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下面这段 Servlet 编译时出错，怎么会事？？---------------------------------------------------import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class SampleServlet extends HttpServlet
{
  
  public void doGet(HttpServletRequest request,
    HttpServletResponse response) throws ServletException,
    IOException
  {
    response.setContentType("text/html");    PrintWriter out = response.getWriter();    out.println("<HTML>");
    out.println("<HEAD><TITLE>Sample Servlet</TITLE></HEAD>");
    out.println("<BODY>");
    out.println("<H1>Hello World!</H1>");
    out.println("</BODY></HTML>");
    out.close();
  }}----------------------------------------------
---------- 编译 JAVA 程序 ----------
SampleServlet.java:1: package javax.servlet does not exist
import javax.servlet.*;
^
SampleServlet.java:2: package javax.servlet.http does not exist
import javax.servlet.http.*;
^
SampleServlet.java:7: cannot resolve symbol
symbol  : class HttpServlet 
location: class SampleServlet
public class SampleServlet extends HttpServlet
                                   ^
SampleServlet.java:10: cannot resolve symbol
symbol  : class HttpServletRequest 
location: class SampleServlet
  public void doGet(HttpServletRequest request,
                    ^
SampleServlet.java:11: cannot resolve symbol
symbol  : class HttpServletResponse 
location: class SampleServlet
    HttpServletResponse response) throws ServletException,
    ^
SampleServlet.java:11: cannot resolve symbol
symbol  : class ServletException 
location: class SampleServlet
    HttpServletResponse response) throws ServletException,
                                         ^
6 errors