介绍二种写法（写法其实类似）：

写法一：

/**
 * cn.jbit.struts2.filter.CharFilter
 * 编码过滤器A
 * 2014-4-21
 * gyy
 */
package cn.jbit.struts2.filter;

import java.io.IOException;
import java.util.Map;
import java.util.Set;

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 class CharFilter implements Filter {

	private FilterConfig filterConfig = null;
	private String encoding;

	// 销毁
	public void destroy() {
		this.filterConfig = null;
	}

	@SuppressWarnings("unchecked")
	public void doFilter(ServletRequest req, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		HttpServletRequest request = (HttpServletRequest) req;
		HttpServletResponse resp = (HttpServletResponse) response;
		request.setCharacterEncoding(encoding);
		resp.setContentType("text/html;charset=" + encoding);
		System.out.println(request.getQueryString());
		// 处理GET参数
		if (request.getMethod().equalsIgnoreCase("get")) {
			Map paramMap = req.getParameterMap();
			System.out.println("paramMap.size=" + paramMap.size());
			System.out.println(((String[]) paramMap.get("id"))[0]);
			System.out.println(((String[]) paramMap.get("name"))[0]);
			String[] queryStringArray = { "" };
			if (request.getQueryString() != null) {
				queryStringArray = request.getQueryString().split("&");
			}
			// 获取参数名 $1为获取"="前(.*)部分
			for (int i = 0; i < queryStringArray.length; i++) {
				queryStringArray[i] = queryStringArray[i].replaceAll(
						"(.*)=(.*)", "$1");
			}
			System.out.println("-----------------------------------");
			for (String str : queryStringArray) {
				System.out.println(str);
			}
			System.out.println("-----------------------------------");
			Set<String> keySet = paramMap.keySet();
			for (String key : keySet) {
				System.out.println("key=" + key);
				boolean isFromGet = false;
				for (String paramFromGet : queryStringArray) {
					if (key.equals(paramFromGet)) {
						isFromGet = true;
					}
				}
				if (!isFromGet) {
					continue;
				}
				String[] paramArray = (String[]) paramMap.get(key);
				// 设置编码方式
				for (int i = 0; i < paramArray.length; i++) {
					paramArray[i] = new String(paramArray[i]
							.getBytes("ISO8859-1"), encoding);
				}
			}
		}
		chain.doFilter(req, resp);
	}

	// 初始化
	public void init(FilterConfig filterConfig) throws ServletException {
		this.filterConfig = filterConfig;
		// 获取编码方式
		this.encoding = this.filterConfig.getInitParameter("encoding");
	}
}

写法二：

/**
 * cn.jbit.struts2.filter.EncodingFilter
 * 编码过滤器B
 * 2014-4-21
 * gyy
 */
package cn.jbit.struts2.filter;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.util.Map;
import java.util.Set;

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 {

	private FilterConfig filterConfig;
	private String encoding;

	class MyRequest extends HttpServletRequestWrapper {

		// 请求包装类
		public MyRequest(HttpServletRequest request) {
			super(request);
		}

		// 编码转换
		protected String encodingConvert(String str) {
			if (null == str) {
				return null;
			} else {
				try {
					byte[] bytes = str.getBytes("ISO8859-1");
					return new String(bytes, encoding);
				} catch (UnsupportedEncodingException e) {
				}
			}
			return null;
		}

		@Override
		public String getParameter(String name) {
			return encodingConvert(name);
		}

		@Override
		public String[] getParameterValues(String name) {
			String[] values = super.getParameterValues(name);
			if (null != values) {
				for (int i = 0; i < values.length; i++) {
					values[i] = encodingConvert(values[i]);
				}
			}
			return values;
		}
	}
	
	// 销毁
	public void destroy() {
		this.filterConfig = null;
		this.encoding = null;
	}

	@SuppressWarnings("unchecked")
	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		HttpServletRequest httpRequest = (HttpServletRequest) request;
		if ("GET".equals(httpRequest.getMethod())) {
			request = new MyRequest(httpRequest);
			// 获取请求参数Map
			Map paramMap = request.getParameterMap();
			Set<String> keySet = paramMap.keySet();
			for (String key : keySet) {
				String[] param = (String[]) paramMap.get(key);
				// System.out.println(paramArray.length);
				// 设置编码方式
				if (param.length == 1) {// 单值
					param[0] = request.getParameter(param[0]);
				} else {// 多值
					param = request.getParameterValues(key);
				}
			}
		}
		chain.doFilter(request, response);
	}

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

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Java编码过滤器