Filter

Posted 2020-06-14

1.什么是Filter

① JavaWeb的一个重要组件,可以对发送到Servlet的请求进行拦截，并对响应也进行拦截。

② Filter是实现了Filter接口的Java类

③ Filter 需要在web.xml文件中进行配置和映射

2.如何创建一个Filter,并跑起来

①.创建一个Filter类：实现Filter接口：public class MyFilter implements Filter 

②.在web.xml文件中配置并映射该Filter

 

<!-- 注册Filter -->
    <filter>
        <filter-name>firstFilter</filter-name>
        <filter-class>com.java.testFilter.MyFilter</filter-class>
    </filter>
    <!-- 映射Filter -->
    <filter-mapping>
        <filter-name>firstFilter</filter-name>
        <url-pattern>/test.jsp</url-pattern>
    </filter-mapping>

3.Filter的API

Filter接口：

@Override
public void destroy() 

释放当前Filter 所占用的资源的方法.在Filter被销毁之前调用,且只被调用一次.　　　

@Override
public void doFilter(ServletRequest arg0, ServletResponse arg1,
FilterChain filterChain) throws IOException, ServletException 

真正Filter的逻辑代码需要写在该方法中.每次拦截都会调用该方法.

*FilterChain ： Filter链，多个Filter可以构成一个Filter链. 

                      filterChain.doFilter(ServletRequest arg0, ServletResponse arg1) 把请求传给Filter链的下一个

　　　　　　　　Filter，若当前Filter是Filter链的最后一个Filter,将把请求给到目标Servlet（或JSP）

*多个Filter的拦截顺序和<filter-mapping>的配置顺序有关，靠前的先被调用.

@Override
public void init(FilterConfig arg0) throws ServletException {
System.out.println("init...");
}

//类似于Servlet的init方法.在创建Filter对象(Filter对象被web应用加载时立即被调用,
　　且只被调用一次.该方法对于当前的Filter进行初始化操作.Filter实例是单例的)

*FilterConfig 类似于 ServletConfig

*可以在web.xml文件中配置当前Filter 的初始化参数.配置方式也和Servlet类似.

<!-- 注册Filter -->
    <filter>
        <filter-name>firstFilter</filter-name>
        <filter-class>com.java.testFilter.MyFilter</filter-class>
        <init-param>
            <param-name>name</param-name>
            <param-value>java</param-value>
        </init-param>
    </filter>

 

*与开发Servlet不同的是,Filter 接口并没有相应的实现类可以继承，要开发过滤器只能实现接口 

同一个web.xml文件中可以为同一个Filter设置多个映射　　　　

example：

一个登陆界面

<font color="red">${message }</font>

    <br><br>
    <form action="${pageContext.request.contextPath }/hello.jsp" method="post">
        UserName:<input type="text" name="username" value="${param.username }">
        <br>
        Password :<input type="password" name="password">
        <br>
        <input type="submit" value="submit">
    </form>

 

两个过滤器

package com.java.testFilter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 拦截/hello.jsp,如果username 不是Tom 则转发到login.jsp,否则放行
 */
public class UserNameFilter implements Filter {

    public UserNameFilter() {
    }

    public void destroy() {
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        String initUser = filterConfig.getInitParameter("username");
        String username = request.getParameter("username");
        if(!initUser.equals(username)){
            request.setAttribute("message", "用户名错误！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
            return;
        }
        chain.doFilter(request, response);
    }
    
    private FilterConfig filterConfig;
    public void init(FilterConfig fConfig) throws ServletException {
        this.filterConfig = fConfig;
    }

}

package com.java.testFilter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

/**
 * 若密码不是1234，则把请求给login.jsp,否则给目标页面
 */
public class PasswordFilter implements Filter {

    public PasswordFilter() {
        // TODO Auto-generated constructor stub
    }

    public void destroy() {
        // TODO Auto-generated method stub
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        ServletContext servletContext = filterConfig.getServletContext();
        String initPass = servletContext.getInitParameter("password");
        String password = request.getParameter("password");
        if(!initPass.equals(password)){
            request.setAttribute("message", "密码错误！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
            return;
        }

        chain.doFilter(request, response);
    }

    /**
     * @see Filter#init(FilterConfig)
     */
    private FilterConfig filterConfig;
    public void init(FilterConfig fConfig) throws ServletException {
        this.filterConfig = fConfig;
    }

}

 

web.xml配置

<context-param>
    <param-name>password</param-name>
    <param-value>1234</param-value>
  </context-param>
<filter>
    <display-name>UserNameFilter</display-name>
    <filter-name>UserNameFilter</filter-name>
    <filter-class>com.java.testFilter.UserNameFilter</filter-class>
    <init-param>
      <param-name>username</param-name>
      <param-value>Tom</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>UserNameFilter</filter-name>
    <url-pattern>/hello.jsp</url-pattern>
  </filter-mapping>
  <filter>
    <display-name>PasswordFilter</display-name>
    <filter-name>PasswordFilter</filter-name>
    <filter-class>com.java.testFilter.PasswordFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>PasswordFilter</filter-name>
    <url-pattern>/hello.jsp</url-pattern>
  </filter-mapping>
  <servlet>
    <description></description>
    <display-name>TestServlet2</display-name>
    <servlet-name>TestServlet2</servlet-name>
    <servlet-class>com.java.testFilter.TestServlet2</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>TestServlet2</servlet-name>
    <url-pattern>/TestServlet2</url-pattern>
  </servlet-mapping>

 

hello.jsp

<h4>Hello Page</h4>
    Hello:${param.username }