Google应用引擎nullpointerexception试图获取列表而不是通过主键搜索
问题描述：
我刚刚开始使用GAE和云端点。我创建了一个示例持久化类，并使用Eclipse中的google菜单生成了enpoint类。现在我只是遵循一些关于如何使用curl并获得很好的json字符串的教程，显然当我试图列出所有值时，它抛出了nullpointerexception，令人惊讶的是，当我提供一个带有url字符串的id时，它神奇地工作。现在，我关心的是这个两分法即listContinent和getContinentGoogle应用引擎nullpointerexception试图获取列表而不是通过主键搜索
这就是产生错误
curl http://localhost:8888/_ah/api/app/v1/continent
被调用listContinent方法提高了错误。
而这一次只是工作
curl http://localhost:8888/_ah/api/app/v1/continent/1
被调用getContinent方法，当正确的Id通过它按预期工作的URL通过。
这是我的端点类 -
@Api(name = "app")
public class ContinentEndpoint {
@SuppressWarnings({ "cast", "unchecked" })
public List<Continent> listContinent() {
PersistenceManager mgr = getPersistenceManager();
List<Continent> result = new ArrayList<Continent>();
try {
Query query = mgr.newQuery(Continent.class);
for (Object obj : (List<Object>) query.execute()) {
result.add(((Continent) obj));
}
} finally {
mgr.close();
}
return result;
}
public Continent getContinent(@Named("id") Long id) {
PersistenceManager mgr = getPersistenceManager();
Continent continent = null;
try {
continent = mgr.getObjectById(Continent.class, id);
} finally {
mgr.close();
}
return continent;
}
private static PersistenceManager getPersistenceManager() {
return PMF.get().getPersistenceManager();
}
}
，这是我的持久化类 -
@PersistenceCapable(identityType = IdentityType.APPLICATION)
@Version(strategy = VersionStrategy.VERSION_NUMBER)
public class Continent {
public long getContinentId() {
return continentId;
}
public void setContinentId(long continentId) {
this.continentId = continentId;
}
public String getContinentName() {
return continentName;
}
public void setContinentName(String continentName) {
this.continentName = continentName;
}
public Date getCreatedOn() {
return createdOn;
}
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private long continentId;
private String continentName;
private Date createdOn = new Date();
}
这是我的堆栈跟踪
Nov 12, 2012 3:07:10 PM com.google.apphosting.utils.jetty.JettyLogger info
INFO: Logging to JettyLogger(null) via com.google.apphosting.utils.jetty.JettyLogger
Nov 12, 2012 3:07:10 PM com.google.apphosting.utils.config.AppEngineWebXmlReader readAppEngineWebXml
INFO: Successfully processed /home/soham/Workspace/cloudcv/war/WEB-INF/appengine-web.xml
Nov 12, 2012 3:07:10 PM com.google.apphosting.utils.config.AbstractConfigXmlReader readConfigXml
INFO: Successfully processed /home/soham/Workspace/cloudcv/war/WEB-INF/web.xml
Nov 12, 2012 8:37:11 PM com.google.appengine.tools.development.DevAppServerImpl start
INFO: The server is running at http://localhost:8888/
Nov 12, 2012 8:37:11 PM com.google.appengine.tools.development.DevAppServerImpl start
INFO: The admin console is running at http://localhost:8888/_ah/admin
Nov 12, 2012 8:37:53 PM com.google.api.server.spi.SystemService invokeServiceMethod
SEVERE: null
java.lang.NullPointerException
at com.google.appengine.datanucleus.query.QueryEntityPKFetchFieldManager.fetchLongField(QueryEntityPKFetchFieldManager.java:74)
at org.datanucleus.identity.IdentityUtils.getApplicationIdentityForResultSetRow(IdentityUtils.java:101)
at com.google.appengine.datanucleus.EntityUtils.entityToPojo(EntityUtils.java:1009)
at com.google.appengine.datanucleus.query.DatastoreQuery$2.apply(DatastoreQuery.java:228)
at com.google.appengine.datanucleus.query.DatastoreQuery$2.apply(DatastoreQuery.java:225)
at com.google.appengine.datanucleus.query.LazyResult.resolveNext(LazyResult.java:96)
at com.google.appengine.datanucleus.query.LazyResult$LazyAbstractListIterator.computeNext(LazyResult.java:229)
at com.google.appengine.datanucleus.query.AbstractIterator.tryToComputeNext(AbstractIterator.java:131)
at com.google.appengine.datanucleus.query.AbstractIterator.hasNext(AbstractIterator.java:126)
at com.google.appengine.datanucleus.query.LazyResult$AbstractListIterator.hasNext(LazyResult.java:183)
at com.cloudcv.ContinentEndpoint.listContinent(ContinentEndpoint.java:23)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at com.google.appengine.tools.development.agent.runtime.Runtime.invoke(Runtime.java:115)
at com.google.api.server.spi.SystemService.invokeServiceMethod(SystemService.java:254)
at com.google.api.server.spi.SystemServiceServlet.execute(SystemServiceServlet.java:135)
at com.google.api.server.spi.SystemServiceServlet.doPost(SystemServiceServlet.java:106)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:123)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:61)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.BackendServersFilter.doFilter(BackendServersFilter.java:97)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:94)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:383)
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)
我做了一些调试，我发现Query query = mgr.newQuery(Continent.class);中的查询没有返回任何内容，并且for each循环是不填充ArrayList。
显示你的代码会让人们更容易帮助你。 – Amber
@Amber刚刚做到了。 –
您是否测试了listContinent方法，并且它是否正常工作？ –