Filter也称之为过滤器，它是Servlet技术中最实用的技术，Web开发人员通过Filter技术，对web服务器管理的所有web资源：比如Jsp, Servlet, 静态图片文件或静态 html 文件等进行拦截，从而实现一些特殊的功能。比如实现URL级别的权限访问控制、过滤敏感词汇、压缩响应信息等一些高级功能。

它主要用于对用户请求进行预处理，也可以对HttpServletResponse进行后处理。使用Filter的完整流程：Filter对用户请求进行预处理，接着将请求交给Servlet进行处理并生成响应，最后Filter再对服务器响应进行后处理。

一、实现拦截器

import javax.servlet.*;import javax.servlet.http.HttpServletRequest;import java.io.IOException;public class ParamsFilter implements Filter {    @Override    public void init(FilterConfig filterConfig) throws ServletException {    }    @Override    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {        HttpServletRequest httpRequest = (HttpServletRequest) servletRequest;        ParamsRequestWrapper requestWrapper = new ParamsRequestWrapper(httpRequest);        filterChain.doFilter(requestWrapper, servletResponse);    }    @Override    public void destroy() {    }}

二、继承HttpServletRequestWrapper

import com.alibaba.fastjson.JSON;import lombok.extern.slf4j.Slf4j;import org.apache.commons.io.IOUtils;import org.apache.commons.lang3.StringUtils;import org.springframework.http.HttpHeaders;import org.springframework.http.MediaType;import javax.servlet.ServletInputStream;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletRequestWrapper;import java.io.ByteArrayInputStream;import java.io.IOException;import java.util.HashMap;import java.util.Iterator;import java.util.Map;import java.util.Set;@Slf4jpublic class ParamsRequestWrapper extends HttpServletRequestWrapper {    private Map<String, String[]> params = new HashMap<>();    private static final String ENCODING = "UTF-8";    private static final String CLASSTYPE = "java.lang.String";    public ParamsRequestWrapper(HttpServletRequest request) {        super(request);        // 将参数表，赋予给当前的Map以便于持有request中的参数        Map<String, String[]> requestMap = request.getParameterMap();        this.params.putAll(requestMap);        this.modifyParameters();        if(!params.isEmpty()){            log.info("请求参数：" + JSON.toJSONString(params));        }    }    /**     * 重写getInputStream方法  post请求参数必须通过流才能获取到值     */    @Override    public ServletInputStream getInputStream() throws IOException {        ServletInputStream stream = super.getInputStream();        //非json类型，直接返回        if (!super.getHeader(HttpHeaders.CONTENT_TYPE).equalsIgnoreCase(MediaType.APPLICATION_JSON_VALUE)) {            return stream;        }        String json = IOUtils.toString(stream, ENCODING);        if (StringUtils.isBlank(json)) {            return stream;        }        Map<String, Object> map = modifyParams(json);        ByteArrayInputStream bis = new ByteArrayInputStream(JSON.toJSONString(map).getBytes(ENCODING));        return new ParamsServletInputStream(bis);    }    private static Map<String, Object> modifyParams(String json) {        Map<String, Object> params = JSON.parseObject(json);        Map<String, Object> maps = new HashMap<>(params.size());        for (String key : params.keySet()) {            Object value = getValue(params.get(key));            maps.put(key, value);        }        return maps;    }    private static Object getValue(Object obj) {        if (obj == null) {            return null;        }        if(obj.equals("undefined,undefined")){            return "";        }        if(obj.equals("undefined")){            return "";        }        String type = obj.getClass().getName();        // 对字符串的处理        if (CLASSTYPE.equals(type)) {            obj = obj.toString().trim();        }        return obj;    }    /**     * 将parameter的值去除空格后重写回去     */    private void modifyParameters() {        Set<String> set = params.keySet();        Iterator<String> it = set.iterator();        while (it.hasNext()) {            String key = it.next();            String[] values = params.get(key);            values[0] = values[0].trim().replaceAll("undefined,undefined","");            params.put(key, values);        }    }    /**     * 重写getParameter 参数从当前类中的map获取     */    @Override    public String getParameter(String name) {        String[] values = params.get(name);        if (values == null || values.length == 0) {            return null;        }        return values[0];    }    @Override    public String[] getParameterValues(String name) {//同上        String[] values = params.get(name);        if (values == null || values.length == 0) {            return null;        }        return values;    }}

import javax.servlet.ReadListener;import javax.servlet.ServletInputStream;import java.io.ByteArrayInputStream;import java.io.IOException;public class ParamsServletInputStream extends ServletInputStream  {    private ByteArrayInputStream bis;    public ParamsServletInputStream(ByteArrayInputStream bis) {        this.bis = bis;    }    @Override    public boolean isFinished() {        return true;    }    @Override    public boolean isReady() {        return true;    }    @Override    public void setReadListener(ReadListener readListener) {    }    @Override    public int read() throws IOException {        return bis.read();    }}