javaWeb过滤器——Filter

一、定义

应用场景
  自动登录
  统一设置编码格式
  访问权限控制
  敏感字符过滤等

二、创建Filter

 1、配置web.xml

    <filter>
        <filter-name>first</filter-name>
        <filter-class>net.pp.Filter.FirstFilter</filter-class>
        <init-param>
            <param-name>param</param-name>
            <param-value>value</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>first</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>REQUEST</dispatcher>
    </filter-mapping>

2、实现Filter接口

FirstFilter.java

package net.pp.Filter;

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 java.io.IOException;

public class FirstFilter implements Filter{

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String filterName = filterConfig.getFilterName();
        String initParameter = filterConfig.getInitParameter("param");
        ServletContext servletContext = filterConfig.getServletContext();
        System.out.println(filterName+"--"+initParameter);

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("FirstFilter已经执行拦截！");
        //解除拦截
        filterChain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {

    }
}

3、在Servlet中测试

TestFilter

package net.pp.servlet;

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

@WebServlet(name = "TestServlet" ,urlPatterns = "/testFilter")
public class TestServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("test/html;charset=utf-8");
        response.getWriter().write("testFilter已经被访问了！");

    }
}

结果：

 三、配置Filter

1）url-Pattern配置
        配置要拦截的资源

1. 以指定资源匹配。例如"/index.jsp"
2. 以目录匹配。例如"/servlet/*"
3. 以后缀名匹配，例如"*.jsp"
4. 通配符，拦截所有web资源。"/*"

    <filter-mapping>
        <filter-name>first</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>REQUEST</dispatcher>
    </filter-mapping>

 2）dispatcherTypes **
配置拦截的类型，可配置多个。

    FORWARD,//转发的
    INCLUDE,//包含在页面的
    REQUEST,//请求的
    ASYNC,//异步的
    ERROR;//出错的

  3）注解方式配置

package filter;

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

@WebFilter(filterName = "CharsetFilter",
        urlPatterns = "/*",/*通配符（*）表示对所有的web资源进行拦截*/
        initParams = {
        @WebInitParam(name = "charset", value = "utf-8")/*这里可以放一些初始化的参数*/
        })
public class CharsetFilter implements Filter {
    private String filterName;
    private String charset;

    public void destroy() {
        /*销毁时调用*/
        System.out.println(filterName + "销毁");
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        /*过滤方法 主要是对request和response进行一些处理，然后交给下一个过滤器或Servlet处理*/
	System.out.println(filterName + "doFilter()");
        req.setCharacterEncoding(charset);
        resp.setCharacterEncoding(charset);
        chain.doFilter(req, resp);
    }

    public void init(FilterConfig config) throws ServletException {

        /*初始化方法  接收一个FilterConfig类型的参数 该参数是对Filter的一些配置*/

        filterName = config.getFilterName();
        charset = config.getInitParameter("charset");

        System.out.println("过滤器名称：" + filterName);
        System.out.println("字符集编码：" + charset);

    }

}

多个Filter的执行顺序

在我们的请求到达Servle之间是可以经过多个Filter的，一般来说，建议Filter之间不要有关联，各自处理各自的逻辑即可。
如果一定要确保执行顺序，就要对配置进行修改了，执行顺序如下

在web.xml中，filter执行顺序跟<filter-mapping>的顺序有关，先声明的先执行
使用注解配置的话，filter的执行顺序跟名称的字母顺序有关，例如AFilter会比BFilter先执行
如果既有在web.xml中声明的Filter，也有通过注解配置的Filter，那么会优先执行web.xml中配置的Filter