1.当前端调用后台接口时，提示如下错误，如下图：

2.解决方案。
第一步：在后台增加一个java类，如下：

package com.manage.filter;import java.io.IOException;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 CorsFilter implements Filter  {@SuppressWarnings("unused")private static String allowOrigin;private static String allowMethods;private static String allowCredentials;private static String allowHeaders;@SuppressWarnings("unused")private static String exposeHeaders;@Overridepublic void destroy() {}/*** Access-Control-Allow-Origin：允许访问的客户端域名，例如：http://web.xxx.com，若为*，则表示从任意域都能访问，即不做任何限制。* Access-Control-Allow-Methods：允许访问的方法名，多个方法名用逗号分割，例如：GET,POST,PUT,DELETE,OPTIONS。* Access-Control-Allow-Credentials：是否允许请求带有验证信息，若要获取客户端域下的cookie时，需要将其设置为true。* Access-Control-Allow-Headers：允许服务端访问的客户端请求头，多个请求头用逗号分割，例如：Content-Type。* Access-Control-Expose-Headers：允许客户端访问的服务端响应头，多个响应头用逗号分割。*//*** !!!!examples* response.setHeader("Access-Control-Expose-Headers", exposeHeaders);  * filterChain.doFilter(servletRequest, servletResponse);* response.setHeader("Access-Control-Allow-Origin", "*");* response.setHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE");* response.setHeader("Access-Control-Allow-Headers", "x-requested-with,Authorization,Content-Type");* response.setHeader("Access-Control-Allow-Credentials", "true");* */@Overridepublic void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)throws IOException, ServletException {HttpServletRequest request = (HttpServletRequest) servletRequest;  HttpServletResponse response = (HttpServletResponse) servletResponse;String origin = request.getHeader("Origin");response.setHeader("Access-Control-Allow-Origin",origin);response.setHeader("Access-Control-Allow-Methods", allowMethods);  response.setHeader("Access-Control-Allow-Credentials", allowCredentials);  response.setHeader("Access-Control-Allow-Headers", allowHeaders);  response.setHeader("Access-Control-Max-Age", "3600");if("OPTIONS".equals(request.getMethod())){return;}filterChain.doFilter(servletRequest, servletResponse);}@Overridepublic void init(FilterConfig filterConfig) throws ServletException {allowOrigin = filterConfig.getInitParameter("allowOrigin");allowMethods = filterConfig.getInitParameter("allowMethods");allowCredentials = filterConfig.getInitParameter("allowCredentials");allowHeaders = filterConfig.getInitParameter("allowHeaders");exposeHeaders = filterConfig.getInitParameter("exposeHeaders");}
}

第二步，在web.xml里增加如下代码：

 <!-- CORS过滤器 start  --><filter><filter-name>cors</filter-name><filter-class>com.manage.filter.CorsFilter</filter-class><init-param><param-name>allowOrigin</param-name><param-value>*</param-value></init-param><init-param><param-name>allowMethods</param-name><param-value>GET,POST,PUT,DELETE,OPTIONS</param-value></init-param><init-param><param-name>allowCredentials</param-name><param-value>true</param-value></init-param><init-param><param-name>allowHeaders</param-name><param-value>Content-Type,x-access-token,x-requested-with</param-value></init-param> </filter><filter-mapping><filter-name>cors</filter-name><url-pattern>*</url-pattern></filter-mapping> <!-- CORS过滤器 end -->

3.如此前端就可以正常访问后台接口了。