Javassist add method and invoke

Question

I'm stuck with javassist. I've added a new method to my object class on runtime.

My object class:

package tmp3;

public class Car {
    public Car(){}
}

My test class:

package tmp3;

import java.lang.reflect.Method;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.CtNewMethod;

public class TestMain {
    public static void main(String[] args) {
        try {

            CtClass ctclass = ClassPool.getDefault().get("tmp3.Car");
            CtMethod newmethod = CtNewMethod.make("public void testPrint() { System.out.println(\"test ok\"); }",ctclass);
            ctclass.addMethod(newmethod);
            ctclass.writeFile();

            for(Method me: ctclass.toClass().getDeclaredMethods()){ //test print, ok
                System.out.println(me.getName());
            }

        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

But after that point, I don't know how to call(invoke) it. I've read that javassist has not the capability to invoke methods. Then how can I invoke the method which I've just added with javassist?

I've tried lots of things in two days but had no success. Could you please help me with this?

Answer 1

Java classes have static interfaces which means, as you probably already know, Java is not designed by default to add methods to a class at runtime so it's a bit tricky, but not that hard to achieve what you want.

You've used Javassist, a bytecode modifier framework, to engineer your compiled class to add more bytecode that represents a new method. You can have one of the two scenarios:

Scenario 1: Code which is compiled along with your Car class before injection

In this case, when your code is being compiled the Java Compiler only knows the Car interface without any injections. So you can't just invoke the injected method directly, like this:

 Car car = new Car();
 car.testPrint();

You have to do it by reflection like @Scorpion correctly commented:

 Car car = new Car();
 Method method = car.getClass().getMethod("testPrint", new Class[]{});
 method.invoke(car,new Object[]{});

But this is not the only way...

Scenario 2: Code which USES your compiled and injected Class

If you compile your Car class, inject it and afterwards write code against the compiled class (for example having the Car class in a jar file) you'll be able to call your injected method as if it were any other regular method.

Do the following exercise:

1. Compile your Car class
2. Run your TestMain which will do the injection
3. Create another project in your IDE and add to that project's classpath the directory with the injected class OR create a jar with only the injected class and add that jar to the classpath
4. Create a class in the new project that creates a new Car instance, notice that you're now able to invoke testPrint method without any hassle.

A few things you should keep attention:

• If you're overwriting your original class with the injected class, you might end up with an invalid class resulting in a java.lang.ClassFormatError with an error message saying that you have a Truncated Class file. This happens if Javassist hasn't loaded all the bytecode to memory and tried to write and read to and from the same class file, which results in a total mess. To avoid this, you can either write to a different path or make sure you load all the bytecode to memory before writing the file (use the toByteCode() from CtClass) .
• If you have two class files, one with the injected code and one with the original code, remember to have only one in your classpath.