步驟：

1. 編寫一個過濾器的類實現Filter介面
2. 實現介面中尚未實現的方法(著重實現doFilter方法)
3. 在web.xml中進行配置(主要是配置要對哪些資源進行過濾)

 EncidingFilter.java

package web.filter;

import java.io.IOException;
import java.io.UnsupportedEncodingException;

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.HttpServletRequestWrapper;

public class EncodingFilter implements Filter{

	
	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		
		
		
		//在傳遞request之前對request的getParameter方法進行增強
		/*
		 * 裝飾者模式(包裝)
		 * 
		 * 1、增強類與被增強的類要實現統一介面
		 * 2、在增強類中傳入被增強的類
		 * 3、需要增強的方法重寫 不需要增強的方法呼叫被增強物件的
		 * 
		 */
		
		//被增強的物件
		HttpServletRequest req = (HttpServletRequest) request;
		//增強物件
		EnhanceRequest enhanceRequest = new EnhanceRequest(req);
		
		
		chain.doFilter(enhanceRequest, response);
		
	}

	@Override
	public void destroy() {
		
	}
	
	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		
	}

}

class EnhanceRequest extends HttpServletRequestWrapper{
	
	private HttpServletRequest request;

	public EnhanceRequest(HttpServletRequest request) {
		super(request);
		this.request = request;
	}
	
	//對getParaameter增強
	@Override
	public String getParameter(String name) {
		String parameter = request.getParameter(name);//亂碼
		System.out.println(parameter);
		try {
			
			parameter = new String(parameter.getBytes("iso8859-1"),"UTF-8");
			System.out.println("進行編碼:"+parameter);
		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		}
		return parameter;
	}
	
}
 

web.xml中加入這個

  <filter>
    <filter-name>EncodingFilter</filter-name>
    <filter-class>web.filter.EncodingFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>EncodingFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
 

經過這個Filter的getparameter方法拿到的都是編碼過的，想用其他方法，直接增強就行