过去几个月我一直在使用 GWT，我需要开始工作的最后一件事是使用 GAE 而不是 SQLITE 将我的应用程序数据存储在 JDO 对象中。
我的应用程序成功地将对象（在本例中为论坛主题）保存到数据存储区（我已在 appspot/appengine 日志查看器中确认了这一点）。当我像这样对主题 ID 进行硬编码时，我设法获取了其中一个对象：
pManager.getObjectById(Topic.class, 1);
但是，当我尝试使用 Query 类并获取包含所有主题的 List 时，我从 execute 方法中得到的所有内容都是空的。如果我清除主题的数据存储区，我可以打印结果列表的大小（因此它不应该为空），但是当数据存储区中有主题时，它总是会生成List<Topic> null.
这是有问题的代码：
// Fetches all Topic-objects from the App Engine storage and
// puts them in an ArrayList that is returned.
public ArrayList<Topic> getAllTopics() {
ArrayList<Topic> queryResults = new ArrayList<Topic>();
PersistenceManager pManager = PMF.get().getPersistenceManager();
pManager.getFetchPlan().setGroup(FetchGroup.ALL);
Query topicQuery = pManager.newQuery(Topic.class);
// Does not make any difference what query I use
//Query topicQuery = pManager.newQuery(Topic.class);
Topic newTopic = null;
try {
System.out.println("Will now try to execute the query...");
List<Topic> results = (List<Topic>)topicQuery.execute();
System.out.println("Result size: " + results.size());
System.out.println("Query now done executing.");
for (Topic t : results) {
// Does not seem to make any difference either
// newTopic = pManager.detachCopy(t);
// queryResults.add(newTopic);
queryResults.add(t);
}
// I can fetch one topic when hard-coding the ID.
// queryResults.add(pManager.getObjectById(Topic.class, 1));
} catch(Exception e) {
System.out.println("Exception when getting all topics on server: " + e.getMessage() + " - ");
System.out.print( e.getStackTrace());
} finally {
topicQuery.closeAll();
pManager.close();
}
return queryResults;
}
这是使主题对象持久化的代码：
// Creates a topic object and stores it in App Engine
// Returns true if it succeeded, otherwise false
public Boolean createTopic(String title_, String text_) {
PersistenceManager pManager = PMF.get().getPersistenceManager();
UserAuthImpl newUserAuth = new UserAuthImpl();
if(newUserAuth.returnUser().isEmpty() || newUserAuth.returnUser() == null )
return false;
Topic newTopic = new Topic(newUserAuth.returnUser(), title_, text_, new Date(Calendar.getInstance().getTimeInMillis()));
try {
// Did not help: pManager.setDetachAllOnCommit(true);
pManager.makePersistent(newTopic);
} catch(Exception e) {
System.out.println("Error while creating topic: " + e.getMessage());
return false;
} finally {
pManager.close();
}
return true;
}
这是主题对象类：
package se.biffnet.tddi15.forum.client;
import java.io.Serializable;
import java.util.Date;
import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.IdentityType;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;
@SuppressWarnings("serial")
@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable = "true")
public class Topic implements Serializable {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
long topicID;
@Persistent
String authorID;
@Persistent
String postTitle;
@Persistent
String postText;
@Persistent
Date postDate;
public Topic(String authorID_, String postTitle_, String postText_,
java.util.Date date) {
authorID = authorID_;
postTitle = postTitle_;
postText = postText_;
}
public Topic() {
authorID = "";
postTitle = "";
postText = "";
postDate = null;
}
public String getPostTitle() {
return postTitle;
}
public void setPostTitle(String postTitle) {
this.postTitle = postTitle;
}
public String getPostText() {
return postText;
}
public void setPostText(String postText) {
this.postText = postText;
}
public long getTopicID() {
return topicID;
}
public String getAuthorID() {
return authorID;
}
public Date getPostDate() {
return postDate;
}
}
所以我想问题是：
为什么execute()总是返回null？非常感谢您的帮助 =)
EDIT2：必须更改 logging.properties 才能看到调试消息。这是我的错。日志太大，无法在此处发布，因此这里是 pastebin 的链接：http: //pastebin.com/CGX1Aug0
EDIT3：异常及其堆栈跟踪：
java.lang.NullPointerException: null
com.google.appengine.datanucleus.query.QueryEntityPKFetchFieldManager.fetchLongField(QueryEntityPKFetchFieldManager.java:74)
org.datanucleus.identity.IdentityUtils.getApplicationIdentityForResultSetRow(IdentityUtils.java:101)
com.google.appengine.datanucleus.EntityUtils.entityToPojo(EntityUtils.java:1011)
com.google.appengine.datanucleus.query.DatastoreQuery$2.apply(DatastoreQuery.java:229)
com.google.appengine.datanucleus.query.DatastoreQuery$2.apply(DatastoreQuery.java:226)
com.google.appengine.datanucleus.query.LazyResult.resolveNext(LazyResult.java:96)
com.google.appengine.datanucleus.query.LazyResult$LazyAbstractListIterator.computeNext(LazyResult.java:229)
com.google.appengine.datanucleus.query.AbstractIterator.tryToComputeNext(AbstractIterator.java:131)
com.google.appengine.datanucleus.query.AbstractIterator.hasNext(AbstractIterator.java:126)
com.google.appengine.datanucleus.query.LazyResult$AbstractListIterator.hasNext(LazyResult.java:183)
se.biffnet.tddi15.forum.server.DbHandlerImpl.getAllTopics(DbHandlerImpl.java:75)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:601)
com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_$1.run(Method_.java:167)
java.security.AccessController.doPrivileged(Native Method)
com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_.privilegedInvoke(Method_.java:164)
com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_.invoke_(Method_.java:125)
com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_.invoke(Method_.java:43)
com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:561)
com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:208)
com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost(RemoteServiceServlet.java:248)
com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:62)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
com.google.apphosting.utils.servlet.ParseBlobUploadFilter.doFilter(ParseBlobUploadFilter.java:125)
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
com.google.apphosting.runtime.jetty.SaveSessionFilter.doFilter(SaveSessionFilter.java:35)
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
com.google.apphosting.utils.servlet.JdbcMySqlConnectionCleanupFilter.doFilter(JdbcMySqlConnectionCleanupFilter.java:57)
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:266)
org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
org.mortbay.jetty.Server.handle(Server.java:326)
org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
com.google.apphosting.runtime.jetty.RpcRequestParser.parseAvailable(RpcRequestParser.java:76)
org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
com.google.apphosting.runtime.jetty.JettyServletEngineAdapter.serviceRequest(JettyServletEngineAdapter.java:146)
com.google.apphosting.runtime.JavaRuntime$RequestRunnable.run(JavaRuntime.java:438)
com.google.tracing.TraceContext$TraceContextRunnable.runInContext(TraceContext.java:480)
com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:487)
com.google.tracing.TraceContext.runInContext(TraceContext.java:774)
com.google.tracing.TraceContext$DoInTraceContext.runInContext(TraceContext.java:751)
com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:342)
com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:334)
com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:484)
com.google.apphosting.runtime.ThreadGroupPool$PoolEntry.run(ThreadGroupPool.java:251)
java.lang.Thread.run(Thread.java:722)