解决方案很简单。

直接上代码：

web.xml中：

添加了 cors来解决跨域

（奇怪的是，credentials设置为false。反正这个是照搬的。。。）

  <filter>

    <filter-name>CorsFilter</filter-name>

    <filter-class>org.apache.catalina.filters.CorsFilter</filter-class>

    <init-param>

      <param-name>cors.allowed.origins</param-name>

      <param-value>*</param-value>

    </init-param>

    <init-param>

      <param-name>cors.support.credentials</param-name>

      <param-value>false</param-value>

    </init-param>

  </filter>

  <filter-mapping>

    <filter-name>CorsFilter</filter-name>

    <url-pattern>/*</url-pattern>

  </filter-mapping>

或者在代码中设置：

response.setHeader("Access-Control-Allow-Origin", "*");

		response.setHeader("Access-Control-Allow-Headers", "*");

		response.setHeader("Access-Control-Allow-Methods", "GET, POST");

		response.setHeader("Access-Control-Allow-Credentials", "true");

ajax：

$.ajax({

                type: "post",

                url: "Get",

                crossDomain: true,                 //加这二行支持ajax跨域，允许跨域

                xhrFields: {withCredentials: true},//加这二行支持ajax跨域，携带凭证

                success: function (data) {

                    alert(data);  

                }

            });