What happens when two Java frameworks need third one but each of the two need different version of third one?

Question

In my Java project I'm using two different frameworks (let's say A.jar and B.jar) and both of them require one common framework (let's say Log4j.jar) but in two different versions. How it is treated by Java if framework A needs Log4J v1.1 and B needs Log4j v1.2? Will it cause some kind of conflict/error or will by somehow (how?) resolved?

If it will not cause conflict/error (my project can be compiled and run) - can I use any version of Log4j myself in this project? Or am I forced to select lower / higher version number of Log4j?

Update: to be more specific... What if some part of Log4j API changed in v1.2 (let's say one single method doIt() signature changed) and both A and B call doIt. What will happend? Will my project run? Will it crash on first use of doIt? What version of Log4j I must put on classpath - v1.2 or both?

Answer 1

In a flat class loading scheme, there is no library versioning support. You need to use the most recent version of the library and remove other versions from the classpath.

Some frameworks, like OSGi, provide mechanisms for handling these cases, but it isn't clear that you're relying on a plug-in framework.

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Edit:

What if some part of Log4j API changed in v1.2 (let's say one single method doIt() signature changed) and both A and B call doIt. What will happen? Will my project run? Will it crash on first use of doIt?

A call relying on a signature that is not present will likely result in a NoSuchMethodError. This probably will not happen until the method is invoked. Other errors may occur if other mechanisms rely on the signature (e.g. class inheritance).

What version of Log4j I must put on classpath - v1.2 or both?

Making two versions of a library on the classpath will just result in one set of classes being loaded randomly. The behaviour will be undefined, but could potentially result in all sorts of unpleasant errors and exceptions.

Answer 2

Java doesn't natively support the management of multiple versions of the same piece of code, that is, you can only use at most one version within the same JVM (with the default class loader). However, checkout question 1705720, which has several answers pointing out possible ways of achieving that (OSGi or custom class loaders).

But I doubt it worth the trouble as multiple log4j versions are not required by your code directly. In you case, I'd suggest to start from using the newer log4j version (v1.2) first and verify if it would cause any problem for framework A. If it does cause conflict, then fall-back to log4j v1.1 and verify again. If you really are out of luck, then you need to get your hands dirty...

Update: for you specific description, there's no way of using either log4j v1.1 or v1.2, as framework A and B each require different signature. You have to roll your own version of any of log4j or framework A, or B.