Compile time vs Run time Dependency - Java

Question

What is the difference between compile time and run time dependencies in Java? It is related to class path, but how do they differ?

Answer 1

• Compile-time dependency: You need the dependency in your CLASSPATH to compile your artifact. They are produced because you have some kind of "reference" to the dependency hardcoded in your code, such as calling new for some class, extending or implementing something (either directly or indirectly), or a method call using the direct reference.method() notation.

• Run-time dependency: You need the dependency in your CLASSPATH to run your artifact. They are produced because you execute code that accesses the dependency (either in a hardcoded way or via reflection or whatever).

Although compile-time dependency usually implies run-time dependency, you can have a compile-time only dependency. This is based on the fact that Java only links class dependencies on first access to that class, so if you never access a particular class at run-time because a code path is never traversed, Java will ignore both the class and its dependencies.

Example of this

In C.java (generates C.class):

package dependencies;
public class C { }

In A.java (generates A.class):

package dependencies;
public class A {
    public static class B {
        public String toString() {
            C c = new C();
            return c.toString();
        }
    }
    public static void main(String[] args) {
        if (args.length > 0) {
            B b = new B();
            System.out.println(b.toString());
        }
    }
}

In this case, A has a compile-time dependency on C through B, but it will only have a run-time dependency on C if you pass some parameters when executing java dependencies.A, as the JVM will only try to solve B's dependency on C when it gets to execute B b = new B(). This feature allows you to provide at runtime only the dependencies of the classes that you use in your code paths, and ignore the dependencies of the rest of the classes in the artifact.