过滤器Filter

 过滤器Filter也具有生命周期：init()->doFilter()->destroy()，由部署文件中的filter元素驱动。在servlet2.4中，过滤器同样可以用于请求分派器，但须在web.xml中声明，<dispatcher>INCLUDE或FORWARD或REQUEST或ERROR</dispatcher>该元素位于filter-mapping中。

一、批量设置请求编码

public class EncodingFilter implements Filter {

    private String encoding = null;

    public void destroy() {
         encoding = null;
     }

    public void doFilter(ServletRequest request, ServletResponse response,
             FilterChain chain) throws IOException, ServletException {
         String encoding = getEncoding();
        if (encoding == null){
             encoding = "gb2312";
         }
         request.setCharacterEncoding(encoding);// 在请求里设置上指定的编码
         chain.doFilter(request, response);
     }

    public void init(FilterConfig filterConfig) throws ServletException {
        this.encoding = filterConfig.getInitParameter("encoding");
     }

    private String getEncoding() {
        return this.encoding;
     }

}

public class EncodingFilter implements Filter {

 private String encoding = null;

 public void destroy() {
  encoding = null;
 }

 public void doFilter(ServletRequest request, ServletResponse response,
   FilterChain chain) throws IOException, ServletException {
  String encoding = getEncoding();
  if (encoding == null){
   encoding = "gb2312";
  }
  request.setCharacterEncoding(encoding);// 在请求里设置上指定的编码
  chain.doFilter(request, response);
 }

 public void init(FilterConfig filterConfig) throws ServletException {
  this.encoding = filterConfig.getInitParameter("encoding");
 }

 private String getEncoding() {
  return this.encoding;
 }

}

 

<filter>
    <filter-name>EncodingFilter</filter-name>
    <filter-class>com.logcd.filter.EncodingFilter</filter-class>
    <init-param>
       <param-name>encoding</param-name>
       <param-value>gb2312</param-value>
    </init-param>
</filter>

<filter-mapping>
   <filter-name>EncodingFilter</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>

<filter>
    <filter-name>EncodingFilter</filter-name>
    <filter-class>com.logcd.filter.EncodingFilter</filter-class>
    <init-param>
       <param-name>encoding</param-name>
       <param-value>gb2312</param-value>
    </init-param>
</filter>

<filter-mapping>
   <filter-name>EncodingFilter</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>

二、用filter控制用户访问权限

public void doFilter(ServletRequest request,
         ServletResponse response,
         FilterChain chain)
        throws IOException, ServletException {

     HttpServletRequest req = (HttpServletRequest) request;
     HttpServletResponse res = (HttpServletResponse) response;

     HttpSession session = req.getSession();
    if (session.getAttribute("username") != null) {//登录后才能访问
         chain.doFilter(request, response);
     } else {
         res.sendRedirect("../failure.jsp");
     }
}

public void doFilter(ServletRequest request,
        ServletResponse response,
        FilterChain chain)
        throws IOException, ServletException {

    HttpServletRequest req = (HttpServletRequest) request;
    HttpServletResponse res = (HttpServletResponse) response;

    HttpSession session = req.getSession();
    if (session.getAttribute("username") != null) {//登录后才能访问
        chain.doFilter(request, response);
    } else {
        res.sendRedirect("../failure.jsp");
    }
}

<filter>
    <filter-name>SecurityFilter</filter-name>
    <filter-class>com.logcd.filter.SecurityFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SecurityFilter</filter-name>
    <url-pattern>/admin/*</url-pattern>
</filter-mapping>

<filter>
    <filter-name>SecurityFilter</filter-name>
    <filter-class>com.logcd.filter.SecurityFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SecurityFilter</filter-name>
    <url-pattern>/admin/*</url-pattern>
</filter-mapping>

三、过滤链

   两个过滤器，EncodingFilter负责设置编码，SecurityFilter负责控制权限，服务器会按照web.xml中过滤器定义的先后循序组装成一条链，然后一次执行其中的doFilter()方法。执行的顺序就如上图所示，执行第一个过滤器的chain.doFilter()之前的代码，第二个过滤器的chain.doFilter()之前的代码，请求的资源，第二个过滤器的chain.doFilter()之后的代码，第一个过滤器的chain.doFilter()之后的代码，最后返回响应。
执行的代码顺序是：

1. 执行EncodingFilter.doFilter()中chain.doFilter()之前的部分：request.setCharacterEncoding("gb2312");
2. 执行SecurityFilter.doFilter()中chain.doFilter()之前的部分：判断用户是否已登录。
3. 如果用户已登录，则访问请求的资源：/admin/index.jsp。
4. 如果用户未登录，则页面重定向到：/failure.jsp。
5. 执行SecurityFilter.doFilter()中chain.doFilter()之后的部分：这里没有代码。
6. 执行EncodingFilter.doFilter()中chain.doFilter()之后的部分：这里也没有代码。

   过滤链的好处是，执行过程中任何时候都可以打断，只要不执行chain.doFilter()就不会再执行后面的过滤器和请求的内容。而在实际使用时，就要特别注意过滤链的执行顺序问题，像EncodingFilter就一定要放在所有Filter之前，这样才能确保在使用请求中的数据前设置正确的编码。