Byte-code Injection with Javassist

import javassist.ClassPool;
import javassist.CtClass;

ClassPool cp = ClassPool.getDefault();
CtClass cc = cp.get("test.Rectangle");

// cc := test.Rectangle class file (CtClass type)

cc.setSuperclass(cp.get("test.Point"));
cc.writeFile();

Reading and Writing Bytecode

// cc := CtClass type Loaded Class file

byte[] buf = cc.toBytecode();
Class clazz = cc.toClass();

Frozen Class

CtClass cc = cp.get("test.Rectangle");

cc.writeFile(); // frozen
cc.setSuperclass(/* ... */); // ERROR

cc.defrost(); // defrost
cc.setSuperclass(/* ... */); // OK

Class Search Path

pool.insertClassPath("/usr/local/javalib");

pool.insertClassPath(new URLClassPath(
	"www.javassist.org",
  80,
  "/java/",
  "org.javassist."
); // www.javassist.org:80/java/org/javassist

pool.insertClassPath(new ByteArrayClassPath("ClassName", buf));

Class Loader

public class Main {
  public static void main(String[] args) throws Throwable {
		ClassPool cp = ClassPool.getDefault();
		Loader cl = new Loader(cp); // javassist.Loader

    // 지금까지 보았던 방법
		CtClass ct = cp.get("test.Rectangle");
    ct.setSuperclass(cp.get("test.Point"));

    // Class Loader를 이용해 클래스를 가져오는 방법
    Class c = cl.loadClass("test.Rectangle");
    Object rect = c.getDeclaredConstructor().newInstance();
  }
}

public interface Translator {
  public void start(ClassPool cp) throws NotFoundException, CannotCompileException;
  public void onLoad(ClassPool cp, String classname)
    throws NotFoundException, CannotCompileException;
}

public class MyTranslator implements Translator {
  @Override
  public void start(ClassPool cp) throws NotFoundException, CannotCompileException { }

  @Override
  public void onLoad(ClassPool cp, String classname)
    throws NotFoundException, CannotCompileException {
    CtClass cc = cp.get(classname);
    cc.setModifiers(Modifier.PUBLIC); // modified class modifier

    // ...
  }
}

• start() : Translator 가 등록될 때 호출

• onLoad() : javassist.Loader 가 클래스를 Load하기 전 호출

public class Main {
  public static void main(String[] args) throws Throwable {
    ClassPool cp = ClassPool.getDefault();
    Loader cl = new Loader(cp);

    Translator t = new MyTranslator();

    cl.addTranslator(cp, t);
    cl.loadClass("test.Rectangle"); // emit => run MyTranslator
  }
}

Introspection and Customization

• Inline: System.outPrintln("Hello");

• Block: { System.out.println("Hello"); }

• Statement: if (i < 0) { i = -1; }

• $0 : this Context

• $1 , $2 , ... : Parameters

• $_ : Return Value

// cm := CtMethod type object

cm.insertAfter(String.join("\n",
  "System.out.println(\"Hello!\");",
  "if ($1 == true) {",
  "  System.out.println(\"Hello2\");",
  "}"
);

// cc := CtClass type Object

cc.addField(CtField.make("private boolean isValid = true;", cc));