Kineolyan / jpms-visibility

jpms-visibility

Prototyping on exposing a public facade to library users UserService while dealing internal with another interface InternalService.

This proves the possibility of an annotation-based approach to extract the internal interface.

The SDK only has to expose the internal interface and the annotation.
See for example that the getter is not available in LibService#createApplication.

The backdoor BackdoorGetter is capable of invoking the marked method in this project because the module explicitly gives access to UserServiceImpl. Otherwise, as the method is private, it is not possible to use it.

module com.kineolyan.jpms.lib {
    // ... 
    // The following line is very important
    opens com.kineolyan.jpms.lib.api to com.kineolyan.jpms.sdk;
}

Would the method be public, we could also restrict the access of the class. This is demonstrated by yet another module _ hlib_. Its HiddenUserServiceImpl is only exposed via IntfOnlyService and not visible from app.

Without the opens ..., running MainApplication generates the following exception.

Using public com.kineolyan.jpms.hlib.internal.OtherInternalServiceImpl com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl.getSecret() to access the inner service
Exception in thread "main" java.lang.reflect.InaccessibleObjectException: Unable to make public com.kineolyan.jpms.hlib.internal.OtherInternalServiceImpl com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl.getSecret() accessible: module com.kineolyan.jpms.hlib does not "opens com.kineolyan.jpms.hlib.api" to module com.kineolyan.jpms.sdk

The core module can use the backdoor to access the internal service and build its Calculator.

The final app module can build an Application using the exposed public API and gets compile errors when trying to access the inner interfaces and classes.

See the attempts of MainApplication#tryingToHackAndAccessService to access the inner classes:

Testing service with public visibility
Using private com.kineolyan.jpms.lib.internal.InternalServiceImpl com.kineolyan.jpms.lib.api.UserServiceImpl.getService() to access the inner service
Using service com.kineolyan.jpms.lib.api.UserServiceImpl
Creating service com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl
Using public com.kineolyan.jpms.hlib.internal.OtherInternalServiceImpl com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl.getSecret() to access the inner service
Service  = 13
Calculator  = 3
Can access the annotation: com.kineolyan.jpms.sdk.internal.intf.Backdoor
Found 1 annotated methods
Cannot access private com.kineolyan.jpms.lib.internal.InternalServiceImpl com.kineolyan.jpms.lib.api.UserServiceImpl.getService()

--------

Testing service with hidden visibility
Service  = -7
Calculator  = 98
Can access the annotation: com.kineolyan.jpms.sdk.internal.intf.Backdoor
Found 1 annotated methods
Cannot access public com.kineolyan.jpms.hlib.internal.OtherInternalServiceImpl com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl.getSecret()

• we must copy-paste the explicit name of the annotation as it is not accessible
• trying to give access is not possible and would result in exception, without an explicit ad-hoc VM option
• it would require something like the following --add-opens com.kineolyan.jpms.lib/com.kineolyan.jpms.lib.api=com.kineolyan.jpms.app to give the access to the magical class, a task even harder with unnamed modules. Note that the previous opens only work for the first service, the opens not being present for the second.

Testing service with public visibility
Using private com.kineolyan.jpms.lib.internal.InternalServiceImpl com.kineolyan.jpms.lib.api.UserServiceImpl.getService() to access the inner service
Using service com.kineolyan.jpms.lib.api.UserServiceImpl
Creating service com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl
Using public com.kineolyan.jpms.hlib.internal.OtherInternalServiceImpl com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl.getSecret() to access the inner service
Service  = 13
Calculator  = 3
Can access the annotation: com.kineolyan.jpms.sdk.internal.intf.Backdoor
Found 1 annotated methods
Giving access to private com.kineolyan.jpms.lib.internal.InternalServiceImpl com.kineolyan.jpms.lib.api.UserServiceImpl.getService()
Accessing inner object com.kineolyan.jpms.lib.internal.InternalServiceImpl@5010be6

--------

Testing service with hidden visibility
Service  = -7
Calculator  = 98
Can access the annotation: com.kineolyan.jpms.sdk.internal.intf.Backdoor
Found 1 annotated methods
Cannot access public com.kineolyan.jpms.hlib.internal.OtherInternalServiceImpl com.kineolyan.jpms.hlib.api.HiddenUserServiceImpl.getSecret()