Servlet/JSP-03 HttpServlet

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);
} }
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