jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to java.net.URLClassLoader

final URLClassLoader urlClassLoader = (URLClassLoader) MyClass.class.getClassLoader();

final Method addURL = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);

addURL.setAccessible(true);

addURL.invoke(urlClassLoader, new File(newpath.trim()).toURI().toURL());
 
在使用Java9之后发的版本汇报如下错误：

Exception in thread "main" java.lang.RuntimeException: java.lang.ClassCastException: jdk.internal.loader.ClassLoaders$AppClassLoader (in module: java.base) cannot be cast to java.net.URLClassLoader (in module: java.base)
 
Java9之后，应用程序和扩展类都不再是 java.net.URLClassLoader 的实例

因此,解决办法如下：

if (classLoader instanceof URLClassLoader) {
System.out.println("DEB: classLoader instanceof URLClassLoader");
URLClassLoader sysloader = (URLClassLoader)ClassLoader.getSystemClassLoader();
Class sysclass = URLClassLoader.class;
try {
Method method = sysclass.getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);
method.invoke(sysloader, url);
} catch (Exception var5) {
var5.printStackTrace();
throw new IllegalStateException(var5.getMessage(), var5);
}
} else {
try {
Field field = classLoader.getClass().getDeclaredField("ucp");
field.setAccessible(true);
Object ucp = field.get(classLoader);

System.out.println("DEB: invoke method!");
Method method = ucp.getClass().getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);

method.invoke(ucp, url);
} catch (Exception exception) {
exception.printStackTrace();
throw new IllegalStateException(exception.getMessage(), exception);
}
}
 
但是此时仍然会报错：

java.lang.reflect.InaccessibleObjectException: Unable to make field final jdk.internal.loader.URLClassPath jdk.internal.loader.ClassLoaders$AppClassLoader.ucp accessible: module java.base does not "opens jdk.internal.loader" to unnamed module @6b143ee9

 
因为在 Java 9+ 以上版本中不允许使用动态加载类，因为想要使用就需要加两个参数给虚拟机
此时的解决办法是：

java --add-opens java.base/jdk.internal.loader=ALL-UNNAMED --add-opens jdk.zipfs/jdk.nio.zipfs=ALL-UNNAMED -jar xx.jar
 
即可

————————————————
版权声明：本文为CSDN博主「Fighting_Boss_Hao」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/Fighting_Boss/article/details/91043555