过滤器浅谈

程序员文章站 2022-03-05 14:15:54

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过滤器的生命周期：
1、实例化
通过web.xml配置，web容器启动时就会加载过滤器，实例化只会实例化一次。Servlet3.0规范中新增了@WebFilter的方式，这种方式用于讲一个类声明为过滤器，该注解将会在部署时被web容器处理，容器将根据具体的属性配置将相应的类部署为过滤器。
2、初始化
调用过滤器的init()，这是过滤器的初始化方法，web容器创建过滤器实例后将调用这个方法。这个方法中可以读取web.xml文件中过滤器的参数。
3、过滤方法
每次请求都会调用过滤器的doFilter()，这个方法完成实际的过滤操作。这个是过滤器的核心方法，当用户请求访问与过滤器关联的URL时，web容器将先调用过滤器的doFilter()。FilterChain参数可以调用chain.doFilter()，将请求传递给下一个过滤器（或目标资源），或利用转发，重定向将请求转发到其他资源。
4、销毁
当web容器在销毁过滤器实例前调用destroy()，在这个方法中可以释放过滤器占用的资源。

过滤器的分类：
指定过滤器的转发模式，对应filter中的dispatchTypes属性，具体属性值包括以下5个，其中Servlet2.5支持REQUEST、FORWARD、INCLUDE、ERROR，Servlet3.0新增ASYNC
1、REQUEST
用户直接访问页面时，web容器将会调用过滤器，过滤器的默认种类。
2、FORWARD
目标资源是通过RequestDispatch的forward访问时，该过滤器将被调用。
3、INCLUDE
目标资源是通过RequestDispatch的include访问时，该过滤器将被调用。
4、ERROR
目标资源是通过声明式异常处理机制调用时，过滤器将被调用。
5、ASYNC
支持异步处理

过滤器示例：
web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">

    <filter>
        <filter-name>loginFilter</filter-name>
        <filter-class>com.pumpkin.filter.LoginFilter</filter-class>
        <init-param>
            <param-name>unfilteredPaths</param-name>
            <param-value>login.jsp;failure.jsp;login</param-value>
        </init-param>
    </filter>
    <filter>
        <filter-name>EncodingFilter</filter-name>
        <filter-class>com.pumpkin.filter.EncodingFilter</filter-class>
        <init-param>
            <param-name>charset</param-name>
            <param-value>UTF-8</param-value>
        </init-param>
    </filter>

    <filter-mapping>
        <filter-name>loginFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>EncodingFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.pumpkin.servlet.LoginServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>

    <welcome-file-list>
        <welcome-file>login.jsp</welcome-file>
    </welcome-file-list>

</web-app>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Filter实例</title>
</head>
<body>
<%
    System.out.println("===login.jsp===");
%>
<form action="login" method="post">
    用户名：<input type="text" name="username"/>
    <br/>
    密码：<input type="password" name="password"/>
    <br/>
    <input type="submit" value="提交"/>
</form>
</body>
</html>

success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>成功页面</title>
</head>
<body>
<%
    System.out.println("===success.jsp===");
%>
${username}，登陆成功！
</body>
</html>

failure.jsp

<%--
  Created by IntelliJ IDEA.
  User: Administrator
  Date: 2017/6/17
  Time: 22:47
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>失败页面</title>
</head>
<body>
<%
    System.out.println("===failure.jsp===");
%>
登录失败！
</body>
</html>

LoginServlet.java

package com.pumpkin.servlet;

import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;


public class LoginServlet extends HttpServlet {
    public LoginServlet() {
        super();
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("===LoginServlet===doGet===");
        doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("===LoginServlet===doPost===start===");
        String username =  req.getParameter("username");
        String password =  req.getParameter("password");
        System.out.println("===username===" + username + "===");

        if ("admin".equals(username) && "admin".equals(password)) {
            HttpSession session = req.getSession();
            session.setAttribute("username",username);
            resp.sendRedirect(req.getContextPath() + "/success.jsp");
        }else {
            resp.sendRedirect(req.getContextPath() + "/failure.jsp");
        }
        System.out.println("===LoginServlet===doPost===end===");
    }
}

LoginFilter.java

package com.pumpkin.filter;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.util.logging.Filter;


public class LoginFilter implements javax.servlet.Filter {
    private FilterConfig config;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("===LoginFilter===init===");
        config = filterConfig;
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("===LoginFilter===doFilter===start===");
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        HttpSession session = request.getSession();

        String unfilteredPaths = config.getInitParameter("unfilteredPaths");
        if (unfilteredPaths != null) {
            String[] strArray = unfilteredPaths.split(";");
            for (int i = 0; i < strArray.length; i++) {
                if (strArray[i] == null || "".equals(strArray[i])) {
                    continue;
                }
                if (request.getRequestURI().indexOf(strArray[i]) != -1) {
                    filterChain.doFilter(request, response);
                    return;
                }
            }
        }

        if (session.getAttribute("username") != null) {
            filterChain.doFilter(request, response);
        } else {
            response.sendRedirect("login.jsp");
        }
        System.out.println("===LoginFilter===doFilter===end===");
    }

    @Override
    public void destroy() {
        System.out.println("===LoginFilter===destroy===");
    }
}

EncodingFilter.java

package com.pumpkin.filter;

import com.sun.net.httpserver.Filter;
import com.sun.net.httpserver.HttpExchange;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import java.io.IOException;


public class EncodingFilter implements javax.servlet.Filter{
    private FilterConfig config;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("===EncodingFilter===init===");
        config = filterConfig;
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("===EncodingFilter===doFilter===start===");

        String charset = config.getInitParameter("charset");
        if (charset == null) {
            charset = "UTF-8";
        }
        servletRequest.setCharacterEncoding(charset);
        filterChain.doFilter(servletRequest,servletResponse);

        System.out.println("===EncodingFilter===doFilter===end===");
    }

    @Override
    public void destroy() {
        System.out.println("===EncodingFilter===destroy===");
    }
}

Servlet3.0特性：
Servlet3.0支持注解的方式注册servlet和filter，我们不需要在web.xml中配置servlet和filter，只需要在servlet和filter类中用@方式注入即可，代码如下：

LoginServlet.java

package com.pumpkin.servlet;

import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;


@WebServlet(name = "LoginServlet",urlPatterns = "/login")
public class LoginServlet extends HttpServlet {
    public LoginServlet() {
        super();
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("===LoginServlet===doGet===");
        doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("===LoginServlet===doPost===start===");
        String username =  req.getParameter("username");
        String password =  req.getParameter("password");
        System.out.println("===username===" + username + "===");

        if ("admin".equals(username) && "admin".equals(password)) {
            HttpSession session = req.getSession();
            session.setAttribute("username",username);
            resp.sendRedirect(req.getContextPath() + "/success.jsp");
        }else {
            resp.sendRedirect(req.getContextPath() + "/failure.jsp");
        }
        System.out.println("===LoginServlet===doPost===end===");
    }
}

LoginFilter.java

package com.pumpkin.filter;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.util.logging.Filter;


@WebFilter(filterName = "Filter1_LoginFilter", value = {"/*"}, initParams = {@WebInitParam(name = "unfilteredPaths", value = "login.jsp;failure.jsp;login")})
public class LoginFilter implements javax.servlet.Filter {
    private FilterConfig config;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("===LoginFilter===init===");
        config = filterConfig;
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("===LoginFilter===doFilter===start===");
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        HttpSession session = request.getSession();

        String unfilteredPaths = config.getInitParameter("unfilteredPaths");
        if (unfilteredPaths != null) {
            String[] strArray = unfilteredPaths.split(";");
            for (int i = 0; i < strArray.length; i++) {
                if (strArray[i] == null || "".equals(strArray[i])) {
                    continue;
                }
                if (request.getRequestURI().indexOf(strArray[i]) != -1) {
                    filterChain.doFilter(request, response);
                    return;
                }
            }
        }

        if (session.getAttribute("username") != null) {
            filterChain.doFilter(request, response);
        } else {
            response.sendRedirect("login.jsp");
        }
        System.out.println("===LoginFilter===doFilter===end===");
    }

    @Override
    public void destroy() {
        System.out.println("===LoginFilter===destroy===");
    }
}

EncodingFilter.java

package com.pumpkin.filter;

import com.sun.net.httpserver.Filter;
import com.sun.net.httpserver.HttpExchange;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import java.io.IOException;


@WebFilter(filterName = "Filter0_EncodingFilter", value = {"/*"}, initParams = {@WebInitParam(name = "charset", value = "UTF-8")})
public class EncodingFilter implements javax.servlet.Filter{
    private FilterConfig config;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("===EncodingFilter===init===");
        config = filterConfig;
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("===EncodingFilter===doFilter===start===");

        String charset = config.getInitParameter("charset");
        if (charset == null) {
            charset = "UTF-8";
        }
        servletRequest.setCharacterEncoding(charset);
        filterChain.doFilter(servletRequest,servletResponse);

        System.out.println("===EncodingFilter===doFilter===end===");
    }

    @Override
    public void destroy() {
        System.out.println("===EncodingFilter===destroy===");
    }
}

LoginServlet.java

@WebServlet(name = "LoginServlet",urlPatterns = "/login")

这样的配置等同web.xml中的配置

<servlet>
	<servlet-name>LoginServlet</servlet-name>
	<servlet-class>com.pumpkin.servlet.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
	<servlet-name>LoginServlet</servlet-name>
	<url-pattern>/login</url-pattern>
</servlet-mapping>

LoginFilter.java

@WebFilter(filterName = "Filter1_LoginFilter", value = {"/*"}, initParams = {@WebInitParam(name = "unfilteredPaths", value = "login.jsp;failure.jsp;login")})

这样的配置等同web.xml中的配置

<filter>
	<filter-name>loginFilter</filter-name>
	<filter-class>com.pumpkin.filter.LoginFilter</filter-class>
	<init-param>
		<param-name>unfilteredPaths</param-name>
		<param-value>login.jsp;failure.jsp;login</param-value>
	</init-param>
</filter>

<filter-mapping>
	<filter-name>loginFilter</filter-name>
	<url-pattern>/*</url-pattern>
</filter-mapping>

EncodingFilter.java

@WebFilter(filterName = "Filter0_EncodingFilter", value = {"/*"}, initParams = {@WebInitParam(name = "charset", value = "UTF-8")})

这样的配置等同web.xml中的配置

<filter>
	<filter-name>EncodingFilter</filter-name>
	<filter-class>com.pumpkin.filter.EncodingFilter</filter-class>
	<init-param>
		<param-name>charset</param-name>
		<param-value>UTF-8</param-value>
	</init-param>
</filter>

<filter-mapping>
	<filter-name>EncodingFilter</filter-name>
	<url-pattern>/*</url-pattern>
</filter-mapping>

Servlet3.0过滤器执行顺序：
Servlet3.0之前Filter过滤的顺序是由用户在web.xml中配置的顺序决定的，如我们的程序会先执行LoginFilter,再执行encodingFilter。
在3.0之后新增@WebFilter注解，当使用注解配置多个Filter时，用户无法控制其执行顺序，此时Filter过滤的顺序是按照Filter的类名来控制的，按自然排序的规则。LoginFilter.java 和 EncodingFilter.java 这两个文件里面分别是“用户登录过滤器”和“字符集过滤器”，因为这两个文件的首字母E排L之前，导致每次执行的时候都是先执行“字符集过滤器”再执行“用户登录过滤器”，所以我们现在修改两个文件的名称分别为
Filter0_EncodingFilter.java
Filter1_LoginFilter.java
这样就能先执行“用户登录过滤器”再执行“字符集过滤器”。

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