GET和POST请求是简单请求，AJAX只发起一次，所以设置了响应头和路由就可以实现。而PUT和DELETE等请求是复杂请求，会提前发起一个 OPTIONS 预请求，再发起PUT或DELETE请求，一共会发起两次请求。

写一个过滤器

实现代码：

先另创一个类：

import javax.servlet.*;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public   class CORSFilter implements Filter {


    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletResponse httpResponse = (HttpServletResponse) servletResponse;
        //*表示允许所有域名跨域
        httpResponse.addHeader("Access-Control-Allow-Origin", "*");
        httpResponse.addHeader("Access-Control-Allow-Headers",
                "Origin, X-Requested-With, Content-Type, Accept");
        //允许跨域的Http方法
        httpResponse.addHeader("Access-Control-Allow-Methods", "GET,POST,PUT,DELETE");
        filterChain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {

    }


}

上述代码只是加了三个参数，接下来在web.xml配置：

<!--为了允许跨域访问-->
<filter>
    <filter-name>CorsFilter</filter-name>
    <filter-class>com.mashang.test.test1.CORSFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>CorsFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

然后问题就解决了