I'm wonder what would happen, if I have an application with couple ears and wars, and in each of them class named "Test"? Is it jar hell? What classes would load first (from ears or from wars)? Please, send me somewhere, where I can read about this tricky case more detailed.
2 Answers
Servlet containers normally use a one-classloader-per-webapp strategy to avoid this hell. So if you have two applications deployed on the same container, the will have different classloaders which satisfy their class resolution and loading needs. Tomcat has a document for this.
Of course, there are some gotchas like working with JDBC drivers (which are one per JVM process) and JNI (a given library can't be loaded more than once by a given JVM or something along those lines) but the general working remains the same.
4 Comments
Denis
Hm. Thanks. So there is no problem to use same classes in different ears/wars. Thanks.
Sanjay T. Sharma
Yes, it shouldn't be a problem. Again, trying out things yourself would be much better. :-)
Denis
Ok. What will be in those (JDBC, JNI) cases? Loads first time from random ear/war and after that just retrieve from cache?
Sanjay T. Sharma
The way JDBC drivers work is by registering themselves with the "DriverManager" class. Since the DriverManager is a core class, it is loaded once by the JVM and is shared across the different deployed applications. Hence only one driver can be registered at any given time. Regarding JNI, I think there was a clause in the JNI specification which said loading a single library more than once results in an exception though I can't locate it right now.
