如何使用 javassist 实例化在运行时创建的 class？

Question

我刚开始使用 javassist，我似乎不知道如何实例化在运行时创建的 class。

makeNewClass() 方法使 NewClass class 变成这样：

public bin.objects.base.NewClass {
    public int quantity = 5;
    private float weight = 30.25f;

    public float getWeight() { return weight; }
    public void setWeight(float weight) { this.weight = weight; }
    public float totalWeight() { return quantity * getWeight(); }
}

这个方法很好用：

public void makeNewClass() throws NotFoundException, IOException, CannotCompileException {
        // ClassMaker maker holds the CtClass object and handles all the "making"
        ClassMaker maker = new ClassMaker("bin.objects.base.NewClass");

        maker.addField(CtClass.intType, "quantity", "5", Modifier.PUBLIC);
        maker.addField(CtClass.floatType, "weight", "30.25f", Modifier.PRIVATE);

        maker.addMethod(Modifier.PUBLIC, CtClass.floatType, "totalWeight", null, null, 
                "{ return quantity * getWeight(); }", null, MethodType.standard);
        maker.getCtClass().writeFile();
    }

现在开始做题。此方法应该实例化 NewClass，访问它的字段并调用它的方法。

    public void testNewClass() 
            throws Throwable {
        CtClass ctclass =  ClassPool.getDefault().get("bin.objects.base.NovaClasse");

        Object testClass = ctclass.toClass(new Loader(), null);

        // Throws NoSuchFieldException
        Field q = testClass.getClass().getDeclaredField("quantity");
        int quantity = (int) q.get(testClass);

        Class[] cArg = new Class[1];
        cArg[0] = Float.class;

        // Throws NoSuchMethodException
        Method m = testClass.getClass().getDeclaredMethod("getWeight", cArg);
        float weight = (float) m.invoke(testClass, null);

        // Throws NoSuchMethodException
        m = testClass.getClass().getDeclaredMethod("totalWeight", cArg);
        float totalWeight = (float) m.invoke(testClass, null);

        System.out.println("quantity = " + quantity +
                "weight = " + weight +
                "totalWeight = " + totalWeight);
    }

现在，我已经弄清楚 testClass 实际上是作为 java.lang.Class 的实例初始化的，而不是 bin.objects.base.NewClass。所以，很明显，它不会找到NewClass的字段和方法。

问题是我该如何解决？我尝试使用 java.lang.Class.cast() 方法，但没有成功。

Answer 1

尝试使用反射将您的 class 对象作为 :

Field q = Class.forName("bin.objetos.base.NovaClasse").getDeclaredField("quantity");

您也可以对其他字段执行相同的操作。

Answer 2

我终于找到问题所在了。

看来是ClassLoader的问题。我找到了答案 in this post。为了解决它，我只包含了这一行：

Class newClasse = ctclass.toClass(this.getClass().getClassLoader(), 
    this.getClass().getProtectionDomain());

@nullpointer 的回答也有帮助。

因此，经过更多更改后，testNewClass() 方法最终变成了这样：

public void testNewClass() 
            throws Throwable {
        CtClass ctclass =  ClassPool.getDefault().get("bin.objects.base.NewClass");
        Class newClass = ctclass.toClass(this.getClass().getClassLoader(), 
            this.getClass().getProtectionDomain());
        Object objNewClass  = newClass.newInstance();

        System.out.println("Accessing the field 'public int quantity'..."); 
        Field q = newClass.getDeclaredField("quantity");        
        int quantity = (int) q.get(objNewClass);
        System.out.println("quantity = " + quantity);

        System.out.println("\nAccessing the field 'private float weight' " +
            "through the method 'public float getWeight()'...");
        Method m = newClass.getDeclaredMethod("getWeight", null);
        float weight = (float) m.invoke(objNewClass, null);
        System.out.println("weight = " + weight);

        System.out.println("\nAccessing the method 'public float totalWeight()'...");
        m = newClass.getDeclaredMethod("totalWeight", null);
        float totalWeight = (float) m.invoke(objNewClass, null);        
        System.out.println("totalWeight = " + totalWeight);
    }

如何使用 javassist 实例化在运行时创建的 class？

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