CodeInjection.java

import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;
import java.util.Random;
import javassist.*;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.invoke.MethodHandles;

public class MainTest {
    
    @Test
    public void test1() {
        Main math = new Main();
        assertEquals(1, math.pow(2, 0));
    }
    
    @Test
    public void test2() {
        Main math = new Main();
        assertEquals(2, math.pow(2, 1));
    }
    
    @Test
    public void test4() {
        Main math = new Main();
        assertEquals(4, math.pow(2, 2));
    }
    
    @Test
    public void test8() {
        Main math = new Main();
        assertEquals(8, math.pow(2, 3));
    }

    @Test
    public void testRandom() {
        Main math = new Main();
    		Random r = new Random();
		    int a = r.nextInt(10);
		    int b = r.nextInt(10);
        assertEquals(Math.pow(a, b), math.pow(a, b), 0);
    }

    @Test
    public void testRecusion() throws Exception {
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.getAndRename("Main", "MainReflect");
        
        CtField f = new CtField(CtClass.intType, "recursion", cc);
        f.setModifiers(Modifier.PUBLIC);
        cc.addField(f);
        
        CtMethod m = cc.getDeclaredMethod("pow");
        m.insertBefore("{ recursion++; }");

        byte[] bytes = cc.toBytecode();
        MethodHandles.Lookup lookup = MethodHandles.lookup();
        Class<?> clazz = lookup.defineClass(bytes);
        Object inst = clazz.getConstructor().newInstance();
        
        Field field = clazz.getField("recursion");
        field.set(inst, 0);
        
        Method pow = clazz.getMethod("pow", int.class, int.class);
          
    		Random r = new Random();
		    final int a = r.nextInt(10);
		    final int b = 2 + r.nextInt(8);
        long result = (long) pow.invoke(inst, a, b);
        
        Object recursion = field.get(inst);
        
        assertEquals((b + 1) + " recursions were expected", b + 1, (int) recursion);
    }
}