java - 从 Google Datastore 获取对象后，该对象为空

Question

我asked question在询问是否可以使用 Java 将复杂的类组合保存到 Google AppEngine 内的 Google Datastore 之前，但我不够清楚，也懒得发布我的所有类，但经过很多小时的挣扎后，我开始放弃。所以这里是代码的更详细问题。

我知道这种东西应该可以正常工作，但由于某种未知的原因，我无法正常工作。问题是，当我保存对象并关闭数据库连接然后再次打开它时，该对象中的数据为空。该对象位于数据库中，当我选择该对象的 ID 时我可以看到它，但其他所有内容都是空白的。不幸的是，Google 没有数据库查看器来查看该数据库中的内容。我尝试搜索并 ask但没有运气。 (有 one 但它是 not working on my computer ) 那么，我们开始吧:

内容

// imports...
@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Content{

 @PrimaryKey
 @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
 @Extension(vendorName="datanucleus", key="gae.encoded-pk", value="true")
 private String id;

 @Persistent protected String title;
 @Persistent protected String thumbnailURL;
 @Persistent protected List<Rating> ratings;
 @Persistent protected List<Tag> tags;
 @Persistent protected Double price;
 @Persistent protected User owner;

 // constructor and getters+setters
}

类(class)

// imports...
@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Course extends Content{

 @Persistent private Video video;
 @Persistent private Document document;
 @Persistent private String notes;
 @Persistent private String summary;

// constructor and getters+setters
    public String toString(){
  return "ID: " + this.getId() + " Title: "+this.getTitle()+", Price: "+this.getPrice()+", No. of Tags: "+this.getTags().size();
 }

}

视频

// imports...
public class Video extends Content {
  @PrimaryKey
  @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
  @Extension(vendorName="datanucleus", key="gae.encoded-pk", value="true")
  String id;

  @Persistent String fileUrl;

  // constructor and getters+setters
}

文档

// imports...
@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Document extends Content /*AthenaObject*/ {
 @PrimaryKey
 @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
 @Extension(vendorName="datanucleus", key="gae.encoded-pk", value="true") 
 private String id;

 @Persistent private String docUrl;

 // constructor and getters+setters
}

标签

// imports...
@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Tag{
 @PrimaryKey
 @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
 @Extension(vendorName="datanucleus", key="gae.encoded-pk", value="true")
 private String id;

 @Persistent private String tagText;

 // constructor and getters+setters
}

用于保存数据并从数据存储区获取数据的代码:

public Boolean testCourse(){
 Boolean isSaved = false;
 PersistenceManager pm = PMF.get().getPersistenceManager();
 Course c = new Course();
 try{
  List<Tag> tags = new ArrayList<Tag>();
  tags.add(new Tag("tag1"));
  tags.add(new Tag("tag2"));
  tags.add(new Tag("tag3"));


  c.setTitle("Course Title - " + new Date().getTime());
  c.setPrice(99.90);
  c.setTags(tags);

  System.out.println(c.toString()); // **Output:** ID: null Title: Course Title - 1247116147858, Price: 99.9, No. of Tags: 3
  pm.makePersistent(c);

  Course cAfter = pm.getObjectById(Course.class, KeyFactory.stringToKey(c.getId()));
  System.out.println(cAfter.toString()); // **Output:** agptYRtzaWL4gZDb3Vy4ErYFgw Title: Course Title - 1247116147858, Price: 99.9, No. of Tags: 3
  isSaved = true;
 }
 catch(Exception e){
  e.printStackTrace();
  isSaved = false;
 }
 finally{
  pm.close();
 }


 pm = PMF.get().getPersistenceManager();

 try{

  Course cAfterClose = pm.getObjectById(Course.class, KeyFactory.stringToKey(c.getId()));
  System.out.println(cAfterClose.toString()); // **Error**: See below
 }
 catch(Exception e){e.printStackTrace();}
 finally{pm.close();}


 return isSaved;
}

这是输出:

ID: nullTitle: Course Title - 1247117389679, Price: 99.9, No. of Tags: 3 ID: agptYWtzaW1zYXBwcgwLEgZDb3Vyc2UYGAwTitle: Course Title - 1247117389679, Price: 99.9, No. of Tags: 3 java.lang.NullPointerException at com.athena.server.entity.Course.toString(Course.java:94) at com.athena.server.CourseServiceImpl.testCourse(CourseServiceImpl.java:146) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:585) at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:527) at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:166) at com.google.gwt.user.server.rpc.RemoteServiceServlet.doPost(RemoteServiceServlet.java:86) at javax.servlet.http.HttpServlet.service(HttpServlet.java:713) at javax.servlet.http.HttpServlet.service(HttpServlet.java:806) at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1093) at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:712) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405) at com.google.apphosting.utils.jetty.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:54) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139) at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:306) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139) at org.mortbay.jetty.Server.handle(Server.java:313) at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:506) at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:844) at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:644) at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211) at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:381) at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:396) at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:442)

我知道这个错误意味着字符串为空。但记录在数据库中。并且应该坚持下去。为什么数据为空？？？

有什么建议为什么会发生在我身上吗？

Answer 1

我不知道到底是什么导致了您的错误，但您可以尝试执行以下几个步骤来帮助排除故障:

首先，尝试将问题范围缩小到尽可能小的测试用例。您在上面发布了很多类(class)，并且很可能仅使用其中的 2 个或 3 个类(class)即可重复此问题。

其次，如果您确实想准确查看实体在数据存储区中的样子，则必须执行类似的操作(或者您可以尝试将应用程序上传到 appengine 并在那里运行，这样您就可以使用他们的数据查看器):

Query q = pm.newQuery(Course.class);
List<Course> list = (List<Course>) q.execute();
for(Course c: list){
  System.out.println("Course id:" + c.getId());
  System.out.println("Course title:" + c.getTitle());
  //...
}