Allow cross origin in leaflet (Access-Control-Allow-Origin)

Step 1.

Copy the zar file at the location-

C:\Program Files (x86)\GeoServer 2.12.1\webapps\geoserver\WEB-INF\lib

 Link-  jetty-servlets-9.2.13.v20150730.jar

Step 2. 

Modify the web.xml file at the locaiton- 

C:\Program Files (x86)\GeoServer 2.12.1\webapps\geoserver\WEB-INF

   <!-- Uncomment following filter to enable CORS -->

   <!--<filter>

        <filter-name>cross-origin</filter-name>

        <filter-class>org.eclipse.jetty.servlets.CrossOriginFilter</filter-class>

    </filter>

  -->

To

   <!-- Uncomment following filter to enable CORS -->

   <!-- -->

   <filter>

        <filter-name>cross-origin</filter-name>

        <filter-class>org.eclipse.jetty.servlets.CrossOriginFilter</filter-class>

    </filter>

and

<!-- Uncomment following filter to enable CORS -->

    <!--<filter-mapping>

        <filter-name>cross-origin</filter-name>

        <url-pattern>/*</url-pattern>

    </filter-mapping>

     -->

To

  <!-- Uncomment following filter to enable CORS -->

    <!--  -->

  <filter-mapping>

        <filter-name>cross-origin</filter-name>

        <url-pattern>/*</url-pattern>

    </filter-mapping> 

Step 3. Restart the geoserver.