CODEWITHSUNDEEP
javajvmclassloader
m0skit0
m0skit0

Reputation: 25863

Understanding ClassLoaders in Java

I'm fairly new to the JVM and ClassLoaders. I have these two classes:

public abstract class CoreModule extends Entity

public final class EventManager extends CoreModule

At the start of the program, I create an instance of EventManager. So I understand that the JVM knows what Entity class is and how to load it (that is, it knows what ClassLoader to use), since EventManager is a grand-child. But when an Entity instance is passed by some serialization mechanism, it throws ClassNotFoundException. I have to manually set the ClassLoader to use (Event.class.getClassLoader()).

How come the JVM does not know what Event class is or how to load it if it already did it?

Upvotes: 4

Views: 1632

Answers (2)

Marko Topolnik
Marko Topolnik

Reputation: 200138

Just by creating an instance of EventManager you didn't show the JVM how to load it. In fact, you're not talking to the JVM here. You're talking to one specific classloader, and when that same classloader is not in charge at the time of deserialization, you can get an error. That's why your problem is all about what clasloader is in charge at what point.

Upvotes: 1

jontro
jontro

Reputation: 10628

Actually the JVM does not figure this out "magically". It is all based on a system class loader which will vary depending on the environment you use. Then each thread has a context ClassLoader which derives from this automatically.

The context ClassLoader you can change by using Thread.setContextClassLoader

If your serialization code should be able to resolve a class not visible from the context ClassLoader you need to set this the way you did.

Upvotes: 2

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