Filter

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Tips

Usually to multiple Servlet Code for unified processing , Can be written in Filter in

effect ：

1. Log messages ： Which needs to be recorded IP, When and what requests were accessed , Which parameters are passed in …
2. Record exception log : Handle global exceptions
3. Control access rights ： Some or all functions of the system can only be used after logging in .
4. Character set settings

Example

 This is the login page  login.jsp  <br>
<form action="/loginServlet" method="get">
	 user name ：<input type="text" name="username"/> <br>
	 The secret   code ：<input type="password" name="password"/> <br>
	<input type="submit" />
</form>

public class LoginServlet extends HttpServlet {
    
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException,IOException {
    
		resp.setContentType("text/html; charset=UTF-8");
		String username = req.getParameter("username");
		String password = req.getParameter("password");
		if ("admin".equals(username) && "123456".equals(password)) {
    
			req.getSession().setAttribute("user",username);
			resp.getWriter().write(" Login successful ！！！");
		} else {
    
			req.getRequestDispatcher("/login.jsp").forward(req,resp);
		}
	}
}

@WebFilter("/*")
public class AdminFilter implements Filter {
    
	@Override
	public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException,ServletException {
    
        
		HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
		HttpSession session = httpServletRequest.getSession();
		Object user = session.getAttribute("user");

        //  If it is equal to  null, It means you haven't logged in yet 
        if (user == null) {
    
            servletRequest.getRequestDispatcher("/login.jsp").forward(servletRequest,servletResponse);
            return;
        } else {
    
            //  Let the program continue to access the user's target resources 
            filterChain.doFilter(servletRequest,servletResponse);
        }
    }
}

If you don't use annotations , You need to in web.xml Middle configuration

<!--filter  The tag is used to configure a  Filter  filter -->
<filter>
	<!-- to  filter  Make up an alias -->
	<filter-name>AdminFilter</filter-name>
	<!-- To configure  filter  Full class name of -->
	<filter-class>com.atguigu.filter.AdminFilter</filter-class>
</filter>

<!--filter-mapping  To configure  Filter  The interception path of the filter -->
<filter-mapping>
	<!--filter-name  Indicates to which... The current interception path is given  filter  Use -->
	<filter-name>AdminFilter</filter-name>
	<!--url-pattern  Configure interception path  /  Indicates that the request address is ：http://ip:port/ Project path /  Mapping to  IDEA  Of  web  Catalog  /admin/*  Indicates that the request address is ：http://ip:port/ Project path /admin/* -->
	<url-pattern>/admin/*</url-pattern>
</filter-mapping>

Life cycle

@WebFilter("/*")
public class MyFilter implements Filter {
    

    public MyFilter() {
    
        System.out.println("1. Construction method , Execute when starting the server ");
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    
        System.out.println("2. Initialization method , Execute when starting the server ");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
        System.out.println("3.doFilter, Accessing any page that passes through this filter will execute ");
        //  Follow up 
        filterChain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {
    
        System.out.println("4.destroy, Execute when shutting down the server ");
    }
}

FilterConfig class

Tips

Filter Filter profile class ,Tomcat Each creation Filter When , It will also create a FilterConfig class

The setting of interception path is the same as Servlet Medium url-pattern Set up

Filter effect ：

1. obtain Filter The name of filter-name The content of
2. To get in Filter Configured in init-param Initialize parameters
3. obtain ServletContext object

Example

@Override
public void init(FilterConfig filterConfig) throws ServletException {
    
    System.out.println("2.Filter  Of  init(FilterConfig filterConfig) initialization ");
    
    // 1、 obtain  Filter  The name of  filter-name  The content of 
    System.out.println("filter-name  The value of is ：" + filterConfig.getFilterName());
    
    // 2、 To get in  web.xml  Configured in  init-param  Initialize parameters 
    System.out.println(" Initialize parameters  username  The value of is ：" + filterConfig.getInitParameter("username"));
    System.out.println(" Initialize parameters  url  The value of is ：" + filterConfig.getInitParameter("url"));
    
    // 3、 obtain  ServletContext  object 
    //org.apache.catalina.core.ApplicationContextFacade@d359b2b
    System.out.println(filterConfig.getServletContext());
}

<!--filter  The tag is used to configure a  Filter  filter -->
<filter>
	<!-- to  filter  Make up an alias -->
	<filter-name>AdminFilter</filter-name>
	<!-- To configure  filter  Full class name of -->
	<filter-class>com.atguigu.filter.AdminFilter</filter-class>

    <init-param>
        <param-name>username</param-name>
        <param-value>root</param-value>
    </init-param>
    
    <init-param>
       <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost3306/test</param-value>
    </init-param>
</filter>

FilterChain Filter chain

1. In more than one Filter When the filter is executed , they Priority of execution , By web.xml The order of configuration in , From top to bottom

2. all filter And target resources are executed in the same thread by default

3. Multiple Filter When jointly executed , They all use the same Request object

4. If a Filter Not written in doFilter, So it's time to Filter Start back to , Don't go any further

   That is, the server thinks that the execution has been completed