How to generate bytecode of existing class at Runtime in Java?

Question

How to generate bytecode of existing class at Runtime in Java?

My Existing class is say Foo.java

public class Foo {

    public String saySomething() {
        return "Hello World";
    }
}

Now I want to generate byte code of this existing class Foo.java and probably use it in another instance of JVM. Please understand I am not creating a class at runtime since I already have an existing class called Foo.java . Open to any libraries such as ASM or ByteBuddy etc.

Answer 1

[Edited following the clarification in the comments]

The following article describes how to compile a class from source code at runtime, using ToolProvider.getSystemJavaCompiler() .

Answer 2

You do not need a library for that, any class file is just a resource just like any other file which can be located by Foo.class.getResource("Foo.class") . You can open a stream of this resource and read its byte just as of any other resource.

If you are already using Byte Buddy, you can get hold of the bytes by: ClassFileLocator.ForClassLoader.read(Foo.class) but if that is all you need, using a library would be an overkill.