java - 自定义类加载器来限制对类的访问

Question

我的应用程序有这个自定义类加载器:

JarByteClassloader loader = new JarByteClassloader(jar);
Class c = loader.loadClass(classToLoad);
Thread.currentThread().setContextClassLoader(loader);
JarEntryObject jarEntry = (JarEntryObject) c.newInstance();

我想问是否可以限制自定义类加载器加载的实例对特定类的访问？

因此，例如，System.getProperty() 或 System.getenv() 应该失败。通过什么方式可以做到这一点？

Answer 1

虽然对于这种情况，SecurityManager 可能是最合适的(尽管自 Java 17 起它已被弃用并标记为删除而无需替换)，但实际上您可以通过类加载器限制对其他类的访问。

这可以通过重写类加载器的 loadClass 方法来完成。
有必要了解类加载器的委托(delegate)模型来限制它:

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.

loadClass方法的作用如下:

Loads the class with the specified binary name. The default implementation of this method searches for classes in the following order:

• Invoke findLoadedClass(String) to check if the class has already been loaded.

• Invoke the loadClass method on the parent class loader. If the parent is null the class loader built-in to the virtual machine is used, instead.

• Invoke the findClass(String) method to find the class.

https://docs.oracle.com/javase/8/docs/api/java/lang/ClassLoader.html

因此，如果我们重写findClass，则仅当父类加载器无法加载/查找该类时才会调用我们的代码。但我们想要拦截所有加载的类，因此我们需要重写 loadClass :

@Override
public Class<?> loadClass(String name) throws ClassNotFoundException {
    synchronized (getClassLoadingLock(name)) {
        if (allowDelegateLoadingOf(name)) {
            System.out.println("Delegate loading of " + name + " to parent class loader.");
            return super.loadClass(name);
        }

        System.out.println("Prohibit class from delegating: " + name);
        return findClass(name);
    }
}

在这种情况下，allowDelegateLoadingOf 只是一个简单的 boolean 方法，用于确定是否应委托(delegate)类并因此加载或不加载。这可以通过简单地根据白名单或黑名单包/类列表检查类名来完成。