List <Object> is not null but cannot call .isEmpty without nullpointer exception
Laerer laerer = (Laerer) session.getAttribute("laererInnlogget");
List<Spoerreskjema> ss = laerer.getSkjemaliste();
if(ss == null)
System.out.println("1");
if(ss != null)
System.out.println("2");
if(ss.isEmpty())
System.out.println("3");
if(!ss.isEmpty())
System.out.println("4");
There are some errors here to show what the problem is.
The console will show 2 and then throw a NullPointerException after reaching ss.isEmpty ()
The laerer object is not null, but contains a list that must be null (or at least empty)
All of this is saved and retrieved from / from the postgresql database.
Any idea what the problem might be?
Btw, calling ss.size () will also throw NullPointerException
Basically I want to check if the list is empty or not.
SEVERE: Servlet.service() for servlet [no.hib.prosjekt01.controllers.laerer.LaererHjemServlet] in context with path [/prosjekt01] threw exception
java.lang.NullPointerException
at org.apache.openjpa.enhance.no$hib$prosjekt01$models$Laerer$pcsubclass.pcReplaceField(Unknown Source)
at org.apache.openjpa.kernel.StateManagerImpl.replaceField(StateManagerImpl.java:3254)
at org.apache.openjpa.kernel.StateManagerImpl.storeObjectField(StateManagerImpl.java:2681)
at org.apache.openjpa.kernel.StateManagerImpl.storeObject(StateManagerImpl.java:2671)
at org.apache.openjpa.jdbc.meta.strats.StoreCollectionFieldStrategy.load(StoreCollectionFieldStrategy.java:596)
at org.apache.openjpa.jdbc.meta.FieldMapping.load(FieldMapping.java:934)
at org.apache.openjpa.jdbc.kernel.JDBCStoreManager.load(JDBCStoreManager.java:681)
at org.apache.openjpa.kernel.DelegatingStoreManager.load(DelegatingStoreManager.java:117)
at org.apache.openjpa.kernel.ROPStoreManager.load(ROPStoreManager.java:78)
at org.apache.openjpa.kernel.StateManagerImpl.loadFields(StateManagerImpl.java:3146)
at org.apache.openjpa.kernel.StateManagerImpl.loadField(StateManagerImpl.java:3226)
at org.apache.openjpa.kernel.StateManagerImpl.fetchObjectField(StateManagerImpl.java:2468)
at org.apache.openjpa.kernel.StateManagerImpl.fetchField(StateManagerImpl.java:890)
at org.apache.openjpa.kernel.StateManagerImpl.fetch(StateManagerImpl.java:852)
at org.apache.openjpa.enhance.RedefinitionHelper$1.invoke(RedefinitionHelper.java:230)
at com.sun.proxy.$Proxy68.isEmpty(Unknown Source)
at no.hib.prosjekt01.controllers.laerer.LaererHjemServlet.doGet(LaererHjemServlet.java:35)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.tomee.catalina.OpenEJBValve.invoke(OpenEJBValve.java:44)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)
Edit1:
Creating subclass for "[class no.hib.prosjekt01.models.Spoerreskjema, class no.hib.prosjekt01.models.Laerer, class no.hib.prosjekt01.models.Svar, class no.hib.prosjekt01.models.Spoersmaal, class no.hib.prosjekt01.models.Kryptering]". This means that your application will be less efficient and will consume more memory than it would if you ran the OpenJPA enhancer. Additionally, lazy loading will not be available for one-to-one and many-to-one persistent attributes in types using field access; they will be loaded eagerly instead.
Edit2:
@Entity
@Table(name = "laerer")
public class Laerer implements Serializable {
private static final long serialVersionUID = 1L;
@Id
private String id;
private String fornavn;
private String etternavn;
@Lob
private byte[] passord;
@OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "laerer")
private List<Spoerreskjema> skjemaliste;
public Laerer(String epost, String fornavn, String etternavn, byte[] passord) {
this.id = epost;
this.fornavn = fornavn;
this.etternavn = etternavn;
this.passord = passord;
this.skjemaliste = new ArrayList<Spoerreskjema>();
}
public Laerer() {
id = null;
fornavn = null;
etternavn = null;
passord = null;
skjemaliste = null;
}
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Ok, now I have it. You load the Laerer object and then put it into the session. Somewhere later, you will try to access the lazy loaded collection. But the problem is that the proxy inside your list that is supposed to call your database to get the data must be in the same transaction as your call to the database when Laerer is loaded.
There are several solutions.
-
When you know that there are not many objects on this list, go to "EAGER" and you are happy.
-
In your DAO, after you got the Laerer object from the entitymanager, invoke the list to load it. Same as loading it directly with "EAGER". (JPA specifies that an object can only have one loaded EAGER list)
-
Do not download the list directly. Instead, load the ids list when loading the Laerer object. When you then access the list of IDs by session, you can call the database again to load the required objects by ID. For example, you can put a comma-separated string of identifiers in a transient variable of your object. This line can be directly used in SELECT a FROM a.class WHERE query a.id IN (: IDS).
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