servlet中反射应用

╰+哭是因爲堅強的太久メ 2021-09-26 18:30 186阅读 0赞

1.首先是BaseServlet

/**
     * 通用的servlet
     */
    public class BaseServlet extends HttpServlet {
    	private static final long serialVersionUID = 1L;
    
    	protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		try {
    			//1.获取方法名称
    			String mName = request.getParameter("method");
    			
    			//1.1判断 参数是否为空  若为空,执行默认的方法
    			if(mName == null || mName.trim().length()==0){
    				mName = "index";
    			}
    			
    			//2.获取方法对象
    			Method method = this.getClass().getMethod(mName, HttpServletRequest.class,HttpServletResponse.class);
    			
    			//3.让方法执行,接受返回值
    			String path=(String) method.invoke(this, request,response);
    			
    			//4.判断返回值是否为空 若不为空统一处理请求转发
    			if(path != null){
    				request.getRequestDispatcher(path).forward(request, response);
    			}
    		} catch (Exception e) {
    			e.printStackTrace();
    			throw new RuntimeException();
    		}
    	}
    
    }

2.然后是很多servlet，这里展示UserServlet，继承BaseServlet

/**
     * 用户模块
     */
    public class UserServlet extends BaseServlet {
    	private static final long serialVersionUID = 1L;
    
    	/**
    	 * 退出
    	 */
    	public String logout(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		request.getSession().invalidate();
    		
    		response.sendRedirect(request.getContextPath());
    		return null;
    	}
    	
    	/**
    	 * 用户登录
    	 * @param request
    	 * @param response
    	 * @return
    	 * @throws ServletException
    	 * @throws IOException
    	 */
    	public String login(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		...
    	}
    	
    	/**
    	 * 跳转到登录页面
    	 * @param request
    	 * @param response
    	 * @return
    	 * @throws ServletException
    	 * @throws IOException
    	 */
    	public String loginUI(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		return "/jsp/login.jsp";
    	}
    	
    	/**
    	 * 用户激活
    	 * @param request
    	 * @param response
    	 * @return
    	 * @throws ServletException
    	 * @throws IOException
    	 */
    	public String active(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		...
    	}
    	
    	/**
    	 * 用户注册
    	 * @param request
    	 * @param response
    	 * @return
    	 * @throws ServletException
    	 * @throws IOException
    	 */
    	public String regist(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		...
    
    	}
    	
    	/**
    	 * 跳转到注册页面
    	 * @param request
    	 * @param response
    	 * @return
    	 * @throws ServletException
    	 * @throws IOException
    	 */
    	public String registUI(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		return "/jsp/register.jsp";
    	}
    
    }

3.理解下这个BaseServlet

Servlet默认是在第一次被访问的时候被创建。

所以当我们通过web.xml的映射关系访问到UserServlet时，UserServlet对象被创建，当然作为父类会先通过构造函数来初始化。每次客户向服务器发送请求时，服务器就会调用service方法。

重写的service方法中：

通过String mName = request.getParameter("method")得到要调用的UserServlet的方法名；

通过Method method = this.getClass().getMethod(mName, HttpServletRequest.class,HttpServletResponse.class)得到方法对象，this是对象；

再通过method.invoke(this, request,response)来执行这个方法。

4.简单例子

父类 Fanshe 和两个子类 AServlet 和 BServlet

public class Fanshe {
    	Fanshe(){
    		System.out.println(this.getClass());
    	}
    	public void say(String s){
    		System.out.println("Fanshe say");
    		String methodName = "eat";
    		try {
    			Method method = this.getClass().getMethod(methodName, String.class);//第一个参数是方法名，第二个参数是方法的参数列表
    			method.invoke(this, s);//第一个参数是对象名，第二个参数是方法的参数列表
    		} catch (Exception e) {
    			e.printStackTrace();
    		} 
    	}
    	public static void main(String[] args) {
    		
    		AServlet a = new AServlet();
    		a.say("A");
    		Bservlet b = new Bservlet();
    		b.say("B");
    		
    	}
    
    }
    
    
    public class AServlet extends Fanshe{
    	public void eat(String s){
    		System.out.println(s+" eat");
    	}
    }
    
    
    public class Bservlet extends Fanshe{
    	public void eat(String s){
    		System.out.println(s+" eat");
    	}
    }

运行结果：

class AServlet  
Fanshe say  
A eat  
class Bservlet  
Fanshe say  
B eat