What I want to do is to modify a method using ASM:
My code in the methodVisitor adapter:
public void visitCode() {
mv.visitCode();
if (needModify){
// package all the method arguments to an Object array and push to the stack
...
// selfReturnTypeDotClassName is the dot class name of return type
mv.visitLdcInsn(selfReturnTypeDotClassName);
// push the object (of class Object)
mv.visitMethodInsn(INVOKESTATIC, MyClass, "getOutputObj",
"([Ljava/lang/Object;Ljava/lang/String;)Ljava/lang/Object;", false);
// cast the object to the return type
castPeekOnStack(selfReturnType);
mv.visitInsn(selfReturnType.getOpcode(IRETURN));
}
}
The method getOutputObj in MyClass (It tries to recover previously recorded Json String to an object using Gson):
public static Object getOutputObj(Object[] args, String methodId, String returnTypeDotClassName){
HashMap<String, String> inOutMap = getInOutMapOfMethod(methodId);
// `GSON` is an instance of class `Gson`
String inputJson = GSON.toJson(args);
String outputJson = inOutMap.get(inputJson);
return recoverObjFromJson(outputJson, returnTypeDotClassName);
}
public static Object recoverObjFromJson(String outputJson, String returnTypeDotClassName){
try{
// the object is previously packaged as an object array with length 1.
Object obj = GSON.fromJson(outputJson, Object[].class)[0];
return obj;
}catch (Exception e){
e.printStackTrace();
MyEkstaziAgent.log(String.format("Gson Error: fromJson failed for arguments %s, %s",
outputJson, returnTypeDotClassName));
return null;
}
}
My first version of method castPeekOnStack:
public void castPeekOnStack(Type targetType){
switch (targetType.getSort()) {
// not sure
case Type.BOOLEAN:
mv.visitTypeInsn(CHECKCAST, "java/lang/Boolean");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Boolean", "booleanValue", "()Z", false);
break;
case Type.BYTE:
mv.visitTypeInsn(CHECKCAST, "java/lang/Byte");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Byte", "byteValue", "()B", false);
break;
case Type.CHAR:
mv.visitTypeInsn(CHECKCAST, "java/lang/Character");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Character", "charValue", "()C", false);
break;
case Type.SHORT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Short");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Short", "shortValue", "()S", false);
break;
case Type.INT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Integer");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Integer", "intValue", "()I", false);
break;
case Type.FLOAT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Float");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Float", "floatValue", "()F", false);
break;
case Type.LONG:
mv.visitTypeInsn(CHECKCAST, "java/lang/Long");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Long", "longValue", "()J", false);
break;
case Type.DOUBLE:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "doubleValue", "()D", false);
break;
case Type.ARRAY:
mv.visitTypeInsn(CHECKCAST, targetType.getDescriptor());
break;
case Type.OBJECT:
mv.visitTypeInsn(CHECKCAST, targetType.getInternalName());
break;
}
}
I tried this code on a benchmark whose methods only have int return type. Then I get java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer. I think when I push the object to the stack, it is of type Double by default if it represents value. So I have the second version:
public void castPeekOnStack(Type targetType){
switch (targetType.getSort()) {
// not sure
case Type.BOOLEAN:
mv.visitTypeInsn(CHECKCAST, "java/lang/Boolean");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Boolean", "booleanValue", "()Z", false);
break;
case Type.BYTE:
mv.visitTypeInsn(CHECKCAST, "java/lang/Byte");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "byteValue", "()B", false);
break;
case Type.CHAR:
mv.visitTypeInsn(CHECKCAST, "java/lang/Character");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Character", "charValue", "()C", false);
break;
case Type.SHORT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Short");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "shortValue", "()S", false);
break;
case Type.INT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Integer");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "intValue", "()I", false);
break;
case Type.FLOAT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Float");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "floatValue", "()F", false);
break;
case Type.LONG:
mv.visitTypeInsn(CHECKCAST, "java/lang/Long");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "longValue", "()J", false);
break;
case Type.DOUBLE:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "doubleValue", "()D", false);
break;
case Type.ARRAY:
mv.visitTypeInsn(CHECKCAST, targetType.getDescriptor());
break;
case Type.OBJECT:
mv.visitTypeInsn(CHECKCAST, targetType.getInternalName());
break;
}
}
However, I got java.lang.VerifyError: (class: com/D, method: p signature: ()I) Incompatible object argument for function call. I am stuck here, I have no idea why this error is thrown.
It seems the problem is: I use mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "intValue", "()I", false); on an object which is not of class java/lang/Double. I need to checkcast java/lang/Double first. I used the third version of method castPeekOnStack, the error has gone:
public void castPeekOnStack(Type targetType){
switch (targetType.getSort()) {
// not sure
case Type.BOOLEAN:
mv.visitTypeInsn(CHECKCAST, "java/lang/Boolean");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Boolean", "booleanValue", "()Z", false);
break;
case Type.BYTE:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "byteValue", "()B", false);
break;
case Type.CHAR:
mv.visitTypeInsn(CHECKCAST, "java/lang/Character");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Character", "charValue", "()C", false);
break;
case Type.SHORT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "shortValue", "()S", false);
break;
case Type.INT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "intValue", "()I", false);
break;
case Type.FLOAT:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "floatValue", "()F", false);
break;
case Type.LONG:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "longValue", "()J", false);
break;
case Type.DOUBLE:
mv.visitTypeInsn(CHECKCAST, "java/lang/Double");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "doubleValue", "()D", false);
break;
case Type.ARRAY:
mv.visitTypeInsn(CHECKCAST, targetType.getDescriptor());
break;
case Type.OBJECT:
mv.visitTypeInsn(CHECKCAST, targetType.getInternalName());
break;
}
}
However I haven't test the method on a wide range of cases, I am not sure if it can work for other return types.
The solution above can only handle cases that the object is a value. When I try to cast a reference type, It throws something like com.google.gson.internal.LinkedTreeMap cannot be cast to .... So the way I recover object from Json must have some problems.
So in method recoverObjFromJson, I directly cast the Json to the type I want. It should be noted that, although by fromJson the object is casted to the type I designate, the return type of method recoverObjFromJson is still Object, so I still need to cast it on the stack.
public static Object recoverObjFromJson(String outputJson, String returnTypeDotClassName){
try{
Object obj = GSON.fromJson(outputJson, getClassObjByName(returnTypeDotClassName));
return obj;
}catch (Exception e){
e.printStackTrace();
MyEkstaziAgent.log(String.format("Gson Error: fromJson failed for arguments %s, %s",
outputJson, returnTypeDotClassName));
return null;
}
}
Finally, this recoverObjFromJson works well with the first version of castPeekOnStack.