Vaadin 7：java.lang.IllegalStateException：推送不可用

Question

为什么我收到此错误：

 javax.servlet.ServletException: com.vaadin.server.ServiceException: java.lang.IllegalStateException: Push is not available. See previous log messages for more information.
com.vaadin.server.VaadinServlet.service(VaadinServlet.java:240)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

当我使用Push注释时：

@Push(PushMode.MANUAL)
public class KopivaadincomponentsUI extends UI {

@Override
protected void init(VaadinRequest request) {...

   }
}

Answer 1

       <dependency>
            <groupId>com.vaadin</groupId>
            <artifactId>vaadin-push</artifactId>
            <version>${com.vaadin.version}</version>
        </dependency>

尝试将此依赖项添加到.pom文件中。