Servlet过滤器

Servlet 过 滤 器拦截请求和响应，以便查看、提取或操作客户机和服务器之间交换的数据，他的用途有：

• 用户认证与授权管理。
• 统计Web应用的访问量，和访问的命中率，形成访问报告。
• 实现Web应用的日志处理功能。
• 实现数据压缩功能。
• 对传输的数据进行加密。
• 实现XML文件的XSLT转换。

javax.servlet.Filter接口

package javax.servlet;
import java.io.IOException;
  public interface Filter {
    public void init(FilterConfig filterConfig) throws ServletException;

    public void doFilter ( ServletRequest request, ServletResponse response,        FilterChain chain ) throws IOException, ServletException;
  
    public void destroy();
}

例子：用过滤器实现reqiest字符集的装换
CharacterEncodingFilter.java

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 {
	private FilterConfig config;
	private String encoding = "ISO8859_1";

	public void destroy() {
		config = null;
	}

	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		request.setCharacterEncoding(encoding);
		chain.doFilter(request, response);
	}

	public void init(FilterConfig config) throws ServletException {
		this.config = config;
		String s = config.getInitParameter("encoding");
		if (s != null) {
			encoding = s;
		}
	}
}

修改web.xml

<filter>
	<filter-name>EncodingFilter</filter-name>
	<filter-class>com.v512.guestbook.CharacterEncodingFilter</filter-class>	
	<init-param>
	  <param-name>encoding</param-name>
	  <param-value>UTF-8</param-value>
	</init-param>
</filter>
<filter-mapping>
	<filter-name>EncodingFilter	</filter-name>
	<url-pattern>/*</url-pattern>
</filter-mapping>