EPractice Labs final exam question 16

[Himai Minh]

About this question:

When the stateful session beans get passivated, the following activities should be done:
...
c. Non transient ,non serializable variables have to be set to null in the ejbPassivate() method.

The correct answer is c.

I think setting non-serializable variables null in ejbPassivate() or @PrePassivate method is a common practice, but not a must.
Also, the specification does not say non transient variables have to set to null.

//On p.71 of the specification:
The objects that are assigned to the instance's non-transient fields.... after PrePassivate method complete must be :
a Serializable object
a null
....

It means non-transient variables must be a serializable object or null in order to be serialized when the stateful bean is passivated.
But it does not mean non-transient have to be set to null in PrePassivate method.

[Kaxhif Khan]

Hi Himai,

If an object is not serializable then it will give exception. The best way to test it is to try it by keeping a non-serializable object in your stateful session bean and let it be passivated and destroyed. I tried it once and i was able to see exception in the server log file.


So technically it is a must to make non-serializable object to 'null'

[Himai Minh]

Really? The container will throw an exception if the object is not serializable?
According to the spec, the container may be able to serialize some non-serializable objects. Or, the container will use externalization to serialize the non-serializable object.
Setting non-serializable object to null before the stateful bean passivated is just a good practice, but not a must.

[Kaxhif Khan]

According to the spec, the container may be able to serialize some non-serializable objects. Or, the container will use externalization to serialize the non-serializable object.
Setting non-serializable object to null before the stateful bean passivated is just a good practice, but not a must.

Its getting confusing for me, here is what i have done

@Local
public interface BookService {

	public void searchBookById(int id);
}

@Stateful(passivationCapable = true)
public class BookServiceImpl implements BookService{

	private SessionContext context;	
	private Employee employee = new Employee("Azimuth");

	@Resource
	public void setContext(SessionContext context) {
		this.context = context;
	}
	
	@Override
	public void searchBookById(int id) {
		// TODO Auto-generated method stub
		System.out.println("Search Book by ID : " + id);
		
		String str = context.getInvokedBusinessInterface() + ":" +
		context.getContextData();

		System.out.println(str);
	}
	
}

public class Employee {

	private String empName;
	
	public Employee(String name)
	{
		empName = name;
	}

	public String getEmpName() {
		return empName;
	}

	public void setEmpName(String empName) {
		this.empName = empName;
	}
	
	
}

Here is stack trace from the server log file

Caused by: java.io.IOException
at com.sun.ejb.EJBUtils.serializeObjectFields(EJBUtils.java:746)
at com.ejb._BookServiceImpl_Serializable.writeObject(Unknown Source)
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:606)
at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1495)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1547)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1508)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347)
at com.sun.ejb.base.io.IOUtils.serializeObject(IOUtils.java:95)
... 49 more
Caused by: java.io.NotSerializableException: com.domain.Employee
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347)
at com.sun.ejb.EJBUtils.serializeObjectFields(EJBUtils.java:744)
... 64 more

I got this behavior with GlassFish 3.1.2 but i was able to run same example correctly with GlassFish 4. I am a bit double minded which server to use.

[Himai Minh]

According to the spec, p.73:

While the container is not required to use the Serialization protocol for the Java programming language to store the state of a passivated session instance, it must achieve the equivalent result.

You can refer to this:
http://www.enthuware.com/forum/viewtopic.php?f=4&t=861
Pay attention to what the admin wrote:

This is basic programming stuff.
1. Serialization doesn't necessarily require implementing Serializable. One can use Externalizable also or have a custom serialization mechanism.

Also, refer to this thread:
https://coderanch.com/t/649564/java-EJB-SCBCD/certification/container-serializes-EntityManager-SessionContext-serializable

Pay attention to Frits' reply:

how can the container serialize those fields which do not implement Serializable ?

Good question for the EJB-container developers. However in the relation to the exam this knowledge is not needed.

Glassfish mandates the objects like Employee to be serializable. But the spec does not mandate it.

[Kaxhif Khan]

Glassfish mandates the objects like Employee to be serializable. But the spec does not mandate it.

That's the point, specification allowed containers to choose different implementation for e.g. glassfish gives an error but other containers may not.

If one got a question in exam like this, what answer he/she should choose ?