URLClassLoader and accessibility of package-private methods

Question

I have a class `Formula`, located in package `javaapplication4`, which I load with a URLClassLoader. However, when I call it from another class `Test1`, located in the same package, I can't access its methods that have a default access modifier (I can access public methods). I get the following exception: java.lang.IllegalAccessException: Class javaapplication4.Test1 can not access a member of class javaapplication4.Formula with modifiers "" How can I access package-private methods of a class loaded at runtime from the same package? I suppose it is a problem with using a different class loader, but not sure why (I have set the parent of the URLClassLoader). SSCCE reproducing the issue (Windows paths) - I suppose the issue is in the `loadClass` method: ``` public class Test1 { private static final Path TEMP_PATH = Paths.get("C:/temp/"); public static void main(String[] args) throws Exception { String thisPackage = Test1.class.getPackage().getName(); String className = thisPackage + ".F…

Accepted Answer

A class at runtime is identified by both its fully qualified name and its ClassLoader.

For example, when you test two `Class<T>` objects for equality, if they have the same canonical name but were loaded from different ClassLoaders, they won't be equal.

For two classes to belong to the same package (and in turn being able to access package-private methods), they need to be loaded from the same ClassLoader too, which is not the case here. In fact `Test1` is loaded by the system classloader, while the Formula is loaded by the URLClassLoader created inside `loadClass()`.

If you specify a parent loader for your URLClassLoader in order to make it load `Test1`, still two different loaders are used (you can check it by asserting loaders equality).

I don't think you can make the `Formula` class loaded by the same `Test1` ClassLoader (you'd have to use a well-known path and put it on the CLASSPATH), but I found a way to do the opposite: loading another instance of `Test1` in the ClassLoader used for loading the formula. This is the layout in pseudocode:

```
class Test1 {

public static void main(String... args) {
    loadClass(formula);
  }

static void loadClass(location) {
    ClassLoader loader = new ClassLoader();
    Class formula = loader.load(location);
    Class test1 = loader.load(Test1);
    // ...
    Method compute = test1.getMethod("compute");
    compute.invoke(test1, formula);
  }

static void compute(formula) {
    print formula;
  }
}
```

Here is the pastebin. A couple of notes: I specifed a `null` parent for the URLClassLoader to avoid the issue listed above, and I manipulated strings to achieve the purpose - but don't know how robust this approach can be in other deployment scenarios. Also, the URLCLassLoader I used only searches in two directories to find class definitions, not all the entries listed in the CLASSPATH

URLClassLoader and accessibility of package-private methods

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