如何使用 ASM 将对象（属于 class 对象）动态转换为方法 return 类型？

Question

我想做的是修改一个使用ASM的方法：

我将一个对象（属于 class 个对象）推入堆栈
我想将该对象转换为该方法的 return 类型
Return 铸造的对象。

methodVisitor 适配器中我的代码：

    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));
        }
    }

MyClass中的方法getOutputObj（尝试使用Gson将之前记录的Json字符串恢复为对象）：

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;
        }
    }

我的第一个方法版本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;
        }
    }

我在基准测试中尝试了这段代码，其方法只有 int return 类型。然后我得到 java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer。我认为当我将对象压入堆栈时，如果它表示值，则默认情况下它是 Double 类型。所以我有第二个版本：

    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;
        }
    }

然而，我得到了java.lang.VerifyError: (class: com/D, method: p signature: ()I) Incompatible object argument for function call。我被困在这里，我不知道为什么会抛出这个错误。

Answer 1

问题似乎是：我在一个不属于 class java/lang/Double 的对象上使用 mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "intValue", "()I", false);。我需要先checkcast java/lang/Double。我用的是第三版的方法castPeekOnStack，错误消失了：

    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;
        }
    }

不过我还没有在广泛的案例中测试过这个方法，我不确定它是否适用于其他 return 类型。

上面的解决方案只能处理对象是一个值的情况。当我尝试转换引用类型时，它会抛出类似 com.google.gson.internal.LinkedTreeMap cannot be cast to ... 的内容。所以我从Json恢复对象的方式一定有问题。

所以在方法recoverObjFromJson中，我直接将Json转换为我想要的类型。需要注意的是，虽然通过fromJson将对象强制转换为我指定的类型，但是方法recoverObjFromJson的return类型仍然是Object，所以我仍然需要将它投到堆栈上。

    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;
        }
    }

最后，这个 recoverObjFromJson 与 castPeekOnStack 的第一个版本配合得很好。

如何使用 ASM 将对象（属于 class 对象）动态转换为方法 return 类型？

How to dynamically cast an object (of class Object) to the method return type using ASM?

java

jvm

gson

instrumentation

java-bytecode-asm