前端出现Access to XMLHttpRequest at ‘xxx’ from origin ‘xxx’ has been been blocked by CORS policy错误
解决方法
在SpringBoot后台接口上加上 @CrossOrigin
例如

import com.example.demo.bean.Admin;
import com.example.demo.service.Imap.AdminService;
import com.example.demo.utils.Message;
import com.example.demo.utils.MessageUtil;
import io.swagger.annotations.*;
import org.springframework.web.bind.annotation.*;

import javax.annotation.Resource;

@Api(description = "用户相关接口")
//@CrossOrigin 跨域请求
@CrossOrigin
@RestController
@RequestMapping("/Admin")
public class AdminController {
    @Resource
    private AdminService adminService;

    @ApiOperation(value ="查询所有用户",notes="" )
    @ApiResponses(value = {
            @ApiResponse(code = 200, message = "请求已完成"),
            @ApiResponse(code = 400, message = "请求中有语法问题，或不能满足请求"),
            @ApiResponse(code = 401, message = "未授权客户机访问数据"),
            @ApiResponse(code = 403, message = "资源不可用"),
            @ApiResponse(code = 404, message = "服务器找不到给定的资源；文档不存在"),
            @ApiResponse(code = 500, message = "服务器不能完成请求")
    })