Filter的常見應用
阿新 • • 發佈：2018-11-11
import 功能 web jsp imp for 因此 讀取配置文件 text
1.字符編碼過濾器
實現功能，在a.jsp中填寫用戶名提交到b.jsp，在b.jsp中讀取參數名。
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>
若需要讀取參數的頁面太多，需要在每一個頁面都添加<% request.setCharacterEncoding("UTF-8");%>，該方法行不通。字符編碼過濾器通過配置參數encoding指明使用何種字符編碼，以處理Html Form請求參數的中文問題。
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，在方法內部把ServletRequest和ServletResponse轉為了HttpServletRequest和HttpServletResponse，
* 並調用了doFilter(HttpServletRequest request,HttpServletResponse response,FilterChain filterchain)方法。
* 若編寫Filter的過濾方法不建議直接繼承該方法，而建議繼承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，為Http請求定制必須實現的方法。
public abstract void doFilter(HttpServletRequest request,HttpServletResponse response,FilterChain filterchain) throws IOException, ServletException;
private FilterConfig filterconfig;
//不建議子類直接覆蓋，若直接覆蓋，將可能會導致filterConfig成員變量初始化失敗。
@Override
public void init(FilterConfig filterconfig) throws ServletException {
this.filterconfig =filterconfig;
init();
}
//供子類繼承的初始化方法，可以通過getFilterConfig()獲取FilterConfig對象。
public void init(){}
//直接返回init(ServletConfig)的ServletConfig對象。
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.指定請求的字符編碼為1中讀取的字符編碼
request.setCharacterEncoding(encoding);
//3.調用chain.doFile()方法放行請求
filterchain.doFilter(request,response);
}
private String encoding;
public void init(){
//1.讀取配置文件web.xml中的字符編碼方式
encoding=getFilterConfig().getServletContext().getInitParameter("encoding");
}
}
在web.xml中進行配置，首先指定當前頁的字符編碼：
<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>
這樣，不用在b.jsp頁面指定<% request.setCharacterEncoding("UTF-8");%>也不會出現輸入中文的亂碼問題。
2.檢測用戶是否登錄過的過濾器
系統中的某些頁面只有在正常登錄後才可以使用，用戶請求這些頁面時要檢查session中有無該用戶的信息，但在所有的頁面加上session的判斷相當麻煩。因此需要重新編寫一個用於檢測用戶是否登錄的過濾器，如果用戶未登錄，則重定向到指定的登錄頁面。
示例代碼：
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();
//檢查uncheckedUrls是否包含請求的URL，若包含，放行
List<String> Urls=Arrays.asList(uncheckedUrls.split(","));
if (Urls.contains(servletPath)){
filterchain.doFilter(request, response);
return;
}
//從sessionKey中檢查是否包含sessionKey對應的值，若值不存在，則重定向到redirectUrl
Object user=request.getSession().getAttribute(sessionKey);
if (user==null){
response.sendRedirect(request.getContextPath()+redirectUrl);
return;
}
//若存在，則放行
filterchain.doFilter(request, response);
}
}
在web.xml中進行配置：
<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頁面不需要登錄即可訪問，
點擊BBB（及之後的）超鏈接時，跳轉到登錄頁面：
填寫登錄名之後再訪問BBB，可以跳轉到BBB頁面：
Filter的常見應用