Java Web filter and listener

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foreword

提示：这里可以添加本文要记录的大概内容：

There are three standard components in javaEE:

1. Servlet component
2. Filter component (filter component)
3. Listener component (listener component)

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提示：以下是本篇文章正文内容，下面案例可供参考

package servlet;

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;

@WebServlet("/s1")
public class Servlet1 extends HttpServlet{
    
    
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
		resp.setContentType("text/html;charset=utf8");
		PrintWriter writer = resp.getWriter();
		writer.print("<h1>第一个servlet</h1>");
		
	}
}

package servlet;

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;

@WebServlet("/s2")
public class Servlet2 extends HttpServlet{
    
    
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
		resp.setContentType("text/html;charset=utf8");
		PrintWriter writer = resp.getWriter();
		writer.print("<h1>第二个servlet</h1>");
		
	}
}

Filter Filter

package filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletResponse;
@WebFilter("/*")
public class WebHttpFilter implements Filter{
    
    

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
    
    
		// TODO 自动生成的方法存根
		System.out.println("请求已拦截");
	}
	
}

Re-run Tomcat and refresh the browser: the browser has a big white screen

Back to the console: the request has been intercepted

Add the following code after the output:

chain.doFilter(request, response);

The control shows that the request has been intercepted.
Browser output:

delete the code in Servlet1 and 2:

resp.setContentType("text/html;charset=utf8");

Garbled characters will be found:

Then add the following code after the output of WebHttpFilter:
force this parameter to http type

		HttpServletResponse resp = (HttpServletResponse)response;
		resp.setContentType("text/html;charset=utf8");

Listener Listener

package listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class WebServerLisener implements ServletContextListener{
    
    
	@Override
	public void contextInitialized(ServletContextEvent sce) {
    
    
		System.out.println("监听到了服务器正在启动");
	}
}