Writing a Java standard class library alternative from the scratch

Question

Hi,

I am just curious but I want to know if it is feasible to remove totally the Java standard class libraries coming with the JVM and start a new one from the scratch [à la ClassPath].

If that is possible, what classes MUST be implemented as minimum? (Object and String come to my mind, but... I do not know).

Such thing breaks some license? Is there any way to say to the "java" command to "not use the rt.jar"?

Thanks in advance,

Ernesto

Answer 1

You can use the -Xbootclasspath option to specify your own set of core classes.

If you do go down this path, you will probably end up with a lot of problems if you intend to also use third party libraries as they will depend on the core API and any inconsistencies in your version will likely cause bugs.

As an absolute minimum you'd probably have to reimplement everything in the java.lang package. As well as Object and String, the primitive wrapper classes need to be present in order for auto-boxing to work. I don't think you can replace java.lang without a fair bit of native code to make things like threads work.

Answer 2

No, it is not feasible at all. I mean, sure, you could do it, but you aren't going to do it better than a large corporation or open source project with years of experience and large numbers of Java gurus. It might be fun to geek it up though.

Answer 3

In theory, "yes" it is possible, though you might also need to implement your own JVM! The relationships between the JVM and some of the low level classes (Object, Class, Thread, etc) are such that these classes have to be implemented as part of the JVM.

In practice, it is such a big task that you'd be working on it for the rest of your life, and the chances are that nobody would use your code even if you succeeded. That doesn't sound like "fun" to me.

Such thing breaks some license?

Not per-say. But if you ever tried to release it calling it "Java", Sun's lawyers would be after you for trademark infringement. You can only legally call your implementation Java if it has been validated against the Sun TCK.

But I don't want to be totally discouraging. If you want to hack on the internals of a JVM or stuff like that, the JNode project is always looking for keen new people.