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;

@Slf4j
public class ParamsRequestWrapper extends HttpServletRequestWrapper {

    private Map 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 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 map = modifyParams(json);
        ByteArrayInputStream bis = new ByteArrayInputStream(JSON.toJSONString(map).getBytes(ENCODING));
        return new ParamsServletInputStream(bis);
    }

    private static Map modifyParams(String json) {
        Map params = JSON.parseObject(json);
        Map 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 set = params.keySet();
        Iterator 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();
    }

}