如何使用 ASM 将 object（class 对象）动态转换为方法返回类型？

Question

我想要做的是使用 ASM 修改方法：

1. 我将 object（class 对象的）推送到堆栈
2. 我想将 object 转换为该方法的返回类型
3. 返回铸造 object。

我在 methodVisitor 适配器中的代码：

    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getOutputObj方法（它尝试使用 Gson 将先前记录的 Json 字符串恢复为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;
        }
    }

我的第一个版本的方法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;
        }
    }

我在一个基准测试中尝试了这段代码，它的方法只有int返回类型。 Then I get java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Integer . 我认为当我将 object 推入堆栈时，如果它代表值，则默认为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;
        }
    }

但是，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

看来问题是：我使用mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Double", "intValue", "()I", false); 在不属于 class java/lang/Double的 object 上。 我需要先checkcast java/lang/Double 。 我使用了第三版的castPeekOnStack方法，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;
        }
    }

但是我没有在广泛的情况下测试该方法，我不确定它是否适用于其他返回类型。

---

上述解决方案只能处理 object 为值的情况。 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...类的东西。 所以我从Json恢复object的方式肯定有一些问题。

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fromJson将 object 强制转换为 I 指定的类型，但方法recoverObjFromJson的返回类型仍然是Object ，所以我仍然需要将其强制转换为堆栈。

    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。