java - 是否由于类加载器而导致ClassCastException？

在使用类加载器时，我遇到以下异常:

Exception in thread "main" java.lang.ClassCastException: xxx.Singleton cannot be cast to xxx.Singleton

这是否意味着某个类加载器中的实例不可转换为另一个类加载器的类？

检查我的代码，即使有了“”安全性，我也可以通过类加载器实例化3个单例。

public static void main(String[] args) throws Exception {
        URL basePath = new URL("file:/myMavenPath/target/classes/");

    Object instance = getClassInstance(Singleton.class);
    System.out.println(instance);
    //
    Object instance2 = getClassInstance(
            new URLClassLoader( new URL[]{basePath} , null )
                    .loadClass("my.Singleton")
    );
    System.out.println(instance2);
    //
    Object instance3 = getClassInstance(
            new URLClassLoader( new URL[]{basePath} , null )
                    .loadClass("my.Singleton")
    );
    System.out.println(instance3);

    // Only the 1st cast is ok
    Singleton testCast1 = (Singleton) instance;
    System.out.println("1st cast ok");
    Singleton testCast2 = (Singleton) instance2;
    System.out.println("2nd cast ok");
    Singleton testCast3 = (Singleton) instance3;
    System.out.println("3rd cast ok");
}

private static Object getClassInstance(Class clazz) throws Exception {
    Method method = clazz.getMethod("getInstance");
    method.setAccessible(true);
    return method.invoke(null);
}


class Singleton {

    private static final Singleton INSTANCE = new Singleton();

    public static Singleton getInstance() {
        return INSTANCE;
    }

    private Singleton() {
        Exception e = new Exception();
        StackTraceElement[] stackTrace = e.getStackTrace();
        if (!"<clinit>".equals(stackTrace[1].getMethodName())) {
            throw new IllegalStateException("You shall not instanciate the Singleton twice !",e);
        }
    }

    public void sayHello() {
        System.out.println("Hello World ! " + this);
    }

}

最佳答案

您不能在类加载器之间进行转换。类标识由完全限定的名称和类加载器组成。检查类身份孤岛危机 here。

关于java - 是否由于类加载器而导致ClassCastException？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/11865917/