Servlet 过滤器 Filter

过滤器是一个实现了 javax.servlet.Filter 接口的 Java 类。javax.servlet.Filter 接口定义了三个方法：

下面是对所有编码过滤器

package 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;

public class CharacterEncodingFilter implements Filter {

    FilterConfig config;
    public void destroy() {
        // TODO Auto-generated method stub
        
    }
    public void doFilter(ServletRequest arg0, ServletResponse arg1,
            FilterChain arg2) throws IOException, ServletException {
        String CharacterEncoding=config.getInitParameter("CharacterEncoding");
        arg0.setCharacterEncoding(CharacterEncoding);
        arg1.setCharacterEncoding(CharacterEncoding);
        arg1.setContentType("text/html;charset="+CharacterEncoding);
        arg2.doFilter(arg0, arg1);
    }

    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub
        this.config=arg0;
    }

}

Web.xml配置

  <filter>
      <filter-name>CharacterEncodingFilter</filter-name>
      <filter-class>filter.CharacterEncodingFilter</filter-class>
      <init-param>
          <param-name>CharacterEncoding</param-name>
          <param-value>UTF-8</param-value>
      </init-param>
  </filter>
  <filter-mapping>
      <filter-name>CharacterEncodingFilter</filter-name>
      <url-pattern>/*</url-pattern>
  </filter-mapping>

使用多个过滤器

Web 应用程序可以根据特定的目的定义若干个不同的过滤器。假设您定义了两个过滤器 AuthenFilter 和 LogFilter。您需要创建一个如下所述的不同的映射，其余的处理与上述所讲解的大致相同：

<filter>
   <filter-name>LogFilter</filter-name>
   <filter-class>LogFilter</filter-class>
   <init-param>
      <param-name>test-param</param-name>
      <param-value>Initialization Paramter</param-value>
   </init-param>
</filter>

<filter>
   <filter-name>AuthenFilter</filter-name>
   <filter-class>AuthenFilter</filter-class>
   <init-param>
      <param-name>test-param</param-name>
      <param-value>Initialization Paramter</param-value>
   </init-param>
</filter>

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

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

过滤器的应用顺序

web.xml 中的 filter-mapping 元素的顺序决定了 Web 容器应用过滤器到 Servlet 的顺序。若要反转过滤器的顺序，您只需要在 web.xml 文件中反转 filter-mapping 元素即可。

例如，上面的实例将先应用 LogFilter，然后再应用 AuthenFilter，但是下面的实例将颠倒这个顺序：

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

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