Which java class file will be called if same class is packed in two jar files?

Question

I am working on opensource project. As of of now I don't have any customization in any class. So using all the jars files provided by opensource project. My question is if I modify one java file, compile it and pack it new jar file with same folder structure, will there be any issue at start up of server or run time? If not which class file will be called(default file or my customize java class file)?

Answer 1

In fact it depends on many factors:

• If both jar files are in the same ClassLoader, for instance the Java classpath (-cp option), normally it should be the first file found in the jar list order.

• If deployed in a JavaEE container, like in an EAR file or in WEB-INF/lib or a WAR file, there is no warranty the container will load the same class between two startups. In that context, the only thing sure is that WEB-INF/classes is searched before WEB-INF/lib

• In a complex ClassLoader hierarchy, the default behavior is parent-first search but JavaEE implementations have introduced mechanisms like parent-last policy (WebSphere) or filtering thanks to deployment descriptors (WebLogic, JBoss/WildFly)

An option may be to declare jar file dependencies in META-INF/MANIFEST.MF file thanks to the Class-Path attribute. It should enforce a loading order at the ClassLoader level, particularly when started with java -jar myapp.jar but it may depend on implementations in a JavaEE context.

Remark: when using an OpenSource project, it may be fair to submit a change request and publish your changes or improvements so that the community takes benefits from it. Then your project may update to main stream without such a difficulty of wild patches in your ClassPath.

Answer 2

Class loader is looking for the first place where needed resource is located. It means that if class with the same name and package appears in 2 jars the first found will be used. Which one is the first? According to the classpath: if for example class A appears in jars one.jar and two.jar and your command line is:

java -cp one.jar;two.jar MyMain`

the version from one.jar will be used. But if command line is

java -cp two.jar;one.jar MyMain`

the class from two.jar will be instantiated.