Filter的常见应用

1.字符编码过滤器

实现功能，在a.jsp中填写用户名提交到b.jsp，在b.jsp中读取参数名。

a.jsp

<body>
  <form action="encoding/b.jsp" method="post">
    name:<input type="text" name="username"/>
    <input type="submit" value="Submit"/>
  </form>
</body>

b.jsp

<body>
<% request.setCharacterEncoding("UTF-8");%>
  hello:${param.username}
</body>

若需要读取参数的页面太多，需要在每一个页面都添加<% request.setCharacterEncoding("UTF-8");%>，该方法行不通。字符编码过滤器通过配置参数encoding指明使用何种字符编码，以处理Html Form请求参数的中文问题。

HttpFilter.java

package com.javaweb.Filter;

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;

public abstract class HttpFilter implements Filter {
	@Override
	public void destroy() { }
    /*
     * 原生的doFilter方法，在方法内部把ServletRequest和ServletResponse转为了HttpServletRequest和HttpServletResponse，
     * 并调用了doFilter(HttpServletRequest request,HttpServletResponse response,FilterChain filterchain)方法。
     * 若编写Filter的过滤方法不建议直接继承该方法，而建议继承doFilter(HttpServletRequest request,HttpServletResponse response,
     * FilterChain filterchain)方法。
     * */
	@Override
	public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterchain)
			throws IOException, ServletException {
		HttpServletRequest request=(HttpServletRequest) req;
		HttpServletResponse response=(HttpServletResponse) resp;
	    doFilter(request,response,filterchain);
	}
	//抽象方法，为Http请求定制必须实现的方法。
	public abstract void doFilter(HttpServletRequest request,HttpServletResponse response,FilterChain filterchain) throws IOException, ServletException;
    private FilterConfig filterconfig;
    //不建议子类直接覆盖，若直接覆盖，将可能会导致filterConfig成员变量初始化失败。
	@Override
	public void init(FilterConfig filterconfig) throws ServletException {
		this.filterconfig =filterconfig;
		init();
	}
	//供子类继承的初始化方法，可以通过getFilterConfig()获取FilterConfig对象。
    public void init(){}
    //直接返回init(ServletConfig)的ServletConfig对象。
    public FilterConfig getFilterConfig(){
    	return filterconfig;
    }
}

encodingFilter.java

package com.javaweb.Filter;
import java.io.IOException;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.javaweb.Filter.HttpFilter;

public class encodingFilter extends HttpFilter {

	@Override
	public void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain filterchain)
			throws IOException, ServletException {
		//2.指定请求的字符编码为1中读取的字符编码
        request.setCharacterEncoding(encoding);
        //3.调用chain.doFile()方法放行请求
        filterchain.doFilter(request,response);
	}
	private String encoding;
	public void init(){
		//1.读取配置文件web.xml中的字符编码方式
		encoding=getFilterConfig().getServletContext().getInitParameter("encoding");
	}
}

在web.xml中进行配置，首先指定当前页的字符编码：

<context-param>
  <param-name>encoding</param-name>
  <param-value>UTF-8</param-value>
</context-param>

配置encodingFilter，

  <filter>
    <filter-name>encodingFilter</filter-name>
    <filter-class>com.javaweb.Filter.encodingFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>encodingFilter</filter-name>
    <url-pattern>/encoding/*</url-pattern>
  </filter-mapping>

这样，不用在b.jsp页面指定<% request.setCharacterEncoding("UTF-8");%>也不会出现输入中文的乱码问题。

2.检测用户是否登录过的过滤器

系统中的某些页面只有在正常登录后才可以使用，用户请求这些页面时要检查session中有无该用户的信息，但在所有的页面加上session的判断相当麻烦。因此需要重新编写一个用于检测用户是否登录的过滤器，如果用户未登录，则重定向到指定的登录页面。

示例代码：

a.jsp

  <body>
    <h4>AAA page</h4>
    <a href="login/list.jsp">return list...</a>
  </body>

b.jsp

  <body>
    <h4>BBB page</h4>
    <a href="login/list.jsp">return list...</a>
  </body>

c.jsp

  <body>
    <h4>CCC page</h4>
    <a href="login/list.jsp">return list...</a>
  </body>

d.jsp

  <body>
    <h4>DDD page</h4>
    <a href="login/list.jsp">return list...</a>
  </body>

e.jsp

  <body>
    <h4>EEE page</h4>
    <a href="login/list.jsp">return list...</a>
  </body>

list.jsp

  <body>
    <a href="login/a.jsp">AAA</a>
    <br><br>
    <a href="login/b.jsp">BBB</a>
    <br><br>
    <a href="login/c.jsp">CCC</a>
    <br><br>
    <a href="login/d.jsp">DDD</a>
    <br><br>
    <a href="login/e.jsp">EEE</a>
    <br><br>
  </body>

  <body>
  <form action="login/doLogin.jsp" method="post">
    username:<input type="text" name="username"/>
    <input type="submit" value="Submit"/>
  </form>
  </body>

doLogin.jsp

  <body>
    <%
    String username=request.getParameter("username");
    if(username!=null){
        session.setAttribute(application.getInitParameter("sessionKey"),username);
        response.sendRedirect("list.jsp");
    }else{
        response.sendRedirect("login.jsp");
    }

     %>
  </body>

loginFilter.java

package com.javaweb.Filter;

import java.io.IOException;
import java.util.Arrays;
import java.util.List;

import javax.servlet.FilterChain;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.javaweb.Filter.HttpFilter;

public class loginFilter extends HttpFilter {
    String sessionKey;
    String redirectUrl;
    String uncheckedUrls;
	@Override
	public void init() {
		ServletContext servletcontext=getFilterConfig().getServletContext();
		sessionKey=servletcontext.getInitParameter("sessionKey");
		redirectUrl=servletcontext.getInitParameter("redirectPage");
		uncheckedUrls=servletcontext.getInitParameter("uncheckedPage");

	}

	@Override
	public void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain filterchain)
			throws IOException, ServletException {
		//b.jsp    c.jsp   d.jsp   ...
		String servletPath=request.getServletPath();
		//检查uncheckedUrls是否包含请求的URL，若包含，放行
		List<String> Urls=Arrays.asList(uncheckedUrls.split(","));
		if (Urls.contains(servletPath)){
			filterchain.doFilter(request, response);
			return;
		}
		//从sessionKey中检查是否包含sessionKey对应的值，若值不存在，则重定向到redirectUrl
		Object user=request.getSession().getAttribute(sessionKey);
		if (user==null){
			response.sendRedirect(request.getContextPath()+redirectUrl);
			return;
		}
		//若存在，则放行
		filterchain.doFilter(request, response);

	}

}

在web.xml中进行配置：

  <context-param>
    <param-name>redirectPage</param-name>
    <param-value>/login/login.jsp</param-value>
  </context-param>
  <context-param>
    <param-name>uncheckedPage</param-name>
    <param-value>/login/a.jsp,/login/doLogin.jsp,/login/list.jsp,/login/login.jsp</param-value>
  </context-param>
  <context-param>
    <param-name>sessionKey</param-name>
    <param-value>sessionValue</param-value>
  </context-param>    <filter>    <filter-name>loginFilter</filter-name>    <filter-class>com.javaweb.Filter.loginFilter</filter-class>  </filter>  <filter-mapping>    <filter-name>loginFilter</filter-name>    <url-pattern>/login/*</url-pattern>  </filter-mapping>