CODEWITHSUNDEEP
javajarclassloader
Michel
Michel

Reputation: 357

Java classloader delegation

I have a question about java ClassLoaders. I am trying to use different ClassLoaders to be able to run different versions of a JAR from within the same program.

I have heard somewhere that if you load one class using one ClassLoader all classes called (being loaded) from within that class would use the same ClassLoader. Is this correct?

If not, is there a neat way to set the context of a ClassLoader (let's say, everything being called from a specific class/library should use the same ClassLoader).

Upvotes: 2

Views: 374

Answers (1)

jtahlborn
jtahlborn

Reputation: 53694

This is not a simple subject and i would advise doing more research online as no answer here will be nearly in depth enough. but, as a quick synopsis:

  • classes loaded via normal class references (i.e. a line of code in Class A which uses a variable of static type B) will be loaded using the same classloader as the initial class.
  • however, due to classloader delegation, a class may not actually be loaded by the ClassLoader from which the search originally started. example, i have Class A loaded by classloader LA with parent classloader LP. Class B is referenced by A, so the search for Class B will start with LA. however, the class bytes for B are actually found in LP, so LP loads the class and hands it to LA which returns it. ultimately, however, B is owned by LP, not LA.
  • with utilities which load classes via reflection (e.g. serialization, JAXB, Hibernate, etc.) or frameworks which are typically used with nested classloaders (e.g. Java EE appservers), all bets are off. typically utilities/frameworks like this load classes using the context classloader, but that is not always the case. each utility may have different priorities and fallbacks regarding which classloader is used. additionally, many have ways of explicitly providing a classloader at runtime.

as a rule of thumb, while executing code which you know is from a nested classloader (probably because you set it up), you should set the current context classloader appropriately.

Upvotes: 4

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