servlet的三种创建模式

当前端页面访问jsp和servlet请求资源时，都是调用其中的service(ServletRequest req, ServletResponse res)方法

    第一种：实现javax.servlet.Servlet接口，覆盖service(ServletRequest req, ServletResponse res)方法                   

public class Sub2Servlet implements Servlet{

	@Override
	public void service(ServletRequest req, ServletResponse res)
			throws ServletException, IOException {
		
		String name=req.getParameter("name");
		
		
	}
           

   第二种：继承javax.servlet.GenericServlet，覆盖 service(ServletRequest req, ServletResponse res)方法

    note:   Servlet中变量config是默认调用GenericServlet 中的init(ServletConfig config)方法赋值的

                当我们覆盖此方法时，会导致config变量为null，在调用getInitParameter时出现异常

               我们应覆盖init()来处理  此时servlet调用：  第一次创建时：构造函数------->GenericServlet.init(ServletConfig config)函数------>覆盖的init()函数                                                

                                                                                                                       ------->service(ServletRequest req, ServletResponse res)方法

<span style="color:#000000;">public class Sub2Servlet extends GenericServlet{

	private String charset;

	@Override
	public void service(ServletRequest req, ServletResponse res)
			throws ServletException, IOException {
		String name=new String(req.getParameter("name").getBytes("ISO8859-1"),charset);
		System.out.println(name);
		
	}

	@Override
	public void init() throws ServletException {
		charset=getServletConfig().getInitParameter("charset");
		System.out.println(charset);
	}</span>
           

  第三种：继承javax.servlet.http.HttpServlet，覆盖doGet/doPost方法

       对于HttpServlet属于servlet，必定访问时调用service(ServletRequest req, ServletResponse res)方法

      代码：

@Override
    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);
    }
           

      会根据请求协议判断是否为http协议，调用service(HttpServletRequest req, HttpServletResponse resp)方法

    代码：

protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {

        String method = req.getMethod();<span style="color:#FF0000;">//获取请求方式，调用相应的doGet/doPost方法/其他方法</span>

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                } catch (IllegalArgumentException iae) {
                    // Invalid date header - proceed as if none was set
                    ifModifiedSince = -1;
                }
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);

        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);

        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);

        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);

        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);

        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);

            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }
           

       然而我们必须要覆盖doGet/doPost方法，否则会报错，是因为源代码程序 的正常报错

       代码：

<span style="color:#000000;">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")) {<span style="color:#FF0000;">//无论是否对错都报错</span>
            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }</span>
           

因此我们要覆盖对应的doGet/doPost方法，使得 service(HttpServletRequest req, HttpServletResponse resp)方法调用我们自己写的代码

     

小知识点：服务器路径问题：

                    当项目src源代码编译后，在启用服务器时   .class字节码文件存放在TOMCAT_HOME/webapps/项目名/WEB-INF/class文件下。

                    我们可以通过类加载器来获取path路径

<span style="color:#000000;">// 学习一下服务器下的资源路径加载方式(因为我们的资源已经从MyEclipse中发布到Tomcat服务器中了，所以跟原来纯Java项目不一样了)
// 利用当前类找到它的类加载器，然后通过该类加载器再去获得资源路径
	path = 类名.class.getClassLoader().getResource("fileName").getPath();</span>