Defining a class while a Java application is running

Question

In Java, is it possible to create a class definition on the fly while an application is running, and then create an object of that class?

For example, a running application would read in a text file that contains a list of class members to include in the new class. The application would then define a class definition based on the list of members, and then insantiate it.

Answer 1

Yes its possible to do so in theory your class file is byte code which is at the end a byte array! you can then use the method defineClass(String, byte[], int, int) to get a Class instance that can be used to instantiate objects via reflection.

In practice you can use something like CGLib or javaassist.

You can also use the long way of generating the java code in a file, invoking the compiler, then loading the file.

Answer 2

You can dynamically generate classes using ASM

Answer 3

You can do this by writing the code for your new class out to a file, then invoking the Java compiler on that file and using a classloader to dynamically load that class into your running application. Apache Tomcat does this for its JSP pages; it takes the code, makes some changes to it, wraps it in a try/catch block in the middle of a class, which it then writes to the filesystem, compiles it, and uses a classloader to get and sue it to serve requests.