编译环境：MAC OS

编译servlet需要servlet API，但servlet API不是JDK的一部分。如果直接用javac去编译会报错: 

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classes javac HelloServlet.java 
HelloServlet.java:3: error: package javax.servlet does not exist
import javax.servlet.*;
^
HelloServlet.java:4: error: package javax.servlet.http does not exist
import javax.servlet.http.*;
^
HelloServlet.java:6: error: cannot find symbol
public class HelloServlet extends HttpServlet {
                                  ^
  symbol: class HttpServlet
HelloServlet.java:8: error: cannot find symbol
   public void doGet(HttpServletRequest request, HttpServletResponse response)
                     ^
  symbol:   class HttpServletRequest
  location: class HelloServlet
HelloServlet.java:8: error: cannot find symbol
   public void doGet(HttpServletRequest request, HttpServletResponse response)
                                                 ^
  symbol:   class HttpServletResponse
  location: class HelloServlet
HelloServlet.java:9: error: cannot find symbol
         throws IOException, ServletException {
                             ^
  symbol:   class ServletException
  location: class HelloServlet
HelloServlet.java:7: error: method does not override or implement a method from a supertype
   @Override
   ^
7 errors

Tomcat提供了一个备份文件<TOMCAT_HOME>/lib/servlet-api.jar。在编译时候我们需要把这个JAR文件用-cp命令包含进来。 

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// Assume that Tomcat is installed in /Applications/tomcat
// Change directory to the source file
cd /Applications/tomcat/webapps/hello/WEB-INF/classes

// Compile
javac -cp .:/Applications/tomcat/lib/servlet-api.jar HelloServlet.java