Question

I have a simple question about the java classloading mechanism.

I think that the default class loader loads user-defined classes. If I specify other jars in the classpath, does the default classloader go through each jar and load classes from each jar at application startup?

Answer 1

No, it loads classes whenever they are first referenced, either through Class.forName() or through direct use in your code.

Example:

public class First {
    static {
        System.out.println("first");
    }
    public static void main(final String[] args) {
        System.out.println("second");
        Second.third();
    }
}
public class Second {
    static {
        System.out.println("third");
    }
    public static void third() {
        System.out.println("fourth");
    }
}

If you run First as a main class, the output is:

first   <-- First is loaded
second  <-- method in First is executed
third   <-- Second is loaded
fourth  <-- Method in Second is executed