Filter的常見應用_ZenDei技術網路在線

1.字元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 b.jsp 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。字元編碼過濾器通過配置參數 ...

1.字元編碼過濾器

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。字元編碼過濾器通過配置參數encoding指明使用何種字元編碼，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請求定製必須實現的方法。
	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成員變數初始化失敗。
	@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.指定請求的字元編碼為1中讀取的字元編碼
        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中的字元編碼方式
		encoding=getFilterConfig().getServletContext().getInitParameter("encoding");
	}
}

在web.xml中進行配置，首先指定當前頁的字元編碼：

<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>

  <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：