Let's say I have a simple
class C$Manipulatables{
public Wrapper get(long v, String f){
if(v == 0){
return new Wrapper(0);
}else{
throw new RuntimeException("unacceptable: v not found");
}
}
}
I now want to redefine the get in C$Manipulatables, s.t. it reads
class C$Manipulatables{
public Wrapper get(long v, String f){
if(v == 1){ return null; }
if(v == 0){
return new Wrapper(0);
}else{
throw new RuntimeException("unacceptable: v not found");
}
}
}
This will trigger a nullpointer exception if I try to use the new value, but that's ok - for now I just want confirmation that the new code is loaded.
We add the code with ASM (I will add the full copy-pastable code at the bottom of this post, so I'm just zooming in on the relevant parts, here):
class AddingMethodVisitor extends MethodVisitor implements Opcodes{
int v;
public AddingMethodVisitor(int v, int api, MethodVisitor mv) {
super(api, mv);
this.v = v;
}
@Override
public void visitCode() {
super.visitCode();
mv.visitVarInsn(LLOAD, 1); //arg 1 of method is version number
/*if arg1 == the new version*/
mv.visitLdcInsn(v);
Label lSkip = new Label();
mv.visitInsn(LCMP);
mv.visitJumpInsn(IFNE, lSkip);
mv.visitInsn(ACONST_NULL);
mv.visitInsn(ARETURN);
/*else*/
mv.visitLabel(lSkip);
mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
}
}
and reload the class with ByteBuddy (again, full code at the bottom of the post):
ClassReader cr;
try {
/*note to self: don't forget the ``.getClassLoader()``*/
cr = new ClassReader(manipsClass.getClassLoader().getResourceAsStream( manipsClass.getName().replace('.','/') + ".class"));
}catch(IOException e){
throw new RuntimeException(e);
}
ClassWriter cw = new ClassWriter(cr, 0);
VersionAdder adder = new VersionAdder(C.latest + 1,Opcodes.ASM5,cw);
cr.accept(adder,0);
System.out.println("reloading C$Manipulatables class");
byte[] bytes = cw.toByteArray();
ClassFileLocator classFileLocator = ClassFileLocator.Simple.of(manipsClass.getName(), bytes);
new ByteBuddy()
.redefine(manipsClass,classFileLocator)
.name(manipsClass.getName())
.make()
.load(C.class.getClassLoader(), ClassReloadingStrategy.fromInstalledAgent())
;
C.latest++;
System.out.println("RELOADED");
}
}
This fails.
got 0
reloading C$Manipulatables class
Exception in thread "main" java.lang.VerifyError
at sun.instrument.InstrumentationImpl.redefineClasses0(Native Method)
at sun.instrument.InstrumentationImpl.redefineClasses(InstrumentationImpl.java:170)
at net.bytebuddy.dynamic.loading.ClassReloadingStrategy$Strategy$1.apply(ClassReloadingStrategy.java:261)
at net.bytebuddy.dynamic.loading.ClassReloadingStrategy.load(ClassReloadingStrategy.java:171)
at net.bytebuddy.dynamic.TypeResolutionStrategy$Passive.initialize(TypeResolutionStrategy.java:79)
at net.bytebuddy.dynamic.DynamicType$Default$Unloaded.load(DynamicType.java:4456)
at redefineconcept.CInserter.addNext(CInserter.java:60)
at redefineconcept.CInserter.run(CInserter.java:22)
at redefineconcept.CInserter.main(CInserter.java:16)
Process finished with exit code 1
In fact, this even fails when I comment the return null stmt generation (as I have in the full code provided below).
Apparently, java just doesn't like the way I've constructed my IF, even though it's essentially the code I got when I used asmifier on
public class B {
public Object run(long version, String field){
if(version == 2) {
return null;
}
return null;
}
}
which yielded
{
mv = cw.visitMethod(ACC_PUBLIC, "run", "(JLjava/lang/String;)Ljava/lang/Object;", null, null);
mv.visitCode();
mv.visitVarInsn(LLOAD, 1);
mv.visitLdcInsn(new Long(2L));
mv.visitInsn(LCMP);
Label l0 = new Label();
mv.visitJumpInsn(IFNE, l0);
mv.visitInsn(ACONST_NULL);
mv.visitInsn(ARETURN);
mv.visitLabel(l0);
mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
mv.visitInsn(ACONST_NULL);
mv.visitInsn(ARETURN);
mv.visitMaxs(4, 4);
mv.visitEnd();
}
I simply left away everything after
mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
because after that, the already existing part of the method follows.
I really do believe that there's something about that visitFrame that java doesn't like.
Let's say I change my visitCode s.t. it reads
public void visitCode() {
super.visitCode();
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitLdcInsn("Work, you ..!");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
// mv.visitVarInsn(LLOAD, 1); //arg 1 of method is version number
//
// /*if arg1 == the new version*/
// mv.visitLdcInsn(v);
// Label lSkip = new Label();
//
// mv.visitInsn(LCMP);
// mv.visitJumpInsn(IFNE, lSkip);
//
//// mv.visitInsn(ACONST_NULL);
//// mv.visitInsn(ARETURN);
//
// /*else*/
// mv.visitLabel(lSkip);
// mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
}
Then the redefinition works. I get the expected exception because the code falls through to the original if-else block that cannot handle the new version number, but I do get the output, at least.
got 0
reloading C$Manipulatables class
RELOADED
Work, you ..!
Exception in thread "main" java.lang.RuntimeException: unacceptable: v not found
at redefineconcept.C$Manipulatables.get(C.java:27)
at redefineconcept.C.get(C.java:10)
at redefineconcept.CInserter.run(CInserter.java:23)
at redefineconcept.CInserter.main(CInserter.java:16)
I'd very much appreciate some help in resolving this. What's the correct way to insert a new if stmt that java will accept?
FULL CODE
C.java
(Note that the class C$Manipulatables is necessary, because ByteBuddy cannot redefine classes that have static initialisers.)
package redefineconcept;
public class C {
public static volatile int latest = 0;
public static final C$Manipulatables manips = new C$Manipulatables();
public int get(){
int v = latest;
return manips.get(v,"").value;
}
}
class Wrapper{
int value;
public Wrapper(int value){
this.value = value;
}
}
class C$Manipulatables{
public Wrapper get(long v, String f){
if(v == 0){
return new Wrapper(0);
}else{
throw new RuntimeException("unacceptable: v not found");
}
}
}
CInserter.java
package redefineconcept;
import net.bytebuddy.ByteBuddy;
import net.bytebuddy.agent.ByteBuddyAgent;
import net.bytebuddy.dynamic.ClassFileLocator;
import net.bytebuddy.dynamic.loading.ClassReloadingStrategy;
import net.bytebuddy.jar.asm.*;
import java.io.IOException;
public class CInserter {
public static void main(String[] args) {
ByteBuddyAgent.install();
new CInserter().run();
}
private void run(){
C c = new C();
System.out.println("got " + c.get());
addNext();
System.out.println("got " + c.get()); //should trigger nullptr exception
}
private void addNext(){
Object manips;
String manipsFld = "manips";
try {
manips = C.class.getDeclaredField(manipsFld).get(null);
}catch(NoSuchFieldException | IllegalAccessException e){
throw new RuntimeException(e);
}
Class<?> manipsClass = manips.getClass();
assert(manipsClass.getName().equals("redefineconcept.C$Manipulatables"));
ClassReader cr;
try {
/*note to self: don't forget the ``.getClassLoader()``*/
cr = new ClassReader(manipsClass.getClassLoader().getResourceAsStream( manipsClass.getName().replace('.','/') + ".class"));
}catch(IOException e){
throw new RuntimeException(e);
}
ClassWriter cw = new ClassWriter(cr, 0);
VersionAdder adder = new VersionAdder(C.latest + 1,Opcodes.ASM5,cw);
cr.accept(adder,0);
System.out.println("reloading C$Manipulatables class");
byte[] bytes = cw.toByteArray();
ClassFileLocator classFileLocator = ClassFileLocator.Simple.of(manipsClass.getName(), bytes);
new ByteBuddy()
.redefine(manipsClass,classFileLocator)
.name(manipsClass.getName())
.make()
.load(C.class.getClassLoader(), ClassReloadingStrategy.fromInstalledAgent())
;
C.latest++;
System.out.println("RELOADED");
}
}
class VersionAdder extends ClassVisitor{
private int v;
public VersionAdder(int v, int api, ClassVisitor cv) {
super(api, cv);
this.v = v;
}
@Override
public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
if(mv != null && name.equals("get")){
return new AddingMethodVisitor(v,Opcodes.ASM5,mv);
}
return mv;
}
class AddingMethodVisitor extends MethodVisitor implements Opcodes{
int v;
public AddingMethodVisitor(int v, int api, MethodVisitor mv) {
super(api, mv);
this.v = v;
}
@Override
public void visitCode() {
super.visitCode();
mv.visitVarInsn(LLOAD, 1); //arg 1 of method is version number
/*if arg1 == the new version*/
mv.visitLdcInsn(v);
Label lSkip = new Label();
mv.visitInsn(LCMP);
mv.visitJumpInsn(IFNE, lSkip);
// mv.visitInsn(ACONST_NULL);
// mv.visitInsn(ARETURN);
/*else*/
mv.visitLabel(lSkip);
mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
}
}
}
One bug I noticed in your code is the following line
mv.visitLdcInsn(v);
The intent of the code is to create and load a long constant, but v has type int, so an integer constant will be created instead, thus creating a type error in the bytecode when you compare it with lcmp on the next line. visitLdcInsn will create a different constant type depending on the type of Object you pass in, so the argument needs to be the exact type you want.
On a side note, you don't need LDC in the first place to create a long constant of value 1, because there is a dedicated bytecode instruction for this, lconst_1. In ASM, this should be something like visitInsn(LCONST_1);