前后端分离session获取为null解决

第一步：

后台添加过滤器，添加过滤器统一处理。

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * @author Administrator
 */
@WebFilter(urlPatterns = "/*", filterName = "CORSFilter")
public class CORSFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        Filter.super.init(filterConfig);
    }

/**
     * 此过滤器只是处理跨域问题
     * @param servletRequest
     * @param servletResponse
     * @param chain
     * @throws ServletException
     * @throws IOException
     */
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) servletRequest;
        HttpServletResponse resp = (HttpServletResponse) servletResponse;
        String origin = req.getHeader("Origin");
        if (origin == null) {
            origin = req.getHeader("Referer");
        }
        resp.setHeader("Access-Control-Allow-Origin", origin);//这里不能写*，*代表接受所有域名访问，如写*则下面一行代码无效。谨记
        resp.setHeader("Access-Control-Allow-Credentials", "true");//true代表允许携带cookie
        filterChain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {
        Filter.super.destroy();
    }
}

springboot2.配置过滤器时，启动类必须加上@ServletComponentScan才会加载过滤器

@SpringBootApplication
@MapperScan("com.mian.mapper")
@ServletComponentScan
public class Game_Box_8001 {
    public static void main(String[] args) {
        SpringApplication.run(Game_Box_8001.class, args);
    }
}

前端配置

使用vue.resource发送请求时配置如下：
main.js中
Vue.http.options.xhr = { withCredentials: true }

使用vue.axios发送请求时配置如下：
axios.defaults.withCredentials = true；

jquery请求带上  xhrFields: {withCredentials: true}, crossDomain: true；
$.ajax({
    type: "post",
    url: "",
    xhrFields: {withCredentials: true},
    crossDomain: true,
    data: {username:$("#username").val()},
    dataType: "json",
    success: function(data){ }
});