java_过滤器|fliter

功能

1. 用于拦截传入的请求和传出的响应
2. 修改或以某种方式处理正在客户端和服务端之间的数据流

如何使用

fliter与使用Servlet类似，是java WEB提供的一个接口，开发者只需要自定义一个类并且实现该接口即可

1. 注解方式

@WebFilter("/filter_Servlet")
public class myfliter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    }
}

@WebServlet("/filter_Servlet")
public class filterServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = (String) req.getAttribute("name");
        System.out.println(name);
    }
}

2. web.xml中配置filter

拦截单个servlet

    <filter>
        <filter-name>myfliter</filter-name>
        <filter-class>com.example.qihonwangjava.myfliter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>myfliter</filter-name>
        <url-pattern>/filter_Servlet</url-pattern>
    </filter-mapping>

拦截多个servlet

    <filter>
        <filter-name>myfliter</filter-name>
        <filter-class>com.example.qihonwangjava.myfliter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>myfliter</filter-name>
        <url-pattern>/filter_Servlet</url-pattern>
        <url-pattern>/myServlet</url-pattern>
    </filter-mapping>

2. filter类和sevlet

filterChain.doFilter(servletRequest,servletResponse);
必须添加 否则请求会一直停留在过滤器中 无法进入servlet

public class myfliter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws    ServletException {
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("utf-8");
        servletRequest.setAttribute("name","祁红旺");
        filterChain.doFilter(servletRequest,servletResponse);
    }
    @Override
    public void destroy() {
    }
}

@WebServlet("/filter_Servlet")
public class filterServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println(req.getAttribute("name"));
    }
}

生命周期

public class myfliter implements Filter {
    public myfliter() {
        System.out.println("通过反射机制调用无参构造创建了对象");
    }
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("调用init");
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("utf-8");
        servletRequest.setAttribute("name","祁红旺");
        filterChain.doFilter(servletRequest,servletResponse);
    }
    @Override
    public void destroy() {
    }
}

打印结果
 1、通过反射机制调用无参构造创建了对象
 2、调用init
总结
1、tomcat服务启动通过web.xml的配置进行反射机制自动就创建对象同时初始化 当服务关闭的时候调用destroy销毁对象
2、servlet是调用请求的时候才去创建对象

实际开发中filter的使用场景

• 统一处理中文乱码
• 屏蔽敏感词

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        //将敏感词替换成“***”
        servletRequest.setCharacterEncoding("utf-8");
        String name =  servletRequest.getParameter("name");
        name= name.replaceAll("敏感词","***");
        //通过attribute替换Parameter
        servletRequest.setAttribute("name",name);
    }

@WebServlet("/filter_Servlet")
public class filterServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name = (String) req.getAttribute("name");
        System.out.println(name);
    }
}

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