java - com.google.appengine.repackaged.org.codehaus.jackson.map.JsonMappingException，同时实现一对多双向关系App Engine

我想为我的应用程序引擎应用程序实现一对多的双向关系。下面是我的实体：

@Entity
public class UserMaster {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Key key;
    private String userName;

    @OneToMany(mappedBy = "user")
    List<FeedMaster> feeds;

    getter()...setter() for all properties
}

@Entity
public class FeedMaster {
     @Id
    private String feedId;

    @ManyToOne(fetch = FetchType.LAZY)
    UserMaster user;

   getter()....setter() for both properties
}

在这里，UserMaster的第一个对象被持久化，然后当我尝试persisFeedMaster的对象时，它抛出java.io.IOException: com.google.appengine.repackaged.org.codehaus.jackson.map.JsonMappingException异常。
错误日志：

    Uncaught exception from servlet
java.io.IOException: com.google.appengine.repackaged.org.codehaus.jackson.map.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: com.sampleregistrationapp.FeedMaster["user"]->com.sampleregistrationapp.UserMaster["key"]->com.google.appengine.api.datastore.Key["appId"])
    at com.google.api.server.spi.response.ServletResponseResultWriter.writeValueAsString(ServletResponseResultWriter.java:187)
    at com.google.api.server.spi.response.ServletResponseResultWriter.write(ServletResponseResultWriter.java:73)
    at com.google.api.server.spi.SystemService.invokeServiceMethod(SystemService.java:386)
    at com.google.api.server.spi.SystemServiceServlet.execute(SystemServiceServlet.java:124)
    at com.google.api.server.spi.SystemServiceServlet.doPost(SystemServiceServlet.java:82)
    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$CachedChain.doFilter(ServletHandler.java:1166)
    at com.google.apphosting.utils.servlet.ParseBlobUploadFilter.doFilter(ParseBlobUploadFilter.java:125)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.apphosting.runtime.jetty.SaveSessionFilter.doFilter(SaveSessionFilter.java:35)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.apphosting.utils.servlet.JdbcMySqlConnectionCleanupFilter.doFilter(JdbcMySqlConnectionCleanupFilter.java:60)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
    at com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:266)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
    at com.google.apphosting.runtime.jetty.RpcRequestParser.parseAvailable(RpcRequestParser.java:76)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at com.google.apphosting.runtime.jetty.JettyServletEngineAdapter.serviceRequest(JettyServletEngineAdapter.java:146)
    at com.google.apphosting.runtime.JavaRuntime$RequestRunnable.run(JavaRuntime.java:446)
    at com.google.tracing.TraceContext$TraceContextRunnable.runInContext(TraceContext.java:437)
    at com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:444)
    at com.google.tracing.CurrentContext.runInContext(CurrentContext.java:188)
    at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:308)
    at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:300)
    at com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:441)
    at com.google.apphosting.runtime.ThreadGroupPool$PoolEntry.run(ThreadGroupPool.java:251)

................................

我找了很多，但找不到任何解决办法。我提到了stackoverflow link，但在这里，他们说要删除UserMaster中FeedMaster的getter setter，但是如果我删除了它，我将无法通过我的UserMaster对象访问FeedMaster数据。请帮我解决这个问题。谢谢您。

最佳答案

这里发生的事情是jackson库正在寻找一个不能转换为json的循环。在返回此对象时，必须决定要返回关系的位置，以及不返回关系的位置。
您的代码意味着您可以从FeedMaster访问UserMaster，从UserMaster访问FeedMaster。在您的应用程序级别，这是非常好的，但是当您将这个对象图序列化为json时，您将不得不放弃其中一个关系，因为它将导致json表示的无休止循环。
为此，您可以使用@JsonIgnore注释，如下所示：

import com.fasterxml.jackson.annotation.JsonIgnore;

@Entity
public class FeedMaster {
   @Id
   private String feedId;

   @ManyToOne(fetch = FetchType.LAZY)
   UserMaster user;

   @JsonIgnore
   public UserMaster getUser() {
      ...
   }
}

在这种情况下，json将允许您访问FeedMaster中的所有UserMaster对象，但不能访问UserMaster中的FeedMaster对象。