googleAppEngine在调用user.getEmail（）时抛出异常

Question

在我调用user.getEmail（）时googleAppEngine中收到异常消息 “尝试联系服务器时发生错误，请检查您的网络连接并重试”

以下是其中的颂歌给错误

  UserService userService = UserServiceFactory.getUserService(); 
     User user = userService.getCurrentUser(); 
     String greet= user.getEmail(); 

以下是堆栈跟踪

SEVERE: javax.servlet.ServletContext log: Exception while dispatching incoming RPC call 
com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract java.lang.String com.dearcircle.tutorials.client.GreetingService.greetServer(java.lang.String) throws java.lang.IllegalArgumentException' threw an unexpected exception: java.lang.NullPointerException 
    at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure(RPC.java:385) 
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:588) 
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:208) 
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost(RemoteServiceServlet.java:248) 
    at com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:62) 
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:637) 
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) 
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511) 
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166) 
    at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:35) 
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
    at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:60) 
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
    at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43) 
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
    at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:122) 
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
    at com.google.appengine.tools.development.BackendServersFilter.doFilter(BackendServersFilter.java:97) 
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388) 
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) 
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182) 
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765) 
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418) 
    at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:78) 
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) 
    at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:363) 
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) 
    at org.mortbay.jetty.Server.handle(Server.java:326) 
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542) 
    at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938) 
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755) 
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218) 
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404) 
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409) 
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582) 
Caused by: java.lang.NullPointerException 
    at com.dearcircle.tutorials.server.GreetingServiceImpl.greetServer(GreetingServiceImpl.java:30) 
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
    at java.lang.reflect.Method.invoke(Unknown Source) 
    at com.google.appengine.tools.development.agent.runtime.Runtime.invoke(Runtime.java:104) 
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:569) 
    ... 34 more 

Answer 1

User user = userService.getCurrentUser(); 

从API docs（重点煤矿）：

如果用户已登录，此方法将返回一个包含他们 信息的用户。请注意，重复呼叫可能不一定会返回相同的用户对象 。

返回：

用户，如果用户登录，否则返回null。

那么是什么在代码段最有可能的情况是，用户没有登录，因此user对象null。

User user = userService.getCurrentUser(); // user object is null 
String greet= user.getEmail();    // Null pointer exception 

Answer 2

退房我已经写在解释了认证的servlet：

http://cloudspring.com/google-app-engine-gae-java-api-part-3-getting-started-with-apis/

您需要授权用户才可以得到他的电子邮件。至于例外情况，请为user对象添加空值检查。如果用户为空，则表示授权未完成。