp);
} else
{
long ifModifiedSince;
try
{
ifModifiedSince = req.getDateHeader("If-Modified-Since");
}
catch(IllegalArgumentException iae)
{
ifModifiedSince = -1L;
}
if(ifModifiedSince < (lastModified / 1000L) * 1000L)
{
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else
{
resp.setStatus(304);
}
}
} else
if(method.equals("HEAD"))
{
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else
if(method.equals("POST"))
doPost(req, resp);
else
if(method.equals("PUT"))
doPut(req, resp);
else
if(method.equals("DELETE"))
doDelete(req, resp);
else
if(method.equals("OPTIONS"))
doOptions(req, resp);
else
if(method.equals("TRACE"))
{
doTrace(req, resp);
} else
{
String errMsg = lStrings.getString("http.method_not_implemented");
Object errArgs[] = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(501, errMsg);
}
} //此为GenericServlet中方法，最终调用HttpServlet内实现的service方法
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequest request;
HttpServletResponse response;
try
{
request = (HttpServletRequest)req;
response = (HttpServletResponse)res;
}
catch(ClassCastException e)
{
throw new ServletException("non-HTTP request or response");
}
service(request, response);
} }