Servlet,GenericServlet,HttpServlet源码解析

继承关系：GenericServlet实现了Servlet接口，HttpServlet抽象类是GenericServlet类的子类



GenericServlet.java

public abstract class GenericServlet
implements Servlet, ServletConfig, java.io.Serializable
{
//私有变量，保存 init()传入的ServletConfig对象的引用
private transient ServletConfig config;

public GenericServlet() { }

public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init(); //调用了无参的 init()方法
}

//无参的init()方法
public void init() throws ServletException {

}

//空实现了destroy方法
public void destroy() { }

//实现了接口中的getServletConfig方法，返回ServletConfig对象
public ServletConfig getServletConfig()
{
return config;
}

//该方法实现接口<Servlet>中的ServletInfo，默认返回空字符串
public String getServletInfo() {
return "";
}

//当HttpServlet继承GenericServlet抽象类时需要实现service方法
public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;

//得到ServletContext实例
public ServletContext getServletContext() {
return getServletConfig().getServletContext();
}

//得到servlet的配置参数
public String getInitParameter(String name) {
return getServletConfig().getInitParameter(name);
}

//得到servlet的配置参数名
public Enumeration getInitParameterNames() {
return getServletConfig().getInitParameterNames();

public String getServletName() {
return config.getServletName();
}

public void log(String msg) {
getServletContext().log(getServletName() + ": "+ msg);
}

public void log(String message, Throwable t) {
getServletContext().log(getServletName() + ": " + message, t);
}
} HttpServlet.java

//继承GenericServlet抽象类
public abstract class HttpServlet extends GenericServlet
implements Serializable
{
public HttpServlet()
{
}

//get请求，自己的扩展类中需要覆写该方法
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);
}
//post请求，自己的扩展类中需要覆写该方法
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_post_not_supported");
if(protocol.endsWith("1.1"))
resp.sendError(405, msg);
else
resp.sendError(400, msg);
}
protected void doHead(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
NoBodyResponse response = new NoBodyResponse(resp);
doGet(req, response);
response.setContentLength();
}

protected void doPut(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_put_not_supported");
if(protocol.endsWith("1.1"))
resp.sendError(405, msg);
else
resp.sendError(400, msg);
}
protected void doDelete(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_delete_not_supported");
if(protocol.endsWith("1.1"))
resp.sendError(405, msg);
else
resp.sendError(400, msg);
}

protected void doTrace(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String CRLF = "\r\n";
String responseString = (new StringBuilder()).append("TRACE ").append(req.getRequestURI()).append(" ").append(req.getProtocol()).toString();
for(Enumeration reqHeaderEnum = req.getHeaderNames(); reqHeaderEnum.hasMoreElements();)
{
String headerName = (String)reqHeaderEnum.nextElement();
responseString = (new StringBuilder()).append(responseString).append(CRLF).append(headerName).append(": ").append(req.getHeader(headerName)).toString();
}
responseString = (new StringBuilder()).append(responseString).append(CRLF).toString();
int responseLength = responseString.length();
resp.setContentType("message/http");
resp.setContentLength(responseLength);
ServletOutputStream out = resp.getOutputStream();
out.print(responseString);
out.close();
}

//重载service(ServletRequest req, ServletResponse resp)方法
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
//判断客户端的请求方式
if(method.equals("GET"))
{
long lastModified = getLastModified(req);
if(lastModified == -1L)
{
doGet(req, resp);
} else
{
long ifModifiedSince = req.getDateHeader("If-Modified-Since");
if(ifModifiedSince < (lastModified / 1000L) * 1000L)
{
maybeSetLastModified(resp, lastModified);
doGet(req, resp); //执行doGet方法，所以需要覆写doGet方法
} 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); //执行doPost方法，所以需要覆写doPost方法
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);
}
}
//重要
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequest request;
HttpServletResponse response;
//将请求ServletRequest强转为HttpServletRequest，调用service(HttpServletReqeust req,HttpServletResponse resp)方法
try
{
request = (HttpServletRequest)req;
response = (HttpServletResponse)res;
}
catch(ClassCastException e)
{
throw new ServletException("non-HTTP request or response");
}
service(request, response); //调用service(HttpServletReqeust req,HttpServletResponse resp)方法
}
private static final String METHOD_DELETE = "DELETE";
private static final String METHOD_HEAD = "HEAD";
private static final String METHOD_GET = "GET";
private static final String METHOD_OPTIONS = "OPTIONS";
private static final String METHOD_POST = "POST";
private static final String METHOD_PUT = "PUT";
private static final String METHOD_TRACE = "TRACE";
private static final Stri
4000
ng HEADER_IFMODSINCE = "If-Modified-Since";
private static final String HEADER_LASTMOD = "Last-Modified";
private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
private static ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");
}