(可怕的标题,我知道)

解释这个的最好方法是,我需要加载一些将在运行时从URLClassLoader加载的类,这些类引用了已经由System ClassLoader加载的类的实例,并且它们无法重新加载,因为其他一些问题.

此外,我愿意更改我的代码,以便我可以使用System ClassLoader从类路径中的jar加载类,但我无法做到这一点.

解决方法:

不是.每个类加载器都将引导类加载器作为祖先,并且尝试加载已经在类加载器中定义的类只会导致使用祖先的版本.这是为了防止重新定义java.lang.Object和其他内在函数.

从javadoc for ClassLoader：

The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. The virtual machine’s built-in class loader, called the “bootstrap class loader”, does not itself have a parent but may serve as the parent of a ClassLoader instance.

您可以定义一个自定义类加载器,它暴露defineClass而不会被findClass调用,并避免在findClass中发生的委托,但是当parent.findClass(我不会依赖于所有JVM上的预期的defineClass(className,bytes)) className)存在.