Entity bean best practices

[ttarhan]

For this question, assume that the web container and EJB container are on seperate machines.

Machine 1 hosts the EJB3 beans. Let's say there's two entity beans: Order and LineItem, the relationship between them is fairly simple -- each Order contains many LineItems. The Order entity is set to LAZY FETCH the LineItems.

Machine 1 also hosts a session bean with a single method getOrderById(). The implementation is obvious.

Machine 2 is the webserver. The webserver calls getOrderById on the session bean. The session bean uses the Entity Manager to find the Order and returns it to the webserver.

Machine 2 now has an entity bean it can work with, however, if the webserver calls order.getLineItems(), there's an exception thrown because the webserver (obviously) can't lazy load the LineItems.

With all that said, does anyone here have recommendations for implementing something like what I've described above. I want to take advantage of lazy loading, but then what's the best way to give clients (the webserver in this case) access to the lazy-loaded data? I imagine there must be a better way than having the session bean also contain a getLineItemsByOrder() method.

Essentially, I want the clients to be able to do { Order x = getOrderById(order_id); Collection y = x.getLineItems() }. All while using lazy loading (so the second call, "x.getLineItems()", should make another RMI call to get the get the LineItems.

[bill.burke]

YOu cannot do what you want. Remember, what you have is a detached POJO with no knowledge of remoteness. EJB 3.0 Entities are POJOs. They have no notion of remoteness.

You can have control over relationships with your query. For instance, if you have lazy for the line item, you can use a fetch to prefetch the relationship:

"from Order o left join fetch o.lineItems where id=55"

This query SHOULD work, but i'm not sure it WILL work because we've not tested it yet.

Bill

[janprill]

In a test in which questions and definitions are associated in a ManyToMany-Relation and in which Macromedia Flex is involved the following query won't work but throw a LazyInitializationException.

Best regards
Jan Prill


The query:

public List findByMarkup(String markup) {
 return manager
 .createQuery("from Question q left join fetch q.definitions where q.markup = :markup")
 .setParameter("markup", markup).listResults();
 }

The full stacktrace:

17:42:27,046 ERROR [LazyInitializationException] failed to lazily initialize a c
ollection - no session or session was closed
org.hibernate.LazyInitializationException: failed to lazily initialize a collect
ion - no session or session was closed
 at org.hibernate.collection.AbstractPersistentCollection.initialize(Abst
ractPersistentCollection.java:179)
 at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPe
rsistentCollection.java:49)
 at org.hibernate.collection.Bag.size(Bag.java:207)
 at flashgateway.io.DataOutput.writeObjectArray(DataOutput.java:703)
 at flashgateway.io.DataOutput.writeObject(DataOutput.java:166)
 at flashgateway.io.DataOutput.writeASObject(DataOutput.java:387)
 at flashgateway.io.JavaBeanSerializer.writeObject(JavaBeanSerializer.jav
a:96)
 at flashgateway.io.DataOutput.writeObject(DataOutput.java:174)
 at flashgateway.io.DataOutput.writeObjectArray(DataOutput.java:713)
 at flashgateway.io.DataOutput.writeObject(DataOutput.java:166)
 at flashgateway.io.DataOutput.writeASObject(DataOutput.java:387)
 at flashgateway.io.JavaBeanSerializer.writeObject(JavaBeanSerializer.jav
a:96)
 at flashgateway.io.DataOutput.writeObject(DataOutput.java:174)
 at flashgateway.io.DataOutput.writeObjectArray(DataOutput.java:659)
 at flashgateway.io.DataOutput.writeObjectArray(DataOutput.java:681)
 at flashgateway.io.DataOutput.writeObject(DataOutput.java:107)
 at flashgateway.io.MessageSerializer.writeBody(MessageSerializer.java:15
7)
 at flashgateway.io.MessageSerializer.writeMessage(MessageSerializer.java
:113)
 at flashgateway.filter.SerializationFilter.invoke(SerializationFilter.ja
va:119)
 at flashgateway.Gateway.invoke(Gateway.java:217)
 at flashgateway.controller.GatewayServlet.service(GatewayServlet.java:69
)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Appl
icationFilterChain.java:237)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationF
ilterChain.java:157)
 at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFi
lter.java:75)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Appl
icationFilterChain.java:186)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationF
ilterChain.java:157)
 at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperV
alve.java:214)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:104)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.jav
a:520)
 at org.apache.catalina.core.StandardContextValve.invokeInternal(Standard
ContextValve.java:198)
 at org.apache.catalina.core.StandardContextValve.invoke(StandardContextV
alve.java:152)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:104)
 at org.jboss.web.tomcat.security.CustomPrincipalValve.invoke(CustomPrinc
ipalValve.java:66)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:102)
 at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(Securit
yAssociationValve.java:153)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:102)
 at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValv
e.java:54)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:102)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.jav
a:520)
 at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.j
ava:137)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:104)
 at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.j
ava:118)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:102)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.jav
a:520)
 at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineVal
ve.java:109)
 at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValv
eContext.java:104)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.jav
a:520)
 at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:929)

 at org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:16
0)
 at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java
:799)
 at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.proce
ssConnection(Http11Protocol.java:705)
 at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java
:577)
 at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadP
ool.java:683)
 at java.lang.Thread.run(Thread.java:595)
17:42:27,062 INFO [STDOUT] Error: failed to lazily initialize a collection - no
 session or session was closed
17:42:27,062 INFO [STDOUT] Error: null

[bill.burke]

Either traverse the relationship on the server side, or change your fetch policy to EAGER. This isn't a bug....Its a feature.

Bill

[camel]

See Hibernate.initialize() (it's in hibernate 2, not sure if hibernate3 / EJB3 has it).