CODEWITHSUNDEEP
javaclassloader
BambooleanLogic
BambooleanLogic

Reputation: 8161

Do shared dependencies between classloaders get loaded twice?

Let's say I have two classloaders, x and y, used for loading ClassX and ClassY, respectively, as well as their dependencies. Everything else in the application is loaded using the default classloaders.

Now assume that ClassX and ClassY share a dependency, ClassZ.

If the main application also has a dependency to ClassZ (whether or not it has actually loaded the class yet), my assumption is that x/y won't be used at all to resolve this as the default classloaders should be able to find it. (Please correct me if I'm wrong.)

Now, my main question is what happens if only x and y can locate ClassZ: If both ClassX and ClassY reference it, will it be loaded twice (i.e. waste twice as much space in PermGen) or is Java clever enough to detect that it's exactly the same class, loaded by two different classloaders, and just keep one copy in memory?

Upvotes: 3

Views: 139

Answers (1)

G_H
G_H

Reputation: 11999

They will both have their own Class instance for ClassZ. Keeping such loading separate is important in various technologies built on Java, such as webservers or Java EE environments that may need to maintain separate contexts for deployed applications.

You are also correct in your assumption that if the parent classloader for ClassX and ClassY (assuming it is the same) has loaded ClassZ, then that one will be used by both its children. At least, that's the default approach. A classloader first delegates lookup to its parent and if that one can't find it, it will try itself. Some custom loaders may override this behaviour, though.

Complex classloader hierarchies can emerge. While this does "waste" some room in the permgen, trying to share resources is often more trouble than it's worth, and will cause compatibility issues or classloader leaks in some frameworks.

Upvotes: 5

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