在玩类加载器时，我遇到了以下异常：
Exception in thread "main" java.lang.ClassCastException: xxx.Singleton cannot be cast to xxx.Singleton
这是否意味着来自类加载器的实例不能转换为另一个类加载器的类？
检查我的代码，感谢类加载器，即使使用“”安全性，我也可以在其中实例化 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);
}
}