The missing “framework level” access modifier

问题:

Here's the scenario. As a creator of publicly licensed, open source APIs, my group has created a Java-based web user interface framework (so what else is new?). To keep things nice and organized as one should in Java, we have used packages with naming convention org.mygroup.myframework.x, with the x being things like components, validators, converters, utilities, and so on (again, what else is new?).

Now, somewhere in class org.mygroup.myframework.foo.Bar is a method void doStuff() that I need to perform logic specific to my framework, and I need to be able to call it from a few other places in my framework, for example org.mygroup.myframework.far.Boo. Given that Boo is neither a subclass of Bar nor in the exact same package, the method doStuff() must be declared public to be callable by Boo.

However, my framework exists as a tool to allow other developers to create simpler more elegant R.I.A.s for their clients. But if com.yourcompany.yourapplication.YourComponent calls doStuff(), it could have unexpected and undesirable consequences. I would prefer that this never be allowed to happen. Note that Bar contains other methods that are genuinely public.

In an ivory tower world, we would re-write the Java language and insert a tokenized analogue to default access, that would allow any class in a package structure of our choice to access my method, maybe looking similar to:

[org.mygroup.myframework.*] void doStuff() { .... }

where the wildcard would mean any class whose package begins with org.mygroup.myframework can call, but no one else.

Given that this world does not exist, what other good options might we have?

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Note that this is motivated by a real-life scenario; names have been changed to protect the guilty. There exists a real framework where peppered throughout its Javadoc one will find public methods commented as "THIS METHOD IS INTERNAL TO MYFRAMEWORK AND NOT PART OF ITS PUBLIC API. DO NOT CALL!!!!!!" A little research shows these methods are called from elsewhere within the framework.

In truth, I am a developer using the framework in question. Although our application is deployed and is a success, my team experienced so many challenges that we want to convince our bosses to never use this framework again. We want to do this in a well thought out presentation of the poor design decisions made by the framework's developers, and not just as a rant. This issue would be one (of several) of our points, but we just can't put a finger on how we might have done it differently. There has already been some lively discussion here at my workplace, so I wondered what the rest of the world would think.

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Update: No offense to the two answerers so far, but I think you've missed the mark, or I didn't express it well. Either way allow me to try to illuminate things. Put as simply as I can, how should the framework's developers have refactored the following. Note this is a really rough example.

package org.mygroup.myframework.foo;
public class Bar {
     /** Adds a Bar component to application UI */
     public boolean addComponentHTML() {
         // Code that adds the HTML for a Bar component to a UI screen
         // returns true if successful
         // I need users of my framework to be able to call this method, so
         // they can actually add a Bar component to their application's UI
     }

     /** Not really public, do not call */
     public void doStuff() {
         // Code that performs internal logic to my framework
         // If other users call it, Really Bad Things could happen!
         // But I need it to be public so org.mygroup.myframework.far.Boo can call
     }
}

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Another update: So I just learned that C# has the "internal" access modifier. So perhaps a better way to have phrased this question might have been, "How to simulate/ emulate internal access in Java?" Nevertheless, I am not in search of new answers. Our boss ultimately agreed with the concerns mentioned above

回答1:

You get closest to the answer when you mention the documentation problem. The real issue isn't that you can't "protect" your internal methods; rather, it is that the internal methods pollute your documentation and introduce the risk that a client module may call an internal method by mistake.

Of course, even if you did have fine grained permissions, you still aren't going to be able to prevent a client module from calling internal methods---the jvm doesn't protect against reflection based calls to private methods anyway.

The approach I use is to define an interface for each problematic class, and have the class implement it. The interface can be documented solely in terms of client modules, while the implementing class can provide what internal documentation you desire. You don't even have to include the implementation javadoc in your distribution bundle if you don't want to, but either way the boundary is clearly demarcated.

As long as you ensure that at runtime only one implementation is loaded per documentation-interface, a modern jvm will guarantee you don't suffer any performance penalty for using it; and, you can load harness/stub versions during testing for an added bonus.

回答2:

The only idea that I can think in order to supply this missing "Framework level access modifier" is CDI and a better design. If you have to use a method from very different classes and packages in various (but few) situations THERE WILL BE certainly a way to redesign those classes in order to make those methods "private" and inacessible.

回答3:

There is no support in Java language for such kind of access level (you would like something like "internal" with namespace). You can only restrict access to package level (or the known inheritance public-protected-private model).

From my experience, you can use Eclipse convention: create a package called "internal" that all class hierarchy (including sub-packages) of this package will be considered as non-API code and could be changed anytime with no guarantee for your users. In that non-API code, use public methods whenever you like. Since it is only a convention and it is not enforced by the JVM or Java compiler, you cannot prevent users from using the code, but at least let them know that these classes were not meant to be used by 3rd parties.

By the way, in Eclipse platform source code, there is a complex plugin model that enforces you not to use internal code of other plugins by implementing custom class loader for each plugin that prevents loading classes that should be "internal" in these plugins.

回答4:

Interfaces and dynamic proxies are sometimes used to make sure you only expose methods that you do want to expose.

However that comes at a fairly hefty performance cost, if your methods are called very often.

Using the @Deprecated annotation might also be an option, although it won't stop external users invoking your "framework private" methods, they can't say they hadn't been warned.

In general I don't think you should worry about your users deliberately shooting themselves in the foot too much, so long as you made it clear to them that they shouldn't use something.