How can I know at runtime if a JAR file is already in the classpath?

Question

What is the best way to know at runtime if a particular JAR file is already in my classpath ? (if that is not the case I should add it at runtime).

I do not know in advance the name of the jar nor the classes in it. A user can select it. The jar represents a runtime plugable component (a driver in my problem).

Answer 1

String classpath = System.getProperty("java.class.path")

this will give you what is on your classpath. you can then parse that for the jar file you want

Answer 2

A pragmatic way: Class.forName("com.myclass") where com.myclass is a class that is inside (and only inside) your target jar; if that throws a ClassNotFoundException, then the jar is not on you current classpath.

Bear in mind, though, that loading a jar at runtime is not very simple, you need to mess with classloaders. As a rule (there are exceptions) that's not the way, you should be able to add explicitly the jar to the classpath before running.

Update: the updated question states that we don't know in advance "the name of the jar nor the classes in it"; if so, this answer obviously does not apply. And the answer will depend on your specific classloader. In the usual scenario, AlexAndas' answer should work.