index.java调用HelloClass.java类方法:

HelloClass.java内容：

package li; //声明包，必须在java文件的第一行声明
  
public class HelloClass{
 
	public String out() throws IOException{ 
		//如果方法内使用File相似可能发生错误的，需要加上 throws IOException 错误捕获
      
		return("hello");
   
	};
  
}



index.java内容：

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import li.HelloClass; //引用包，包名+类名

@WebServlet("/index")
public class index extends HttpServlet {
  private static final long serialVersionUID = 1L;
   
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    PrintWriter out = response.getWriter();
    
    //实例化类
    HelloClass o = new HelloClass();
    out.println(o.out());
  }

  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doGet(request, response);
  }

}