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>
login.jsp
<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>
AAA页面不需要登录即可访问，
点击BBB（及之后的）超链接时，跳转到登录页面：
填写登录名之后再访问BBB，可以跳转到BBB页面：
